Category: link to climate change

blogs that relate P&P or invasive plants to climate change; or discuss assisted migration; CO2 storage; etc.

Is this a way to overcome difficulties detecting invasive pathogens? Is APHIS applying these ideas?

SOD-infected rhododendron in a nursery; photo by Jennifer Parke, ODF

A group of scientists (See Khusnitdinova et al., 2026; full reference at the end of this blog.) contend that landscape interfaces—e.g., crop–forest edges, riparian zones, abandoned agricultural fields and orchards, and nursery–wildland transitions—are active zones of pathogen exchange. Biological and abiotic vectors collectively move pathogens from crops to wild plants, and vice versa. These exchanges create conditions speed up the evolution of pathogen aggressiveness and dispersal traits and promote the selection of generalist pathogen lineages capable of infecting both cultivated and wild hosts. In this way, crop-natural ecotones become not just passive transition zones but centers of adaptation.

The stronger or novel pathogens don’t stay in the specific local area; they are spread by a variety of human activities. Establishing large monocultures of crops and simplifying biological diversity at the landscape level boost inoculum production, limit host genetic diversity, and diminish natural regulation. Pathogens present in irrigation water can be spread during floods. Improperly sanitized green waste and compost can harbor viable oomycete propagules. Foot traffic and heavy equipment can move contaminated soil. Movement of infested plants for planting can transport the disease to a different continent. One example cited by Khusnitdinova et al. (2026) is the spread of numerous Phytophthora spp. from nurseries to forests and shrublands. A second example is rapid ʻōhiʻa death. They say it demonstrates that 1) a combination of human movement, forestry activities, and animal vectors can enable rapid local and landscape-scale spread; and 2) management measures (biobarriers, access control, restriction of animal movements, and phytosanitary inspection of planting material) can curtail that spread.

Meanwhile, the changing climate is causing shifts in the latitudinal and elevational distribution of plants and their associates; changing reproduction rates and latent periods; altering ranges and connectivity; and affecting disease incidence and severity. The direction is not always predictable; while drought or heat might reduce fungal and oomycete epidemics, the same conditions increase host stress and so might worsen disease outcomes.

Plant health scientists can use these concentrated geographic areas to focus plant disease surveillance. By integrating molecular and genomic tools with remote sensing and Geographic Information System (GIS)-based monitoring, plant health agencies can more quickly detect newly emerging diseases and implement effective action to counter the threat. 

However, Khusnitdinova et al. (2026) warn that surveillance employing these technological advances can reduce the risk that a pathogen will “spill over” from an anthropogenic to a natural ecosystem or vice versa only if pertinent sectors are transformed. Yes, they need resources: funding, staff, facilities. Also required is unification – or at least coordination. Khusnitdinova et al. (2026) advocate abandoning the compartmentalization that currently separatesforest health studies from invasive-plant and infectious-disease ecology studies. Instead, agencies should consider managed and natural systems together. They should conduct joint surveillance programs, share data standards, and coordinate management of the transition zones. In other words, apply a “One Health” landscape-based approach to the entire landscape.

Khusnitdinova et al. (2026) add that implementing such combined surveillance programs is especially vital in biodiversity-rich regions which have limited monitoring capacity. Might I suggest Hawai’i? 

ohia trees killed by ROD; photo by J.B. Friday, UH

Other facts that challenge traditional phytosanitary practices

Khusnitdinova et al. (2026) provide strong evidence that pathogens change – sometimes quickly. Is the current regulatory system sufficiently flexible and agile to effectively address these developments?

First, pathogens’ host range is not fixed. Instead, it is a trait that changes quickly under the influence of alterations in effector repertoires, plant immunity genes, and environmental conditions (including those driven by human actions). Even small genetic changes—such as mutations, gene losses or gains, or horizontal gene transfers—can enable pathogens to infect new hosts or weaken previous infection barriers. They suggest that plant pathogens with broad host ranges, e.g., Phytophthora cinnamomi, can easily move between hosts in agricultural plantings, ornamental landscapes, and semi-natural vegetation within a relatively small region. Such frequent spillovers maintain inoculum in landscape mosaics and complicating eradication or containment efforts.

Khusnitdinova et al. (2026) note that host-range expansions have especially long-term consequence in forest ecosystems, where loss of a single tree species can change understory makeup, light and moisture patterns, related fungi and invertebrate communities, and ultimately, landscape diversity and function. They cite chestnut blight and sudden oak death in North America and ash dieback in Europe as examples.

In addition, Khusnitdinova et al. (2026) maintain that genetic recombination is now recognized as a fundamental driver of innovation in plant pathogen populations. Table 2 of their publication lists pathogens exhibiting well-documented and experimentally confirmed cases of recombination, hybridization, or other forms of genome exchange. Forest-related examples include several Phytophthora hybrids and the ash decline fungus, Hymenoscyphus fraxineus.

Phytophthora dieback in Western Australia

Khusnitdinova et al. (2026) add their voices to a growing chorus decrying a global forest health crisis. They say that repeated pathogen introductions—often via trade in plants and wood—have shifted many temperate and boreal forests into states characterized by higher tree mortality, increased dominance of opportunistic or disturbance-adapted species, and reduced functional diversity. These changes lead to reduced resistance [defined as the capacity to limit damage during a new outbreak] and resilience [defined as the speed and trajectory of post-disturbance regeneration and ecosystem reorganization]. They note that increasing tree species diversity is one of the few management interventions that succeeds in strengthening both forest resistance and resilience to pathogens—by decreasing host density for specialist pathogens and reducing continuous “fuel” for epidemics.

One step toward improving scientific understanding on the scale they advocate, in their view, is the European Holistic Management of Emerging Forest Pests and Diseases (HOMED) effort. HOMED combines plant pathology, forest ecology, and biosecurity. The emphasis is on early detection, risk assessment, and management of human-mediated pathways, incl plant trade and nursery systems. The initiative aims to limit pathogen establishment and spread while strengthening forest resistance and resilience under global change. Participants also try to provide practical solutions for stakeholders to manage emerging native and non-native pests and pathogens threatening European trees not only in forests, but also in nurseries, urban and rural areas.

USDA Secretary Brooke Rollins

I am inspired by the proposals in Khusnitdinova et al. (2026). In hopes that USDA will explore how to implement them, I presented a poster presentation at the annual USDA Research Forum on Invasive Species. In that poster I suggested that these ideas complement USDA Secretary Rollins’ Memorandum on departmental research priorities. The need for research to clarify scientific puzzles is particularly acute regarding tree-killing pathogens nematodes, etc.

I suggested prioritizing research on the following issues:

  • Setting up intensive monitoring programs targetting the agriculture/natural system interfaces, as recommended by Khusnitdinova et al. (2025). These authors describe useful technologies in molecular diagnostics, genomic surveillance, environmental DNA, and remote sensing to detect fungi, oomycetes, rusts, bacteria, and viruses. Kantor et al. (2025) define techniques applicable for nematodes.
  • Rapid analysis of potentially invasive species and their pathways of entry revealed by “early warning” systems [e.g., APHIS’ “PestLens” website; “door knocker” introductions; academic studies; and “unimportant” species introduced to the U.S. (e.g., Leptosillia pistaciae in California)].  
  • Exploring ways (in addition to those suggested by Khusnitdinova et al. 2025) to shorten the time lag between introduction of a pathogen and its detection.
  • Incorporating into risk analyses information from sentinel garden program. Fund expansion of data collection and analysis to address asymptomatic plants, sampling techniques, and seasonality, as outlined by Drs. Eliana Torres Bedoya and Enrico Bonello (at the 2025 USDA Research Forum) and Raffa et al. (2023).

Over a somewhat longer-term, I suggested that research address these topics:   

  • Find techniques to speed up determination of disease causal agents – which often remain obscure for years or decades. The International Plant Protection Convention (IPPC) link requires countries to name the causal agent before regulating disease hosts and vectors.
  • Determine which components of a “systems approach” are most effective against each type of pathogen – fungi, oomycetes, rusts, bacteria, viruses, nematodes, etc.
  • With state counterparts, explore ways to better curtail domestic spread of organisms once they have established in the United States.
  • Integrate socio-economic drivers of pest introductions into studies. E.g., why do some organisms suddenly spread to numerous countries over a period of a few years?
  • Greatly expand efforts (in house and by collaborators) to breed trees resistant to established and newly detected pathogens.
  • Increase research supporting biocontrol.

As I have frequently complained in the past, the international phytosanitary system has failed to protect Earth’s forests and other natural ecosystems from non-native plant pests (or invasive plants). This failure has been documented by Weed, Ayres, and Hicke (2013), Fei et al. (2019), Quirion et al. (2021) for North America; and Gougherty (2023), Wu (2023), Sitzia et al. (2021), Martinac et al. (2025) and Khusnitdinova et al. (2025) from a global perspective.

Challenges:

  • Most microorganisms are unknown to science – “unknown unknowns”.
  • Scientists usually cannot predict the impact of known micro-organisms on new hosts under novel environmental conditions.
  • The World Trade Organization’s SPS Agreement and the International Plant Protection Convention (IPPC) demand unachievable levels of specificity re: a potential pest’s impact.
  • Most tree-killing pathogens are detected after they have entered the forest.
  • Agencies assign a low priority to protecting natural ecosystems from bioinvasion.
  • Resources (funds, staffing, etc.) are unreliable for agencies carrying out the full range of efforts, from assessing various risks to restoring pest-resistant trees to the forest.

SOURCES

Fei, S., R.S. Morin, C.M. Oswalt, & A.M. 2019. Biomass losses resulting from insect & disease invasions in United States forests

Gougherty, A.V. (2023) Emerging tree diseases are accumulating rapidly in the native & non-native ranges of Holarctic trees. NeoBiota 87: 143–160. https://doi.org/10.3897/neobiota.87.103525

Kantor, C., Teixeira, M., Kantor, M., and Gleason, C. 2025. Tiny Invaders, Big Trouble: Emerging Nematode Threats in the United States. Phytopathology 2025   115:587-595  https://doi.org/10.1094/PHYTO-09.-24-0290-IA

Khusnitdinova, M., V. Kostyukov, G. Nizamdinova, A. Pozharskiy, Y. Kydyrbayev and D. Gritsenko. 2026. Cross-Ecosystem Transmission of Pathogens from Crops to Natural Vegetation. Forests 2026, 17, 76

Martinac, M-L., F. Ningre, A. Dowkiw, N.Le Goff, B. Marcais. 2025.  High host density favour ash dieback Preprint  Plant Pathology

Quirion BR, Domke GM, Walters BF, Lovett GM, Fargione JE, Greenwood L, Serbesoff-King K, Randall JM & Fei S (2021) Insect and Disease Disturbances Correlate With Reduced Carbon Sequestration in Forests of the Contiguous United States. Front. For. Glob. Change 4:716582.  [Volume 4 | Article 716582] doi: 10.3389/ffgc.2021.716582

Sitzia, T., T. Campagnaro, G. Brundu, M. Faccoli, A. Santini & B.L. Webber. 2021.  Routledge Handbook of Biosecurity & invasive species. Chapter 7. Forest Ecosystems. ISBN 9780367763213

Weed, A.S., M.P. Ayers, J.A. Hicke. 2013. Consequences of CC for biotic disturbances in North American forests. Ecological Monographs, 83(4), 2013, pp. 441–470

Wu, H. 2023/24. Modelling Tree Mortality Caused by Ash Dieback in a Changing World: A Complexity-based Approach MSc/MPhil Dissertation Submitted August 12, 2024. School of Geography & the Enviro, Oxford University

Posted by Faith Campbell

We welcome comments that supplement or correct factual information, suggest new approaches, or promote thoughtful consideration. We post comments that disagree with us — but not those we judge to be not civil or inflammatory.

For a detailed discussion of the policies and practices that have allowed these pests to enter and spread – and that do not promote effective restoration strategies – review the Fading Forests report at http://treeimprovement.utk.edu/FadingForests.htm

Or https://fadingforests.org/

Tree-killing pests can undermine conservation programs on tropical islands

an aye-aye – one of the highly endangered lemurs dependent on moist tropical forests of Madagascar; photo by Andrew Ciscel via Wikimedia

A forthcoming study examines two important issues: interactions of pathogens’ spread and changing climate, and invasive species threats to tropical islands’ forests.

Underwood et al. (in press) analyzed how an introduced vascular wilt pathogen — Leptographium calophylli – is likely to affect a tree endemic to Madagascar’s already threatened mid-level elevation humid & subhumid forests, Calophyllum paniculatum (sorry; I can find no photographs of the tree species).

Climate change is expected to cause substantial shifts in temperature and precipitation patterns on the island. These temperature and moisture regimes in turn govern pathogen sporulation, infection efficiency, and survival. They also affect the host’s levels of stress and defenses. The direction of change is not certain, however. In some cases, warming and other changes to the climate might facilitate a pathogen’s spread, allowing it to track shifts in the host’s range and expand into previously unoccupied refugia. In other cases, these changes might erect environmental thresholds that limit the pathogen’s survival and spread, thereby creating spatial refugia for the host.

diademed lemur, courtesy of Animalia

Environmental change increases the area of suitable landscape, that is, it weakens climatic barriers to establishment. Continued anthropogenic movement of some vector (biological or not) generates multiple introductory events over time. As a result, the likelihood of a successful establishment also increases, even if the probability per individual introduction is unchanged. Underwood et al. say that invasion outcomes thus become increasingly dependent on propagule pressure.

On many other tropical islands the threat from climate change is exacerbated by deforestation. On Madagascar, clearing driven by slash-and-burn agriculture and fuelwood harvesting has already reduced natural forest cover to less than 10% of its original extent. [For more on this topic, see e.g., Mittermeier et al. (2011).]  Underwood et al. cite a determination by the ForestAtRisk model that humid forest in Madagascar could be almost entirely lost by 2100.

Loss of Madagascar’s forest has global implications. The island is one of 36 global biodiversity hotspots for both flora and fauna (e.g., lemurs). Its flora exceeds 12,000 plant species, of which 83% are endemic. In this case, the host tree species — Calophyllum paniculatum — is already considered vulnerable by the International Union for the Conservation of Nature (IUCN). Thus it is of global importance to understand the relative importance of several threats so that conservations can adopt the most effective countermeasures.

