Category: invasive species

Proposed new surveillance approach: focus on the receiving ecosystem

Whitebark pine – a foundational & keystone species in high elevation mountains of the West; photo by Walter Siegmund

A British scientist has proposed a new way to conduct early pest detection surveillance that she thinks will better serve resource managers: prioritize ecosystems which would suffer the greatest alteration if a non-native plant pest decimated one or more plant species. She says scientists should focus on foundational species and maintaining habitat resilience.

Dr. Ruth J. Mitchell leads the Biodiversity and Ecosystems Group within the Ecological Sciences Department at the James Hutton Institute in Aberdeen, Scotland. The Institute works on issues relevant to sustainable management of natural resources. I provide a full citation of her article at the end of the blog.

Dr. Mitchell’s focus is on protecting biological diversity. She worries that introduced plant pests can drive large-scale declines in native plant species. She mentions several examples, including chestnut blight and ash decline. Those declines, in turn, can cause a range of cascading effects on associated species that use the host plant for feeding, breeding and shelter, and on ecosystem functioning. To be prepared to counter this level of risk, managers of natural habitats need to know which habitats and plants are at greatest risk in order to prioritize surveillance of the most likely human actions and sites; and allocate resources to address the most damaging invasions.

Her proposal: prioritize host plant species or habitats which ecological theory indicates an invasion would have the greatest ecological impact. In other words, focus on “foundational species” — plant species that drive key ecosystem functions; or low (plant) diversity habitats — based on the assumption that diverse communities are more stable and resilient than less diverse communities.

Mitchell notes that ecological theory posits that if a foundation species is lost or declines, its disappearance will have a greater effect on the ecosystem than if non-foundation species are impacted. She believes that although there is no list of foundation species, scientific staff can develop appropriate lists for their site. For her study, she made the simplistic assumption that those species that occur at high abundance are most likely to be foundation species. Regarding the second, habitat-resilience criterion, Mitchell assumed that a pest which eliminates a plant species in a low-diversity habitat is likely to have a greater ecological impact on that habitat’s functioning than would extinction of a species in a high-diversity habitat, which is likely to have redundancies.

Mitchell asserts that these approaches to surveillance take account of an invasion’s impacts on broader associated species and ecosystem functions – on biodiversity broadly. These suggested methods have other advantages, too. They avoid the bias in existing lists of pests, which consist predominantly plants of commercial importance; and they don’t need to be updated frequently.

Mitchell identifies four ways to prioritize surveillance efforts based on the potential host rather than the potential pest. The surveillance monitoring might target:

(1) Plant genera known to host the pests (including pathogens) most likely to establish (Host-pest);

(2) Habitats harboring hosts for the greatest number of pests most likely to establish (Habitat-pest);

(3) Plants classed as foundation species (Foundation-species);

(4) Habitats with low plant species diversity and hence low resilience (Habitat-resilience).

Mitchell analyzed the damage that 91 pest species might cause to plant species which occur at 25% or higher cover in 12 broad habitat types in the United Kingdom. As a case study, she also looked at 22 vegetation communities within one of those habitat types (heathland). (See the article for a discussion of how she derived her list of 91 pests, their hosts, and the entity responsible for designating the habitat types.)

For both hosts and habitats, Mitchell compared results of two approaches: (a) assessment based on lists of known known pests; and (b) assessment based on potential ecological impact. Surveillance based on known risks i.e. lists of plant pests(i.e., the Hosts-pest and Habitat-pest methods) assumes that scientists have a complete list of pests, their risk of establishment, and their impacts. We know that is not the case. As an illustration, Mitchell’s review of the literature identified 142 insects or pathogens  hosted by plant genera present on British moorlands that are not listed as pests by the appropriate British authority, the UK Plant Health Risk Register (PHRR).

To conduct a “Foundation-species” surveillance program, one must first identify foundational plant species. Mitchell defined those as species that constitute more than 75% cover in any plant community. (While this is admittedly an oversimplification, Mitchell says that the loss or severe decline of such abundant species will have a major impact on community composition.) One then prioritizes surveillance of these species – regardless of whether they are at risk from a known pest. This method emphasizes attention to potential impact to the habitat or plant community. Furthermore, this approach accommodates detection of the ‘known unknown’ pests.

To conduct a “Habitat-resilience” surveillance program, one must first identify the number of species in each habitat or vegetation community that occur at more than 25% cover. One then prioritizes surveillance of those habitats with the lowest average species diversity.

Differences in results

When basing the analysis on lists of known pests threatening all 12 habitat types, two genera stood out as at particular risk: Prunus and Solanum. Each consists of hosts supporting more than 20 of the 91 pests. Another 17 genera comprised hosts of six or more pests. Many of these genera include species that are important in ornamental horticulture or production forestry. Mitchell considers this a flaw. She points out that different genera ranked highest under this system when the focus narrowed to heathland communities. In heathlands, the genera comprising hosts of the most pests were Calluna, Erica, Festuca and Vaccinium.

American elm – a deserving priority for pest surveillance! Photo by F.T. Campbell

I note that from my perspective – concern about pests that kill native trees – several of the 17 genera included in the “known pests” analysis do raise alarm: Acer, Salix, Ulmus, Fraxinus, Pinus, Quercus, Betula, Viburnum, and Juniperus.

Mitchell then tested the results of focusing on habitat types where the highest number of pests were likely to become established. This method gave highest priority to woodlands – because plants in this habitat type can host 87 of the 91 pests. The second priority should be open habitats (defined as disturbed habitats, arable weed communities, weedy pastures, paths, verges, wasteland and urban habitats). Plants in the “open habitat” type can host 54 pests. (While Mitchell did not specify whether she excluded non-native plant species from her calculations, she does write generally about impacts on native flora – so I believe she did.)

Looking specifically at the 22 heathland vegetation communities, Mitchell identified four communities as able to host the greatest number of pests so deserving surveillance priority.

