‘Ohi‘a (Metrosideros polymorpha) is the most abundant native forest tree in Hawai`i and of enormous ecological, cultural, and economic importance. Five species of endemic Metrosideros are recognized on the Hawaiian islands. Only one — M. polymorpha — is found throughout the state. Eight varieties are recognized. These varieties inhabit different environments and have adapted to selective pressures characteristic of these locations. There are at least five other species in the Metrosideros genus, each endemic to one or a few nearby islands. Blaine et al. (2022) [full citation at end of this blog] provide a helpful summary of the tree’s ecological importance and its apparently on-going speciation.
‘Ohi‘a provides habitat for endemic birds, insects, and plants, many of which are endangered. Thus, conservation of this species — and all Hawaiian Metrosideros – is vital for the conservation of countless other taxa. In addition, high elevation ʻōhiʻa forests protect vitally important watersheds across the state. For Native Hawaiians, ʻōhiʻa is a physical manifestation of multiple Hawaiian deities so is the subject of many proverbs, chants, stories, and a foundation of scared hula. Finally, the tree is beautiful!
Native Hawaiian forests face multiple threats — invasive animals and plants, wildfire, and land-use changes. Due to such threats, natural ʻōhiʻa regeneration is largely absent in most lower-elevation forests. In this case, competition with invasive species and the presence of diseases such as ʻōhiʻa rust (Austropuccinia psidii) are probably the specific causes. Multiple government and non-governmental entities have made substantial effort to mitigate these threats.
ROD-infected ʻōhiʻa; photo by J.B. Friday
The disease Rapid ‘Ohi‘a Death (ROD) is an unprecedented threat to this species and the forests it constitutes. The disease is caused by two newly described fungal pathogens: Ceratocystis lukuohia and C. huliohia. The disease caused by C. lukuohia is more severe. To date it has been detected on the two islands farthest apart in the chain — Hawai`i (the Big Island) and Kaua‘i. C. huliohia causes a canker disease that kills trees more slowly. It is more widespread, found on Maui and O‘ahu in addition to Hawai`i and Kaua‘i. Blaine et al. (2022) and the profile here describe the two diseases’ epidemiologies, progression, impacts, and challenges.
Because of the clear threat to Hawaiian ecosystems, ecosystem services, and cultural assets, considerable effort has put into delimitation and research on possible mitigation actions since ROD was discovered in 2010. The first strategic plan covered the period 2017–2019. It focused on expanded efforts to map outbreaks, research on the epidemiology of the pathogens, and most-promising management practices. The second strategic plan covers 2020–2024. It provides for continued surveillance and improvement of these technologies; expanding outreach and public engagement; research on possible vectors of the pathogens; collection and preservation of seeds for research and future restoration; and comprehensive evaluation and development of disease resistance in ʻōhiʻa.
Soon after the causal agents were clarified, the USDA Agriculture Research Service (ARS) began screening for disease resistance. By 2016, ARS had demonstrated that five individuals from two varieties of M. polymorpha had survived inoculation by the more virulent pathogen, C. lukuohia. Their survival raised hopes that natural resistance might be present in wild populations of at least some varieties. However, more comprehensive screening of trees from throughout the species’ range is needed to provide an accurate baseline on the frequency, level, and distribution of genetic resistance to both pathogens. The goal is to produce material resistant to both pathogens that can be used to preserve the ecology, culture, and biotic communities that are dependent on this tree species.
To carry the expanded effort forward, in 2018 a collaborative partnership of state, federal, and non-profit groups was formed. Participants in the ‘Ohi‘a Disease Resistance Program (‘ODRP) include: the Akaka Foundation for Tropical Forests; USDA’s Forest Service and Agriculture Research Service; the state’s Division of Forestry and Wildlife and Agriculture Research Center; programs of the University of Hawai‘i at Manoa and at Hilo; Purdue and Arizona State universities; the Tropical Hardwood Tree Improvement and Regeneration Center; and Kalehua Seed Conservation Consulting.
Blaine et al. (2022) have now outlined a framework to guide the overall effort to identify and develop ROD resistance in M. polymorpha and, possibly, all Hawaiian Metrosideros species. The framework calls for the following activities:
(1) evaluating and operationalizing methods for inoculation-based screening and greenhouse-based production of test plants; and
(2) short-term greenhouse screenings of seedlings and rooted cuttings sampled from native Metrosideros throughout Hawai’i.
Once these tasks have been achieved, the effort is expected to expand to address:
(3) establishing field trials to validate the short-term greenhouse assays and monitor durability and stability of resistance;
drivers of susceptibility and resistance to characterize the durability and stability of genetic resistance to ROD;
(5) developing remote sensing and molecular methods to rapidly detect ROD-resistant individuals;
(6) if necessary, conducting breeding to increase the efficacy of resistance and improve durability of ROD resistance; and
(7) supporting already established and ongoing Metrosideros conservation, including state-wide seed collection and banking, with information on not only genotypes resistant to ROD but also production of ROD-resistant seed.
Blaine et al. (2022) outline how to proceed on each step, and describe the challenges that must be overcome. Challenges range from building growing and screening capacity to handle the thousands of plants required, to developing the remote sensing tools to identify diseased trees in the forest, to identifying sites for seed orchards. Actions by ‘ODRP will focus on Stage II screening in the field to examine the durability of resistance under the wide variety of ecological conditions in which ʻōhiʻa grows and in the presence of a potentially evolving pathogen. Resistance studies must expand beyond M. polymorpha varieties from only one island (the Big Island) to include the other Hawaiian Metrosideros taxa.
Once ROD-resistant M. polymorpha trees are discovered and groundwork has been laid to satisfy initial needs for resistant tree seedlings for forest restoration, scientists can begin research into the genetic basis of ROD resistance. This knowledge will assist breeding efforts which might be necessary if resistance to one of the pathogens does not confer resistance to the other, since the goal is to provide seedlings that are resistant to both.
Blaine et al. (2022) note that the state and others continue efforts to address other aspects of ROD management. These include
1) controlling the spread of the pathogen through local quarantines on movement of infected material and increased public education on bio-sanitation for forest users;
2) testing repellants to reduce beetle attack on infected trees and subsequent frass production.
3) reducing wounding of trees by fencing more pristine forests and removing feral ungulates
SOURCE
Blaine C. Luiz, Christian P. Giardina, Lisa M. Keith, Douglass F. Jacobs, Richard A. Sniezko, Marc A. Hughes, James B. Friday, Philip Cannon, Robert Hauff, Kainana Francisco, Marian M. Chau, Nicklos Dudley, Aileen Yeh, Gregory Asner, Roberta E. Martin, Ryan Perroy, Brian J. Tucker, Ale.alani Evangelista, Veronica Fernandez, Chloe Martins-Keli.iho.omalu, Kirie Santos, Rebekah Ohara. 2022. A framework for establishing a rapid ‘Ohia death resistance program. New Forests. https://doi.org/10.1007/s11056-021-09896-5
See also the video at https://www.bigislandvideonews.com/2019/06/16/video-to-save-ohia-a-genetic-resistance-program-will-be-built/
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
The Invasive Species Prevention and Forest Restoration Act (H.R. 1389) is before Congress. It is co-sponsored by Reps. Peter Welch [VT], Ann Kuster and Chris Pappas [NH], Chellie Pingree [ME], Elise Stefanik and Antonio Delgado [NY], Brian Fitzpatrick [PA], Mike Thompson [CA], Deborah Ross [NC].
Ask your Member of Congress/Representative to co-sponsor this bill. Ask your Senators to sponsor a companion bill.
In summary, this bill will:
Expand USDA APHIS’ access to emergency funds to eradicate or contain newly detected pest outbreaks.
Establish a pair of grant programs to support strategies aimed at restoring tree species decimated by non-native plant pests or noxious weeds. Such strategies include biological control of pests and enhancement of a tree host’s pest resistance.
One grant program supports research to explore and develop these strategies.
The second program support application of resistance breeding and other measures to restore forest tree species. Funded programs must incorporate a majority of the following components: collection and conservation of native tree genetic material; production of sufficient numbers of propagules; preparation of planting sites in the species’ former habitat; planting and post-planting maintenance.
Mandate a study to identify actions to overcome the shortfall of mission, leadership, and prioritization; identify agencies’ expertise and resources; improve coordination among agencies and with partners; and develop national strategies for saving tree species.
Organizations eligible for these grants include federal agencies; state cooperative institutions; colleges or universities offering a degree in the study of food, forestry, and agricultural sciences; and nonprofit entities with non-profit status per §501(c)(3) of the Internal Revenue Code.
Endorsements: Vermont Woodlands Association, American Forest Foundation, The Association of Consulting Foresters (ACF), Audubon Vermont, Center for Invasive Species Prevention, Ecological Society of America, Entomological Society of America, Maine Woodland Owners Association, Massachusetts Forest Alliance, National Association of State Foresters (NASF), National Woodland Owners Association (NWOA), The Nature Conservancy (TNC) Vermont, New Hampshire Timberland Owners Association, North American Invasive Species Management Association (NAISMA), Pennsylvania Forestry Association, Reduce Risk from Invasive Species Coalition, The Society of American Foresters (SAF), and a broad group of university professors and scientists.
Legislative Point of Contact: Alex Piper, Legislative Assistant, office of Rep. Welch. Contact me – providing your email! – if you wish me to send you Alex’ contact information. [The “contact” form does not provide your email and I will not reply in a public way.]
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
American beech (Fagus grandifolia) is a widespread and beautiful tree of the eastern deciduous forest. Its range reaches from Nova Scotia to eastern Wisconsin, then south to Mississippi and Louisiana and east to mid-Georgia. It is an important food source for 40 wildlife species, particularly in the northern parts of its range where few other species produce hard mast. (See Lovett et al. 2006.)
Threats
Unfortunately American beech is under threat from three non-native organisms or complexes: 1) beech bark disease, 2) beech leaf disease, and 3) beech leaf mining weevil. A fourth pest, a previously unknown – and still unnamed bark beetle in the genus Agrilus – has been detected in New York City on European beech trees. It is not yet known whether it will attack American beech and, if so, whether it will also cause serious damage (Michael Bohne, USFS, pers. comm.)
symptoms of beech bark disease; photo by Linda Haugen, USFS; via Bugwood
Beech bark disease (BBD) results from the interaction of the introduced European beech scale insect (Cryptococcus fagisuga) and several fungi in the Neonectria genus – some of which are also introduced. The resulting disease has been killing American beech trees since the beginning of the 20th Century. It has spread from Nova Scotia to much of the tree’s range. It has dramatically altered the composition and structure of stands containing beech.
symptoms of beech leaf disease; photo provided by Jennifer Koch, USFS
Beech leaf disease (BLD) was initially detected in 2012, near Cleveland. As of December, 2021, it has spread due east across New York, Pennsylvania, and New Jersey to the Atlantic, then up the coast through Connecticut and eastern Massachusetts, with a separate outbreak in central Maine. The disease is apparently associated with a nematode, Litylenchus crenatae ssp. mccanni, although additional pathogens, like bacteria, might also play a role. The origin of the North American population of the nematode is unknown; it is a related but separate subspecies from a Japanese nematode (Reed et al. 2022).
American beech defoliated by leaf mining weevil; photo courtesy of Jon Sweeney, CFS
Beech leaf mining weevil (Orchestes fagi) is, so far, limited to Nova Scotia. However, it is expected that the weevil will continue spreading throughout the range of American beech through both natural dispersal and human-assisted movement. Repeated defoliation by the weevil might increases mortality rates in forests that are surviving in the “aftermath” stage of BBD (Sweeney et al. 2020).