While they do not say so explicitly, it appears that Underwood et al. worry that too few of the conservationists active on Madagascar are paying attention to the possible impact of introduced pathogens. They note that pathogen-driven mortality of dominant or functionally unique trees can rapidly alter community structure and ecosystem function, potentially triggering local extinctions and cascading ecological consequences. For example, if an infection removes mature trees, their loss reduces fruit and nectar availability and so depresses populations of dependent wildlife. The trees’ death also diminishes above-ground carbon stocks and litter inputs. In combination, these impacts can shift community composition toward disturbance-tolerant states and heighten susceptibility at forest margins. These changes difficult to reverse once thresholds crossed.

red-bellied lemur in Ranomafana National Park – site of the first detection of Leptographium calphylli; via Flickr

This threat is not hypothetical. Since 2016 mature C. paniculatum at one site – a National Park – have been dying from a vascular wilt disease caused by a species in the Leptographium genus, probably Leptographium (formerly Verticillium) calophylli. While the species hasnot yet officially been recorded in Madagascar, it is established on neighboring Indian Ocean islands and across much of mainland Africa. Various species in the fungal genus are known to cause disease in other woody hosts. Underwood et al. suggest it was probably transported to Madagascar on infected wood, although they present no data.

Inside forests, Leptographium spp. are vectored by bark beetles in the Cryphalus genus. At least 25 Cryphalus species occur on the African Continent; some are vectoring disease on Seychelles and Mauritius.

The analysis by Underwood et al. indicates that future climatic conditions are likely to worsen the Leptographium calophylli infection over coming decades. The causal agent is likely to retain two-thirds of its current probable distribution and expand into previously uninhabited regions. The suitable habitat is expected to stretch across the entire north-south humid belt – the entire distribution of the host tree. Underwood et al. (in press) say it is even possible that the pathogen might remain in the forest, subsisting on other hosts, after C. paniculatum becomes functionally extinct across its range.

Meanwhile, that host – Calophyllum paniculatum – is projected to experience severe range shifts, with an overall net contraction across all climate change scenarios. It is forecast up to 67% of its current area by 2100. This range contraction will be compounded by fragmentation and dispersal limitation resulting from from deforestation. The refugia will be few and geographically isolated by late in the 21st century.

red-veined swallowtail; photographed in Ranomafana National Park by Frank Vassen, via Wikimedia

Are conservationists considering the implications of Leptographium calophylli’s probable persistence? Underwood et al. imply they are not; they say the impact of this and related pathogens on Madagascar & nearby islands is “still an unknown to the conservation community”. They urge their colleagues to conduct a set of research actions to identify, monitor, & limit the fungus’ spread – – and thereby improve the effectiveness of conservation efforts.

  1. Host range & other targets: determine whether L. calophylli infects other taxa in Madagascar – especially the endemic species and genera. They suggest systematic field sampling of multiple species across sites within the core probable range of L. calophylli. A trained pathologists should be consulted to officially identify the pathogen.
  • Determine the spread phase of the pathogen. They suggest random sampling of species & sites within & outside of the fungus’ probable distribution, mapping the possible start point & dispersal patterns, including both anthropogenic & natural spread routes.
  • Assess applicability of IPBES tools & suggestions for invasive species management to the case of a fatal pathogen in the context of tropical islands’ characteristics. How might Madagascar implement prevention, early detection & rapid response systems?

I applaud Underwood et al. for trying to alert the conservation community active on tropical islands to the simultaneous impacts of multiple global & regional change drivers on vulnerable species. Probably other host-pathogen systems are experiencing the same diverging trajectories that might intensify their biodiversity loss, particularly when compounded by deforestation.

SOURCES

Mittermeier, R.A., E.E. Louis Jr., M. Richardson, C. Schwitzer, O. Langrand, A.B. Rylands. 2010. Lemurs of Madagascar. Conservation International, Arlington, USA. ISBN 9781934151235

Underwood, E.L., K.A Brown, A. Ronnfeldt, M. Mulligan, N. Walford, R. Allgayer. In press. Climate change facilitates fungal pathogen expansion while driving endemic host range contractions in a tropical biodiversity hotspot. Research Square.

Posted by Faith Campbell

We welcome comments that supplement or correct factual information, suggest new approaches, or promote thoughtful consideration. We post comments that disagree with us — but not those we judge to be not civil or inflammatory.

For a detailed discussion of the policies and practices that have allowed these pests to enter and spread – and that do not promote effective restoration strategies – review the Fading Forests report at http://treeimprovement.utk.edu/FadingForests.htm

Or

https://fadingforests.org

Act Now!!! Administration Proposes to “0 out” key USFS Programs

The Trump Administration’s budget for Fiscal Year 2026 [which begins at the end of September 2025] proposes to eliminate funding for nearly all  USFS research & Forest Health Protection.

Proposed Cuts to USFS Research: Timber the Sole Aim

 In a letter from Office of Management and Budget (OMB) to Senate Appropriations Committee Chair Susan Collins (R-Maine, Director Russell Vought says the Administration wants to manage National forests “for their intended purpose of producing timber” and that the research and development program “is out of step with the practical needs of forest management for timber production.” The Administration proposes to eliminate funding for USFS research projects other than the small portion covering Forest Inventory and Analysis.

I understand that the USFS Chief told various NGOs that his job is to run the National Forest System, increase timber production by 40%, and do nothing else.

This single aim conflicts with the 1897 legislation founding and authorizing the USFS. It also violates provisions of subsequent legislation such as the Multiple-Use Sustained-Yield Act of 1960 and the National Forest Management Act of 1976. It also departs from long-standing US Forest Service policy – which is the intention. 

The “intended purpose” of establishing “forest reserves” [which were later renamed National forests] has never been solely for timber production. The “Organic Act” of 1897 provided that any new forest reserves would have to meet the criteria of forest protection, watershed protection, and timber production.

Specifically, the ORGANIC ACT OF 1897 [PUBLIC–No.2.] says:

“[All public lands heretofore designated and reserved by the President of the US under the provisions of the Act [of] March 3rd 1891, the orders for which shall be and remains in full force and effect, unsuspended and unrevoked, and all public lands that may hereafter be set aside as public forest reserves under said act, [these were the “forest reserves,”predecessors of “National Forests]” shall be as far as practicable controlled and administered in accordance with the following provisions:

“No public forest reservation shall be established, except to improve and protect the forest within the reservation, or for the purpose of securing favorable conditions of water flows, and to furnish a continuous supply of timber for the use and necessities of [US] citizens; but it is not the purpose or intent of these provisions, or of the Act providing for such reservations, to authorize the inclusion therein of lands more valuable for the mineral therein, or for agricultural purposes, than for forest purposes.”

The Department of the Interior, which then managed these forest reserves, promptly issued implementing regulations. The regulations stated that the “object” of forest reservations was:

“2. Public forest reservations are established to protect and improve the forests for the purpose of securing a permanent supply of timber for the people and insuring conditions favorable to continuous water flow.”

Therefore, I think the Administration has exaggerated the emphasis on timber production by calling it “the” intended purpose of the original establishment of National forests. The Administration has also chosen to ignore subsequent legislation, such as the Multiple-Use Sustained-Yield Act of 1960 and the National Forest Management Act of 1976.

Sec. 13 of the NFMA limits the sale of timber from each national forest to a quantity equal to or less than a quantity which can be removed from such forest annually in perpetuity on a sustained-yield basis. This limit might be exceeded under certain circumstances, but such excess must still be consistent with the multiple-use management objectives of the land management plan. Further, Sec. 14 requires public input into any decision to raise timber allowances.

During his period as Chief (1905 – 1910), Gifford Pinchot invented and applied the concept of “conservation” of natural resources. As a result “wise use” became accepted as the national goal.

Culminating more than a century of legislation and informed policy, the mission of the USDA Forest Service is to “sustain the health, diversity, and productivity of the nation’s forests and grasslands to meet the needs of present and future generations.”

Proposed Cuts to State, Private, and Tribal Forests

The budget also cuts $303 million from the State, Private, and Tribal Forests program. (I understand this zeroes out the entire program). The OMB Director alleges that the program has been “plagued by oversight issues, including allegation of impropriety by both the Agency and State governments.” I understand that this would eliminate the cooperative projects managed by the Forest Health Protection program, too.

Implications for Non-native Insects and Pathogens

Remember that USFS’s research and development program is intended to improve forest managers’ understanding of ecosystems, including human interactions and influences, thereby enabling improvements to the health and use of our Nation’s forests and grasslands. Most importantly to me, this program provides foundational knowledge needed to develop effective programs to prevent, suppress, mitigate, and eradicate the approximately 500 non-native insects and pathogens that are killing America’s trees.

The Forest Health Program provides technical and financial assistance to the states and other forest-management partners to carry out projects (designed based on the above research) intended to prevent, suppress, mitigate, and eradicate those non-native insects and pathogens. The program’s work on non-federal lands is crucial because introduced pests usually start their incursions near cities that receive imports (often transported in crates, pallets, or imported plants).

Eliminating either or both programs will allow these pests to cause even more damage to forest resources – including timber.

Both supporting research and on-the-ground management must address pest threats across all U.S. forests, including the more than 69% that are located on lands managed by others than the USFS. Already, the 15 most damaging of these pests threaten destruction of 41% of forest biomass in the “lower 48” states. This is a rate similar in magnitude to that attributed to fire (Fei et al. 2019). It is ironic that the Administration considers the fire threat to be so severe that it has proposed restructuring the government’s fire management structure.

I remind you that the existing USFS R&D budget allocates less than 1% of the total appropriation to studying a few of the dozens of highly damaging non-native pests. I have argued that this program should be expanded, not eliminated. Adequate funding might allow the USFS to design successful pest-management programs for additional pests (as suggested by Coleman et al.).

As a new international report (FAO 2025) notes, genetic resources underpin forests’ resilience, adaptability, and productivity. Funding shortfalls already undercut efforts to breed trees able to thrive despite introduced pests and climate change (the latter threat is still real, although the Administration disregards it). I have frequently urged the Congress to increase funding for USFS programs – which are sponsored primarily by the National Forest System and State, Private, and Tribal, although some are under the R&D program.

Please ask your Member of Congress and Senators to oppose these proposed cuts. Ask them to support continued funding for both USFS R&D and its State, Private, and Tribal Programs targetting non-native insects and pathogens. America’s forests provide resources to all Americans – well beyond only timber production and they deserve protection.

Contacting your Representative and Senators is particularly important if they serve on the Appropriations committees.

House Appropriations Committee members:

Republicans: AL: Robert Aderholt, Dale Strong; AR: Steve Womack; AZ: Juan Ciscomani; CA: Ken Calvert, David Valadao, Norma Torres; FL: Mario Diaz-Balart, John Rutherford, Scott Franklin; GA: Andrew Clyde; ID: Michael Simpson; IA: Ashley Hinson; KY: Harold Rogers; LA: Julia Letlow; MD: Andy Harris; MI: John Moolenaar; MO: Mark Alford; MS: Michael Guest;  MT: Ryan Zinke; NC: Chuck Edwards; NV: Mark Amodei; NY: Nick LaLota; OH: David Joyce; OK: Tom Cole, Stephanie Bice; PA: Guy Reschenthaler TX: John Carter, Chuck Fleishmann, Tony Gonzales, Michael Cloud, Jake Ellzey; UT: Celeste Maloy; VA: Ben Cline; WA: Dan Newhouse; WV: Riley Moore

Democrats: CA: Pete Aguilar, Josh Harder, Mike Levin; CT: Rosa DeLauro; FL: Debbie Wasserman Schultz, Lois Frankel; GA: Sanford Bishop; HI: Ed Case IL: Mike Quigley, Lauren Underwood; IN: Frank Mrvan; MD: Steny Hoyer, Glenn Ivey; ME: Chellie Pingree; MN: Betty McCollum; NJ: Bonnie Watson Coleman NY: Grace Meng, Adriano Espaillat, Joseph Morelle; NV: Susie Lee;  OH: Marcy Kaptur;  PA: Madeleine Dean; SC: James Clyburn;  TX: Henry Cuellar, Veronica Escobar; WA: Marie Gluesenkamp Perez; WI: Mark Pocan

Senate Appropriations Committee members:

Republicans: AK: Lisa Murkowski; AL: Katie Britt; AR: John Boozman (AR); KS: Jerry Moran; KY: Mitch McConnell; LA: John Kennedy; ME: Susan Collins; MS: Cindy Hyde-Smith; ND: John Hoeven; NE: Deb Fischer; OK: Markwayne Mullin; SC: Lindsey Graham; SD: Mike Rounds TN: Bill Hagerty; WV: Shelley Moore Capito;

Democrats: CT: Chris Murphy; DE: Chris Coons; GA: Jon Ossof; HI: Brian Schatz; IL: Richard Durbin; MD: Chris van Hollen; MI: Gary Peters; NH: Jeanne Shaheen; NM: Martin Heinrich; NY: Kirsten Gillibrand; OR: Jeff Merkley; RI: Jack Reed; WA: Patty Murray; WI: Tammy Baldwin

SOURCES

Coleman, T.W, A.D. Graves, B.W. Oblinger, R.W. Flowers, J.J. Jacobs, B.D. Moltzan, S.S. Stephens, R.J. Rabaglia. 2023. Evaluating a decade (2011–2020) of integrated forest pest management in the United States. Journal of Integrated Pest Management, (2023) 14(1): 23; 1–17

FAO. 2025. The Second Report on the State of the World’s Forest Genetic Resources. FAO Commission on Genetic Resources for Food and Agriculture Assessments, 2025. Rome.

Fei, S., R.S. Morin, C.M. Oswalt, and A.M. 2019. Biomass losses resulting from insect and disease invasions in United States forests. PNAS August 27, 2019. Vol. 116 No. 35  17371–17376

Posted by Faith Campbell

We welcome comments that supplement or correct factual information, suggest new approaches, or promote thoughtful consideration. We post comments that disagree with us — but not those we judge to be not civil or inflammatory.

For a detailed discussion of the policies and practices that have allowed these pests to enter and spread – and that do not promote effective restoration strategies – review the Fading Forests report at  https://treeimprovement.tennessee.edu/

or

www.fadingforests.org

Does a long-established non-native insect threaten America’s cedars?

Guest blog by Kristy M. McAndrew, Department of Forestry, Mississippi State University

Virginia juniper (Juniperus virginiana) preforming its ecological role: succession in a field (in Ohio); photo by Greg Hume via Wikimedia

Spread of non-native species is a facet of global change that is an unintended consequence of the modern global trade network. Despite efforts put in place to limit such transport, such as International Standards for Phytosanitary Measures (ISPMs), unintentional spread of species continues, and thus, an important part of forest health research and management includes non-native monitoring and control efforts. As other aspects of global change, such as climate and weather patterns, shift, the dynamics between native landscapes and introduced pests may unexpectedly shift as well. For example, increased climate stress of tree hosts may weaken tree defenses, allowing species that historically have not been pests of concern to reach pest status.