When she focused on “foundation species”, Mitchell found a range of plant species that occur at 75% or greater cover in each habitat. Again, the highest number (71 species) occur in woodlands; the lowest (11 species) grow in Calcicolous grasslands. In the 22 heathland plant communities, the number of plant species meeting this criterion numbered fewer than five in each. Two communities have no “foundation” species for surveillance since no vascular plant species that occur at 75% cover. In both the habitat and community cases, the surveillance priority of managers of each habitat type would concentrate on the species that fit this criterion for the appropriate biome.

Finally, Mitchell identified those habitats or communities with the lowest species richness / fewest species as being at greatest risk of unravelling if they lose one or more species to an introduced pest. The data indicated these to be the Salt Marsh and Swamps and tall-herb fens systems. At the other end of the spectrum, Mesotrophic grasslands and Woodlands have the lowest priority for surveillance because they are species-rich. Of course, communities within a habitat type vary greatly in their species richness and associated resilience. For example, the one heathland community which has only two species occurring at 25% or greater cover has a higher priority than the communities with more such species.

heath – Erica carnea ; photo by H. Zell

Mitchell asserts that prioritizing plant species or habitats for surveillance based on potential ecological impact rather than risk (known pests) provides a less biased process and allows for the detection of the known unknowns pests. The resulting set of priority surveillance targets differs significantly from the set developed by reliance on pest lists. For example, looking at heathland communities, the Host-pest and Foundation-species methodologies share only three of 24 host genera. The differences arise from the PHRR’s bias oflisting predominantly species relevant to agriculture, horticulture, or forestry. None of these genera is listed under the Foundation-species methodology.

Since trade in plants for planting is the main pathway of introduction of non-native pests, Mitchell concedes that plant species in natural habitats that are closely related to species of commercial importance might be more threatened than other species. However, such an approach takes no account of the potential for a pest to jump hosts.

Prioritization based on potential ecological impact rather than known risk has many advantages. The Foundation-species method prioritizes those plant species whose decline would have the greatest impact on wider biological diversity, ecosystem function and service delivery. That is, it incorporates consideration of the wider risks to the whole ecosystem rather than just the risk to a specific plant species. The Habitat-resilience method similarly takes account of the wider ecosystem level impacts, targeting those habitats or communities that might recover less quickly

On a practical level, these approach do not require surveyors (who might be citizen scientists or land manager) to identify specific pests. Instead, the surveyors report signs of unhealthy-looking plants to the relevant authorities, who then identify the cause.

These methods address a universal problem for plant health: the many pests that are previously unknown before their emergence in new regions and on naïve hosts. Mitchell briefly mentions scientists’ continue struggle to identify traits that can forecast potential pest impacts. [See my blogs re: studies by Mech, Schulz, Raffa]

redbay tree killed by laurel wilt disease – a pathogen unknown until it was introduced to southeastern U.S. Photo by F.T. Campbell

Mitchell suggests several ways to adapt these approaches to other countries or improve their targetting. First, scientists can link various pest/host databases (e.g., EPPO or CABI databases) to landcover or biome data and national or regional vegetation classification systems to make the system appropriate for their country or region. Incorporating attention to dirty equipment and movement of soil &/or plants is fitting at sites undergoing habitat restoration.

It is possible to refine the “foundation species” approach by applying a trait-based approach. She names two examples.

Finally, the Habitat-resilience method could be enhanced by integrating metrics of plant phylogenetic and functional diversity to the idea functional redundancy.

Mitchell stresses the need to unite efforts by many agencies and stakeholders within each country, as well as across political boundaries. She asserts that such collaborative efforts are more efficient / less costly, so lessening the restrictions imposed by resource limits. She also advocates reliance on citizen science and “passive surveillance” or chance observations by professionals agents, land-users and owners. These steps can facilitate large-scale surveillance that would otherwise be financially infeasible.

Mitchell highlights the difficulties imposed by the division of responsibilities. Usually the National Plant Protection Organization (NPPO) is responsible for early detection surveillance. The agency’s goal is to detect pests sufficiently early to facilitate eradication – or at least effective control. Its program  is linked to regulatory requirements under the international plant health system. link to blogs & FF reports While the NPPO’s responsibilities include both cultivated and uncultivated (wild) plants, in many countries the NPPO prioritizes plants with commercial value. (This is certainly true in the United States – see my previous blogs & the Fading Forest reports – links provided below; and apparently the United Kingdom [Dr. Mitchell’s article] and Australia.) Protecting plant health in habitats is usually the task of conservation organizations. Mitchell calls for unifying these programs. CISP is advocating draft legislation that aims to fix this gap in the U.S.  link to Welsh bill

What do you think? Is this approach as promising as Dr. Mitchell believes? Is it feasible?

I certainly concur that pest-based surveillance ignores the various categories of “unknown” pests and focus on commercially important species to the detriment of ecologically important ones. However, can such a system provide “early detection” of introduced pests? We have learned that insects and pathogens causing noticeable damage in natural environments have probably been present in a country or region for years – or decades. Perhaps these ecosystem-based criteria should be applied as guidance for selecting species to be monitored in “sentinel plant” programs. The plantings would be established in situations likely to receive pests early in their invasion process, e.g., warehouse districts (for pests in wood packaging) and ornamental nurseries that import growing stock.

Mitchell says the same issues pertain with regard to wildlife disease. See her article for sources.

SOURCE

Mitchell, R.J. 2024. A host-based approach for the prioritization of surveillance of plant pests and pathogens in wild flora and natural habitats in the UK. Biol Invasions (2024) 26:1125–1137 https://doi.org/10.1007/s10530-023-03233-x

Posted by Faith Campbell

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

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

or

www.fadingforests.org

Read both: a short call to action (41 pp) based on a long report (952 pp!) Then Act!!!

U.S. Department of Agriculture headquarters; lets lobby these people! photo by Wikimedia

Twenty-three  scientists based around the world published a Letter to the Editor titled “Overwhelming evidence galvanizes a global consensus on the need for action against Invasive Alien Species” It appears in the most recent edition of Biological Invasions (2024) 26:621–626.

The authors’ purpose is to draw attention to the release of a new assessment by the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services’ (IPBES).  