A new study (Reed et al. 2022) concludes that, despite being detected only 10 years ago, BLD has already become pervasive in forests surrounding Lake Erie in the U.S. and Ontario. While somewhat more prevalent in U.S. states on the eastern side of the Great Lakes (on 54% of trees) than in Ontario (on 46% of trees), BLD is spreading rapidly and affecting every canopy layer. Mortality is highest in seedlings and saplings; understory saplings die within 2 – 5 years. The occasional mortality of overstory trees occurs within seven years of [observed] infection. Defoliation and mortality of saplings allow more light to pass through to the understory; this is expected to alter plant communities on the forest floor.
Beech scale is more widespread in Ontario (found on 60% of trees) than in the U.S. (38% of trees). This is not surprising since the scale was detected in Ontario in 1960, 24 years before it was detected in portions of Ohio, New York and Pennsylvania included in the study (in 1984). Throughout this region, beech scale is disproportionately affecting overstory trees.
Only 4% of trees throughout the study area are infected with Neonectria cankers. In other words, full-scale beech bark disease is not yet widespread and is spreading surprisingly slowly. Scientists do not understand this phenomenon.
These findings are based on a network of monitoring plots a network of monitoring plots set up in 2019 set up in areas surrounding the Great Lakes. They comprise 34 plots at 17 locations in southwest Ontario and 30 plots at 25 locations in Ohio, Pennsylvania, and New York. In total the plots hold 646 live American beech trees — 412 saplings; 85 in the intermediate/suppressed (subcanopy) category; and 149 in the dominant/codominant (canopy) class.
Forest composition is similar throughout the study area. The most common species in association with American beech are sugar and red maples (Acer spp.), and white and green ash (Fraxinus spp.). Other tree species present include eastern hemlock (Tsuga canadensis), white pine (Pinus strobus), oaks (Quercus spp.), and birches (Betula spp.). Study plots had few invasive plants – although the invasive species present are well-documented to invade forests.
Ontario disease assessment
In Ontario, BLD was identified in 25 of the 34 plots. It was present on 171 saplings, 53 intermediate trees, and 70 dominant trees. Both prevalence and severity were greatest on intermediate trees. Beech scale was present at all 34 plots. While scales were found on trees of all sizes, they were almost two times more prevalent and were more severe on mature trees than saplings. Neonectria cankers were detected at 34 plots. Neonectria was rare but most severe on dominant trees. Fewer than one third of saplings and one-sixth of mature trees were pest free.
U.S. disease assessment
BLD was present in 17 of the 30 plots. It was found on 75 saplings, 30 intermediate trees, and 38 dominant trees. Saplings and dominant trees had similar levels of disease; intermediate trees had significantly less. However, BLD severity was twice as high on saplings compared to mature trees. BLD was present on more than half of the seedlings assessed – 46 out of 82. Beech scale was present in 20 of the 30 plots. It was significantly less common and severe on saplings than on mature trees. Neonectria cankers were present in only 4 of 30 plots. Canker prevalence and severity did not differ significantly among size classes.
Distribution and Effects of Beech Scale and BBD
While beech scale attack facilitates invasion by the Neonectria fungi, the disease – BBD complex – had the most limited distribution of the three pests in this study. It was found on only ~4% of beech trees throughout the study area. The disease was first reported there in the early 2000s. Although no one knows why, it has spread more slowly there than in areas to the east (Reed et al. 2022).
As is the case with beech scale, BBD disproportionately affects large diameter trees. Typically, BBD kills more than half of mature beech within 10 years of its arrival. Dying trees produce prolific root sprouts resulting in dense beech sapling understories that impede regeneration of less shade-tolerant tree species. The persistence of thickets of disease-vulnerable small beech perpetuates the disease. BBD is the only forest disease in North America that can inadvertently intensify itself by increasing densities of its host while suppressing other species.
Beech Forest Community Change in Response to Combined Impacts of BBD and BLD
It is unclear how forests will change as beech die. Some expect saplings of species already present — red maple, white ash, and, especially sugar maple — to exploit the canopy gaps. Of course, white and green ash are under attack by the emerald ash borer; DMF their ability to reach the canopy will depend on the success of biocontrol agents.
However, if BBD or BLD resistant beech survive or if BLD fails to persist, future forests might instead consist of beech thickets that would prevent all but the most shade tolerant species from establishing. Heavy deer predation on maple seedlings and saplings might also play a role. A third possibility is that morbidity from BBD and BLD might lead to uneven-aged conditions that allow younger trees — perhaps even shade intolerant species e.g., oaks — to establish.
Invasive plants also have the potential to fill gaps left by declining beech. While maple-beech forests often have sparse understories due to low understory light levels, pest-caused canopy gaps are expected to increase the abundance of invaders, especially in small woodlots and forests near urban areas. Several shade-tolerant invasive shrubs are already present in low numbers: Japanese barberry (Berberis thunbergii), tatarian honeysuckle (Lonicera tatarica), multiflora rose (Rosa multiflora), and buckthorn (Frangula sp.). Reed et al. (2022) note that these species, plus privet and autumn olive, can take advantage of small canopy gaps, especially when soils are disturbed, e.g., by active intervention to counteract the loss of beech.
Precautionary Research and Management
Reed et al. (2022) call for enhanced monitoring of beech forests focused on
the timing of BLD presence relative to tree age and size – which might affect competitiveness of sprouting beech in the understory; and
compositional and structural change in forests with BLD or to which it is likely to spread
They also recommend abandoning the management approach for BBD currently recommended by foresters. It calls for removing scale-susceptible beech so that resistant genotypes increase in prevalence. In forests with both BBD and BLD, they conclude, management of natural regeneration is unlikely to succeed because BLD will kill sprouts and saplings that might be resistant to scale. They recommend instead active management of the forest to promote mast-producing, shade intolerant species, such as oaks and hickories.
They also recommend increased support for resistance-breeding programs. Such programs already target BBD, based on the estimated 1% of American beech that show some resistance. Now those programs need to incorporate BLD resistance. (Reed et al. note that small numbers of beech show few or no BLD symptoms so might possess resistance or tolerance.)
grafted beech for resistance breeding; photo by Rachel Kappler, then USFS (now Great Lakes Basin Initiative & Holden Arboretum)
Unfortunately, the Canadian beech breeding program’s future funding is highly uncertain. To counter this threat, in part, Reed et al. (2022) suggest cryopreserving beech embryos from Canada to develop a beech conservation collection that would be available for a more robust, future Canadian breeding program. The USFS is trying to develop methods to screen trees for resistance to BLD, specifically to the nematode (J. Koch, USFS, pers. comm.)
Another approach would actively manage beech stands in which potentially BLD-resistant beech grow to help these trees reach the canopy and reproduce. In the absence of management, any BLD-resistant beech seedlings might be overtopped by faster growing, shade-intolerant species – especially if the gaps promote soil drying or sun scald.
Finally, breeding programs need to factor in the beech leaf mining weevil, DMF which — as I noted in the beginning — is spreading across Nova Scotia and could spread to the rest of the native range of beech (Sweeney et al., 2020).
The Department of Agriculture has created a website on the Department’s plant-breeding efforts. It includes a subwebsite on USFS efforts. However, I did not find much useful information there.
SOURCES
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)
Reed, S.F., D. Volk, D.K.H. Martin, C.E. Hausman, T. Macy, T. Tomon, S. Cousins. 2022. The distribution of beech leaf disease and the causal agents of beech bark disease (Cryptoccocus fagisuga, Neonectria faginata, N. ditissima) in forests surrounding Lake Erie and future implications Forest Ecology and Management 503 (2022) 119753
Sweeney J.D., Hughes, C., Zhang, H., Hillier, N.K., Morrison, A. and Johns R. (2020) Impact of the Invasive Beech Leaf-Mining Weevil, Orchestes fagi, on American Beech in Nova Scotia, Canada. Frontiers in Forests and Global Change | www.frontiersin.org 1 April 2020 | Volume 3 | Article 46
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
I often blog about the forest pest situation in the UK because its scientists provide lots of easily accessible information. This blog has new information on Britain and – I am pleased to add – on Ireland!
European ash trees alongside the Malham Cove path in Yorkshire; KA via geograph
UK Overview
British woodlands cover just 13% of total land area (just over one-third of the European average of 37%). Their value is increasingly recognized—especially their role in combating climate change through carbon sequestration, flood mitigation and urban cooling. Realization of these benefits is driving new policy to increase woodland cover. In 2019–2020, 13,700 ha of new woodland was created in the United Kingdom (Green et al. 2021). The U.K. government has pledged to plant 30,000 ha of broadleaf and coniferous woodland every year as part of its climate change mitigation strategy (Donald et al. 2021). One example, the ‘northern forest’ scheme, involves planting 50 million native trees over 25 years (Green et al. 2021).
Risks Associated with Conservation Plantings or Translocations
On-going conservation planting efforts plus these ambitious new plans prompted Donald et al. (2021) to assess risk that pathogens might be introduced into the environment as a result of putting native plants in natural habitats. The focus of their study is the planting of common juniper (Juniperus communis)in habitats throughout England, Scotland, and Wales.
Juniper killed by Phytophthora austrocedri; photo from British Forest Research
Juniper is one of three native conifers in the UK. It has been in decline for decades, which conservationists hope to reverse. Now, though, juniper populations are experiencing significant mortality from disease. In 2012, the causal agent was determined to be the non-native pathogen Phytophthora austrocedri. The presence of a single genotype in ~60 geographically separate locations across Scotland and England lends support to idea that the pathogen is being introduced to these sites through some human mechanism. Phytophthora austrocedri has not [yet] been detected in Northern Ireland.
British and European nurseries have contained many Phytophthora pathogens. Hence Donald et al. (2021) sought to determine whether the pathogen is being introduced through use of nursery stock in these well-intentioned plantings. (I have blogged about a similar problem in California restoration plantings.)
The authors found that 19% of P. austrocedri detections are within 2 km of a known planting. The more frequently junipers were planted at a site, and the more cuttings planted during each planting effort, the higher the likelihood that nearby junipers would be infested by P. austrocedri. They conclude that transplanting material is a significant risk pathway for the introduction of disease. The key factor appeared to be the origin of the material. A higher percentage of stock at sites with P. austrocedri outbreaks for which data were available had been raised in a central location by the organization doing the planting or obtained from commercial nurseries. No P. austrocedri was detected in Wales. There, unlike in Scotland or England, the majority of plants were sourced from a commercial nursery that only grew juniper collected from Welsh populations and did not trade with other retailers.
Planting juniper has risen rapidly since the mid-1990s. The highest percentage of planting events co-occurring with disease outbreaks were conducted in 2000–2009. It is likely that there is a time lag between planting and disease detectability. If so, the even larger planting effort since 2010 probably will produce many more P. austrocedri outbreaks that will become visible in the future. And that might not be the end. Planting guidelines have been revised based on pathogen detection. However, the entities doing the planting have not changed their approach, especially regarding site selection.
Donald et al. (2021) also found serious data gaps in these programs beyond the health of propagules. They found:
1) very incomplete knowledge of which organizations are doing the planting;
2) poor attention to traceability of source material; and
3) very little follow-up to check the success of planting projects.
The authors concluded that planting projects have had mixed success in restoring juniper populations. They called for changes in planting strategy to reduce the risks of pathogen introduction.
They also note that efforts to slow the spread of P. austrocedri – for which there is no treatment – are more expensive and less likely to succeed than measures aimed at ensuring that nurseries are free of Phytophthoras. California native plant nurseries have shown that nurseries can maintain Phytophthora-free stock.