Japanese cedar longhorned beetle (Callidiellum rufipenne; JCLB) is a wood boring beetle in the longhorned beetle family, Cerambycidae. The adults are reddish brown in color, and relatively small for longhorned beetles, at only around 1 cm in length. Japanese cedar longhorned beetle has a long history of establishing outside of its native range but has largely been considered a non-issue. It has long been disregarded as a pest because it feeds primarily on dead or dying trees in both the native and invaded ranges. However, there are more examples of these beetles feeding on stressed, but alive, trees in North America. Therefore, I think it is an important insect to take a closer look at.

Life cycle

These beetles have a one-year life cycle, most of which is spent inside a host tree. Adults emerge from host trees in the early spring and seek out other adults to mate with and trees to lay eggs on. Eggs are laid on thin parts of bark or in bark crevices, and when the eggs hatch larvae chew beneath the bark where they feed on the phloem until they have completed larval development. Once larvae are fully developed, they burrow further into the tree, into the xylem tissue, where they pupate, overwinter as fully formed adults, and continue the cycle the following spring.

Native range

The native range of JCLB is eastern Asia. It is common throughout the Korean peninsula and across the islands of Japan. It is also considered native to Eastern China and Russia. Within the native range JCLB is found primarily on dead and/or dying trees and is thus considered a secondary pest. On dead trees they can be found on any diameter of dead woody material, but on declining trees they will likely be in the small diameter branches and stems.

Arborvitae (Thuja occidentalis); photo by James St. John via Flickr

Invasion history

Japanese cedar longhorned beetle was first documented as an invasive pest in the early 1900s in France, and since then has established in at least fifteen countries (Clément 2023). Most of these countries are in Europe, but the United States and Argentina also have established populations. As with most woodboring insects, the invasion pathway is believed to have been wood packaging material being transported via global trade routes. Between 1914 and 2022 it was intercepted over 700 times (reviewed by KM). Since the implementation of ISPM No. 15, only six interceptions have been reported up to 2022 (USDA APHIS data reviewed by K.M.). [For Faith’s view on the regulation of wood packaging, see Fading Forests II and III (links provided at the end of this blog) and earlier blogs posted here under the category “wood packaging”. esp. 1 from 2015].

A USDA risk assessment completed in 2000 suggested other possible pathways of introduction, including balled nursery stock, green logs, and pruned branches (USDA APHIS and Forest Service, 2000). 

In terms of establishments in North America, JCLB was first detected in natural forests in North Carolina in 1997. It was soon discovered in Connecticut in 1998; in neighboring New York in 1999; and in Massachusetts, New Jersey, and Rhode Island in 2000. It was quickly discovered feeding on live arborvitae (also called northern white cedar; Thuja occidentalis) in these invaded regions. JCLB has since been found in Pennsylvania (in 2010) and Maryland (in 2011). It is important to note that it is not clear when this species truly established, because of its previously discussed long history of being intercepted in ports of entry.

Most introduced populations of JCLB are found in either dead hosts or in the damaged/dead limbs of live hosts. In Buenos Aires, for example, storm-damaged trees with broken limbs are often where beetles are collected (Turienzo 2007). In the United States, eastern red cedar (Juniperus virginiana) and common juniper (Juniperus communis ) are the two native species most commonly affected, but so far there is no evidence of live trees of these species being infested (Maier 2007). However, a growing concern in the United States is that JCLB has been documented on live trees – particularly in urban environments. These trees are typically arborvitae, and they are typically stressed urban trees that have been overwatered and often show signs and symptoms of other health issues.

Host breadth

The host breadth of JCLB encompasses much of the family Cupressaceae. Maier (2007) identified 19 potential hosts from the literature and research, with the vast majority (14) of the hosts being Cupressaceae species, which is indicative of JCLB being a relative generalist, especially when considering species in the cypress family. This is important, because there are over 130 species within Cupressaceae worldwide that could be suitable hosts for JCLB, meaning host will not be a limiting factor in many invasion scenarios for this insect. Most often trees infested by JCLB need to be either stressed or dead, which limits suitability to an extent. However, many landscape trees are inherently stressed, whether it be from a history of roots being balled and wrapped in burlap, being planted in less than ideal scenarios, or being overwatered.

A few reports from research in Japan record JCLB feeding on plants in Pinaceae, primarily Pinus and Abies species. One article reports use of Larix kaempferi; another documented JCLB on the Taxaceae species, Taxus cuspidata. North American pine (Pinus spp.) and fir (Abies spp.) species have not been tested, but if they are revealed as suitable that would increase the availability of hosts in North America significantly.

In southern New England at least nine species have been confirmed as suitable, all of which are in the family Cupressaceae. Native and abundant junipers, such as Juniperus virginiana, appear to be highly suitable hosts. Additional host testing would be beneficial – especially Cupressaceae species that are either threatened or have a limited range. Within the United States there are a total of 28 native Cupressaceae species. Thus the suitable range (in terms of hosts) covers the entire Eastern half of North America through central Texas, most of the Pacific Coast, and widespread but spotty/disjunct areas throughout the Intermountain West and High Plains regions.

Atlantic white cedar swamp (Chamaecyparis thyoides) in Brendan Byrne State Forest, New Jersey; photo by Famartin via Wikimedia

Suitability

Tools such as environmental niche models can give helpful estimates of suitability. For species that are typically secondary pests, such as JCLB, it can be difficult to obtain non-biased data with good coverage to make reliable predictions. Preliminary research (unpublished) has been completed to estimate suitable habitat with limited occurrence records from the native range. Despite limited occurrences, models performed well and estimated moderate to high suitability in most temperate regions globally. These preliminary models are still being optimized by working with collaborators within the native range of JCLB to increase the number of occurrences. It is also important to note that these models are only accounting for climate data. Host data was not included, but Cupressaceae species are abundant globally, and therefore host availability is not likely a limiting factor for JCLB in establishing in regions.

Importance of monitoring species

While JCLB is still mostly limited to dead, dying trees, many of the species it may affect in the Eastern United States are already of heightened conservation concern. Wetland Cupressaceae, such as bald cypress (Taxodium distichum) and Atlantic White Cedar (Chamaecyparis thyoides), are valuable in terms of ecosystem services they provide in coastal, and inland, wetlands. These wetlands are encountering heightened stress in the form of increasing saltwater intrusion, increased storm strength, and changing landscapes, all of which may predispose trees to insect attack. Japanese cedar longhorned beetle has been successfully reared out of logs of Atlantic White Cedar, but thankfully has not been documented on live trees of this species (Maier 2009)[Ma1] . Bald cypress has not yet been tested for suitability. It is unknown if the stressors these trees are facing and will continue to face will impact JCLB’s ability to infest these landscapes, or if they will remain restricted to dead trees in these coastal forests. Regardless, given JCLB already has an established foothold in the Eastern United States, it is important to better understand the potential impacts of this insect.

First steps to understanding those impacts include 1) better documenting the host range in the regions and 2) determining the climate that may support the species. Hopefully we can continue research in these areas to best manage this non-native pest.

Much of the research conducted on JCLB in North America took place almost 20 years ago (Maier 2007, 2009), so updated sampling has potential to provide a wealth of information regarding spread rate, suitable climate, and establishment patterns.

bald cypress(Taxodium distichum); photo by Kej605 via Wikimedia; it is unknown whether this species is vulnerable to the Japanese cedar longhorned beetle

Sources

Clément F. 2023. Le point sur la distribution en France et en Europe de Callidiellum rufipenne (Motschulsky, 1861)(Coleoptera, Cerambycidae, Cerambycinae, Callidiini). Le Coléoptériste. 26(3):188–203.

Maier CT. 2007. Distribution and Hosts of Callidiellum rufipenne (Coleoptera: Cerambycidae), an Asian Cedar Borer Established in the Eastern United States. JOURNAL OF ECONOMIC ENTOMOLOGY. 100(4).

Maier CT. 2009. Distributional and host records of Cerambycidae (Coleoptera) associated with Cupressaceae in New England, New York, and New Jersey. Proceedings of the Entomological Society of Washington. 111(2):438–453. https://doi.org/10.4289/0013-8797-111.2.438

Turienzo P. 2007. New records and emergence period of Callidiellum rufipenne (Motschulsky, 1860) [Coleoptera:Cerambycidae: Cerambycinae: Callidiini] in Argentina. Boletín de Sanidad Vegetal, Plagas. 33:341–349.

United States Department of Agriculture Animal and Plant Health Inspection Service and Forest Service 2000. (Pasek, J.E., H.H. Burdsall, J.F. Cavey, A. Eglitis, R.A. Haack, D.A. Haugen, M.I. Haverty, C.S. Hodges, D.R. Kucera, J.D. Lattin, W.J. Mattson, D.J. Nowak, J.G. O’Brien, R.L. Orr, R.A. Sequeira, E.B. Smalley, B.M. Tkacz, W.W. Wallner) Pest Risk Assessment for Importation of Solid Wood Packing Materials into the United States. USDA APHIS and Forest Service. August 2000.

Posted by Faith Campbell

We welcome comments that supplement or correct factual information, suggest new approaches, or promote thoughtful consideration. We post comments that disagree with us — but not those we judge to be not civil or inflammatory.

For a detailed discussion of the policies and practices that have allowed these pests to enter and spread – and that do not promote effective restoration strategies – review the Fading Forests report at  https://treeimprovement.tennessee.edu/

or

www.fadingforests.org

 [Ma1]another old source

“Ecological memory” determines a forest’s resilience — implications of bioinvasion to New Zealand’s unique flora

kauri dieback

Scientists in New Zealand are saying explicitly that a forest’s unique mixture of species matters when considering the future. This mixture is the result of the forest’s evolutionary history. Losing members of the biological community reduces the forest’s ability to respond to current and future stresses – its resilience.

New Zealand’s forests are part of the broader legacy of the ancient supercontinent of Gondwanaland – the island nation’s plants have close relatives in South America, the Pacific Ocean islands, and Australia. Still, these forests are unique: 80% of New Zealand’s plant species are endemic. The forests are also species-rich. The warm temperate evergreen rain forests of the North Island are home to at least 66 woody plant species that can reach that reach heights above six meters (Simpkins et al. 2024).

These forests have been severely changed by human activity. In just ~ 750 years people have cut down approximately 80% of the original forest cover! (Simpkins et al. 2024) Of the eight million hectares of surviving native forest, a little over five million hectares is managed for the conservation of biodiversity, heritage, and recreation.  Another 2 million hectares are plantations of non-native species.

sites in New Zealand where pine plantations are “wilding”

All these forests are challenged by introduced mammals – from European deer to Australian possums. Climate change is expected to cause further disturbance, both directly (through e.g., drought, extreme weather) and indirectly (e.g., by facilitating weed invasion and shifting fire regimes) (Simpkins et al. 2024).

Pathogen threats are also common threats to the native trees of the Pacific’s biologically unique island systems. For example, Ceratocystis lukuohia and C. huliohia (rapid ‘ōhi‘a death, or ROD). The latter is killing ‘ōhi‘a (Metrosideros polymorpha) on the Hawaiian Islands. More than 40% of native plant species in Western Australia are susceptible to Phytophthora cinnamomi. Here I focus on two pathogens, kauri dieback and myrtle rust, now ravaging New Zealand’s native flora. No landscape-level treatment is available for either pathogen.

When considering this suite of challenges, Simpkins et al. focus on these two pathogens’ probable impact on forest carbon sequestration. They worry in particular about erosion of the forests’ resilience due to loss of “ecological memory” – the life-history traits of the species (e.g., soil seed banks) and the structures left behind after individual disturbances.

one of the largest remaining kauri trees, “Tane Mahuta”, in Waipoua Kauri Forest; photo by F.T. Campbell

Kauri Dieback

The causal agent of Kauri dieback, Phytophthora agathidicida, is a soil-borne pathogen that spreads slowly in the absence of animal or human vectors. The disease affects a single species, Agathis australis (kauri, Araucariaceae). However, kauri is a long-lived, large tree that is a significant carbon sink. It probably modifies local soil conditions, nutrient and water cycles, and associated vegetation. Also, kauri has immense cultural significance.

Simpkins et al. note that kauri dieback threatens stand-level loss of A. australis – that is, local extinctions. In the absence of disturbance Kauri trees can grow to awe-inspiring size. In the 19th Century, before widespread logging, some were measured at 20 meters or more in circumference. Consequently, kauri dieback might cause a decline in aboveground live carbon storage of up to 55%. This loss would occur over a period of hundreds of years, not immediately.

Huge kauri are not likely to be replaced by other long-lived emergent conifers (based on an analysis of one species, Dacrydium cupressinum). Instead, kauri are probably going to be replaced by late-successional angiosperms. The authors discuss the ecological implications for levels of carbon storage and proportions of trees composed of Myrtaceae – exacerbating damage caused by myrtle rust (see below).

The expectation of Simpkins et al. that kauri will suffer at least local extinctions is based on an assumption that no kauri trees are resistant to the pathogen. Fortunately, this might not be true: different Agathis populations show various levels of tolerance to Agathis dieback. Identification and promotion of some levels of resistance could enable A. australis to retain a diminished presence in the landscape.

However, Lantham, et al. make clear that containing kauri dieback remains “challenging,” despite its discovery nearly 20 years ago (in 2006). Scientists and land managers have little information on the distribution of symptomatic trees, much less of the pathogen itself. This means they don’t know where infection foci are or how fast the disease is spreading.

As is often true, the pathogen is probably present in a stand for years, possibly a decade or more, before symptoms are noticed. This means that the current reliance on public reports of diseased trees, or targetting surveillance on easy-to-access sites (e.g., park entrances and along existing track networks), or at highly impacted areas readily identified through aerial methods, fails to detect early stages of infection. Indeed, it seems probable that P. agathidicida had been present in New Zealand’s ecosystems for decades before its formal identification.

The Waipoua forest is one of the largest areas of forest with old kauri stands in the country. A new analysis of aerial surveys done between 1950 and 2019, shows how the forest is changing. The number of dead trees increased more than four-fold and the number of unhealthy-looking trees increased 16-fold over these 70 years. Kauri dieback is now widespread in this forest, especially in areas near human activities like clearing for pasture or planting commercial pine plantations).