The report was issued in September 2023. It is described as the most comprehensive global synthesis of the current knowledge on the bioinvasion process and the impacts of invasive alien species (952 pages!). Its preparation took nearly a decade. Most important, it represents the first consensus among governments and scientists worldwide on the magnitude and extent of the threats that bioinvasions pose to nature, people, and the economy.

The proposed solutions are astoundingly broad and ambitious: transformation of how governments and societies perform. I don’t disagree! However, we need interim steps – “bites of the elephant.”  In my view, the report falls short on providing these.

Our challenge: join others in bringing this analysis to decision-makers’ attention. Can we pull out information that will help persuade U.S. decision-makers – governmental and non-governmental – that the  threat is both urgent and solvable? How do we more effectively advocate for the aggressive, science-based action that we all know is needed?

(I hope that the fact that the report was prepared under the auspices of the Convention on Biodiversity, to which the U.S. is not a party, does not intensify the challenge for us.)

Why we need to restructure the behavior of governments and societies

Bioinvasions are facilitated by policies, decision-making structures, institutions, and technologies that are almost always focused on achieving other goals. Species transport and introduction are driven by policies aimed at promoting economic growth – especially trade. Later stages of invasions, i.e., establishment and some spread, are accelerated by certain uses of land and sea plus climate change. For example, activities that fragment habitats or cause widespread habitat disturbance provide ready places for bioinvasions. Rarely are those who gain by such policies held accountable for the harms they produce via bioinvasions.

To address these unintended consequences, the IPBES report calls for “integrated governance.” Its authors want coordination of all policies and agencies that touch on the indirect drivers, e.g., conservation; trade; economic development; transport; and human, animal, and plant health. Policy instruments need to reinforce – rather than conflict with — strategic invasive species management across sectors and scales. This involves international agreements, national regulations, all governmental sectors, as well as industry, the scientific community, and ordinary people – including local communities and Indigenous Peoples.

The report also calls for establishment of open and inter-operable information systems. This improved access to information is critical for setting priorities; evaluating and improving regulations’ effectiveness; and reducing costs by avoiding duplication of efforts.

Critically important information that is often unspoken:

  • Indirect causes underlying the usual list of human activities that directly promote bioinvasions are the rapid rise of human population and even more rapid rise in consumption and global trade.
  • Biosecurity measures at international borders have not kept pace with the growing volume, diversity, and geographic origins of goods in trade.
  • Continuation of current patterns is expected to result in one-third more invasive species globally by 2050. However, this is an underestimate because today’s harms reflect the consequences of past actions – often from decades ago. Drivers of invasions are expected to grow in both volume and impact.
  • We can prevent and control invasive alien species – but that success depends on the availability of adequate, sustained resources, plus capacity building; scientific cooperation and transfer of technology; appropriate biosecurity legislation and enforcement; and engaging the full range of stakeholders. These require political will.
  • A major impact of bioinvasion is increased biotic homogenization (loss of biological communities’ uniqueness). This concerns us because we are losing the biotic heterogeneity that provides insurance for the maintenance of ecosystem functioning in the face of ongoing global change.
  • The IPBES study asserts that successfully addressing bioinvasions can also strengthen the effectiveness of policies designed to respond to other drivers, especially programs addressing conservation of biological diversity, ensuring food security, sustaining economic growth, and slowing climate change. All these challenges interact. The authors affirm that evidence-based policy planning can reflect the interconnectedness of the drivers so that efforts to solve one problem do not exacerbate the magnitude of others and might even have multiple benefits.

More Key Findings

  • Overall, 9% (3,500) of an estimated 37,000 alien species established in novel environments are invasive (those for which scientists have evidence of negative impacts). Proportions of invasives is high among many taxonomic groups: 22% of all 1,852 alien invertebrates; 14% of all 461 alien vertebrates; 11% of all 141 alien microbes; and 6% of all 1,061 alien plants. (The discussion of probable undercounts relates to aquatic systems and certain geographic regions. However, I believe these data are all undermined by gaps in studies.)
  • Invasive alien species – solely or in combination with other drivers – have contributed to 60% of recorded global extinctions. Invasive species are the only driver in 16% of global animal and plant extinctions. Some invasive species have broader impacts, affecting not just individual species but also communities or whole ecosystems. Sometimes these create complexoutcomes that push the system across a threshold beyond which ecosystem restoration is not possible. (No tree pests are listed among the examples.)

dead whitebark pine in Glacier National Park; photo by National Park Service

  • The benefits that some non-native – even invasive – species provide to some groups of people do not mitigate or undo their negative impacts broadly, including to the global commons. The report authors note that beneficiaries usually differ from those people or sectors that bear the costs. The authors cite many resulting inequities.
  • There are insufficient studies of, or data from, aquatic systems, and from Africa; Latin America and the Caribbean; and parts of Asia.
  • The number of alien species is rising globally at unprecedented and increasing rates. There are insufficient data specifically on invasive species, but they, too, are thought to be rising at similar rates.
  • Horticulure is a major pathway for introducing 46% of invasive alien plant species worldwide.
  • Regarding invasive species’ greater impact on islands,the IPBES report mentions brown tree snakes on Guam and black rats on the Galapagos Islands. It also notes that on more than a quarter of the world’s islands, the number of alien plants exceeds the total number of native ones. See my blogs on non-native plants on Hawai`i and Puerto Rico. In addition, I have posted several blogs regarding disease threats to rare bird species in Hawai`. The IPBES report does not mention these.  

Where the Report Is Weak: Interim Steps

  • The report endorses adoption of regulated species (“black”) lists.
  • The report emphasizes risk analysis of species. Unfortunately IPBES’ analysis was completed before publication of the critique of risk analysis methods by Raffa et al. ( (2023) (see references). However, we must take the latter into consideration when deciding what to advocate as U.S. policy.
  • The report authors call for more countries to adopt national legislation or regulations specifically on preventing and controlling invasive species. (They note that 83% of countries lack such policies). They also list the many international agreements that touch on invasive species-relevant issues. However, Raffa et al. found that the number of such agreements to which a country is a party bears no relationship to the numbers of alien species detected at its border or established on its territory.
  • The challenge to risk assessment posed by multiple sources of uncertainty can be managed by recognizing, quantifying, and documenting the extent of that uncertainty.