Risk of Nursery-spread Pathogens & Willingness of UK Nurseries to Adopt BMPs
Great Britain has experienced an accelerating series of Phytophthora outbreaks and disease epidemics affecting British trees. Introductions detected just since early 2000s include P. ramorum, P. kernoviae, P. lateralis, P. austrocedri and P. pseudosyringae. In all the above cases, imported planting material either is confirmed or strongly implicated as the likely route of intro (Green et al. 2021).
To address this threat – and with massive planting projects proposed – in 2016 the British forest research entity initiated the multidisciplinary ”Phyto-threats” project. Its goal was to understand the drivers of rising Phytophthora infestations and opportunities for mitigating them. The project:
(i) examined Phytophthora distribution and diversity in different nursery management systems;
(ii) assessed the social and economic feasibility of a nursery accreditation programs to curb the risk; and
(iii) identified Phytophthora risks by modelling introduction, establishment and spread of species in relation to biological characteristics, environmental factors and trade flows.
The assessment of Phytophthora presence in nurseries involved collecting 3,624 water and root samples from 163 host genera growing in plant nurseries across the U.K. over a three-year period. Sampling was not random but targetted to facilities thought to harbor Phytophthora. About half of the samples tested positive. They identified 63 species of Phytophthora. Among the most commonly detected species are several that are considered pathogenic — P. cinnamomi, P. cryptogea/pseudocryptogea, P. syringae, P. cactorum, P. cambivora, P. plurivora and P. nicotianae. P. ramorum was found in 12 samples; P. lateralis and P. austrocedri were each found in 10 samples. Several Phytophthora species are potential new records for the U.K. (i.e., P. castanetorum, P. palmivora, P. pseudotsugae,P. tentaculata,P. terminalis, P. uliginosa).
They also saw evidence for Phytophthora root infections in newly arrived plants imported from the European Union.
Their finding raised question about whether Phytophthora can be transported in peat-free potting media, that is, coconut fiber or coir.
The widespread presence and the diversity of Phytophthora found in nurseries was linked to high-risk management practices. These included: careless disposal of culled plants, the near presence of trees along nursery boundaries, and, especially, open water sources. [These factors are essentially identical to infection-facilitating factors found by researchers in California, Oregon, and Washington State. See advisory issued by Oregon State University Extension.]
The project also assessed the feasibility of nursery accreditation programs. The authors consulted widely with nursery owners and customers and conducted a cost-benefit analysis. Regarding nursery practices, owners claimed they were already addressing issues related to water storage in enclosed tanks, clean/covered storage of growing media, installation of drains or free-draining gravel beds, raised benches, and tool disinfestation stations. Therefore the new analysis focused on seven other topics: water testing for pathogens; water treatment s; quarantine holding areas for imported plants; composting or incineration of culled plants; boot and vehicle washing stations; and purchase from only trusted or accredited UK suppliers.
The study found that nurseries would support an accreditation program. However, their support required that costs not be “prohibitive”, actions required not be “unreasonable”, the scheme provide a safety net; and that measures exist to deter non-compliance. Nursery staff wanted to see evidence of consumer demand – a willingness to drive farther to buy “clean” plants, or to pay higher prices for them. The cost-benefit analysis reached a worrying conclusion: nurseries would benefit financially from introducing best practices only when the program would prevent introduction of a wider range of pests and pathogens, not only Phytophthoras. Green et al. (2021) note that the overall net benefit to society from nurseries adopting best practices would be much more substantial. That is, healthy trees are important in meeting carbon sequestration goals. They did not explore whether society should subsidize nurseries’ participation in BMP accreditation programs.
Ireland and Northern Ireland
The island of Ireland (Ireland and Northern Ireland) is thought to have fewer plant pests than other European countries due to its island status and because of its national and international phytosanitary regulations. O’Hanlon et al. (2022) do not mention another possible factor: the likelihood that import volumes to Ireland were probably much lower until the recent vitalization of the Republic’s economy.
O’Hanlon et al. (2022) sought to establish baseline information so scientists can track changes as trade increases and the climate changes. Their search of the literature and unpublished sources identified 396 forest pests on the island, including 11 bacteria, 20 oomycetes, 150 fungi and 215 arthropods. They believe these figures are all probably underestimates. At least 44 of the pests or pathogens are probably non-native to Ireland. (Determining original ranges is difficult, especially for pathogens.)
The Republic of Ireland is one of the least forested countries in Europe. Forests cover ~ 11% of the land area. In Northern Ireland, it is even less: ~ 8%. These forests are predominantly plantations of exotic species. In the Republic, Picea sitchensis makes up 51% of the forest area, Pinus contorta another 10%. Other exotic species planted are Picea abies (4%) and Larix kaempferi. In Northern Ireland, ~ 62% of the forest area is composed of conifer mixtures. Planting of P. sitchensis has accelerated recently, probably as a result of removal of ash and larch because of their vulnerability to pests already established on the island.
Sitka spruce plantation in U.K. Adam Ward, Geograph.org.uk
O’Hanlon et al. (2022) note the great vulnerability of these monocultures to pests. They found 51 pests native to Ireland that are associated with non-indigenous tree genera. They are also concerned about pests introduced from other parts of Europe. For example, green spruce aphid (Elatobium abietinum, native to Central and Eastern Europe) is already attacking Sitka spruce. A second pest of spruce, Ips typographus, which is native to much of Europe but not the British/Irish isles, is not yet established on the island. Northern Ireland imports bark and wood from Europe for processing. Ips typographus has been associated with at least one such shipment.
Non-native forest pests and pathogens also threaten tree species native to Ireland. These include:
Dutch elm disease caused by fungi from the genus Ophiostoma vectored by bark beetles of the genus Scolytus. The second outbreak, caused by the more aggressive pathogen 0. novo-ulmi, was detected in Britain in 1965 and in Ireland in 1977. It caused considerable mortality of elms in Northern Ireland throughout 1970s.
Phytophthora ramorumwas recognized as a threat to forests in Europe only in 2010, when extensive mortality of Japanese larch was detected in Britain. The Republic of Ireland has only the EU1 lineage of the species. Northern Ireland has both the EU1 and EU2 lineages – the former only in nurseries.
Phytophthora disease of alder (caused by several Phytophthora species) was confirmed in Ireland in 2001. However, symptoms of the disease were noted as far back as 1995. It is likely that there are many other Phytophthora species present but not yet recorded.
Ash dieback disease (causal agent Hymenoscyphus fraxineus) on European ash (Fraxinus excelsior) has spread across Europe from Poland beginning in the 1990s. It was confirmed on the Irish island in 2012. Authorities made significant attempts to eradicate the disease, but were not successful. It is now recorded in every county in both Northern Ireland and Ireland. Damage to the economy, environment, and society are expected to be large. The Irish government had helped plant more than 13,000 ha of ash between 1992 and 2012. An estimated 2.9 million ash trees are in Northern Irish hedgerows. British scientists say more than 1,000 fauna species are associated with ash trees.
A second pest on ash — ash sawfly (Tomostethus nigritus) — was detected in Northern Ireland in 2016; it has defoliated hundreds of trees in Belfast.
In recent years, forest pest incursions have increased at a relatively steady rate, comparable to other countries, including Britain. In the 1970s, 26 species were reported; in the 1980s, 27; in the 1990s, 16; in the 2000s, 37; between 2010 and 2017, 28. See the graph in Fig. 2
There is a strong link between pest and pathogen findings in Britain and Ireland. O’Hanlon et al. (2021) list 16 insects and pathogens detected in Britain after 1960 which were later detected in Ireland. The list includes H. fraxineus, 0. novo-ulmi, Phytophthora ramorum, and Phytophthora lateralis. The average delay was 10 years. The authors note that the two islands share similar ecological conditions and hosts, are nearby, plus there is substantial travel and exchange of goods between them. For example, in 2018 an estimated 30,000 metric tonnes of conifer roundwood was sent from Scotland to Northern Ireland for processing.
There are very limited physical checks on plants or plant products moving between Ireland and Northern Ireland. The exception is conifer wood that is not bark-free. European Union regulations require that such shipments be accompanied by a plant passport that certifies that the wood has been inspected by a professional operator authorized by the NPPO of the exporting country. What rules will apply now, after BREXIT, remains unclear. Because of concerns about re-igniting sectarian conflict, most political figures want the border on the island to be almost invisible.
The Europhyt database for the period February 2006 – November 2016 documented interception of numerous high-risk pests at the British and Irish borders, including Anoplophora chinensis and A. glabripennis; I. typographus; Monochamus alternatus; H. fraxineus; and P. ramorum O’Hanlon et al. (2021). believe many more go undetected. O’Hanlon et al. (2021) report specifically on detections on commodities from China, especially on wood packaging. One detection on imported plants of interest to me is that of Discula destructiva(dogwood anthracnose). The article does not mention the origin of the shipment. The native British dogwood, Cornus sanguinea, would presumably be vulnerable to this Asian fungus, which has already caused widespread mortality of woodland dogwoods in North America.
Cornus sanguinea; photo by Hans Hillewaert
In addition to reviewing the current situation, O’Hanlon et al. (2021) note pertinent facts about current policy and future science. First, while the two political units on the island have a history of plant pathology expertise, there has recently been a reduction in the number of practicing forest pathologists, mycologists and entomologists. (I and others have complained about the same deterioration in expertise in the United States.)
Second, they describe the years of delay before official recognition that the pathogen Gremmeniella abietina was present in Northern Ireland. This delay resulted from officials refused to accept data from molecular detection tools.
O’Hanlon et al. (2021) add their voice to others criticizing the international phytosanitary system (they cite six major publications: Brasier 2008; Liebhold el. al. 2012; Santini et al. 2012; Eschen et al. 2015; Jung et al. 2016; Meurisse et al. 2019). The failures are (i) visual inspections can miss asymptomatic infections, (ii) limited resources mean only a small proportion of commodities can be inspected, (iii) allowing the use of fungicides masks disease symptoms on plants, (iv) list-based regulations don’t address undescribed organisms and (v) countries vary in how aggressively they carry out the required phytosanitary procedures. O’Hanlon et al. (2021) conclude that “Until these issues are addressed it is likely further increases in the numbers of non-native pests and pathogens of trees will increase.”
The authors note that Eschen et al. (2018) suggested that risk analysis should focus on the commodity (commodity risk assessment) rather than on an individual pest. I have made a similar suggestion, although less clearly worded.
Finally, O’Hanlon et al. (2021) note that climate change is expected to increase the island’s vulnerability to tree-killing pests and pathogens due to fewer frost days, more rain in winter, increased chance of drought in summer, increased average annual temperatures, and more frequent weather extremes. These changes are likely to affect the amount of damage caused by both native and introduced pests organisms. Range shifts in both pests and pathogens and their natural enemies; physiological or behavioral responses in the pests; phenological changes in the hosts; and increased stress on the trees will combine to affect damage.