Lantham et al. have developed a model which they believe will help identify areas of higher risk so as to prioritize surveillance and inform responses. These could delimit the disease front and help implement quarantines or other measures aimed at limiting the spread of P. agathidicida to uninfected neighboring sites.

I hope New Zealand devotes sufficient resources to expand surveillance and management to levels commensurate with the threat to this ecologically and culturally important tree species.

Leptospermum scoparia; photo by Brian Gatwicke via Flickr

Myrtle Rust

Myrtle rust is a wind-borne disease that affecting numerous species in the Myrtaceae, including some of the dominant early successional species (e.g., Leptospermum spp.). Simpkins et al. expect that myrtle rust might hasten the decline of two such tree species (L. scoparium and Kunzea ericoides). However, these trees’ small size and rapid replacement by other species during succession minimizes the effect of their demise on carbon storage.

Because I am concerned about the irreplaceable loss to biodiversity, I note that Simpkins et al. also feared immediate threats to some trees in the host Myrtaceae family, specifically highly susceptible species such as Leptospermum bullata.

As I reported in a recent blog, a second group of scientists (McCarthy et al.) explored the threat from myrtle rust more broadly. Austropuccinia psidii has spread through Myrtaceae-dominated forests of the Pacific islands for about 20 years.

Trees in the vulnerable plant family, Myrtaceae, are second in importance (based on density and cover) in New Zealand’s forests. Successional shrub communities dominated by the two species named above, Kunzea ericoides and Leptospermum scoparium, are widespread in the northern and central regions of the North Island and in northeastern and interior parts of the South Island. These regions’ vulnerability is exacerbated by the area’s climate, which is highly suitable for A. psidii infection (Simpkins et al. 2024).

McCarthy et al. concluded that if Leptospermum scoparium and Kunzea ericoides prove to be vulnerable to myrtle rust, their loss would cause considerable change in stand-level functional composition across these large areas. They probably would be replaced by non-native shrubs, which are already common on the islands. Any resulting forest will differ from that formed via Leptospermeae succession.

These authors also worry that the risk to native ecosystems would increase if more virulent strains of the myrtle rust pathogen were introduced or evolved. They note that A. psidii is known to have many strains and that these strains attack different host species.

SOURCES

Latham, M.C., A. Lustig, N.M. Williams, A. McDonald, T. Patuawa, J. Chetham, S. Johnson, A. Carrington, W. Wood, and D.P. Anderson. 2025.  Design of risk-based surveillance to demonstrate absence of Phytophthora agathidicida in New Zealand kauri forests. Biol. Invasions (2025) 27, no. 26

McCarthy, J.K., S.J. Richardson, I. Jo, S.K. Wiser, T.A. Easdale, J.D. Shepherd, P.J. Bellingham. 2024. A Functional Assessment of Community Vulnerability to the Loss of Myrtaceae from Myrtle Rust. Diversity and Distributions, https://doi.org/10.1111/ddi.13928

Simpkins, C.E., P.J. Bellingham, K. Reihana, J.M.R. Brock, G.L.W. Perry. 2024. Evaluating the effects of two newly emerging plant pathogens on North Aotearoa-New Zealand forests using an individual-based model.  Ecological Modelling, www.elsevier.com/locate/ecolmodel

Posted by Faith Campbell

We welcome comments that supplement or correct factual information, suggest new approaches, or promote thoughtful consideration. We post comments that disagree with us — but not those we judge to be not civil or inflammatory.

For a detailed discussion of the policies and practices that have allowed these pests to enter and spread – and that do not promote effective restoration strategies – review the Fading Forests report at  https://treeimprovement.tennessee.edu/

or

www.fadingforests.org

Coming to an Ecosystem Near You??

Xylotrechus chinensis; EPPO photograph

Europe has been invaded by two insect species that North Americans should be watching out for. First, a Cerambycid, the wasp-mimicking tiger longicorn beetle, Xylotrechus chinensis. And second,the Buprestid cypress jewel beetle, Lamprodila festiva. We should also ensure that none of the other 500+ beetles introduced to Europe poses a threat to our trees. These are summarized in a 2024 paper by Bunescu et al.

Tiger Longicorn Beetle

This beetle is native to eastern Asia. It feeds on and kills mulberry trees (Moraceae: Morus spp.). It might also attack apple and pear trees and grapevines – Asian sources report these as hosts. The status of grapevines has been questioned by a Spanish experiment, in which artificial inoculations failed. I have seen no further information about the vulnerability of apple (Malus spp.) and pear (Pyrus spp.) (Saarto i Monteyu, Costa Ribeu, and Savin 2021)

In Europe, the pest threatens mulberry trees which are commonly planted for shade and ornamentation, especially in southern France, Spain and Greece (Saarto i Monteyu, Costa Ribeu, and Savin 2021). For example, there are more than 20,000 mulberry trees in Athens (EFSA 2021). The trees’ abundance contributes to spread of any associated pests, the level of damage caused by falling branches, and the expense of tree removal. Economic damages are those typically associated with wood-borer invasions of urban areas. That is, the cost of tree removals, loss of shade and amenity values, and increased risk of injury from falling branches.

We Americans should be concerned, too. Wild red mulberry (Morus rubra) occupies much of the eastern United States, from southern New England west to southeastern Minnesota, then south along the eastern edge of the Great Plains to central Texas, and east to southern Florida. It is also found in Bermuda. It grows primarily in flood plains and low moist hillsides. . Presumably it would also be attacked by Xylotrechus chinensis, although I don’t know whether anyone has tested this. As a native tree, red mulberry plays a role in natural ecosystems, including wildlife food supplies. Thus, America would see even more significant losses if Xylotrechus chinensis were to establish.

Morus rubra in Fairfax County, Virginia; photo by Fmartin via Wikipedia

Red mulberry is already declining in parts of its central range, possibly due to a bacterial disease. The effects and extent of this disease have not been investigated thoroughly.

Apples and pears are important crops across North America; the farm-gate value is estimated at $3.2 billon.

Introductions of the beetle to Spain, France, and Greece might have resulted from inadequately-treated wood packaging or other wood products. Detections of the species in wood imports were reported in Germany in 2007 and 2017 (Saarto i Monteyu, Costa Ribeu, and Savin 2021). The U.S. has also intercepted X. chinensis at least once, at the port of Philadelphia, in 2011 (EFSA 2021).

These detections have raised questions to which no-one yet has answers. First, can X. chinensis develop in cut logs? The European Food Safety Agency concluded that it can (EFSA 2021). Second, one detection involved a shipment of wooden items made from birch (Betula spp.) and willow (Salix spp). It is not yet clear whether these taxa are also hosts (EFSA 2021). (The wood species were not specified in the case of the other interceptions.) I have blogged often about how “leaky” the wood packaging pathway has been; to see these blogs, scroll below the “archives” section of the webpage, then click on the category “wood packaging”.

European scientists believe X. chinensis might also be transported in shipments of plants for planting. However, the beetle prefers to oviposit on large trees. This pathway is less viable for the United States since USDA APHIS allows imports of mulberries (Morus) and pears (Pyrus) only from Canada. Apple trees (Malus spp.), however, may be imported from France – which hosts an introduced population of X. chinensis – and other European countries.  

Detection of any invasion by X. chinensis will pose the usual difficulties associated with woodborers. In some European cities, hundreds or even a thousand trees were infested before the outbreak was detected (EFSA 2021).

I am concerned that the Europeans appear to have been slow to respond to the threat from Xylotrechus chinensis. After several outbreaks were discovered in Greece, France, and Spain in 2017 and 2018, the European and Mediterranean Plant Protection Organization (EPPO) added X. chinensis to its Alert List. This action requires member states (which are not limited to European Union members) to report new outbreaks and inform about efforts to either stop or eradicate them (Saarto i Monteyu, Costa Ribeu, and Savin 2021).

Shortly afterwards the European Union Commission requested the European Food Safety Agency (EFSA) to conduct a risk assessment. This analysis was completed in 2021. (It contains lots of photos of the insect and its damage.) The analysis concluded that Xylotrechus chinensis could probably infest most areas in the Union and cause significant damage. The species meets the criteria for designation as a quarantine pest in the Union. However, as of December 2024, this action had not been taken. As a result, control measures for this species are not mandatory.

Introductions continue; an outbreak in Lombardy, Italy, was found in June 2023 (Sarto i Monteys, Savin, Torras i Tutusaus & Bedós i Balsach 2024).  European regulations – following IPPC standards – also are linked to named pests and known outbreak locations. Such restrictions almost guarantee that the pest will continue to spread from not-yet-detected outbreaks. (Decades ago, after the emerald ash borer invasion, Michigan’s State Plant Regulatory Official, Ken Rasher, noted that, to be effective, “slow the spread” efforts must apply to areas beyond the known limits of the pest’s range.) The EFSA risk assessment did suggest delimitation of buffer zones around known European outbreaks. I don’t know whether such zones have been set up.

The risk assessment also recommended [true?] improving detection of this insect by developing male pheromones as lures. These have not been acted on. Guidance on best timing for treatment [trunk injections of systemic insecticides] also appears to have been taken up by Greece but not by Spain (Sarto i Monteys, Savin, Torras i Tutusaus & Bedós i Balsach 2024).

These authors include more information about the Xylotrechus chinensis life cycle and trajectory of the invasion,. They note that climate change appears to be altering the insect’s phenology. Especially, the adult flight period is beginning earlier in the spring.

Lamprodila festiva; Udo Schmidt via flickr

Cypress jewel beetle

This second pest of concern is a buprestid that attacks trees in the Cupressaceae. Infested trees generally die within a few years.

In its native Mediterranean range, the beetle feeds on native Juniperus, Cupressus and Tetraclinis. In invaded urban landscapes of Europe it attacks primarily introduced Cupressaceae , particularly Thuja, Chamaecyparis, Platycladus, Callitris, and some hybrids (Cupressocyparis). It has also been recorded as damaging Sequoia sempervirens (Brunescu, et al., 2024). (Genera in bold are native to North America.)

Thuja occidentalis; photo by H. Zell via Wikimedia

White cedar, Thuja occidentalis is the focus of Brunescu, et al.’s article. It is native to eastern Canada and much of the north-central and northeastern United States. The European and Mediterranean Plant Protection Organization (EPPO) has identified eight species in the Lamprodila genus as important pests, (Brunescu et al. 2024) so the danger might be more widespread. The invasion of Europe is probably the result of adult flight or other short-range transport. The article does not suggest pathways that the species might exploit to cross oceans.

SOURCES

Bunescu, H., T. Florian, D. Dragan, A. Mara, I-B. Hulujan, X-D. Rau. 2024  The Cypress Jewel Beetle Lamprodila Festiva Linné, 1767 (Coleoptera: Buprestidae), an Invasive Major Pest of Thuja Occidentalis Linné in Romania Hop and Medicinal Plants, 2024 XXXII, No. 1-2, 2024.

Saarto i Monteyu V., A. Costa Ribeu. I. Savin. 2021a. The invasive longhorn beetle Xylotrechus chinensis, pest of mulberries, in Euro: Study on its local spread & efficacy of abamectin control Plos One January 29, 2021. https://doi.org/10.1371/journal.pone.0245527

Sarto i Monteys, V., I. Savin, G. Torras i Tutusaus & M. Bedós i Balsach. 2024b. New evidence on the spread in Catalonia of the invasive longhorn beetle, Xylotrechus chinensis, & the efficacy of abamectin control. Scientific Reports | (2024) 14:26754 | https://doi.org/10.1038/s41598-024-78265-x  www.nature.com/scientificreports/

Posted by Faith Campbell

We welcome comments that supplement or correct factual information, suggest new approaches, or promote thoughtful consideration. We post comments that disagree with us — but not those we judge to be not civil or inflammatory.

For a detailed discussion of the policies and practices that have allowed these pests to enter and spread – and that do not promote effective restoration strategies – review the Fading Forests report at  https://treeimprovement.tennessee.edu/

or

www.fadingforests.org

Again – analysts of changing forests leave out key factors

oak & beech seedlings; photo by F.T. Campbell

Yet again, studies focusing on issues of regeneration and mortality failing to consider all aspects.

Two studies focused on persistence of oak forests – a topic of great concern because of economic and ecological importance of oak-dominated forests. Since they dominate forests covering 78.5 million ha (51% of all forestland in the eastern United States) (Dey 2013), oaks shape stand structure and composition; their extensive crowns support many bird and arboreal mammal species; their acorns and leaf litter are the foundation of complex food webs; they live in symbiotic relationships with mycorrhizal fungi that enhance nutrient cycling and uptake within forest ecosystems. Deep roots prevent soil erosion. Oaks play a pivotal role in carbon sequestration (Khadka, Hong, and Bardhan 2024).

Until recently concern has focused on mortality of species in the red oak group (Section Lobatae). Now there is increasing concern about white oak (Quercus alba) mortality. Forest managers reported elevated mortality not just in resource-limited sites,e.g., those characterized by drought conditions, poor drainage, and soil nutrient deficiencies. Deaths are also occurring in higher-quality mesic sites, especially in forests with high stand density and advanced maturity stages. While white oaks go through a self-thinning phase – when dense stands of younger trees compete intensely for limited resources –it appears that some of the concern is focused on this stage (Khadka, Hong, and Bardhan 2024).

I think much of the concern is driven by economic rather than ecological considerations. None of oak species mentioned by Duana et al. (2024) is considered at risk by the authors of the recent conservation gap analysis (Beckman et al. 2019). (This is not surprising since presumably these species are sufficiently numerous to support commercial harvests). Furthermore, complaints about forest regeneration in the East are broader than oaks. A multi-author examination of the future of the northern forest projected decreases for four forest types = aspen-birch, elm-ash-cottonwood, oak-hickory, and spruce-fir. One type –maple-beech-birch – was expected to expand (Shifley and Moser 2016).

Regarding oaks specifically, Khadka, Hong, and Bardhan (2024) found that 30% of FIA plots in ten states composed primarily of white oak met their criteria for considering white oaks to be “declining”. However, higher mortality was limited to scattered areas (see map in Fig. 2B in the article). They suggested that contributing factors included higher elevation and distance from water in the north, intense competition in central regions, and drought stress in oak-hickory forests in the south. They also mentioned mature stands which are not replacing themselves in the southern region. Khadka, Hong, and Bardhan (2024) noted that oak decline complex is a factor in the southern region, and localized non-native insect pests (apparently spongy moth) in the northern region. (I will discuss both regeneration failures and the impacts of non-native pests below.) Still, these authors focus most attention to environmental stresses, e.g., droughts or water logging, poor soils, extreme weather events; and to human management, e.g., fire suppression, logging intensity, edge effects. They suggest strategies for mitigating these factors.