Beech leaf disease – one of many non-native pests that were unknown before introduction to a naive ecosystem. Photo by Jennifer Koch, USDA Forest Service

  • I appreciate the report’s emphasis on the importance of public awareness and engagement, but I thought the discussion of effective campaigns lacked original ideas.

The report did not fulfill its own goal of fully exploring unappreciated impacts of policies in its discussion of habitat fragmentation. For example, the report notes that grazing by feral alien ungulates facilitates the spread of invasive alien plant species. However, it does not mention the similar impact by livestock grazing (Molvar, et al. 2024).

SOURCES

Molvar, E.M., R. Rosentreter, D. Mansfield, and G.M. Anderson. 2024. Cheat invasions: History, causes, consequences, and solutions. Hailey, Idaho: Western Watersheds Project, 128 pp.

Raffa, K.F., E.G. Brockerhoff, J-C. GRÉGOIRE, R.C. Hamelin, A.M. Liebhold, A. Santini, R.C. Venette, and M.J. Wingfield. 2023. Approaches to forecasting damage by invasive forest insects and pathogens: a cross-assessment. BioScience 85 Vol. 73 No. 2 (February 2023) https://academic.oup.com/bioscience  

Posted by Faith Campbell

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

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

or

www.fadingforests.org

IUCN Leaders: “We Cannot Solve One Problem by Creating Others”

ash trees killed along Mattawoman Creek in Maryland; photo by Leslie A. Brice

Two important players published documents pressing for “nature-based” solutions to climate change in response to the December 2023 24th Convention of the Parties to the UN Framework Convention on Climate Change.

First, chairs of seven IUCN expert Commissions released a joint statement calling for addressing both climate change and biodiversity loss simultaneously. The elected Commission Chairs represent over 15,000 scientists, scholars, policy makers, economists, lawyers, and other experts who work on issues related to this mission (including me, as well as current and former CISP board members!).

Second, the U.S. Department of the Interior issued detailed guidelines on how to do this.

In this blog, I review the IUCN pronouncement. I will discuss the DOI’s guidelines in a separate blog.

I welcome this statement because I have seen examples of climate “solutions” that worsen the biodiversity crisis. For example, Lugo et al. (2022; full citation at end of this blog) claim to assess the abundance, geographic distribution, contribution to forest structure (including carbon), & temporal trends of non-native tree species. However, they focus almost exclusively on levels of carbon storage. They do not discuss other impacts of non-native tree invasions.

More informative is the 2019 study by Fei et al. ; full citation at end of the blog) that estimated that 41% of total live (woody) biomass in forests of the “lower 48” states was at risk from the most damaging of introduced pests. I pointed out link to blog 159 that elms and beech began dying decades before the underlying (Forest Inventory and Analysis; FIA) data began to be collected. Consequently, the reported mortality rates underestimate the actual loss in biomass associated with these pests. In that blog, I noted that USFS scientists are shifting to new models that will result in a slight bump in overall biomass for the U.S. largely because of increased recognition of the biomass in crowns and limbs.  That methodology has now been published.

the “survivor elm” at Longwood Botanical Garden; photo by F.T. Campbell

I also summarized findings by Badgley et al. (2022) that the California cap-and-trade program does not adequately incorporate sequestration losses tied to mortality of tanoak (Notholithocarpus densiflorus) caused by sudden oak death. I noted that California — and North America as a whole – are home to other tree-killing pathogens and insects.

As the IUCN statement clearly demonstrates, climate change and biodiversity loss are inseparable, interdependent, and mutually reinforcing. However, countries’ and businesses’ approaches now fall short of what scientific evidence indicates is needed.  We must have bold, transformative, and holistic efforts by scientists – and everyone else.

The IUCN’s full statement has 10 points, which the organization’s blog compresses to four:

1. Integrate Climate and Biodiversity Efforts

The climate and biodiversity challenges require coherent, consistent, and integrated actions that simultaneously limit global warming to a maximum of 1.5oC, conserve and sustainably use biodiversity, and restore degraded ecosystems. Only by considering climate and biodiversity as parts of the same complex, systemic challenge can decision-makers develop effective solutions that maximize benefits while minimizing risks.

“green” infrastructure in urban spaces; Washington, D.C.

2. Enhance Ecosystem Integrity

We humans must maintain, enhance, and restore ecosystem integrity in order to halt biodiversity decline and species extinctions and to maintain the ecosystem services that underpin human well-being. Appropriate actions to conserve and restore terrestrial and marine ecosystems also support climate change mitigation, adaptation, and limits on temperature increases. This is true, however, only as long as chosen actions complement—and are not in lieu of—ambitious reductions of greenhouse gas emissions from fossil fuels, industrial processes, and land-use change.

The full IUCN statement also notes that the effects of “nature-based solutions” must be verified through a robust accounting system. IUCN has released separately a Global Standard for Nature-based Solutions  which provides eight specific criteria.

3. Equitably transforming the way we live

Addressing the biodiversity and climate crises will require systemic changes in the way we live. These demand rapid and far-reaching actions across all sectors of a type, scale, and speed never before attempted. IUCN notes, several times, that these transformations must be realized in ways that are equitable and consider impacts on the most vulnerable populations, e.g., indigenous peoples, women, and youth.

IUCN calls for a rapid phase out of fossil fuels, paired with an accelerated and equitable deployment of sustainable clean or renewable energy generation and distribution. In the full statement, IUCN urges countries to avoid relying on unproven — and untested — geoengineering technologies.