SOURCES
Donald, F.; Purse, B.V.; Green, S. 2021. Investigating the Role of Restoration Plantings in Introducing Disease—A Case Study Using Phytophthora [UK] Forests 2021, 12, 764
Green, S., D.E.L. Cooke, M. Dunn, L. Barwell, B. Purse, D.S. Chapman, G. Valatin, A. Schlenzig, J. Barbrook, T. Pettitt, C. Price, A. Pérez-Sierra, D. Frederickson-Matika, L. Pritchard, P. Thorpe, P.J.A. Cock, E. Randall, B. Keillor and M. Marzano. 2021. PHYTO-THREATS: Addressing Threats to UK Forests and Woodlands from Phytophthora; Identifying Risks of Spread in Trade and Methods for Mitigation. Forests 2021, 12, 1617 https://doi.org/10.3390/f12121617ý
O’Hanlon, R., Ryan, C., Choiseul, J., Murchie, A.K. and Williams, C. D. 2021 Catalogue of P&P of trees on the island of Ireland. Biology and Enviro
Proceedings of the Royal Irish Academy 2021. Vol. 121, No. 1.12-45 DOI: 10.3318/ BIOE.2021.02
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
Scott Schlarbaum collecting butternuts; photo by F.T. Campbell
I have blogged several times about the need to enhance efforts to breed trees resistant to the most damaging of the hundreds of introduced insects and pathogens. Others concur – see reports by the National Academy of Sciences in 2018; several publications by USFS scientists Richard Sniezko and Jennifer Koch; a workshop hosted by Purdue w/ USFS support, the creation and efforts of several consortia – Whitebark Pine Ecosystem Foundation, Great Lakes Basin Forest Health Collaborative, Forest Restoration Alliance …
Also, Richard J. A. Buggs, of the Royal Botanic Gardens, Kew, recently summarized barriers to tree breeding. It was published as an especially thoughtful editorial in Plants People Planet in anticipation of the International Year of Plant Health in 2020 (see reference at the end of this blog). That issue included several related articles, also noted below.
R.J.A. Buggs’ Perspective on Tree Breeding
Buggs says the need for tree resistance research is greater than ever before. First, damage caused by introduced insects and pathogen is rising. Plus, we now recognize trees’ importance in capturing atmospheric carbon. He sees encouraging signs of growing public awareness of both factors. Also, he thinks citizen science might reduce the cost of some activities … although he doesn’t name which they are.
Dr. Buggs lists six major hindrances to breeding programs, including some aspects that I, at least, have not considered:
1) Trees’ size and long generation times mean research is necessarily slow. One result is it is hard to formulate research proposals that match funding cycles. This in turn means a dependence on long-term institutional commitment from well-funded organizations, and such institutions are rare.
I point out that the U.S. government – especially the USFS – is one such institution. Unfortunately, it has so far been reluctant to take commit major resources to breeding pest-resistant trees. Every year I urge you to lobby Congress on appropriations for the agency. In this context, do you understand that while the USFS Research budget receives approximately $300 million each year, less than $5 million of that total is allocated to researching invasive species (of all taxa)? Some gaps are filled by projects funded by the Forest Health Program. You will have a new opportunity to lobby Congress for Fiscal Year 2023 in the spring!
2) On the other hand, reliance on long-term institutional funding shelters projects from multidisciplinary peer-review that could introduce improved technology or methods. This lack of peer review also contributes to a perception among other scientists that tree resistance research is a scientific backwater.
3) Similarly, studies requiring a long time horizon don’t fit publication schedules. Again, the result is that the findings often appear only in institutional reports or conference proceedings. This means they are hard to find and often lack external peer review at not only the proposal stage but also before publication.
4) The long decades without clear success in dealing with Dutch elm disease (but see recent encouraging developments here) and chestnut blight (see The American Chestnut Foundation here) gave the impression that resistance breeding of forest trees is impossible. Buggs says pest resistance problems are easier to tackle for other trees.
TACF American chestnut; photo by F.T. Campbell
5) Those considering what efforts to fund might demand complete resistance to the pest. This goal is not only unrealistic – it is often unnecessary. Often lower levels of resistance or tolerance can result in trees that can be self-sustaining. Dr. Sneizko concurs; see his article appearing in this issue.
6) Forest stakeholders differ over the goal of developing resistant trees. Some think any human intervention is unwarranted in wilderness areas. Some want a tree as similar as possible to pre-epidemic trees. Others want a tree that produces more timber.
Other Significant Articles
A second article in the same issue of Plants People Planet (Federman and Zankowski) discusses the USDA’s commitment to new approaches in tree resistance research.
I found a third article that discusses British approaches to mitigating tree pests to be more informative than Federman and Zankowski – although somewhat worrying. Spence, Hill and Morris praise the U.K.’s Plant Health Risk Register, which they say has enhanced vigilance on possible new pest introductions. However, the authors describe resistance breeding as a strategy to be considered “when a pest has established such that a tree population is unable to recover, and where a genetic basis for resistance is demonstrable in a proportion of the tree population.” Dr. Sneizko, and others – and I! – call for initiating exploration of the potential for resistance breeding much earlier in an invasion.
A fourth article – by Richard Sniezko and colleagues — describes encouraging levels of partial resistance to white pine blister rust in two western white pines and evidence for both qualitative and quantitative resistance to Phytophtohora lateralis in Port-Orford Cedar.
Port-Orford test seedlings; photo courtesy of Richard Sniezko
A fifth – by Showalter et al. — reports encouraging levels of resistance to both emerald ash borer DMF and ash dieback in European ash. The authors conclude that a breeding program might be a viable solution to both pests.
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
Sean Connery as Hotspur, Shakespeare Henry IV Part I (BBC, “Age of Kings”)
[Starlings – one of the agricultural pests that prompted adoption of the Lacey Act – were introduced to the U.S. because they were mentioned by Shakespeare: Hotspur says “Nay, I’ll have a starling shall be taught to speak nothing but ‘Mortimer,’ and give it him, to keep his anger still in motion.”]
Americans are increasingly aware of the damage caused by invasive species. The law that ostensibly protects our environment from most potentially invasive animals is the Lacey Act – more specifically, the “injurious wildlife” sections of the law, now known as 18 U.S.C. 42 or title 18.
When it was adopted 120 years ago, the Lacey Act was not intended to protect the environment from the full range of possible animal bioinvaders. While Congress amended it several times in the first 60 years of its existence, the law still has many gaps that impede its usefulness for that purpose.
Rep. John F. Lacey via Wikimedia Commons
When first adopted in 1900, the injurious wildlife provisions of the Lacey Act prohibited importation only of wild mammals and birds that posed a threat to agriculture and horticulture. The statute was quite broad in that it prohibited importation of any wild bird or mammal without a permit; there was no requirement that a species be designated as “injurious” to be regulated. The Act was then administered by the U.S. Department of Agriculture. [For a detailed discussion of the Lacey Act’s changing provisions, see Jewell 2020; full reference at the end of this blog.]
In 1960 the Act was amended to expand the list of taxa eligible for designation as “injurious” to include fishes, mollusks, crustaceans, reptiles, and amphibians. Congress also expanded the justifications for listing a species as injurious. It added harm to people, to forestry, or to wildlife or US wildlife resources to the law’s original concerns for agriculture and horticulture. This second change brought the purposes of the Lacey Act closer to the mandate of the U.S. Fish and Wildlife Service (USFWS) – which had assumed responsibility for implementing the Act in 1939.
Unfortunately, Congress simultaneously took other action that greatly weakened USFWS’ ability to use the Act to protect the environment from introduced animals. First, it dropped the requirement that the Secretary approve, with a permit, any importation of a wild bird or mammal.
Second, the 1960 amendment clouded the originally clear prohibition of movement of listed species across state lines. The new language prohibits “any shipment between the continental United States, the District of Columbia, Hawaii, the Commonwealth of Puerto Rico, or any possession of the United States …”
For the next 57 years, the USFWS and Congress sometimes interpreted that language as continuing to prohibit transport between the states within the continental United States. However, this situation could not last. In 2017, acting in a case that had challenged the 2012 listing of several nonnative constrictor snakes as “injurious,” the D.C. Circuit court found that the plain language of §18 U.S.C. 42(a)(1) does not prohibit the transportation of injurious wildlife between states within the continental United States. So now, transportation of injurious wildlife among the continental states is not prohibited by the statute in most circumstances.
Burmese python; photo by R. Cammauf, Everglades National Park via Flickr
The Law’s Strengths
Some aspects of the law have been strengths. Since the term “injurious” has never been defined, the USFWS has been able to use its discretion to list species that are not necessarily invasive themselves but that might cause harm in some other way. For example, the salmon family and 20 genera of salamanders have been listed because they are vectors of harmful wildlife pathogens.
In addition, USFWS has listed entire genera or families of organisms – as long as each species within the taxon has been shown to possess the “injurious” trait(s). This flexibility has probably helped listings aimed at precluding importers from switching from the species that initially raised concerns to related species.
The Law’s Inherent Weaknesses
1) Legal shortfalls
Due to the confusion created by the 1960 amendment, the USFWS now lacks authority to prohibit interstate transport of species listed as “injurious”. This gap undermines the law’s efficacy in controlling spread of listed species once they are established within the U.S.
Also, the law does not prohibit other human actions that pertain to the presence and spread of species listed as “injurious,” e.g., sale, possession, or intra-state transport. Addressing these other aspects of invasive species policy was left to other players, such as states or resource managers.
2) Funding shortfall
Neither the Executive Branch nor Congress has ever provided specific funding for implementation of the Lacey Act. Only one USFWS staffer has the job of listing species under the Act. This situation might change now, since the American Rescue Plan Act adopted in spring 2021 does provide funding over the next five years for listing species that can vector pathogens harmful to people.
Staff’s Evaluation of Its Implementation of the Lacey Act
Since USFWS took over implementation of the Lacey Act in 1939, 36 taxonomic groups have been added to the “injurious wildlife” list. Seven of these listings comprise multiple species – either as genera or families.
Two mammals have been listed since the late 1960s – brushtail possum in 2002 and raccoon dog in 1983. Recent listings have strongly focused on aquatic organisms. This is because the staff is housed in the Fish and Aquatic Conservation program and their expertise is in these species.
silver carp; photo by University of Illinois
Listing activity appeared to be building in the second decade of the 21st Century, with multi-species listings of fish, snakes, and salamanders between 2012 and 2016. However, there has been only one listing action since 2016 – and that was by an act of Congress (listing of the quagga mussel).
In two peer reviewed papers, the USFWS’ Jewell and Fuller provide a history of the Lacey Act’s injurious wildlife title and analyze the effects of listing of 307 species (those listed since 1952). They conclude that 98% of the species listings were “effective” because the listed species either had not been introduced subsequent to listing [288 species; 94% of the total number of listed species] or had not spread to additional states [12 species, 4% of the total]. Another way to calculate the latter figure is to say that 63% of all established species have remained within the state(s) where they were established at the time of listing. Only three species have been spread to additional states by human actions. In these cases, Jewell and Fuller considered the Lacey Act measures to be “ineffective”. For further details on the Jewell and Fuller evaluations of listing efficacy, see their article – full citation given at the end of this blog.
Jewell and Fuller do not evaluate the impacts of animal species introduced to the U.S. after 1960 that have never been listed under the Lacey Act, or speculate about whether listing those species might have minimized the risk of their introduction.
Jewell and Fuller consider listing of species not yet established in the U.S. to be most effective for two reasons. First, listing minimizes the probability that the species will be imported intentionally or unintentionally. Second, listing provides states with risk analyses and other information on which to rely in adopting their own restrictions, including possible prohibitions on sale or possession.
Jewell and Fuller also argue that even in the absence of legal authority to regulate interstate transport of listed species among the continental states, it is still worthwhile to list species that are already established in the U.S. They give six reasons. I summarize those reasons (placing them in my order, not Jewell and Fuller’s):
1) Listing can protect the islands of Hawai`i, Puerto Rico, and the Caribbean and Pacific territories. All are extremely vulnerable to invasive species.
2) If a species shares the traits of injuriousness with other species, particularly those in the same genus or family, then including the already-invasive species demonstrates why the related species should also be listed.