A second study, published by Duana et al. (2024), considered stocking levels of several species of oaks (Q. alba, Q. coccinea, Q. prinus, Q. rubra, and Q. velutina) but limited themselves to a large, temperate hardwood forest landscape in southeastern Ohio. Their purpose was to evaluate the efficacy of two levels of silvicultural intervention in sustaining oaks and restraining maples over the long-term, defined as 150-years (to 2060).

red oak (Quercus rubra); photo by F.T. Campbell

Their model suggested that continuing “business as usual” management would result in oaks shrinking from 22.8% dominance in 2010 to 12% dominance in 2160. Many of the remaining oaks would be large — in the 70 cm DBH class. The undesired maples would rise from 23% of total relative dominance in 2010 to 58% in 2160. The maples grew to almost the same size as the oaks: 50–65 cm DBH. As a result of these developments, the maple basal area increase by more than five times. The basal area of early successional species, e.g., poplars and aspens, decreased from 25% dominance to 11% dominance by 2160. Shade-tolerant species like elms, hickories, beech, and hemlock were suppressed by more competitive maples, occupying 17% of the total dominance.

Under the more manipulative alternative management strategy, oaks’ relative dominance on private land would stay above 20% of total relative dominance; all ages and sizes would be present. Maples would hold steadier at 23% to 33%. Shade-tolerant species would also rise, reaching a quarter of relative dominance on private some site (private public lands).

Duana et al. (2024) explained the outcome of “business as usual” management on maples’ ability to thrive in shaded conditions while oak regeneration requires sunlight to reach the forest floor. Another factor is the prevalence of high-grading harvesting practices. These factors result in a significant absence of oak trees in the sapling and midstory sizes, reflecting challenges to both oak seedlings and saplings. In other words, despite the continued growth of mature overstory oaks, the trees cannot reproduce. As Duana et al. (2024) point out, these results are supported by other field-based studies — including ones I have blogged about. Duana et al. (2024) discuss barriers and incentives to private landowners adopting more active management.

However, as I pointed out above, many tree species are regenerating poorly, not just oaks. Indeed, none of the eastern species fulfilling Potter and Riitters’ (2022) criteria for species threatened by poor regeneration was an oak. See Table 2 in Potter and Riitters (2022).

American sycamore (Platanus  occidentalis) – one of the tree species not regenerating adequately; photo by F.T. Campbell

Hanberry et al. (2020) found that actual changes in forest species composition and density do not conform to expectations arising from three factors proposed as drivers: increased precipitation, increased white-tailed deer densities, and functional extinction of American chestnut. They found disappearance of frequent low-intensity fires to be determinative.  However, Hanberry et al. (2020) also do not mention invasive plants or non-native pests other than chestnut blight.

Here I review others’ discussion of browsing by overabundant deer and competition from non-native plants as factors widely recognized as impeding regeneration of canopy trees, including oaks.

Deer

There is widespread agreement that browsing by overabundant deer is a major cause of poor regeneration of deciduous forests, especially but not limited to oaks (Quercus species.). Sources cited in my previous blogs include most studies discussed at the 2023 Northern Hardwood research forum (USDA, FS 2023b Proceedings),  Spicer et al. (2023), Miller et al., and two studies based in either Ohio (the location of the study by Duana et al. [2024]) or neighboring Pennsylvania: Yaccuci et al. (2023) and Reed et al. 2023. Yacucci et al. reported that stem density of red (Q. rubra) and pin oaks (Q. palustris) was 13 times higher in canopy gaps located in areas with low densities of deer than in gaps in high-deer-density locations. In these gaps, oak saplings were growing into the subcanopy. Reed et al. said deer herbivory might be one of the most important drivers of forest composition and canopy structure over long time-scales.

Deer might be less important in New England. Stern et al. (2023), working in Vermont, focused on the importance of changing precipitation patterns in shifting numbers of red maple (Acer rubrum), sugar maple (Acer saccharum), American beech (Fagus grandifolia), and yellow birch (Betula alleghaniensis). Northern red oak was described as a common co-occurring dominant species in their plots, but was not discussed. In New Hampshire, Ducey et al. reported changing species composition as the forest ages but did not mention deer.

Some of these authors advocated wide-scale efforts to reduce deer populations in order to restore forest ecosystems. Yacucci et al. proposed enlisting those military posts that regularly cull deer into efforts to conserve and regenerate native plants. Otherwise, they say, the prognosis for regeneration is poor. Blossey et al. urged creation of a nation-wide lethal removal program.

Some of these studies indicated that additional biological entities were also important. Miller et al. stressed the role of invasive plants in suppressing forest regeneration in National parks from Virginia to Maine. Reed et al. focused on invading earthworms. One study – again, conducted in Ohio – Hovena et al. (2022), found that interactions between non-native shrubs and soil wetness overshadowed even the impact of deer herbivory on the species richness and abundance of seedlings.

Invasive Plants

FIA data indicate that 46% of forests in the eastern United States are invaded by alien plant species (Oswalt et al. 2016). Across the region, hundreds of non-native plant species are established in forests and woodlands. (See lists compiled by the Southeast Exotic Pest Plant Council, Mid-Atlantic Invasive Plant Council, Midwest Invasive Plants Network). Forests of the northern Midwest are among the most heavily invaded; in Ohio specifically, two studies found that more than 90% of FIA plots harbor at least one invasive plant species (Oswalt et al. [2016] and Kurtz (USDA NRS 311).

Many of these invaders are herbs, shrubs, or trees which can invade shaded environments. I remind you that a high proportion of these invasive plant species have been deliberately planted either directly in “natural” areas or in yards and gardens throughout the region.

Invasive plants can reduce native diversity, alter forest structure, suppress tree regeneration, alter nutrient cycling, and modify disturbance regimes (Miller et al. 2023).

Japanese stiltgrass (Microstegium vimineum) is widespread in forests of both Northeast (Oswalt et al. (2016) and Southeast. Stiltgrass invasions can suppress oak regeneration – at least as part of interactions with herbivore browsing and harvest history (Johnson et al. 2015).

Amur honeysuckle; via Flickr

Several non-native shrub and vine species are also widespread. For example, multiflora rose (Rosa multiflora) is the most frequently recorded invasive plant, present on 16.6% of surveyed plots in 39 states and five Canadian provinces. Again, the state with the highest proportion of plots invaded is Ohio – 85% (USDA Forest Service NRS-109). A study in central Ohio found that the presence of Amur honeysuckle (Lonicera mackii) had a stronger influence on tree species diversity than on the size or number of trees. Removing honeysuckle from heavily invaded areas promoted native tree growth (e.g., the height of tallest trees) and increased the tree canopy’s structural complexity for up to 10 years. Forest recovery began within two years of honeysuckle removal Fotis et al. (2022). (To access earlier blogs, visit www.nivemnic.us; scroll below “archives” to “categories”, click on “invasive plants.)

This impediment to forest regeneration is expected to get worse: non-native plant species are already more widely distributed than native species although the average invasive plant inhabits only about 50% of its expected range (Bradley, Early and Sorte 2015). From Virginia and West Virginia north to Maine, 80% of National Park units have experienced a significant increase in at least one trend measuring abundance of invasive plants in recent decades. In 10 parks (a quarter of all parks studied), total invasives increased significantly in two of three metrics (Miller et al. 2023).

Non-native Pests

Another set of biological factors affecting forest persistence and possibly regeneration is non-native pests that kill North American trees. I have complained that too few of the studies of regeneration discuss implications of these bioinvasions. So Khadka, Hong, and Bardhan (2024), Duana et al. (2024), and Hanberry et al. (2020) continue a tradition that I think is most unfortunate.

American elm in full glory; photo by F.T. Campbell

In Ohio specifically, Hovena et al. and Yacucci et al. did not mention loss of canopy elms, or ash, or the impending threat from beech leaf disease. All these trees are – or used to be – quite common in Ohio. More understandable, perhaps, is lack of attention to laurel wilt disease, which is just now at the state’s southern border. It might decimate an important native shrub, Lindera benzoin. American chestnut was also present in Ohio before its near disappearance following introduction of the chestnut blight fungus early in the 20th Century.

Another possibly damaging pest that has recently turned up in Ohio is the elm zigzag sawfly Aproceros leucopoda. This Asian insect was first detected in North America in 2020 in Ontario. It quickly became apparent that it was more widespread. The Ohio detection came in 2023 – too recent to be discussed by Hovena et al. or Yacucci et al. Its impact several elm species is currently unknown.

There are exceptions. Both Stern et al. (2023) and Ducey at al. (2023) reported robust growth rates of American beech (Fagus grandifolia) despite decades-long establishment of beech bark disease. DMF Neither mentioned beech leaf disease – to be fair, this bioinvader is just starting to appear in New England. Stern et al. (2023) did not discuss hemlock woolly adelgid although Eastern hemlock (Tsuga canadensis) is also a common co-occurring dominant species in their plots. Ducey et al. did anticipate pest-driven reversals of increased numbers of eastern hemlock (Tsuga canadensis) and of white ash (Fraxinus americana). Stern et al. (2023) also did not mention oak wilt, despite a vulnerable host — northern red oak — being a common co-dominant species in his study site in Vermont. To be fair, oak wilt is not yet established in New England, although it is in New York and in western Ontario.

The most complete discussion of non-native pests is by Payne and Peet, working in the Piedmont of North Carolina. They state that several “specialist” pathogens have caused loss of important tree species, resulting in drastic and long-lasting shifts in community dynamics. They mention elms and dogwoods plus impending insect-caused widespread mortality of ash.

flowering dogwood (Cornus florida); photo by F.T. Campbell

Miller et al. describe the impact of EAB on ash resources in the National parks and express concern that BLD will cause considerable damage to some units of the system.

I think the failure of scientists to integrate invasive species’ impacts into assessments of changes in forest tree composition will mean that recommendations for management will be – at best – incomplete; at worst – wrong.

SOURCES

Beckman, E., Meyer, A., Denvir, A., Gill, D., Man, G., Pivorunas, D., Shaw, K., and Westwood, M. (2019). Conservation Gap Analysis of Native U.S. Oaks. Lisle, IL: The Morton Arboretum.

Blossey. B., D. Hare, and D.M. Waller, 2024. Where have all the flowers gone? A call for federal leadership in deer management in the US. Front. Conserv. Sci. 5:1382132. doi: 10.3389/fcosc.2024.1382132

Bradley, B.A., R. Early and C. J. B. Sorte. 2015. Space to invade? Comparative range infilling and potential range of invasive and native plants. Global Ecology and Biogeography

Dey, D.C. 2013. Sustaining Oak Forests in Eastern North America: Regeneration and Recruitment, the Pillars of Sustainability. For. Sci. 60(5):926–942 October 2013. http://dx.doi.org/10.5849/forsci.13-114  

Duana, S., H.S. He, L.S. Pile Knapp, T.W. Bonnot, J.S. Fraser. 2024. Private land management is more important than public land in sustaining oaks in temperate forests in the eastern U.S. Journal of Environmental Management 352 (2024) 120013

Ducey, M.J, O.L. Fraser, M. Yamasaki, E.P. Belair, W.B. Leak. 2023. Eight decades of compositional change in a managed northern hardwood landscape. Forest Ecosystems 10 (2023) 100121

Fotis, A., Flower, C.E.; Atkins, J.W. Pinchot, C.C., Rodewald, A.D., Matthews, S. 2022. The short-term and long-term effects of honeysuckle removal on canopy structure and implications for urban forest management. Forest Ecology and Management. 517(6): 120251. 10 p. https://doi.org/10.1016/j.foreco.2022.120251

Hanberry, B.B., M.D. Abrams, M.A. Arthur & J.M. Varner. 2020. Reviewing Fire, Climate, Deer, & Foundation Spp as Drivers of Historically Open Oak & Pine Forests & Transition to Closed Forests. Front. For. Glob. Change 3:56. doi: 10.3389/ffgc.2020.00056

Hovena, B.M., K.S. Knight, V.E. Peters, and D.L Gorchov. 2022. Woody seedling community responses to deer herbivory, intro shrubs, and ash mortality depend on canopy competition and site wetness. Forest Ecology and Management. 523 (2022) 120488

Johnson, D.J., S.L. Flory, A. Shelton, C. Huebner and Keith Clay. 2015 Interactive effects of a non-native invasive grass Microstegium vimineum and herbivore exclusion on experimental tree regeneration under differing forest management. Journal of Applied Ecology 2015, 52, 210–219 doi: 10.1111/1365-2664.12356

Khadka, H.S. Hong,  S. Bardhan. 2024. Investigating the Spatial Pattern of White Oak (Q. alba L.) Mortality Using Ripley’s K Function across the Ten States of the eastern United States. Forests 2024, 15, 1809. https://doi.org/10.3390/f15101809

Miller, K.M., S.J. Perles, J.P. Schmit, E.R. Matthews, and M.R. Marshall. 2023. Overabundant deer and invasive plants drive widespread regeneration debt in eastern United States national parks. Ecological Applications. 2023;33:e2837. https://onlinelibrary.wiley.com/r/eap  Open Access

Oswalt, C.M., S. Fei, Q. Guo, B.V. Iannone III, S.N. Oswalt, B.C. Pijanowski, K.M. Potte. 2916. A subcontinental view of forest plant invasions. NeoBiota. 24: 49-54 http://www.srs.fs.usda.gov/pubs/48489

Payne, C.J. and R.K. Peet. 2023. Revisiting the model system for forest succession: Eighty years of resampling Piedmont forests reveals need for an improved suite of indicators of successional change. Ecological Indicators 154 (2023) 110679

Pinchot, C.C., A.A. Royo, J.S. Stanovick, S.E. Schlarbaum, A.M. Sharp, S.L. Anagnostakis. YEAR

Deer browse susceptibility limits c’nut restoration success in northern hardwood forests PUBLIC

Potter, K.M and Riitters, K. 2022. A National Multi-Scale Assessment of Regeneration Deficit as an Indicator of Potential Risk of Forest Genetic Variation Loss. Forests 2022, 13, 19.

https://doi.org/10.3390/f13010019.