4. Prop the Window Open

The window of opportunity to address climate change and biodiversity loss is closing rapidly. Protecting 30% of the Earth’s terrestrial and marine areas by 2030 — a goal adopted by the parties to the Global Biodiversity Convention in late 2022 — will require significant expansion of protected areas in only seven years. I note that while the U.S. is not a party to the biodiversity convention, the Biden Administration has accepted this goal. The IUCN states that achieving this goal depends on greater collaboration across the international agreements on biodiversity, climate change, desertification, and the United Nations’ Sustainable Development Goals. The full statement notes that the United Nations Environment Program (UNEP) calls for tripling expects that funding for nature-based solutions.

old-growth forest in the Pacific Northwest; photo by Richard Orr, via Wikimedia

The IUCN commission chairs warn that delegates at COP28 – and presumably others focused on the climate crisis — must be alert to possible conflicts between biodiversity conservation and climate change mitigation. They cite particularly actions aimed at transitioning energy supplies to “green” sources. This risk arises during choices of sites for solar facilities, wind farms, hydropower dams, and the locations and methods for deep-sea mining for minerals. The IUCN Standard  provides guidance for navigating these conflicts.

SOURCES

Fei, S., R.S. Morin, C.M. Oswalt, and A.M. Liebhold. 2019. Biomass losses resulting from insect and disease invasions in US forests. Proceedings of the National Academy of Sciences Vol. 116 No. 35. August 2019

Lugo, A.E., J.E. Smith, K.M. Potter, H. Marcano Vega, and C.M. Kurtz. 2022. The Contribution of NIS Tree Species to the Structure and Composition of Forests in the Conterminous United States in Comparison with Tropical Islands in the Pacific & Caribbean. USDA USFS General Technical Report IITF-54.

Posted by Faith Campbell

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

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

or

www.fadingforests.org

U.S. Department of the Interior’s Guidance on Nature-Based Solutions

whitebark pine in Glacier National Park killed by white pine blister rust; National Park Service photo

As I noted in the accompanying blog, the U.S. Department of Interior has also weighed in on how to mitigate climate change as part of the Nation’s response to COP24 of the UN Framework Convention on Climate Change.

Interior’s Nature-Based Solutions “Roadmap” (citation at the end of the blog) is 480 pages long! It includes lots of pictures and extensive lists of examples of various types of projects. The document reviews “nature-based” restoration techniques, the benefits they provide in various realms (ecosystem, economy, social values); and the challenges or barriers likely to be encountered. These analyses cover six types of ecosystems – coastal (further divided into five subgroups), forests, grasslands (two types), inland wetland habitats, riverine habitats (three subgroups), and built environments. The obvious emphasis on aquatic and semi-aquatic habitats reflects the Department’s responsibilities. The threat from invasive species is recognized in each case. Plus there are separate chapters discussing management/removal of invasive pests and pathogens, plants, and vertebrates in all types of ecosystems.

The document’s purpose is to provide Interior’s staff – and others who are interested – with reliable information on determining the conditions and goals under which “nature-based” strategies perform best, the benefits they are likely to provide, instructive examples, and additional resources. Much of the information is intended to help staff persuade skeptics that a “nature-based” approach can solve a climate-related problem, such as sea level rise, as well as, or better than, “grey” infrastructure. This includes discussion of: construction and maintenance costs, efficacy in solving a specific problem, and managing conflicts over land use. Also, it considers benefits to other realms, for example, protecting biodiversity and providing opportunities for recreation and mental and physical well-being.

I will focus on aspects dealing with forests. These occur in several chapters. Each chapter has a brief description of the climate and other services provided by that ecosystem type, followed by sections on ways forward (“Technical Approach”), factors affecting site suitability, tools and training resources, likely benefits and outcomes (economic and ecological), barriers and solutions, and examples of projects.

The forest chapter (Chapter 10) discusses forest conservation and restoration with an emphasis on improving forest health, including fuels management, reforestation, and addressing threats from native and non-native pests. One proposed solution is thinning. This measure is said to enhance tree health and promote invasive plants. The “Roadmap” does not recognize that experts consider thinning is helpful in managing native pests such as mountain pine beetle but not non-native pests.

I was startled to find another suggestion – to plant native tree species that are resistant to non-native pests to restore stands. The “Roadmap” refers readers to the National Park Service Resilient Forests Initiative for Region 1 [which reaches from Virginia to Maine]. The Initiative encourages collaboration among parks with similar issues; provides park-specific resource briefs for 39 parks in the Region; and offers management strategies for a host of problems. These include invasive species control, prescribed fire, deer management, silvicultural treatments, tree planting, and fencing. My confusion is that – as far as I know – there are no sources of trees resistant to the non-native pests plaguing forests of the Northeast, e.g., beech, butternut, chestnut, hemlocks, ash, and oaks.

test planting of pathogen-resistant whitebark pine seedlings in Glacier National Park; photo by Richard Sniezko

In the “Tools” section Chapter 10 lists forest restoration guides published by the U.S. Forest Service (USFS) and the International Union of Forest Research Organizations. The “Examples” section includes a few thinning projects.

Chapter 16 advises on enhancing urban forests, which provide many benefits. The chapter stresses the importance of ensuring that projects’ budgets can support protecting trees from such risks as flooding, fire, pests, disease, “invasive species” (presumably other than insects or pathogens), and climate change. The authors note that urban trees are often more susceptible to pests because of their proximity to human activities that facilitate pests’ spread. However, there is no mention that such pests spread to nearby natural forests. They warn against planting a single tree species. An issue noted but not discussed in detail is the use of non-native species in urban forests, some of which have already become invasive.

Three chapters discuss invasive species per se — insects and pathogens (Chap. 26), plants (Chap 27), and vertebrates (Chap. 28) Each chapter summaries invasion stages and stresses the importance of preventing new introductions, detecting them early, and responding rapidly. Most of the text deals with managing established populations – with the emphasis on applying integrated pest management (IPM).  Each raises caveats about biological control agents possibly attacking non-target organisms. Again, the authors emphasize the necessity of ensuring availability of adequate resources to carry out the program.