3) Many imported animals carry parasites and pathogens harmful to native species, and stopping the continued importation can reduce those threats that cause disease.
4) Prohibiting further importation of the invasive species can prevent individuals from being introduced to new areas where the species would not otherwise have arrived and can reduce propagule pressure that could introduce hardier individuals.
5) Listing can provide states and other jurisdictions with the technical information they need to pursue additional restrictions not federally authorized under 18 U.S.C. 42, such as transport into a state, possession, and sale.
6) Listing reduces propagule pressure and might enhance the efficacy of any eradication or control measures.
How to Improve the Lacey Act
1) Amend the Lacey Act to restore authority to regulate interstate movement of listed species – including among the continental states and emergency listing authority. Also establish a more streamlined listing process.
2) Strengthen implementation of the law by providing a specific, adequate appropriation to hire additional staff. Utilize the enhanced resources to assess species proactively using risk assessment tools.
It is not yet clear whether the Biden Administration will initiate a more active listing process, especially beyond the zoonotic disease vectors that are the subject of the American Rescue Plan Act.
Note: The “injurious wildlife” section of the Lacey Act (18 U.S.C. 42, or title 18) is separate from another part of the Lacey Act (16 U.S.C. 3371-3378) that is has always been more widely known. This provision regulates wildlife trafficking across State lines. It was later broadened to include plants and trafficking of wildlife and plants from foreign countries.
SOURCES
Jewell S.D. (2020) A century of injurious wildlife listing under the Lacey Act: a history. Management of Biological Invasions. Volume 11, Issue 3: 356–371, https://doi.org/10. 3391/mbi.2020.11.3.01 https://www.reabic.net/journals/mbi/2020/3/MBI_2020_Jewell.pdf
Alternative view – that Lacey Act implementation has failed to protect the U.S. – presented by the following authors:
Fowler, A.J., D.M. Lodge and J. Hsia. 2007. Failure of the Lacey Act to protect US ecosystems against animal invasions. Frontiers in Ecology and the Environment.
Springborn, M. C.M. Romagosa and R.P. Keller. 2011. The value of nonindigenous species risk assessment in international trade. Ecological Economics
Jenkins, P.T. 2012. Invasive animals and wildlife pathogens in the United States: the economic case for more risk assessments and regulation. Biological Invasions
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.
lodgepole pines killed by mountain pine beetle in British Columbia; photo courtesy of Wikipedia
Natural systems, especially forests, could provide as much as 37% of the near-term mitigation necessary to meet Paris global climate goals. In the US, conservation, restoration, and improved land management could provide carbon sequestration equivalent to an estimated 21% of current net annual emissions.
However, the current U.S. forest carbon sink, which includes soils and standing and downed wood as well as live trees, might be in jeopardy due to increasing levels of disturbance, conversion, and/or declining sequestration rates in old growth stands.
Insects and plant diseases are one such disturbance agent. Acting alone or in combination with other forest stressors, they can damage or kill large numbers of trees in short periods of time, thereby reducing carbon sequestration and increasing emissions of stored carbon through decomposition of wood in dead or injured trees.
Historically, native and introduced insects and diseases have impacted an estimated 15% of the total U.S. forest cover annually. This impact is likely to increase. One study (Feiet al., 2019) found that an estimated 41% of the live forest biomass in the contiguous U.S. could be impacted by the 15 most damaging introduced pests already established in the U.S. Continuing introductions of new pests and exacerbated effects of native pests associated with climate change portend worsening losses of live trees. These rising impact of pests, combined with more frequent and severe fires and other forest disturbances, are likely to negate efforts to improve forests’ carbon sequestration capacity.
Sources of information about introduced pests’ impacts is available from, inter alia Campbell and Schlarbaum Fading Forests II and III, Lovett et al 2016, Poland et al. 2021, many blogs on this site, and pests’ profiles posed here under “invasive species” tab. Chapter 4 of Poland et al. (2021) provides a summary of what is known about interactions between invasive species and climate change – both climate impacts on bioinvaders and bioinvaders’ effect on carbon sequestration.
The United States and other major polluting countries have certain advantages. Their strong economies have the scientific and financial resources needed to implement effective invasive species prevention and forest management strategies. At the same time, many of them receive the most new forest pests – because they are major importers. These introduced pests pose the most serious and urgent near-term ecological threat to their forests and all the ecosystem services forests provide.
So, reducing insect and disease impacts to forests can simultaneously serve several goals—carbon sequestration, biodiversity conservation, and protecting the myriad economic and societal benefits of forests. See the recent IUCN report on threatened tree species.
A Major New Study
A new study by Quirion et al. (2021) takes another step in quantifying the threat to U.S. forests’ ability to sequester carbon by analyzing data from National Forest Inventory plots. Unfortunately, the re-measurement data for the period 2001 – 2019 are not available in the NFI for the Rocky Mountain states, which represents a critical data gap in the NFI program. This gap might not have had a significant impact on the national findings, however, because while the insect damage level (measured by an earlier inventory round) was quite severe in the Rocky Mountain States, the relatively slow growth of trees in that region means carbon sequestration rates are low.
Forest stand productivity – and carbon sequestration — will typically decline immediately after pest outbreaks, then recover or even increase beyond pre-outbreak levels depending on the productivity and maximum achieved biomass of replacement plant species and related soil characteristics. However, when prevalence of the disturbance increases, by, for example, more frequent pest outbreaks, carbon stocks in standing trees and sequestration rates can be reduced for extended periods.
Findings
Nationally, insects and diseases have decreased carbon sequestration by live trees on forest land by 12.83 teragrams carbon per year. This equals ~ 9% of the contiguous states’ total annual forest carbon sequestration and equivalent to the CO2 emissions from over 10 million passenger vehicles driven for one year.
This estimate includes the impacts of both native and introduced insects and diseases, because the NFI database does not distinguish between them.
Insect-caused mortality had a larger impact than disease-caused mortality (see below). Forest plots recently impacted by insect disturbance sequestered on average 69% less carbon in live trees than plots with no recent disturbance. Plots recently impacted by disease disturbance sequestered on average 28% less carbon in live trees than plots with no recent disturbance.
Ecoprovinces in which the greatest annual reductions in live tree carbon sequestration due to pests were the Southern Rocky Mountain Steppe, Cascade Mixed Forest, Midwest Broadleaf Forest, and Laurentian Mixed Forest. (Ecoprovinces are outlined – but not named – in Quirion et al. 2021; more complete information is provided in the supplementary material.)
If this study had been carried out in the 1920’s, when chestnut blight and white pine blister rust were spreading across vast areas and killing large trees, the impact of diseases would have been much higher. Today, the most widespread impacts of diseases are on either small trees (e.g., redbay succumbing to laurel wilt) or slow-growing, high-elevation trees (e.g., whitebark and limber pine to white pine blister rust). As long as no equivalents of those earlier diseases are introduced, insects will probably continue to have the larger impacts.
western white pine killed by blister rust; photo from National Archives
Quirion et al. 2021 note that their estimates should be considered conservative. The USFS’s inventory records only major disturbances. That is, when mortality or damage is equal to or exceeds 25% of trees or 50% of an individual tree species’ count on an area of at least 0.4 ha. This criterion largely excludes less severe pest disturbances, including those from which trees recover but which might have temporary negative effects on carbon sequestration.
The study’s authors note that their work has important limitations. The dearth of data from the Rocky Mountain states is one. Other factors not considered include transfers of carbon from live biomass to dead organic matter, soils, and salvaged or preemptively harvested wood products. As trees die from pests or diseases, their carbon becomes dead wood and decays slowly, producing a lag in the carbon emissions to the atmosphere. A small fraction of the carbon in dead wood might be incorporated into soil organic matter, further delaying the emissions. A full accounting of the carbon consequences of pests and diseases would require assessment of these lags, probably through a modeling study.
affects of mountan pine beetle on lodgepole pine in Rocky Mountain National Park, Colorado photo from Wikimedia
Actions to Maintain Carbon Sequestration
Quirion et al. (2021) outline several actions that would help protect the ability of America’s forests to sequester carbon. These suggestions address both native and introduced pests, since both contribute to the threatened reduction in capacity.
Concerning native pests, the authors call for improved forest management, but warn that measures must be tailored to species and environmental context.
Concerning introduced insects and pathogens, Quirion et al. (2021) call for strengthening international trade policies and phytosanitary standards, as well as their enforcement. The focus should be on the principal pathways: wood packaging (click on “wood packaging” category for on this blog site) and imported plants (click on “plants as vectors” category for on this blog site). Specific steps to reduce the rate of introduction of wood-boring insects include enforcement to increase compliance with the international treatment standard (ISPM#15), requiring trade partners – especially those which have repeatedly shipped infested packaging – to switch to packaging made from alternative materials. Introductions via the plant trade could be reduced by requiring foreign shippers to employ integrated management and critical control point systems (per criteria set by the U.S.) and using emergency powers (e.g., NAPPRA) to further restrict imports of the plants associated with the highest pest risk, especially plant species that are congeneric with native woody plants in North America. See Lovett et al 2016; Fading Forests II & III
As backup, since even the most stringent prevention and enforcement will not eliminate all risk, the authors urge increased funding for and research into improved inspection, early detection of new outbreaks, and strategic rapid response to newly detected incursions.
To reduce impacts of pests established on the continent – both recently and years ago – they recommend increasing and stabilizing dedicated funding for classical biocontrol, research into technologies such as sterile-insect release and gene drive, and host resistance breeding.
Thinning is useful in reducing damage by native bark beetles to conifers. However, it has not been successful in controlling introduced pests for which trees do not have an evolved resistance. Indeed, preemptive harvesting of susceptible species can harm forest ecosystems directly through impacts of the harvesting operation and indirectly as individual trees that may exhibit resistance are removed, reducing the species’ ability to develop resistance over time.
Further research is needed to clarify several more issues, including whether introduced pests’ impacts are additive to, or interact with, those of native species and/or other forest stressors.
SOURCE
Quirion BR, Domke GM, Walters BF, Lovett GM, Fargione JE, Greenwood L, Serbesoff-King K, Randall JM & Fei S (2021) P&P Disturbances 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
SOURCES of additional information
Campbell, F.T. and S.E. Schlarbaum. Fading Forest reports at http://treeimprovement.utk.edu/FadingForests.htm
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. Available for download at no cost at https://www.fs.usda.gov/treesearch/pubs/61982
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
Kew Gardens U.K., home to Botanic Gardens Conservation International; Wikipedia
A massive international effort has completed a “Global Tree Assessment: State of Earth’s Trees”. This is the result of five years’ effort; it aims at a comprehensive assessment of the conservation status of all the Earth’s trees. As a result of their work, the authors issue a call to action and include specific recommendations.
The leads were the Botanic Gardens Conservation International (BGCI) and International Union for Conservation of Nature’s (IUCN) Species Survival Commission (SSC) Global Tree Specialist Group. They were assisted by about 60 cooperating institutions and more than 500 individual experts. The Morton Arboretum was a major U.S. contributor. Here, my focus is on the global assessment. An accompanying blog contains my analysis of reports on the Morton Arboretum report for the U.S.
The Global Tree Assessment is the largest initiative in the history of the IUCN Red List process. (This process is described in Box 3 of the report, on p. 12; and on p. 40.) As of the end of 2020, IUCN Red List assessments evaluated 28,463 tree species, representing half of all known tree species. Organizers hope to complete comprehensive conservation assessments of all tree species for inclusion on the IUCN Red List by 2023. Other sources utilized included draft Red List profiles and national-level assessments of those species that are found in only one country.