Reed, S.P., D.R. Bronson, J.A. Forrester, L.M. Prudent, A.M. Yang, A.M. Yantes, P.B. Reich, and L.E. Frelich. 2023. Linked disturbance in the temperate forest: Earthworms, deer, and canopy gaps. Ecology. 2023;104:e4040. https://onlinelibrary.wiley.com/r/ecy

Shifley, S.R. and W.K. Moser, editors. 2016. Future Forests of the Northern United States

Simpson, A., and Eyler, M.C., 2018, First comprehensive list of non-native species established in three major regions of the United States: U.S. Geological Survey Open-File Report 2018-1156, 15 p., https://doi.org/10.3133/ofr20181156.

ISSN 2331-1258 (online)

Spicer, M.E., A.A. Royo, J.W. Wenzel, and W.P. Carson. 2023. Understory plant growth forms respond independently to combined natural and anthropogenic disturbances. Forest Ecology and Management 543 (2023) 12077

Stern, R.L., P.G. Schaberg, S.A. Rayback, C.F. Hansen, P.F. Murakami, G.J. Hawley. 2023.

Growth trends and environmental drivers of major tree species of the northern hardwood forest of eastern North America J. For. Res. (2023) 34:37–50 https://doi.org/10.1007/s11676-022-01553-7

Stout, S.L., A.T. Hille, and A.A. Royo. 2023. Science-Management Collaboration is Essential to Address Current and Future Forestry Challenges. IN United States Department of Agriculture. Forest Service. 2023. Proceedings of the First Biennial Northern Hardwood Conference 2021: Bridging Science and Management for the Future. Northern Research Station General Technical Report NRS-P-211 May 2023

United States Department of Agriculture, Forest Service. 2023a. Proceedings of the First Biennial Northern Hardwood Conference 2021: Bridging Science and Management for the Future. Northern Research Station General Technical Report NRS-P-211 May 2023

USDA Forest Service Northern Research Station Rooted in Research ISSUE 18 | SEPTEMBER 2023

Kurtz, C.M. 2023. An assessment of invasive plant species in northern U.S. forests. Res. Note NRS-311. http://doi.org/10.2737/NRS-RN-311

United States Department of Agriculture Forest Service General Technical Report NRS-109.  An Assessment of Invasive Plant Species Monitored by the Northern Research Station

Forest Inventory and Analysis Program, 2005 through 2010.

Yacucci, A.C., W.P. Carson, J.C. Martineau, C.D. Burns, B.P. Riley, A.A. Royo, T.P. Diggins, I.J. Renne. 2023. Native tree species prosper while exotics falter during gap-phase regeneration, but only where deer densities are near historical levels New Forests https://doi.org/10.1007/s11056-023-10022-w

Posted by Faith Campbell

We welcome comments that supplement or correct factual information, suggest new approaches, or promote thoughtful consideration. We post comments that disagree with us — but not those we judge to be not civil or inflammatory.

For a detailed discussion of the policies and practices that have allowed these pests to enter and spread – and that do not promote effective restoration strategies – review the Fading Forests report at  https://treeimprovement.tennessee.edu/

or

www.fadingforests.org

Hawaiian Efforts to Restore Threatened Trees

ʻŌhiʻa trees killed by ROD; photo by Richard Sniezko, USFS

Several Hawaiian tree species are at risk due to introduced forest pests. Two of the Islands’ most widespread species are among the at-risk taxa. Their continuing loss would expose watersheds on which human life and agriculture depend. Habitats for hundreds of other species – many endemic and already endangered – would lose their foundations. These trees also are of the greatest cultural importance to Native Hawaiians.

I am pleased to report that Hawaiian scientists and conservationists are trying to protect and restore them.

Other tree species enjoy less recognition … and efforts to protect them have struggled to obtain support.

1) koa (Acacia koa)

Koa is both a dominant canopy tree and the second-most abundant native tree species in Hawai`i in terms of areas covered. The species is endemic to the Hawaiian archipelago. Koa forests provide habitat for 30 of the islands’ remaining 35 native bird species, many of which are listed under the U.S. Endangered Species Act. Also dependent on koa forests are native plant and invertebrate species and the Islands’ only native terrestrial mammal, the Hawaiian hoary bat. Finally, koa forests protect watersheds, add nitrogen to degraded soils, and store carbon [Inman-Narahari et al.]

Koa forests once ranged from near sea level to above 7000 ft (2100 m) on both the wet and dry sides of all the large Hawaiian Islands. Conversion of forests to livestock grazing and row-crop agriculture has reduced koa’s range. Significant koa forests are now found on four islands – Hawai’i, Maui, O‘ahu, and Kauaʻi. More than 90% of the remaining koa forests occur on Hawai`i Island (the “Big Island) [Inman-Narahari et al.]

In addition to its fundamental environmental role, koa has immense cultural importance. Koa represents strength and the warrior spirit. The wood was used traditionally to make sea-going canoes. Now Koa is widely used for making musical instruments, especially guitars and ukuleles; furniture, surfboards, ornaments, and art [Inman-Narahari et al.]

Koa timber has the highest monetary value of any wood harvested on the Islands. However, supplies of commercial-quality trees are very limited (Dudley et al. 2020). Harvesting is entirely from old-growth forests on private land. [Inman-Narahari et al.]

Koa forests are under threat by a vascular wilt disease caused by Fusarium oxysporum f. sp. koae (FOXY). This disease can kill up to 90% of young trees and – sometimes — mature trees in native forests. The fungus is a soil-dwelling organism that spreads in soil and infects susceptible plants through the root system (Dudley et al. 2020).

Conservation and commercial considerations have converged to prompt efforts to breed koa resistant to FOXY. Conservationists hope to restore native forests on large areas where agriculture has declined. The forestry industry seeks to enhance supplies of the Islands’ most valuable wood. Finally, science indicated that a breeding program would probably be successful. Field trials in the 1990s demonstrated great differences in wilt-disease mortality among seed sources (the proportion of seedlings surviving inoculation ranged from 4% to 91.6%) [Sniezko 2003; Dudley et al. 2009].

In 2003, Dudley and Sniezko outlined a long-term strategy for exploring and utilizing genetic resistance in koa. Since then, a team of scientists and foresters has implemented different phases of the strategy and refined it further (Dudley et al. 2012, 2015, 2017; Sniezko et al. 2016]

First, scientists determined that the wilt disease is established on the four main islands. Having obtained more than 500 isolates of the pathogen from 386 trees sampled at 46 sites, scientists tested more than 700 koa families from 11 ecoregions for resistance against ten of the most highly virulent isolates (Dudley et al. 2020). 

The Hawaiian Agricultural Research Center (HARC), supported by public and private partners, has converted the field-testing facilities on Hawai`i, Maui, and Oahu into seed orchards. The best-performing tree families are being grown to maturity to produce seeds for planting. It is essential that the seedlings be not just resistant to FOXY but also adapted to the ecological conditions of the specific site where they are  to be planted [Dudley et al. 2020; Inman-Narahari et al. ] Locally adapted, wilt-resistant seed has been planted on Kauaʻi and Hawai`i. Preparations are being made to plant seed on Maui and O‘ahu also. Scientists are also exploring methods to scale up planting in both restoration and commercial forests [R. Hauff pers. comm.].  

koa; photo by David Eickhoff via Flickr

Restoration of koa on the approximately half of lands in the species’ former range that are privately owned will require that the trees provide superior timber. Private landowners might also need financial incentives since the rotation time for a koa plantation is thought to be 30-80 years. [Inman-Narahari et al.]

Plantings on both private and public lands will need to be protected from grazing by feral ungulates and encroachment by competing plants. These management actions are intensive, expensive, and must be maintained for years.

Some additional challenges are scientific: uncertainties about appropriate seed zones, efficacy of silvicultural approaches to managing the disease, and whether koa can be managed for sustainable harvests. Human considerations are also important: Hawai`i lacks sufficient professional tree improvement or silvicultural personnel, a functioning seed distribution and banking network — and supporting resources. Finally, some segments of the public oppose ungulate control programs. Inman-Narahari et al.

Finally, scientists must monitor seed orchards and field plantings for any signs of maladaptation to climate change. (Dudley et al. 2020).

2) ʻŌhiʻa Metrosideros polymorpha)   

ʻŌhiʻa  lehua is the most widespread tree on the Islands. It dominates approximately 80% the biomass of Hawaii’s remaining native forest, in both wet and dry habitats. ʻŌhiʻa illustrates adaptive radiation and appears to be undergoing incipient speciation. The multitude of ecological niches and their isolation on the separate islands has resulted in five recognized species in the genus Metrosideros. Even the species found throughout the state, Metrosideros polymorpha, has eight recognized varieties (Luiz et al. (2023) (some authorities say there are more).

Loss of this iconic species could result in significant changes to the structure, composition, and potentially, the function, of forests on a landscape level. High elevation ‘ohi‘a forests protect watersheds across the state. ʻŌhiʻa forests shelter the Islands’ one native terrestrial mammal (Hawaiian hoary bat), 30 species of forest birds, and more than 500 endemic arthropod species. Many species in all these taxa are endangered or threatened (Luiz et al. 2023). The increased light penetrating interior forests following canopy dieback facilitates invasion by light-loving non-native plant species, of which Hawai`i has dozens. There is perhaps no other species in the United States that supports more endangered taxa or that plays such a geographical dominant ecological keystone role [Luiz et al. 2023]

For many Native Hawaiians, ‘ōhi‘a is a physical manifestation of multiple Hawaiian deities and the subject of many Hawaiian proverbs, chants, and stories; and foundational to the scared practice of many hula. The wood has numerous uses. Flowers, shoots, and aerial roots are used medicinally and for making lei. The importance of the biocultural link between ‘ōhi‘a and the people of Hawai`i is described by Loope and LaRosa (2008) and Luiz et al. (2023).

In 2010 scientists detected rapid mortality affecting ‘ōhi‘a on Hawai‘i Island. Scientists determined that the disease is caused by two recently-described pathogenic fungi, Ceratocystis lukuohia and Ceratocystis huliohia. The two diseases, Ceratocystis wilt and Ceratocystis canker of ʻōhiʻa, are jointly called “rapid ‘ōhi‘a death”, or ROD. The more virulent species, C. lukuohia, has since spread across Hawai`i Island and been detected on Kaua‘i.  The less virulent C. huliohia  is established on Hawai`i and Kaua‘i and in about a dozen trees on  O‘ahu. One tree on Maui was infected; it was destroyed, and no new infection has been detected [M. Hughes pers. comm.] As of 2023, significant mortality has occurred on more than one third of the vulnerable forest on Hawai`i Island, although mortality is patchy.  

 [ʻŌhiʻa is also facing a separate  disease called myrtle rust caused by the fungus Austropuccinia psidii; to date this rust has caused less virulent infections on ‘ōhi‘a.]

rust-killed ‘ōhi‘a in 2016; photo by J.B. Friday

Because of the ecological importance of ‘ōhi‘a and the rapid spread of these lethal diseases, research into possible resistance to the more virulent pathogen, C. lukiohia began fairly quickly, in 2016. Some ‘ōhi‘a survive in forests on the Big Island in the presence of ROD, raising hopes that some trees might possess natural resistance. Scientists are collecting germplasm from these lightly impacted stands near high-mortality stands (Luiz et al. 2023). Five seedlings representing four varieties of M. polymorpha that survived several years’ exposure to the disease are being used to produce rooted cuttings and seeds for further evaluation of these genotypes.

ʻŌhiʻa flowers

Encouraged by these developments, and recognizing the scope of additional work needed, in 2018 stakeholders created a collaborative partnership that includes state, federal, and non-profit agencies and entities, ʻŌhiʻa Disease Resistance Program (‘ODRP) (Luiz et al. 2023). The partnership seeks to provide baseline information on genetic resistance present in all Hawaiian taxa in the genus Metrosideros. It aims further to develop sources of ROD-resistant germplasm for restoration intended to serve several purposes: cultural plantings, landscaping, and ecological restoration. ‘ODRP is pursuing screenings of seedlings and rooted cuttings sampled from native Metrosideros throughout Hawai`i while trying to improve screening and growing methods. Progress will depend on expanding these efforts to include field trials; research into environmental and genetic drivers of susceptibility and resistance; developing remote sensing and molecular methods to rapidly detect ROD-resistant individuals; and support already ongoing Metrosideros conservation. If levels of resistance in wild populations prove to be insufficient, the program will also undertake breeding (Luiz et al. 2023).

To be successful, ‘ODRP must surmount several challenges (Luiz et al. 2022):

  • increase capacity to screen seedlings from several hundred plants per year to several thousand;
  • optimize artificial inoculation methodologies;
  • determine the effects of temperature and season on infection rates and disease progression;
  • find ways to speed up seedlings’ attaining sufficient size for testing;
  • develop improved ways to propagate ʻōhiʻa from seed and rooted cuttings;
  • establish sites for field testing of putatively resistant trees across a wide range of climatic and edaphic conditions;
  • establish seed orchard, preferably on several islands;
  • establish systems for seed collection from the wide variety of subspecies/varieties;
  • if breeding to enhance resistance is appropriate, it will be useful to develop high-throughput phenotyping of the seed orchard plantings.

  [See DMF profile for more details.]

Developing ROD-resistant ‘ōhi‘a is only one part of a holistic conservation program. Luiz et al. (2023) reiterate the importance of quarantines and education to curtail movement of infected material and countering activities that injure the trees. Fencing to protect these forests from grazing by feral animals can drastically reduce the amount of disease. Finally, scientists must overcome the factors there caused the almost complete lack of natural regeneration of ‘ōhi‘a in lower elevation forests. Most important are competition by invasive plants, predation by feral ungulates, and the presence of other diseases, e.g., Austropuccinia psidii.

Hawaii’s dryland forests are highly endangered: more than 90% of dry forests are already lost due to habitat destruction and the spread of invasive plant and animal species. Two tree species are the focus of species-specific programs aimed at restoring them to remaining dryland forests. However, support for both programs seems precarious and requires stable long-term funding; disease resistance programs often necessitate decades-long endeavors.

naio in bloom; photo by Forrest & Kim Starr via Creative Commons

1) naio (Myoporum sandwicense)

Naio grows on all of the main Hawaiian Islands at elevations ranging from sea level to 3000 m. While it occurs in the full range of forest types from dry to wet, naio is one of two tree species that dominate upland dry forests. The other species is mamane, Sophora chrysophylla. Naio is a key forage tree for two endangered honeycreepers, palila (Loxioides bailleui) and `akiapola`au (Hemignathus munroi). The tree is also an important host of many species of native yellow-face bees (Hylaeus spp). Finally, loss of a native tree species in priority watersheds might lead to invasions by non-native plants that consume more water or increase runoff.