Chapter 26 addresses Invasive and Nuisance Insects and Pathogens. Examples listed include Asian longhorned beetle, emerald ash borer, hemlock woolly adelgid, spongy moth, Dutch elm disease, sudden oak death, laurel wilt, white pine blister rust, chestnut blight and butternut canker. (All these invaders are profiled under the “invasive species” tab here). The examples also include several native pests, e.g., mountain pine beetle, southern pine beetle, and several pathogens, including Swiss needlecast. I am confused by a statement that priorities for management should be based on pests’ traits; my understanding of the science is that other factors are more important in determining a pest’s impact. See, for example, Lovett et al. 2006.This chapter reiterates the impractical advice to plant trees resistant to the damaging pest. I also wonder at the following statement:

“The process of detection and prevention will need to continue over time to prevent reintroductions or reinvasions of nuisance or invasive pests and pathogens. In some cases, long-term management will be required to contain and prevent spread.” [p. 425] I believe long-term management will required in all cases!

The tools listed in the chapter include various DOI websites re: training and funding; the USDA website listing states’ plant diagnostic laboratories; a USDA IPM “road map”; The Nature Conservancy’s guidebook for assessing and managing invasive species in protected areas; the DOI Strategic Plan; and the University of Georgia’s Center for Invasive Species and Ecosystem Health.

Chapter 27 discusses invasive and nuisance plants. It starts by noting that an estimated 5,000 non-native plant species are stablished in the US. While not all are invasive, there is still potential for these plants to spread and cause harm. The authors state that controlling such plants reduces fire risk and lowers demand for water in arid areas.

The authors say early management is crucial to eradicate or control invasive plant species. Because plant invasions cross property lines, agencies must form partnerships with other agencies and private landowners. Because invasive and nuisance plant species are so widespread, managers must set priorities. The “Roadmap” suggests focusing on sites at the highest risk, e.g., heavily trafficked areas. Continued effort will be necessary to prevent reinvasions or reintroductions. However, long-term management and containment can be incredibly costly and labor-intensive.

lesser celandine invade bottomlands of Delaware Water Gap National Recreation Area

The “Roadmap” complains that many invasive and nuisance plant species are still offered for sale; in fact, that this is the primary pathway by which invasive plants enter the US, (While which we have known this for decades, it is encouraging to see a U.S. government report say:  “Advocating for federal regulation and cohesive local policies for preventing invasive [plant] sales is essential to avoid disjointed state rulings.” – even if it does not specify which agencies should take the lead.  

In the “Tools” section the chapter lists two USFS guides on managing invasive plants; two California Invasive Plant Council guides; the Interior Department’s 2021 Invasive Species Strategic Plan; EDDMapS (a University of Georgia site on which members of the public can report invasive species); and the TNC guidebook for Assessing and Managing Invasive Species in Protected Areas.

Chapter 28 addresses invasive & nuisance vertebrates (called “wildlife”). It notes that invasive animals are present in more than half of all US National parks. It briefly mentions the Lacey Act as providing legal power to curb the introduction and spread of these animals. It does not discuss strengths and weaknesses of this statute, both of which are substantial. This chapter repeats the odd wording from the pest and pathogen chapter – that in some cases long-term management will be required to contain and prevent spread of invasive species. I find it doubtful that short-term actions will be effective in virtually all cases.

Tools listed include Interior guides on IPM, funding sources, and protecting aquatic systems along with the Department of Interior’s 2021 Invasive Species Strategic Plan. Other tools include the USDA guide on IPM, EDDMapS, and the TNC guidebook.

Forests were also mentioned in the discussion of assisted migration of coastal wetlands to avoid drowning by rising seas (Chapter 1). The text notes that forests upland from coastal wetlands might be killed – either as a result of waterlogging as sea levels rise or as deliberate action to make room for the new marsh. Mortality in either case will reduce carbon sequestration. The authors also note the probability that invasive plants – shrubs in the woods, Phragmites on the edge of the wetland — will be present and have to be controlled.

SOURCES

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

Warnell, K., S. Mason, A. Siegle, M. Merritt, & L. Olander. 2023. Department of the Interior Nature-Based Solutions Roadmap. NI R 23-06. Durham, NC: Nicholas Institute for Energy, Environment & Sustainability, Duke University. https://nicholasinstitute.duke.edu/publications/department-interior-nature-based-solutions-roadmap.

Posted by Faith Campbell

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

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

or

www.fadingforests.org

SOD – 3 strains spreading in the West …

locations of P. ramorum in forests of Oregon in 2023

In a recent blog I offered several critiques of APHIS’ new Phytophthora ramorum risk assessment regarding possible establishment of the causal agent of sudden oak death, in the eastern U.S. states. One of my objections was the brevity of its discussion of the likelihood of sexual combination of the recently introduced EU1 strain with the strain established in North America, NA1 and – more recently – NA2.

This blog provides updates on the status of the Phytophthora ramorum invasion in California and Oregon. My information comes primarily from the newsletter posted by the California Oak Mortality Task Force (COMTF),  supplemented by presentations at the recent on-line meeting.  

Research by several scientists, including Tyler Bourret, now with USDA Agricultural Research Service, [summarized in the November 2023 COMTF annual meeting] reported that 216 species are now recognized in the genus Phytophthora.

Establishment of Additional Strains of the Pathogen

Scientists now recognize 12 strains of P. ramorum (Sondreli et al., summarized in COMTF newsletter for August 2023). Three of these strains are established in western North American forests. All three – NA1, NA2, & EU1 – are established in southern Curry County, Oregon. Two of the three – EU1 & NA1– are established in neighboring Del Norte County, California. The genetic lineage of the EU1 population in Del Norte points to a link to the Oregon outbreak.  [Robinson/Valachovic presentation to COMTF annual meeting November 2023] Given the poor record of efforts to prevent additional introductions of P. ramorum to the United States (the APHIS risk assessment notes that the pathogen has been introduced eight to14 times – or more! — in California), continued introductions of strains not yet established in the U.S. appear likely. Once a strain is established in a North American nursery, it is very likely to spread to nurseries – and possibly forests – in other parts of the country. Remember, the risk assessment reported that P. ramorum has probably been moved over a thousand times on nursery stock from West Coast nurseries across the U.S.