SUMMARY OF FINDINGS
Using these sources, the Global Tree Assessment evaluated 58,497 tree species worldwide. The study determined that 30% are threatened with extinction. This number could change significantly if a large proportion of the 7,700 species (13.2%) recorded as “Data Deficient” turn out to be at risk. At least 142 species are recorded as already extinct in the wild. Two-fifths (41.5%) are considered to be not at risk. Detailed species’ evaluations are provided at GlobalTreeSearch or GlobalTree Portal.
Brazilian forest converted to cattle pasture
The principal threats to trees globally are forest clearance and other forms of habitat loss (at least 65% of species) and direct exploitation for timber and other products (27% or more). The spread of non-native pests is said to affect 5% of the species.Climate change is having a measurable impact on 4% of the species and is expected to increase. (The situation in the United States differs significantly. Overexploitation plays almost no role and on-going habitat loss is important for only a few of the at-risk species.)
The authors decry the lack of attention, historically, to tree endangerment given trees’ ecological, cultural and economic importance. They hope that increased attention to the biodiversity crisis — an estimated 1 million animal and plant species threatened with extinction — and trees’ importance as carbon sinks will lead to increased conservation of trees and forests. They warn, however, that tree-planting programs must put the right species in the right place, including utilizing species that are under threat. In other words, tree planting practices need to change. They note that a community of botanists and conservationists is ready to assist.
Centers of tree species diversity – and of species under threat – are in Central and South America, followed by the other tropical regions of Southeast Asia and Africa. Fifty-eight percent of tree species are single country endemics. The highest proportion of endemism is found in New Zealand, Madagascar and New Caledonia. The region with the highest proportion of native tree species under threat is tropical Africa, especially Madagascar. The highest numbers of species “Not Evaluated” or “Data Deficient” are in IndoMalaya (tropical Asia) and Oceania. In those regions, about a third of species fall in one of those categories.
forest in Central America
The assessment authors fear ecosystem collapse caused by major, large-scale disturbance events. Examples are recent unprecedented fires in California, southern Australia, Indonesia, and the Amazon (although they don’t mention Siberia). They also note mass mortality events over large areas of forest caused by other factors, including drought and heat stress and the increased incidence of pests. These events have led to a worrying decline of dominant tree species currently evaluated as “Least Concern.” Citing a 2010 report, they list as examples spruce in Alaska, lodgepole pine in British Columbia, aspen in Saskatchewan and Alberta, and Colorado pinon pine (Pinus edulis) in the American southwest.
The authors emphasize the importance of preventing extinction of monotypic tree families. Such events would represent a disproportionate loss of unique evolutionary history, biological diversity, and potential for future evolution. Of the 257 plant families that include trees, 12 are monotypic. They are scattered around the tropics and former Gondwanaland; none is found in the Neo- or Paleoarctic regions. While extinctions to date have rarely affected plants above the rank of genus, the global assessment authors worry that the on-going sixth extinction wave might result in extinctions at the genus or family level.
In this context, the assessment made a particular effort to evaluate the status of species representing the survival of Gondwanian Rainforest lineages. They found that 29% of these tree species are threatened with extinction. Two case studies focus on Australia. They mention habitat conversion but not two non-native pathogens widespread in Australia, Phytophthora cinnamomi andAustropucciniapsidii.
formerly common, now endangered, Australian tree Rhodamnia rubescens, infected by Austropuccinia psidii; photo courtesy of Flickr
The proportion of total tree diversity designated as threatened is highest on island nations, e.g., 69% of the trees on St. Helena, 59% of the trees on Madagascar, 57% of the trees on Mauritius. Hawai`i is not treated separately from the United States as a whole. According to Megan Barstow of BGCI (pers. comm.), the just updated IUCN Red List includes 214 threatened tree species in Hawai`i.
[For the U.S. overall, the IUCN reports 1,424 tree species, of which 342 (24%) are considered threatened. In the companion U.S. assessment, the Morton Arboretum and collaborators found that 11% of 841 continental U.S. tree species are threatened.]
MAIN THREATS TO TREES
Habitat loss
Over the past 300 years, global forest area has decreased by about 40%. Conversion of land for crops and pasture continues to threaten more tree species than any other known threat. Additional losses are caused by conversion for urban and industrial development and transport corridors, and by changes in fire regimes. In total, these factors cumulatively threaten 78% of all tree species, 84% if one includes conversion to wood plantations.
Caribbean mahogany (Swietenia mahogani); photo by Miguel Vieria
Forest Exploitation
Exploitation, especially for timber, is the second greatest threat globally, affecting 27% of tree species (more than 7,400 tree species). The report focuses on centuries of harvest of valuable tropical timbers and exploitation for fuelwood, with an emphasis on Madagascar, where nearly half of all tree species (117 out of 244 tree species) are threatened.
Pests and diseases
Tree species are impacted by a wide range of pests and diseases that are spread by natural and artificial causes. Invasive and other problematic species are recorded as threats for 1,356 tree species (5%) recorded on the IUCN Red List. This figure might be low because some of the information is outdated (see my discussion of American beech in the companion blog about the North American report, here.) Also, climate change is altering the survival opportunities for many pests and diseases in new environments. The example given is the ash genus (Fraxinus), under attack by not only the emerald ash borer in North America and now Russia and Eastern Europe but also the disease Ash Dieback across Europe. The report refers readers to the International Plant Sentinel Network for early warning system of new and emerging pest and pathogen risks, as well as help in coordinating responses.
black ash (Fraxinus nigra) swamp; Flickr
Climate Change
Climate change is impacting all forest ecosystems and is emerging as a significant recorded threat to individual tree species. In the IUCN Red List assessments, climate change and severe weather is recorded as a threat in 1,080 (4%) cases. Trees of coastal, boreal and montane ecosystems are disproportionately impacted. The authors note that the actual impact of climate change is probably more widespread, as it is also impacting fire regimes and the survival, spread, and virulence of pests.
CURRENT CONSERVATION EFFORTS
In Protected areas
Currently, 15.4% of the global terrestrial surface has formal protection status. The IUCN study authors recognize in situ conservation of trees through protection of existing natural habitats as the best method for conserving tree diversity. It is therefore encouraging that at least 64% of all tree species are included in at least one protected area. However, representation is higher for species that are not threatened – 85% are represented in a conservation area while only 56% of threatened trees species are. Nor does the report assess the effectiveness of protection afforded by the various in situ sites. The authors express hope that the parallel IUCN Red List of Ecosystems will contribute to understanding of the efficacy of conservation efforts targetting forests.
The Global Trees Campaign is a joint initiative of Fauna & Flora International (FFI) and BGCI. Since 1999 the campaign has worked to conserve more than 400 threatened tree species in more than 50 countries. The current focus is on six priority taxa = Acer, Dipterocarps, Magnolia, Nothofagus, Oak, and Rhododendron.
Rhododendron in Cook Forest State Park, PA; photo by F.T. Campbell
In Botanic gardens and seed banks
Especially for species under threat, conservation outside their native habitat – ex situ conservation – is an essential additional component. Currently 30% of tree species are recorded as present in at least one botanic garden or seed bank. Again, representation is higher for species that are not threatened – 45% are represented compared to only 21% of threatened tree species. For 41 species, ex situ conservation provides the only hope of survival, since they are extinct in the wild.
AN URGENT CALL FOR ACTION
The authors and collaborators who prepared the Global Tree Assessment hope that this report will help prompt action and better coordination of priorities and resources to better ensure that all tree species are supported by in situ conservation sites and by appropriate management plans. They state several times the importance of restoration plantings relying on native species. The purpose of plantings needs to include conservation of biological diversity, not just accumulation of carbon credits. The Ecological Restoration Alliance of Botanic Gardens (https://www.erabg.org/) is demonstrating that forest restoration can benefit biodiversity conservation. In many cases, propagation methods need to be developed. Also, projects must include aftercare and monitoring to ensure the survival of planted seedlings.
The IUCN assessment notes that ex situ conservation is an important backup. Education, capacity-building and awareness-raising are needed to equip, support, and empower local communities and other partners with the knowledge and skills to help conserve threatened trees.
Policy
The report say it does not address policy and legislation – a gap that fortunately is not quite true. The report both summarizes pertinent international agreements but also provides specific recommendations.
The international agreements that pertain to tree and forest conservation include:
Convention on Biological Diversity (CBD) and several specific programs: the Forestry Programme, Protected Area Programme and Sustainable Use Programme.
Global Strategy for Plant Conservation (GSPC), which is now developing post-2020 targets.
United Nations Framework Convention on Climate Change (UNFCCC) and countries’ implementing pledges to conserve carbon sinks, e.g., REDD+ (Reducing Emissions from Deforestation and Forest Degradation)
United Nations Strategic Plan for Forests 2017-2030
Global Plan of Action for the Conservation and Sustainable Use of Forest Genetic Resources
Convention on International Trade in Endangered Species, which currently protects 560 tree species, including 308 of the most threatened timbers
The report also mentions the voluntary New York Declaration on Forests, under which more than 200 entities – including governments, businesses, and Indigenous communities — have committed to eliminating deforestation from their supply chains. The supply chains touched on include those for major agricultural commodities, production of which is one of the greatest threat to trees.
SPECIFIC RECOMMENDATIONS
1. Strengthen tree conservation action globally through the formation of a new coalition that brings together existing resources and expertise, and applies lessons from the Global Trees Campaign to radically scale up tree conservation.
2. Use information in the GlobalTree Portal on the conservation status of individual tree species and current conservation action to plan additional action at local, national, and international levels, and for priority taxonomic groups. Build on the Portal by strengthening research on “Data Deficient” tree species, and collating additional information threatened species to avoid duplication of efforts and ensure conservation action is directed where it is needed most.
3. Ensure effective conservation of threatened trees within the protected area network by strengthening local knowledge, monitoring populations of threatened species and, where necessary, increasing enforcement of controls on illegal or non-sustainable harvesting of valuable species. Extend protected area coverage for threatened tree species and species assemblages that are currently not well-represented in protected areas.
4. Ensure that all globally threatened tree species are conserved in well-managed and genetically representative ex situ living and seed bank collections, with associated education and restoration programs.
5. Align work with the UN Decade on Ecosystem Restoration 2021–2030, engaging local communities, government forestry agencies, the business community, and other interested parties to ensure that the most appropriate tree species, including those that are threatened, are used in tree planting and restoration programs.
6. Improve data collection for national inventory and monitoring systems and use this information to reduce deforestation in areas of high tree diversity in association with REDD+ and Nationally Determined Contributions (NDCs).
7. Increase the availability of government, private and corporate funding for threatened tree species, and ensure that funding is directed to species and sites that are in greatest need of conservation.
SOURCE
Global Tree Assessment State of Earth’s Trees September 2021 Botanic Gardens Conservation International available here
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
habitat of the Florida torreya tree; photo via Creative Commons
In August, the Morton Arboretum announced completion of a series of reports on the conservation status of major tree genera native to the continental United States. It is available here. The series of reports provides individual studies on Carya, Fagus, Gymnocladus, Juglans, Pinus, Taxus, and selected Lauraceae (Lindera, Persea, Sassafras). (Links to the individual reports are provided at the principal link above.)
The project was funded by the USDA Forest Service and the Institute of Museum and Library Services, The Morton Arboretum and Botanic Gardens Conservation International U.S.
Each report provides a summary of the ecology, distribution, and threats to species in the genus, plus levels of ex situ conservation efforts. The authors hope that the data in these reports will aid in setting conservation priorities and coordinating activities among stakeholders. The aim is to further conservation of U.S. keystone trees.