The invasive non-native Myoporum thrips, Klambothrips myopori, was detected on Hawai‘i Island in December 2008 (L. Kaufman website). In 2018 the thrips was found also on Oahu (work plan). The Myoporum thrips feeds on and causes galls on plants’ terminal growth. This can eventually lead to death of the plant.

Aware of thrips-caused death of plants in the Myoporum genus in California, the Hawaii Department of Lands and Natural Resources Division of Forestry and Wildlife and the University of Hawai‘i began efforts to determine the insect’s distribution and infestation rates, as well as the overall health of naio populations on the Big Island. This initiative began in September 2010, nearly two years after the thrips’ detection. Scientists monitored nine protected natural habitats for four years. This monitoring program was supported by the USFS Forest Health Protection program. This program is described by Kaufman.

naio monitoring sites from L. Kaufman article

The monitoring program determined that by 2013, the thrips has spread across most of Hawi`i Island, on its own and aided by human movement of landscaping plants. More than 60% of trees being monitored had died. Infestation and dieback levels had both increased, especially at medium elevation sites. The authors feared that mortality at high elevations would increase in the future. They found no evidence that natural enemies are effective controlling naio thrips populations on Hawai`i Island.

Kaufman was skeptical that biological control would be effective. She suggested, instead, a breeding program, including hybridizing M. sandwicensis with non-Hawaiian Myoporum species that appear to be resistant to thrips. Kaufman also called for additional programs: active monitoring to prevent thrips from establishing on neighboring islands; and collection and storage of naio seeds.

Ten years later, in February 2024, DLNR Division of Forestry and Wildlife adopted a draft work plan for exploring possible resistance to the Myoporum thrips. Early steps include establishing a database to record data needed to track parent trees, associated propagules, and the results of tests. These data are crucial to keeping track of which trees show the most promise. Other actions will aim to hone methods and processes. Among practical questions to be answered are a) whether scientists can grow even-aged stands of naio seedlings; b) identifying the most efficient resistance screening techniques; and c) whether K. myopori thrips are naturally present in sufficient numbers to be used in tests, or – alternatively – whether they must be augmented. [Plan]

Meanwhile, scientists have begun collecting seed from unaffected or lightly affected naio in hotspots where mortality is high. They have focused on the dry and mesic forests of the western side of Hawai`i (“Big”) Island, where the largest number of naio populations still occur and are at high risk. Unfortunately, these “lingering” trees remain vulnerable to other threats, such as browsing by feral ungulates, competition with invasive plants, drought, and reduced fecundity & regeneration.

Hawai`i DLNR has secured initial funding from the Department of Defense’s REPI program to begin a pest resistance project and is seeking a partnership with University of Hawai`i to carry out tests “challenging” different naio families’ resistance to the thrips [R. Hauff pers. comm.]

wiliwili; photo by Forrest & Kim Starr

2) wiliwili (Erythrina sandwicensis)

Efforts to protect the wiliwili have focused on biological control. The introduced Erythrina gall wasp, Quadrastichus erythrinae (EGW) was detected on the islands in 2005. It immediately caused considerable damage to the native tree and cultivated nonnative coral trees.

A parasitic wasp, Eurytoma erythrinae, was approved for release in November 2008 – only 3 ½ years after EGW was detected on O‘ahu. The parasitic wasp quickly suppressed the gall wasp’s impacts to both wiliwili trees and non-native Erythrina. By 2024, managers are once again planting the tree in restoration projects.

However, both the gall wasp and a second insect pest – a bruchid, Specularius impressithorax – can cause loss of more than 75% of the seed crop. This damage means that the tree cannot regenerate. By 2019, Hawaiian authorities began seeking permission to release a second biocontrol gent, Aprostocitus nites.Unfortunately, the Hawai’i Department of Agriculture still has not approved the release permit despite five years having passed. Once they have this approval, the scientists will then need to ask USDA Animal and Plant Health Inspection Service (APHIS) for its approval [R. Hauff, pers. comm.]

SOURCES

www.RapidOhiaDeath.org

Dudley, N., R. James, R. Sniezko, P. Cannon, A. Yeh, T. Jones, & Michael Kaufmann. 2009? Operational Disease Screening Program for Resistance to Wilt in Acacia koa in Hawai`i. Hawai`i Forestry Association Newsletter August 29 2009

Dudley, N., T. Jones, K. Gerber, A.L. Ross-Davis, R.A. Sniezko, P. Cannon & J. Dobbs. 2020. Establishment of a Genetically Diverse, Disease-Resistant Acacia koa Seed Orchard in Kokee, Kauai: Early Growth, Form, & Survival. Forests 2020, 11, 1276; doi:10.3390/f11121276 www.mdpi.com/journal/forests

Friday, J. B., L. Keith, and F. Hughes. 2015. Rapid ʻŌhiʻa Death (Ceratocystis Wilt of ʻŌhiʻa). PD-107, College of Tropical Agriculture and Human Resources, University of Hawai‘i, Honolulu, HI. URL: https://www.ctahr.HI.edu/oc/freepubs/pdf/PD-107.pdf  Accessed April 3, 2018.

Friday, J.B. 2018. Rapid ??hi?a Death Symposium -West Hawai`i (“West Side Symposium”) March 3rd 2018,  https://vimeo.com/258704469 Accessed April 4, 2018 (see also full video archive at https://vimeo.com/user10051674)

Inman-Narahari, F., R. Hauff, S.S. Mann, I. Sprecher, & L. Hadway. Koa Action Plan: Management & research priorities for Acacia koa forestry in Hawai`i. State of Hawai`i Department of Land & Natural Resources Division of Forestry & Wildlife no date

Kaufman, L.V, J. Yalemar, M.G. Wright. In press. Classical biological control of the erythrina gall wasp, Quadrastichus erythrinae, in Hawaii: Conserving an endangered habitat. Biological Control. Vol. 142, March 2020

Loope, L. and A.M. LaRosa. 2008. ‘Ohi’a Rust (Eucalyptus Rust) (Puccinia psidii Winter) Risk Assessment for Hawai‘i.

Luiz, B.C. 2017. Understanding Ceratocystis. sp A: Growth, morphology, and host resistance. MS thesis, University of Hawai‘i at Hilo.

Luiz, B.C., C.P. Giardina, L.M. Keith, D.F. Jacobs, R.A. Sniezko, M.A. Hughes, J.B. Friday, P. Cannon, R. Hauff, K. Francisco, M.M. Chau, N. Dudley, A. Yeh, G. Asner, R.E. Martin, R. Perroy, B.J. Tucker, A. Evangelista, V. Fernandez, C. Martins-Keli’iho.omalu, K. Santos, R. Ohara. 2023. A framework for establishlishing a rapid ‘Ohi‘a death resistance program  New Forests 54, 637–660. https://doi.org/10.1007/s11056-021-09896-5

Additional information on the koa resistance program is posted at http://www.harc-hspa.com/forestry.html 

Sniezko, R.A., N. Dudley, T. Jones, & P. Cannon. 2016. Koa wilt resistance & koa genetics – key to successful restoration & reforestation of koa (Acacia koa). Acacia koa in Hawai‘i: Facing the Future. Proceedings of the 2016 Symposium, Hilo, HI: www.TropHTIRC.org , www.ctahr.HI.edu/forestry 

Posted by Faith Campbell

We welcome comments that supplement or correct factual information, suggest new approaches, or promote thoughtful consideration. We post comments that disagree with us — but not those we judge to be not civil or inflammatory.

For a detailed discussion of the policies and practices that have allowed these pests to enter and spread – and that do not promote effective restoration strategies – review the Fading Forests report at  https://treeimprovement.tennessee.edu/

or

www.fadingforests.org

Scientists: Introduced forest pest reshaping forests, with many bad consequences … will regulators step up?

Jarrah dieback in Western Australia

The number of introduced forest pathogens are increasing – creating a crisis that is recognized by more scientists. These experts say tree diseases are reshaping both native and planted forests around the globe. The diseases are threatening biodiversity, ecosystem services, provision of products, and related human wellbeing. Some suggest that bioinvasions might threaten forests as much as climate change, while also undermining forests’ role in carbon sequestration.

Unfortunately, I see little willingness within the plant health regulatory community to tackle improving programs to slow introductions. Even when the scientists documenting the damage work for the U.S. Department of Agriculture – usually the U.S. Forest Service — USDA policy-makers don’t act on their findings. [I tried to spur a conversation with USDA 2 years ago. So far, no response.]

counties where beech leaf disease has been detected

What the scientists say about these pests’ impacts

Andrew Gougherty (2023) – one of the researchers employed by the USDA Forest Service – says that emerging infectious tree diseases are reshaping forests around the globe. Furthermore, new diseases are likely to continue appearing in the future and threaten native and planted forests worldwide. [Full references are provided at the end of the blog.] Haoran Wu (2023/24) – a Master’s Degree student at Oxford University – agrees that arrival of previously unknown pathogens are likely to alter the structure and composition of forests worldwide. Weed, Ayers, and Hicke (2013) [academics] note that forest pests — native and introduced — are the dominant sources of disturbance to North American forests. They suggest that, globally, bioinvasions might be at least as important as climate change as threats to the sustainability of forest ecosystems. They are concerned that recurrent forest disturbances caused by pests might counteract carbon mitigation strategies. 

Scientists have proclaimed these warnings for years. Five years ago, Fei et al. (2019) reported that the 15 most damaging pests introduced to the United States — cumulatively — had already caused tree mortality to exceed background levels by 5.53 teragrams of carbon per year. As these 15 pests spread and invasions intensify, they threaten 41.1% of the total live forest biomass in the 48 coterminous states. Poland et al. (2019) (again – written by USFS employees) document the damage to America’s forest ecosystems caused by the full range of invasive species, terrestrial and aquatic.

Fei et al. and Weed, Ayers, and Hicke (2013) also support the finding that old, large trees are the most important trees with regard to carbon storage. This understanding leads them to conclude that the most damaging non-native pests are the emerald ash borer, Dutch elm disease fungi, beech bark disease, and hemlock woolly adelgid. As I pointed out in earlier blogs, other large trees, e.g., American chestnut and several of the white pines, were virtually eliminated from much of their historical ranges by non-native pathogens decades ago. These same large, old, trees also maintain important aspects of biological diversity.

It is true that not all tree species are killed by any particular pest. Some tree genera or species decrease while others thrive, thus altering the species composition of the affected stands (Weed, Ayers, and Hicke). This mode of protection is being undermined by the proliferation of insects and pathogens that cumulatively attack ever more tree taxa. And while it is true that some of the carbon storage capacity lost to pest attack will be restored by compensatory growth in unaffected trees, this faster growth is delayed by as much as two or more decades after pest invasions begin (Fei et al.).

ash forest after EAB infestation; Photo by Nate Siegert, USFS

Still, despite the rapid rise of destructive tree pests and disease outbreaks, scientists cannot yet resolve critical aspects of pathogens’ ecological impacts or relationship to climate change. Gougherty notes that numerous tree diseases have been linked to climate change or are predicted to be impacted by future changes in the climate. However, various studies’ findings on the effects of changes in moisture and precipitation are contradictory. Wu reports that his study of ash decline in a forest in Oxfordshire found that climate change will have a very small positive impact on disease severity through increased pathogen virulence. Weed, Ayers, and Hicke go farther, making the general statement that despite scientists’ broad knowledge of climate effects on insect and pathogen demography, they still lack the capacity to predict pest outbreaks under climate change. As a result, responses intended to maintain ecosystem productivity under changing climates are plagued by uncertainty.

Clarifying how disease systems are likely to interact with predicted changes in specific characteristics of climate is important — because maintaining carbon storage levels is important. Quirion et al. (2021) estimate that, nation-wide, native and non-native pests have decreased carbon sequestration by live forest trees by at least 12.83 teragrams carbon per year. This equals approximately 9% of the contiguous states’ total annual forest carbon sequestration and is equivalent to the CO2 emissions from more than 10 million passenger vehicles driven for one year. Continuing introductions of new pests, along with worsening effects of native pests associated with climate change, could cause about 30% less carbon sequestration in living trees. These impacts — combined with more frequent and severe fires and other forest disturbances — are likely to negate any efforts to improve forests’ capacity for storing carbon.

Understanding pathogens’ interaction with their hosts is intrinsically complicated. There are multiple biological and environmental factors. What’s more, each taxon adapts individually to the several environmental factors. Wu says there is no general agreement on the relative importance of the various environmental factors. The fact that most forest diseases are not detected until years after their introduction also complicates efforts to understand factors affecting infection and colonization.

The fungal-caused ash decline in Europe is a particularly alarming example of the possible extent of such delays. According to Wu, when the disease was first detected – in Poland in 1992 – it had already been present perhaps 30 years, since the 1960s.  Even then, the causal agent was not isolated until 2006 – or about 40 years after introduction. The disease had already spread through about half the European continent before plant health officials could even name the organism. The pathogen’s arrival in the United Kingdom was not detected until perhaps five years after its introduction – despite the country possessing some of the world’s premier forest pathologists who by then (2012) knew what they to look for. 

Clearly, improving scientific understanding of forest pathogens will be difficult. In addition, effective policy depends on understanding the social and economic drivers of trade, development, and political decisions are primary drivers of the movement of pathogens. Wu calls for collaboration of ecologists, geneticists, earth scientists, and social scientists to understand the complexity of the host-pathogen-surrounding system. Bringing about this new way of working and obtaining needed resources will take time – time that forests cannot afford.  

However, Earth’s forests are under severe threat now. Preventing their collapse depends on plant health officials integrating recognition of these difficulties into their policy formulation. It is time to be realistic: develop and implement policies that reflect the true level of threat and limits of current science.

Background: Rising Numbers of Introductions

Gougherty’s analysis of rising detections of emerging tree diseases found little evidence of saturation globally – in accord with the findings of Seebens et al. (2017) regarding all taxa. Relying on data for 24 tree genera, nearly all native to the Northern Hemisphere, Gougherty found that the number of new pests attacking these tree genera are doubling on average every 11.2 years. Disease accumulation is increasing rapidly in both regions where hosts are native and where they are introduced, but more rapidly in trees’ native ranges.This finding is consistent with most new diseases arise from introductions of pathogens to naïve hosts.