P. ramorum-infected Rhododendron; photo by Jennifer Parke, Oregon State University

Why this matters

Phytophthora ramorum can reproduce sexually only when gametes of the two different mating types (A1 & A2) combine. Most of the North American populations are A2 mating type and most European populations are A1. Establishment of the European EU1 in Oregon and California increases the likelihood that sexual reproduction will occur, which in turn increases the probability that the pathogen will evolve. Sexual combination between NA2 (mating type A2) & EU1 (mating type A1) has occurred at least once – in a nursery in British Columbia. Authorities believe this hybrid has been eradicated. However, the possibility of such matings remains.

The most widespread strain in North America is NA1. It was first detected in the forests north of San Francisco in the middle 1990s; and in Oregon in 2001. Infestations of NA1 are now found from central Curry County, Oregon to Monterey County, California.

The EU1 lineage was first detected in Oregon in 2015. How did it get there since it was previously known only in Europe? The outbreak in Del Norte County, California – detected in 2020 – apparently is associated with the Oregon infection. [Robinson/Valachovic presentation to COMTF annual meeting November 2023] Both states attempted eradication, but the strain is well established. By 2023, the Oregon infestation was detected spreading at sites where intensive surveys in previous years detected no symptomatic trees. In California, new centers of infection have been detected along additional tributary creeks in the area. Scientists expect these infections to spread downhill. Control efforts and even surveys have been hampered by a large fire in the area, which diverted needed personnel and funding. [COMTF newsletter for October 2023 & Robinson/Valachovic]

The NA2 lineage has been found in some nurseries in the Pacific Northwest since 2005. The first detection in forests occurred near Port Orford, Oregon in 2021. Port Orford is 30 miles north of Gold Beach – the hitherto northern extent of the SOD infestation. Oregon authorities believed this signaled a new introduction to the state. By 2023, three sites in the state are now infested with this strain. [Ritokova presentation to COMTF annual meeting November 2023] Oregon now focuses its control efforts on NA2 outbreaks near Port Orford.

In California’s Del Norte County, there are now infestations of two strains of opposite mating types ~ 6 miles apart.The forests between them are conducive to infection, so interactions are likely. Robinson & Valahovic [COMTF annual meeting November 2023] ask how land managers should deal with any interactions. I ask – given the likelihood of hybrids forming – shouldn’t the APHIS risk assessment have tried harder to analyze this risk to the East?

Meanwhile, the NA1 strain continues to spread

In Oregon, the NA1 strain has spread 18 miles to the north and eight miles to the east since 2001 [Ritakova COMTF newsletter October 2023]. In California, spread after the wet winter of 2022-2023 has so far been less than expected. The SOD Blitz [Garbelotto at COMTF annual meeting November 2023] found that the statewide rate of positive trees rose from 7.1% in 2022 to 8.8% in 2023. In the Big Sur region some canyons now test negative that once were positive. Scientists think the negative tests reflect the multi-year drought. Scientists expect the spread will be more visible next year – especially if there is a second wet winter.

As noted above, the exception is in Del Norte County – an area described by CAL FIRE forester Chris Lee as a very wet “pathology” site. SOD (NA1 strain) was first detected in the area north of Crescent City in 2019 [Robinson and Valachovic]. This outbreak could not be re-confirmed for three years, despite intensive surveys. But, in 2022, scientists detected a new concentration of dying tanoak. The infected area is near both rare plants associated with serpentine soils and Jedediah Smith State Park, a unit of Redwood National Park. [Robinson] Meanwhile, the infestation of EU1 strain was first detected in 2020; it has expanded in 2022 and 2023.

In addition to spread facilitated by weather, we also see a continuing role in pathogen transfer via movement of shrubs intended for planting. In fall 2022 Oregon authorities were alerted by a homeowner to an outbreak in Lincoln City, Oregon. This was alarming for four reasons:

  • it was 201 miles north of the generally infested area in southern in Curry County.
  • it was well established and had apparently been present for many years.
  • P. ramorum was not detected in any associated waterways, raising questions about the efficacy of this standard detection method for use in community detections.
  • one of the infected plants was a new host: western sword fern (Polystichum munitum).

Fortunately, the infection has not (yet) been detected in nearby natural forests. Perhaps this is because there are no tanoaks this far north.

Detection Difficulties

Forest pathologists report several examples of outbreaks involving dozens of trees or plants suddenly being detected in areas which had been surveyed intensively in preceding years with no detections. See Robinson/Valachovic presentation [COMTF annual meeting November 2023, re: both EU1 & NA1 strains in Del Norte County]. I noted above that streams near the Lincoln City, Oregon neighborhood outbreak did not test positive. Nor did water associated with a positive nursery in Oregon[description of Oregon Department of Agriculture nursery regulatory program in COMTF newsletter for August 2023]. Stream baiting is an important component of detection surveys, so I worry about the possible implications of these negative results.

Identification of Additional Hosts  [all from COMTF newsletter for August 2023.]

  • silverleaf cotoneaster Cotoneaster pannosus (an invasive non-native plant species) 
  • “Mountain Moon” dogwood Cornus capitata [host previously identified in the United Kingdom]
  • western swordfern (Polystichum munitum) (discussed above)
Oregon P. ramorum eradication attempt; photo by Oregon Department of Forestry

Management

Oregon has tried to manage SOD in the forest since its first detection, but the pathogen’s spread and the recent appearance of two additional strains have overwhelmed the program. One hope was to find a less expensive eradication or containment method. For 20 years, attempts to suppress the disease has focused on eradicating local populations of tanoaks (Notholithocarpus densiflorus) because they are the principal host supporting sporulation in Oregon. When an outbreak has been detected and delimited, they first kill the tanoaks with herbicides to prevent resprouting from the roots. The trees are then felled, piled, and burned. This treatment costs $3,000 – $5,000 / acre. Scientists tested whether they could greatly reduce the cost of the suppression programs by leaving tree boles standing after they have been killed by herbicide. Unfortunately, leaving dead, herbicide-killed trees standing increased sporulation, so this approach would probably exacerbate pathogen spread. [See Jared LeBoldus presentation to COMTF annual meeting November 2023]

Worrying Developments in Europe

In Ireland, sudden larch death – caused by the EU1 strain on Japanese larch (Larix kaempferi) – has spread to several counties. This strain is also causing disease on European beech (Fagus sylvatica) & Noble fir(Abies procera) in locations where these tree grow in association with nearby heavily infected Japanese larch. The EU2 lineage was found in late 2021, infecting L. kaempferi at one site.