These reports are part of the overall “Global Tree Assessment: State of Earth’s Trees” compiled under the auspices of Botanic Gardens Conservation International (BGCI) and IUCN SSC Global Tree Specialist Group. I discuss the global assessment in a separate blog to which I will link. The global report evaluates species’ status according to both the International Union of Conservation of Nature’s (IUCN) Red List and NatureServe. The process used is explained in each both the international and U.S. reports. For the U.S. overall, the global assessment identifies 1,424 species of tree, of which 342 (24%) are considered threatened. Hawai`i specifically is home to 241 endangered tree species (Megan Barstow, BGCI Conservation Officer, pers. comm.). See my blogs about threats to Hawaiian trees.
Quercus lobata (valley oak) at Jack London State Park, California
Like the global assessment, these individual studies of nine genera–carried out by the Morton Arboretum–are a monumental accomplishment. They vary in size and format. The report on oaks was completed first and is the most comprehensive. It is 220 pages, incorporating individual reports on 28 species of concern. The report on pines is 40 pages. It contains summary information and tables on all 37 pine species native to the United States, but lacks write-ups on individual species. The report on Lauracae is 25 pages; it evaluates the threat to five species in three genera from laurel wilt disease. The report on walnuts is 23 pages. It includes brief descriptions of six individual species, including butternut. The report on hickories (Carya spp.) is 20 pages. It provides brief description of 11 species. The report on yews is 18 pages. It covers three species. The report on Fagus addresses the single species in the genus, American beech. It is 17 pages. The shortest report is on another single species, Kentucky coffeetree; it is 15 pages.
Coverage of Threats from Non-Native Insects and Diseases in the Morton Arboretum Reports
In keeping with my focus, I concentrated my review of these nine reports on their handling of threats from non-native insects and pathogens. Six of the reports make some reference to pests – although the discussion is not always adequate, in my view. There are puzzling failures to mention some pathogens.
Genera subject to minimal threats from pests (native or non-native) include the monotypic Kentucky coffeetree (Gymnocladus dioicus), whichis considered by the IUCN to be Vulnerable due habitat fragmentation, rarity on the landscape, and population decline.
A second such genus is Carya spp., the hickories. The entire genus is assessed by the IUCN as of Least Concern. The Morton study ranked two species, C. floridana and C. myristiciformis, as of conservation concern.
Three evaluators – the IUCN, the Morton Arboretum, and Potter et al. (2019) – agree that one of the three U.S. yew species, Florida torreya (Taxus floridana or Torreya taxifolia), is Critically Endangered because of its extremely small range, low population, and deer predation. Indeed, Potter et al. (2019) ranked Florida torreya as first priority of all forest trees in the continental United States for conservation efforts. However, the Morton Arboretum analysis makes no mention of the canker disease reported by, among others, the U.S. Forest Service.
A third of the 28 oak (Quercus spp.) species considered to be of conservation concern per the Morton study criteria are reported to be threatened by non-native pests. Pest threats to oak species not considered to be of conservation concerned were not evaluated in the report.
The Morton report records 37 pine species (Pinus spp.) as native to the U.S. Native and introduced insects and pathogens are a threat to many, especially in the West.
Two reports – those on the Lauraceae and beech – focus almost exclusively on threats from non-native pests. The report on walnuts (Juglans spp.) divides its attention between non-native pests and habitat conversion issues. This approach comes into some question as a result of the recent decision by state plant health officials to that thousand cankers disease does not threaten black walnut (J. nigra) in its native range.
black walnut (J. nigra) photo by F.T. Campbell
Here I examine five of the individual genus reports in greater detail.
Oaks
The Morton report says that more than 200 oak species are known across North America, of which 91 are native in the United States. The study concludes that 28 of these native oaks are of conservation concern based on extinction risk, vulnerability to climate change, and low representation in ex situ collections. [The IUCN Red List recognizes 16 U.S. oak species as globally threatened with extinction.] Nearly all of the Morton’s report 28 species are confined to small ranges. In the U.S., regional conservation hotspots are in coastal southern California, including the Channel Islands; southwest Texas; and the southeastern states.
The summary opening section of the Morton report says 10 (36%) of the threatened oaks face a threat by a non-native pathogen. It admits that lack of information probably results in an underestimation of the pest risk. I found it difficult to confirm this overall figure by studying the detailed species reports because in some cases the threatening pathogen is not currently extant near the specific tree species’ habitat. I appreciate the evaluators’ concern about the potential for the pathogen, e.g., Phytophthora ramorum or oak wilt, to spread from its current range to vulnerable species growing on the other side of the continent. However, I wish the overview summary at the beginning of the report were clearer as to which species are currently being infected, which face a potential threat.
The report emphasizes the sudden oak death pathogen (SOD; Phytophthora ramorum), stating that it which currently poses a significant risk to wild populations of Q. parvula. However, the situation is more complex. As I note in my blog on threats to oaks, Q. parvula is divided into two subspecies. In the view of California officials, one, Q. p. var. shrevei, is currently threatened by SOD but the other, Q. p. var. parvula, (Santa Cruz Island oak) is currently outside the area infested by the pathogen. Perhaps the Morton Arboretum evaluators consider the potential risk to the second subspecies to be sufficient to justify stating that the pathogen poses a significant threat to the entire species; but I would appreciate greater clarity on this matter.
The report also mentions the potential threat to several rare oak species in the Southeast if SOD spreads there. While the Morton report rarely discusses species that have not been assessed as under threat, it does note that two species ranked as being of Least concern – coast live oak (Q. agrifolia) and California black oak (Q. kelloggii) – have been highly affected by SOD.
The Fusarium disease vectored by the polyphagous and Kuroshio shot hole borers is mentioned as a threat to Engelmann (Q. engelmannii)and valley (Q. lobata) oaks. The latter, in particular, is considered by the Morton Arboretum assessors to be already much diminished by habitat conversion.
In the East, hydrological changes have facilitated serious damage to Ogelthorpe oak (Q. oglethorpensis) by the fungus that causes chestnut blight–Cryphonectria parasitica.
The Morton study mentions oak wilt (Ceratocystis or Bretziellafagacearum) as an actual or potential factor in decline of oaks in the red oak clade (Sect. Lobatae). Only one of the oak species discussed – Q. arkansana – is in the East, were oak wilt is established. The rest are red oaks in California, where oak wilt is not yet established. Again, there is no discussion of the impact of oak wilt on widespread species not now considered to be of conservation concern.
In the individual species profiles making up the bulk of the Morton report on oaks, but not in the summary, the Morton report also mentions the goldspotted oak borer (Agrilus auroguttatus) as an actual or potential factor in decline of the same oaks in the red oak group. The following species – Q. engelmanni, Q. agrifolia, Q. parvula, Q. pumila — are in California and at most immediate threat.
The Morton study also mentions several native insects that are attacking oaks, and oak decline. It calls for further research to determine their impacts on oak species of concern.
For analyses of the various pests’ impacts on oaks broadly, not focused on at-risk tree species, see my recent blog updating threats to oaks, posted here, and the pest profiles posted at www.dontmovefirewood.org
Pines
The Morton report lists 12 pine species as priorities out of the total of 37 species native to the United States. The report notes that the majority of the at-risk species in the West are threatened primarily by high mortality from one or more pests, in particular native bark beetles.
Six of the 12 priority species are five-needle pines affected by white pine blister rust (WPBR; Cronartium ribicola). The report contains maps showing the distribution of WPBR. In some cases, the native mountain pine beetle (Dendroctonus ponderosae) contributes to immediate mortality. Presentation of recommendations is scattered and sometimes seems contradictory. Thus, P. longaeva (bristlecone pine) is said by the IUCN to be stable and is not listed among the 12 threatened species, but the Morton Arboretum assessors called for its receiving high conservation priority. P. albicaulis (whitebark pine) is a candidate for listing as Threatened under the Endangered Species Act, but the Morton Arboretum authors did not single it out for priority action beyond listing it among the dozen at-risk species.
P. albicaulis (whitebark pine) at Crater Lake National Park; photo courtesy of Richard Sniezko, USFS
The report also notes impacts by Phytopthora cinnamomi on pines; a maps shows the distribution of this non-native pathogen. A third non-native pathogen — pitch canker (Fusarium circinatum) — is mentioned as affecting Monterrey pine (P. radiata). Torrey pine (Pinus torreyana) is also affected by pitch canker, but this pathogen is ranked by the Morton study as causing only moderate mortality in association with other factors. Torrey pine is ranked as critically endangered and decreasing in populations.
The report also publishes the rankings developed by Potter et al. (2019). P. torreyana was the top-ranked pine, ranked at 18 (less urgent than, eastern hemlock).
The Morton study authors concluded that native U.S. pines are under serious threat. However, their economic, ecological, and cultural importance makes them obvious targets for continued conservation priority.
For my analysis of the various pests’ impacts on pines broadly, see the pest profiles posted at www.dontmovefirewood.org
Lauraecae
The Morton group analyzed five of the 13 species native to the United States, chosen based on three factors – tree-like habit, susceptibility to laurel wilt disease, and distribution in areas currently affected by the disease. They note the importance of Sassafras as a monotypic genus.
Horton House before death of the redbay trees; photo by F.T. Campbell
The Morton study notes the conservation status of several species needs changing due to the rapid spread of laurel wilt disease. I applaud this willingness to adjust, although I would be inclined to assign a higher ranking based on the most recent data from Olatinwo et al. (2021), cited here.
Redbay (Persea borbonia) was assessed in 2018 as IUCN Least Concern; it is now being re-assessed, with a probable upgrade to Vulnerable. The Morton study says that recent evidence points towards the ecological extinction of P. borbonia from coastal forest ecosystems. Potter et al. (2019) ranked redbay as fifth most deserving of conservation effort overall.
Silk bay (Persea humilis), endemic to Florida, is currently being assessed for the IUCN; it is recommended that it be designated as Near Threatened.
Swamp bay (Persea palustris) is widespread. It is being assessed for the IUCN; it is recommended for the Vulnerable category.
Sassafras (Sassafras albidum) is widely distributed. Sassafras had been assessed as of Least Concern as recently as the 2020 edition of the IUCN Red List. The Morton study notes that the current distribution of laurel wilt disease spans only a small percent of its range, so it does not pose an imminent threat to sassafras. However, cold-tolerance tests for the disease’s vector indicate the possibility of northward spread into more of the sassafras’ distribution. I note that laurel wilt is currently present in northern Kentucky and Tennessee.
American Beech
The Morton report notes that beech (Fagus grandifolia) is very widespread and a dominant tree in forests throughout the Northeastern United States and Canada. It is the only species in the genus native to North America, so presumably of high conservation interest. The report also notes its ecological importance (see also Lovett et al. 2006).
Beech bark disease is reported by the Morton Arboretum to have devastated Northeastern populations. The disease is well established in all beech-dominated forests in the United States, though it occurs on less than 30% of American beech’s full distribution. After mature beech die, thickets of young, shade-tolerant root sprouts and seedlings grow up, preventing regeneration of other tree species. Nevertheless, American beech was listed as of Least Concern by the IUCN in 2017.
The report makes no mention of beech leaf disease, which came to attention after the Morton assessment project had been almost completed. I think this is a serious gap that undermines the assessment not just of the species’ status in the wild but also of the efficacy of conservation efforts.
healthy American beech; photo by F.T. Campbell
Walnuts
The Morton team evaluated five species of walnut (Juglans californica, J. hindsii, J. major, J. microcarpa, and J. nigra); and butternut (J. cinerea). Thousand cankers disease – caused by the fungus Geosmithia morbida, which is vectored by the walnut twig beetle (Pityophthorus juglandis) – is reported by the Morton team as second in importance to butternut canker. However, as I noted in a recent blog, the states that formerly considered the disease to pose a serious threat no longer think so and are terminating their quarantine regulations. This decision too recent for consideration by the Morton team.