Gougherty says his estimates are almost certainly underestimates for a number of reasons. Countries differ in scientific resources and their scientists’ facility with English. Scientists are more likely to notice and report high-impact pathogens and those in high-visibility locations. Where national borders are closer, e.g., in Europe, a minor pest expansion can be reported as “new” in several countries.  New pathogens in North America appear to occur more slowly, possibly because the United States and Canada are very large. He suggests that another possible factor is the U.S. (I would add Canada) have adopted pest-prevention regulations that might be more effective than those in place in other regions. (See my blogs and the Fading Forest reports linked to below for my view of these measures’ effectiveness.)

ash dieback in the UK

Wu notes that reports of tree pathogens in Europe began rising suddenly after the 1980s. He cites the findings by Santini et al. (2012) that not only were twice as many pathogens detected in the period after 1950 than in the previous 40 years, the region of origin also changed. During the earlier period, two-thirds of the introduced pathogens came from temperate North America. After 1950, about one-third of previously unknown disease agents were from temperate North America. Another one-third was from Asia. By 2012, more than half of plant infectious diseases were caused by introduction of previously unknown pathogens.

What is to be done?

Most emerging disease agents do not have the same dramatic effects as chestnut blight in North America, ash dieback in Europe, or Jarrah dieback in Australia. Nevertheless, as Gougherty notes, their continued emergence in naïve biomes increases the likelihood of especially damaging diseases emerging and changing forest community composition.

Gougherty calls for policies intended to address both the agents being introduced through trade, etc., and those that emerge from shifts in virulence or host range of native pathogens or changing environmental conditions. In his view, stronger phytosanitary programs are not sufficient.

Wu recommends enhanced monitoring of key patterns of biodiversity and ecosystem functioning, He says these studies should focus on the net outcome of complex interactions. Wu also calls for increasing understanding of key “spillover” effects – outcomes that cannot be currently assessed but might impact the predicted outcome. He lists several examples:

  • the effects of drought–disease interactions  on tree health in southern Europe,
  • interaction between host density and pathogen virulence,
  • reproductive performance of trees experiencing disease,
  • effect of secondary infections,
  • potential for pathogens to gain increased virulence through hybridization.
  • potential for breeding resistant trees to create a population buffer for saving biological diversity. Wu says his study of ash decline in Oxfordshire demonstrates that maintaining a small proportion of resistant trees could help tree population recovery.

Quirion et al. provide separate recommendations with regard to native and introduced pests. To minimize damage from the former, they call for improved forest management – tailored to the target species and the environmental context. When confronting introduced pests, however, thinning is not effective. Instead, they recommend specific steps to minimize introductions via two principal pathways, wood packaging and imports of living plants. In addition, since even the most stringent prevention and enforcement will not eliminate all risk, Quirion et al. advocate increased funding for and research into improved strategies for inspection, early detection of new outbreaks, and strategic rapid response to newly detected incursions. Finally, to reduce impacts of established pests, they recommend providing increased and more stable funding for classical biocontrol, research into technologies such as sterile-insect release and gene drive, and host resistance breeding.

USDA HQ

Remember: reducing forest pest impacts can simultaneously serve several goals—carbon sequestration, biodiversity conservation, and perpetuating the myriad economic and societal benefits of forests. See Poland et al. and the recent IUCN report on threatened tree species.

SOURCES

Barrett, T.M. and G.C. Robertson, Editors. 2021. Disturbance and Sustainability in Forests of the Western United States. USDA Forest Service Pacific Northwest Research Station. General Technical Report PNW-GTR-992. March 2021

Clark, P.W. and A.W. D’Amato. 2021. Long-term development of transition hardwood and Pinus strobus – Quercus mixedwood forests with implications for future adaptation and mitigation potential. Forest Ecology and Management 501 (2021) 119654

Fei, S., R.S. Morin, C.M. Oswalt, and A.M. 2019. Biomass losses resulting from insect and disease invasions in United States forests. Proceedings of the National Academy of Sciences. www.pnas.org/cgi/doi/10.1073/pnas.1820601116  

Gougherty AV (2023) Emerging tree diseases are accumulating rapidly in the native and non-native ranges of Holarctic trees. NeoBiota 87: 143–160. https://doi.org/10.3897/neobiota.87.103525

Lovett, G.M., C.D. Canham, M.A. Arthur, K.C. Weathers, and R.D. Fitzhugh. 2006. Forest Ecosystem Responses to Exotic Pests and Pathogens in Eastern North America. BioScience Vol. 56 No. 5 May 2006

Lovett, G.M., M. Weiss, A.M. Liebhold, T.P. Holmes, B. Leung, K.F. Lambert, D.A. Orwig, F.T. Campbell, J. Rosenthal, D.G. MCCullough, R. Wildova, M.P. Ayres, C.D. Canham, D.R. Foster, S.L. Ladeau, and T. Weldy. 2016.  Nonnative forest insects and pathogens in the United States: Impacts and policy options.  Ecological Applications, 26(5), 2016, pp. 1437-1455

Poland, T.M., Patel-Weynand, T., Finch, D., Miniat, C. F., and Lopez, V. (Eds) (2019), Invasive Species in Forests and Grasslands of the United States: A Comprehensive Science Synthesis for the United States Forest Sector.  Springer Verlag.

Quirion, B.R., G.M. Domke, B.F. Walters, G.M. Lovett, J.E. Fargione, L. Greenwood, K. Serbesoff-King, J.M. Randall, and S. Fei. 2021 Insect and Disease Disturbance Correlate With Reduced Carbon Sequestration in Forests of the Contiguous US. Front. For. Glob. Change 4:716582.  [Volume 4 | Article 716582] doi: 10.3389/ffgc.2021.716582

Weed, A.S., M.P. Ayers, and J.A. Hicke. 2013. Consequences of climate change for biotic disturbances in North American forests. Ecological Monographs, 83(4), 2013, pp. 441–470

Wu, H. 2023/24. Modelling Tree Mortality Caused by Ash Dieback in a Changing World: A Complexity-based Approach MSc/MPhil Dissertation Submitted August 12, 2024. School of Geography and the Environment, Oxford University.

Posted by Faith Campbell

We welcome comments that supplement or correct factual information, suggest new approaches, or promote thoughtful consideration. We post comments that disagree with us — but not those we judge to be not civil or inflammatory.

For a detailed discussion of the policies and practices that have allowed these pests to enter and spread – and that do not promote effective restoration strategies – review the Fading Forests report at  https://treeimprovement.tennessee.edu/

or

www.fadingforests.org

Non-Native Moths in England: Ever Upward

Platyperigea kadenii — one of the moth species that feeds on native plant species introduced recently to Great Britain. Photo by Tony Morris via Flickr

Will phytosanitary agencies and the international system respond to continuing introductions of non-native species?

A new study confirms that introductions of insects continue apace, links this pattern to the horticultural trade, and examines the role of climate change in facilitating introductions. This study focuses on moths introduced to the United Kingdom (Hordley et al.; full citation at the end of the blog). The study sought to detect any trends in numbers of species establishing and the relative importance of natural dispersal vs. those assisted – intentionally or inadvertently – by human activities.

The authors determined that moths continue to be introduced by both processes; there is no sign of “saturation”. This finding agrees with that of Seebens and 44 others (2017; citation below), which analyzed establishments of all types of non-native species globally. The British scientists found that rapidly increasing global trade is the probable driver of the recent acceleration of human-assisted introductions. They emphasize the horticultural trade’s role specifically. Climate change might play a role in facilitating establishment of species entering the UK via human activities.

Hordley et al. found that long-term changes in climate, not recent rapid anthropogenic warming, was important in facilitating introductions of even those moth species that arrived without human assistance. As they note, temperatures in Great Britain have been rising since the 17th Century. These changes in temperature have probably made the British climate more suitable for a large number of Lepidoptera. The data show that the rate of natural establishments began rising in the 1930s, 60 years before anthropogenic changes in temperatures became evident. Hordley et al. point out that an earlier study that posited a more significant role for climate change did not distinguish between insect species which have colonized naturally and those benefitting from human assistance.

The authors expect introductions to continue, spurred by ongoing environmental and economic changes. Fortunately, very few of the introduced moths had any direct or indirect negative impacts. (The box-tree moth (Cydalima perspectalis) is the exception. [Box-tree moth is also killing plants in North America.]

boxtree moth; photo by Tony Morris via Flickr

Still, they consider that introductions pose an ongoing potential risk to native biodiversity and related human interests. Therefore, they advocate enhanced biosecurity. Specifically, they urge improved monitoring of natural colonizations and regulation of the horticultural trade.

Hordley et al. estimated the rate of establishment during the period 1900 – 2019 for (i) all moth species; (ii) immigrants (i.e., those introduced without any human assistance); (iii) immigrants which feed on native hosts; (iv) immigrants which feed on non-native hosts; (v) adventives (i.e., species introduced with human assistance); (vi) adventives which feed on native hosts; and (vii) adventives which feed on NIS hosts.

Their analysis used data on 116 moth species that have become established in Great Britain since 1900. Nearly two-thirds of these species – 63% – feed on plant species native to Great Britain; 34% on plant species that have been imported – intentionally or not. Data were lacking on the hosts of 3 species.

Considering the mode of introduction, the authors found that 67% arrived through natural colonization; 33% via human assistance. Sixty-nine percent of the 78 species that were introduced through natural processes (54 species) feed on plant species native to Great Britain; 31% (24 species) feed on non-native plants. Among the 38 species whose introduction was assisted by human activities, one-half (19 species) feed on native plant species; 42% (16 species) feed on introduced hosts.

Regarding trends, they found that when considering all moth species over the full period, 21.5% more species established in each decade than in the previous decade. This average somewhat obscured the startling acceleration of introductions over time: one species was reported as established in the first decade (1900–1909) compared to 18 species in the final decade (2010–2019).   

The rate of introduction for all immigrant (naturally introduced) species was 22% increase per decade. Considering immigrant species that feed on native plants, the rate of establishment was nearly the same – 23% increase per decade – when averaged over the 120-year period. However, a more detailed analysis demonstrated that these introductions proceeded at a steady rate until 1935, then accelerated by 11% per decade thereafter. In contrast, immigrants that feed on non-native plants have maintained a steady rate of increasing establishments – 13% per decade since 1900.

Adventive species (those introduced via human assistance) increased by 26% per decade. The data showed no signs of saturation. The rates of introduction were similar for adventives that feed on both native plants (22%) and non-native hosts (26%). Again, additional analysis demonstrated a break in rates for adventives that feed on native hosts. The rate was steady until the 1970s, then significantly increased during the years up to 2010. (The scientists dropped data from the final decade since lags in detection might artificially suppress that number.)

In summary, Hordley et al. found no significant differences in trends between

  • the number of species that established naturally (20%) vs. adventives (26%).
  • immigrant or adventive species that feed on native vs. non-native hosts.

The authors discuss the role of climate change facilitating bioinvasion by spurring natural dispersal, changing propagule pressure in source habitats, changing the suitability of receiving habitat, and changing in pathways for natural spread, e.g., altered wind and ocean currents. They recognize that the two modes of colonization – adventives and immigrants – can interact. They stress, however, that the two colonization modes require different interventions.

Although their findings don’t support the premise that a surge of natural colonizers has been prompted by anthropogenic warming, Hordley et al. assert that climate clearly links to increased moth immigration to Britain and increased probability of establishment. They note that even so assisted, colonists still must overcome both the natural barrier of the English Channel and find habitats that are so configured as to   facilitate breeding success. They report that source pools do not appear to be depleted — moth species richness of neighboring European countries greatly exceeds that in Great Britain.

I would have liked to learn what factors they think might explain the acceleration in both natural and human-assisted introductions of species that feed on plant species native to Great Britain. In 2023 I noted that scientists have found that numbers of established non-native insect species are driven primarily by diversity of plants – both native and non-indigenous.

Hordley et al. assert that Great Britain has advantages as a study location because as a large island separated from continental Europe by the sea – a natural barrier – colonization events are relatively easy to detect. However the English Channel is only 32 km across at its narrowest point. I wonder, whether this relatively narrow natural barrier might lead to a misleadingly large proportion of introduced species being natural immigrants. I do agree with the authors that moths are an appropriate focal taxon because they are sensitive to climate and can be introduced by international trade. Furthermore, Britain has a long tradition of citizen scientists recording moth sightings, so trends can be assessed over a long period.

Hordley et al. stress that they measured only the temporal rate of new species’ establishments, not colonization pressure or establishment success rate. They had no access to systematic data regarding species that arrived but failed to establish. Therefore, they could not deduce whether the observed increase in establishment rates are due to:

(1) more species arriving—due either to climate-driven changes in dispersal or to accessibility of source pools; or

(2) higher establishment success due to improved habitat and resource availability; or

(3) both.

Hordley et al. noted two limitations to their study. First, they concede that there is unavoidably some subjectivity in classifying each species as colonizing naturally or with human assistance. They tried to minimize this factor by consulting two experts independently and including in the analysis only those species on which there was consensus.

Second, increases in detection effort and effectiveness might explain the recent increases in establishment rates. They agree that more people have become “citizen scientists” since 1970. Also, sampling techniques and resources for species identification have improved considerably. They note, however, that Seebens et al. (2018) tested these factors in their global assessment and found little effect on trends.

Hordley et al. believe that they have addressed a third possible limitation – the lag between introduction and detection – by running their analyses both with and without data from final decade (2010-2019). The results were very similar qualitatively.

SOURCE

Hordley, L.A., E.B. Dennis, R. Fox, M.S. Parsons, T.M. Davis, N.A.D. Bourn. 2024. Increasing rate of moth species establishment over 120 years shows no deceleration. Insect Conserv. Divers. 2024;1–10. DOI: 10.1111/icad.12783

Seebens, H. et al. 2017. No saturation in the accumulation of alien species worldwide. Nature Communications. January 2017. DOI: 10.1038/ncomms14435

Seebens, H. et al. 2018. Global rise in emerging IAS results from increased accessibility of new source pools. Proceedings of the National Academy of Sciences. www.pnas.org/cgi/doi/10.1073/pnas.1719429115

Posted by Faith Campbell

We welcome comments that supplement or correct factual information, suggest new approaches, or promote thoughtful consideration. We post comments that disagree with us — but not those we judge to be not civil or inflammatory.

For a detailed discussion of the policies and practices that have allowed these pests to enter and spread – and that do not promote effective restoration strategies – review the Fading Forests report at  https://treeimprovement.tennessee.edu/

or

www.fadingforests.org