One of the walnuts — Juglans californica (Southern Calif walnut) — is considered threatened by habitat loss. The rest of the walnuts are categorized by the IUCN as of Least Concern.
cankered butternut in New England; photo by F.T. Campbell
Butternut (Juglans cinerea), however, is considered by the IUCN to be Endangered. Although present across much of the Eastern deciduous forest, it is uncommon. It has suffered an estimated 80% population decline as a result of the disease caused by the butternut canker fungus Ophiognomonia clavigignenti-juglandacearum.
SOURCES
Beckman, E., Meyer, A., Pivorunas, D., Hoban, S., & Westwood, M. (2021). Conservation Gap Analysis of Am beech. Lisle, IL: The Morton Arboretum. August 2021
Beckman, E., Meyer, A., Pivorunas, D., Hoban, S., & Westwood, M. (2021). Conservation Gap Analysis of Native U.S. Hickories. Lisle, IL: The Morton Arboretum.
Beckman, E., Meyer, A., Pivorunas, D., Hoban, S., & Westwood, M. (2021). Conservation Gap Analysis of Kentucky Coffeetree. Lisle, IL: The Morton Arboretum.
Beckman, E., Meyer, A., Denvir, A., Gill, D., Man, G., Pivorunas, D., Shaw, K., & Westwood, M. (2019). Conservation Gap Analysis of Native U.S. Oaks. Lisle, IL: The Morton Arboretum.
Beckman, E., Meyer, A., Pivorunas, D., Hoban, S., & Westwood, M. (2021). Conservation Gap Analysis of Native U.S. Pines. Lisle, IL: The Morton Arboretum.
Beckman, E., Meyer, A., Pivorunas, D., Hoban, S., & Westwood, M. (2021). Conservation Gap Analysis of Native U.S. Laurels. Lisle, IL: The Morton Arboretum. August 2021
Beckman, E., Meyer, A., Pivorunas, D., Hoban, S., & Westwood, M. (2021). Conservation Gap Analysis of Native U.S. Walnuts. Lisle, IL: The Morton Arboretum. August 2021
Beckman, E., Meyer, A., Pivorunas, D., Hoban, S., & Westwood, M. (2021). Conservation Gap Analysis of Native U.S. Yews. Lisle, IL: The Morton Arboretum.
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)
Olatinwo, R.O., S.W. Fraedrich & A.E. Mayfield III. 2021. Laurel Wilt: Current and Potential Impacts and Possibilities for Prevention and Management. Forests 2021, 12, 181.
Potter, K.M., M.E. Escanferla, R.M. Jetton, G. Man, B.S. Crane. 2019. Prioritizing the conservation needs of United States tree species: Evaluating vulnerability to forest insect and disease threats. Global Ecology and Conservation (2019), doi: https://doi.org/10.1016/
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
Quercus lobata in Alameda County, California; photo by Belinda Lo via Flickr
Five years ago I posted a blog about the threat to oak trees from non-native insects and pathogens. I am prompted to update what I said then by the publication of a monumental new analysis of endangered oak species (Beckman et al. 2021; full citation at end of blog). This report is packed with maps and graphics displaying centers of endemism, geographic areas with highest threat levels, etc., and individual profiles of all species it deems at risk.
The new study, led by the Morton Arboretum, says there are more than 200 oak species in North America – including Mexico; but only 91 species native to the United States. Of these, 28 species qualify as “of conservation concern” – defined as facing a moderate or greater threat. The principal threats to oak species are small populations or ranges and conversion of habitats for human use. Overall, 10 (36%) of the oak species “of conservation concern” have some actual or potential exposure to established non-native pests.
The report states that two species are significantly threatened by a non-native pathogen: Shreve oak (Quercus parvula) by the sudden oak death pathogen Phytophthora ramorum and Ogelthorp oak (Q. ogelthorpensis) by the chestnut blight pathogen Cryphonectria parasitica.
Several other California oaks are under some level of attack by the polyphagous and Kuroshio shot hole borers. The goldspotted oak borer (GSOB) is mentioned only in the individual species’ profiles, and largely as a potential or undetermined threat. For example, Engelmann oak (Quercus engelmannii) is reported to have suffered some damage from GSOB but that mortality is “likely a result of a complex of factors (e.g., drought and root diseases).” The potential threat from both SOD and oak wilt is mentioned for several of the oaks that are in the red oak subgenus (Erythrobalanus).
The Morton Arboretum’s determination is based on the fact that the non-native insects and pathogens that I described five years ago are attacking primarily widespread species and have not – to date – caused sufficient damage to imperil those species. This situation contrasts sharply with certain Lauraceae (e.g., redbay) threatened by laurel wilt disease; five-needle pines killed by white pine blister rust; eastern or Canadian hemlock killed by hemlock woolly adelgid; and American beech, which now faces threats from beech bark disease, beech leaf disease, and possibly European beech leaf weevil.
Meanwhile, the non-native pests of oaks that I described five years ago continue to spread.
My Update Incorporating Morton Arboretum’s Analysis: Threats in the East
In the East (from the Atlantic Ocean to the Great Plains), oaks are under serious attack from two non-native pests; a third pest has been suppressed by biological control.
oaks killed by European gypsy moth, Shenandoah National Park; photo by F.T. Campbell
The European gypsy moth (Lymantria dispar). The APHIS quarantine map shows its spread to be largely contained. The moth is currently present throughout the Northeast as far west as Wisconsin and neighboring parts of Minnesota and Illinois; and as far south as Currituck and Dare counties in North Carolina. The European gypsy moth continues to be the target of major containment and suppression programs operated by USDA Animal and Plant Health Inspection Service (APHIS), the US Forest Service and the states. For years US Forest Service spent half of its entire budget for studying and managing non-native pests on the European gypsy moth. By FY2021, this allocation had been reduced to a quarter of the total budget. The European gypsy moth is the most widespread non-native pest (see map, linked to above) and attacks a wide range of tree and shrub species. Still, it rarely causes death of the trees.
Oak wilt (caused by the fungus Ceratocystis fagacearum) is widespread from central Pennsylvania to eastern Minnesota and across Iowa, down the Appalachians in West Virginia and North Carolina-Tennessee border, in northern Arkansas and with large areas affected in central Texas. There are several outbreaks in New York State. The most recent map I can find is from 2016 so it is difficult to assess more recent status. In that year, the US Forest Service called oak wilt one of the most serious tree diseases in the eastern U.S. It attacks primarily red oaks and live oaks. It is spread by both bark-boring beetles and root grafts.
In 2016 I also listed the winter moth (Operophtera brumata) as a threat. Now, its presence in coastal areas of New England and Nova Scotia (and British Columbia) has been reduced to almost nuisance levels by action of the biological control agent Cyzenis albicans. (See this report.)
SOD-infested rhododendron plant; photo by Indiana Department of Natural Resources
The most significant potential threat to eastern oaks identified to date is the sudden oak death (SOD) pathogen, Phytophthora ramorum. Several oak species have been shown in laboratory studies to be vulnerable to infection by this pathogen. Furthermore, the climate in extensive parts of the East is considered conducive to supporting the disease. SOD has not been established in the East. However, too frequently SOD-infected plants have been shipped to eastern nurseries, where some are sold to homeowners before regulatory officials learn about the situation and act to destroy the plants.
My Update Incorporating Morton Arboretum’s Analysis: Threats in the West
In the West, millions of oaks have been killed by several pathogens and insects that are established and spreading. Another has been introduced since my earlier blog (see Mediterranean oak beetle, below). Additional threats loom, especially Asian species of tussock moths.
Coast live oaks, canyon live oaks, California black oaks, Shreve’s oaks, and tanoaks growing in coastal forests from Monterey County north to southern Oregon that catch fog/rain are being killed by sudden oak death (SOD). In this region, SOD has killed an estimated 50 million trees. While the preponderance of dead trees are not true oaks, but tanoaks (Notholithocarpus densiflorus), significant numbers of coast live oak (Quercus agrifolia), Shreve oak (Q. parvula var. shrevei), and California black oaks (Q. kelloggii) have also been killed. SOD continues to intensify in this region, and to expand. Sixteen California counties are now infected, and the infection in Curry County, Oregon has spread farther North. More worrying, two additional strains of the pathogen have been detected in forests of the region.
The Morton Arboretum analysis singled out Q. parvula as particularly threatened by SOD. Californians note that it is the subspecies Q. parvula var. shrevei that is threatened by SOD; the other subspecies, Q. parvula var. parvula (Santa Cruz Island oak) is – so far – outside the area infested by SOD.
California black oak killed by GSOB; photo by F.T. Campbell
Also in California, coast live oaks, black oaks, and canyon oaks in the southern part of the state are being killed by goldspotted oak borer. Confirmed infestations are now in San Diego, Orange, Riverside, San Bernardino, and Los Angeles counties. See the map here. At least 100,000 black oaks have been killed in less than 20 years. Neither the State of California nor USDA APHIS has adopted regulations aimed at preventing spread of the goldspotted oak borer.
The Morton Arboretum analysis considers California black oak (Q. kellogii) to be secure.
Two more wood-boring beetles threaten oaks in southern California – the Polyphagous and Kuroshio shot hole borers. One or both of the invasive shot hole borers are known to be present in San Diego, Orange, Los Angeles, Riverside, San Bernardino, Ventura, and Santa Barbara counties. The beetles feed on coast live oaks, canyon live oaks, Engelmann oaks, and valley oaks – as well as many other kinds of trees. In the process, the beetles transmit a fungus that kills the tree. Many of the vulnerable tree species anchor the region’s riparian areas and urban plantings. See a map of the shot hole borers’ distribution here.
In November 2019, scientists discovered a new ambrosia beetle in symptomatic valley oaks (Quercus lobata) trees in Calistoga, Napa County. The insect was determined to be a European species, Xyleborus monographus. The common name is Mediterranean oak borer, or MOB. Within a few months it was known that this beetle is fairly widespread in Napa and neighboring Lake counties. The beetle had never been intercepted at ports in California or found in traps designed to detect bark beetles deployed in the San Francisco Bay area but not including Napa or Sonoma. Like other beetles in the Scolytinae subfamily, MOB can transmit fungi. One of the fungal species detected in the Calistoga infestation is Raffaelea montetyi, which is reported to be pathogenic on at least one European oak species.
The California Department of Food and Agriculture proposed assigning a pest rank to the beetle in December 2020. In their draft document ranking risk, state officials note that a proven host — Q. lobata — is widespread in California and the insect is probably capable of establishing over much of the state. The possible economic impact was described as possibly affecting production of oaks in California nurseries and triggering quarantines.
Therefore, X. monographus could exacerbate the effects of SOD on California oaks.
The Morton Arboretum has singled out Q. lobata as at risk because of conversion of more than 90% of its habitat to agriculture.
Asian gypsy moths swarm a ship in Nakhodka, Russian Far East; USDA APHIS photo
A looming potential threat to oaks on the West coast is the risk that tussock (gypsy) moths could be introduced to the area. The risk is two-fold – the Asian gypsy moth continually is carried to the area on ships bearing imports from Asia (as discussed in my blog in April). The European gypsy moth is sometimes taken across the country from its widespread introduced range in the East on travellers’ vehicles, outdoor furniture, or firewood. Both the West Coast states and USDA search vigilantly for any signs of gypsy moth arrival.
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