Several other Phytophthora species are causing disease on trees, including P. lateralis on Lawson’s cypress, Port-Orford cedar (Chamaecyparis lawsoniana); P. pseudosyringae on Japanese larch; and P. austrocedri on trees in the Juniperus and Cupressus genera.

[information about Ireland from R. O’Hanlon, summarized in COMTF newsletter for August 2023]

Regulation

The European Union has relaxed phytosanitary regulation of Phytophthora ramorum. Previously the species – all strains – was considered a quarantine pest. Now its regulatory status depends on the origin of the infected material. “Non-EU isolates” of Phytopththora ramorum are still quarantine pests (presumably the two North American strains [NA1 & NA2] and the eight other strains identified in Asian forests). These pests are treated as the most serious pests in the Union; when they are detected, extensive control actions must be taken. “EU isolates” (presumably EU1 & EU2) are now treated as regulated non-quarantine pests. The focus is to limit the spread of these on plants for planting only.

The European Union and USDA APHIS regulatory emphases differ to some extent (APHIS does not regulate P. ramorum in natural settings, only interstate movement via, inter alia, the nursery trade). However, I am worried that both seem intent on minimizing their regulatory programs.

Arbutus canariensis; photo by Moreno José Antonio via Plantnet

Another region at risk

Macaronesia is a group of several North Atlantic islands,e.g., Madeira and the Azores, Canary, and Cape Verde islands. The islands have climates similar to areas affected by P. ramorum. The Macaronesian laurel forest is a remnant subtropical evergreen forest which shares some plant taxa with those that host the pathogen elsewhere. Moralejo et al. found that, overall, plant species showed considerable tolerance of the pathogen. However, P. ramorum was “rather aggressive” on Viburnum tinus, Arbutus canariensis and Ilex canariensis. Furthermore, mean sporangia production on five Macaronesian laurel forest species was similar to levels on Umbellularia californica, a key host driving the SOD epidemics in California.Moralejo et al. concluded that there is a moderate to high risk of establishment if Phytophthora ramorum were introduced in the Macaronesian laurel forest. [Study summarized in October 2023 COMTF newsletter.]

Important Research

The COMTF August newsletter reports exciting work developing improved detection tools for Phytophthora species, especially P. ramorum. Sondreli, Tabima, & LeBoldus have developed a method to quickly distinguish among the four most common clonal lineages (NA1, NA2, EU1 and EU2). These assays are sensitive to weak concentrations and effective in testing a variety of sample types including plant tissue and cultures. Oregon State University is already using in its diagnostic laboratory.

YuFang, Xia, Dai, Liu, Shamoun, and Wu have developed a simple, rapid, sensitive detection system for the molecular identification of P. ramorum that does not require technical expertise or expensive ancillary equipment. It can be used in laboratory or using samples collected from the field.

Quiroga et al. found that thinning – with or without burning of the slash – significantly reduced stand density and increased average tree size without significantly decreasing total basal area. This effect persisted for five years after treatments – especially when supported by follow-up basal sprout removal. Preventative treatments also significantly increased dominance of tree species not susceptible to Phytophthora ramorum.

In a study summarized in the October 2023 COMTF newsletter, Bourret et al. reported results of nearly 20 years of leaf baiting in watersheds covering an 800-mile section of the Pacific Coast in northern and central California. They found 22 Phytophthora & Nothophytophthora species. Several – including P. ramorum — were abundant and widespread. Some isolates in northern California differ from those found elsewhere. Mitochondrial sequences revealed multiple hybridization events between P. lacustris and P. riparia.

Bourret et al. also found that P. pluvialis is probably native to Western North America. The strain invasive on conifers in New Zealand probably originated in California rather than Oregon or Washington.

Jared LeBoldus and colleagues are studying the ecological impact of tanoak mortality in Oregon forests. [Summarized in November 2023 COMTF newsletter.] They expect impacts at various trophic levels and functions. Preliminary findings regarding the plant community show increases in understory and herbacious species diversity; a shift away from tanoak to Douglas-fir; and increased coarse woody debris. These findings are similar to results from studies in central California by Dave Rizzo and colleagues at UC Davis. LeBoldus is now studying the microbiome of plant leaves; soil mycorrhizal diversity; invertebrates and pollinators (loss of the large annual flower crop of tanoaks presumably affects pollinators). They hope in the future to study small mammal communities (which they expect to be affected by the loss of acorns).

Jared LeBoldus and colleagues also reported early results of genomic studies exploring disease resistance in tanoaks. Various scientists started such studies in the past, but so far all efforts have petered out due to absence of sustained funding, support from agency management, and links to facilities with the necessary tree improvement/breeding resources. (See Richard Sneizko’s description of requirements for resistance breeding, here.) I hope this project proves more sustainable.

Posted by Faith Campbell

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

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

or

www.fadingforests.org

Invasing deer, earthworms, shrubs & more! Managing Forests with Many Risks

a West Virginia forest; photo by Jarek Tuszyński

The Eastern deciduous forest is large and important ecologically. The forest is important for biological diversity: it shelters many endangered species, especially plants, molluscs and fish, mammals, and reptiles. In addition, the majority of forest carbon stocks in the U.S. are those of the eastern states.

But the Eastern deciduous forest is also under many anthropogenic stresses – including high numbers of non-native insects and pathogens, Liebhold map high numbers of invasive plants, blog invasive earthworms, blog browsing by overabundant deer, and timber extraction. In the southern portions of the forest, human populations are expanding, resulting in landscape fragmentation (USDA FS 2023b RPA, full reference at end of this blog).

map showing number of non-native pests in each county, as of ~2010

Agency and academic scientists in the USDA Forest Service Eastern Region (Maine to Minnesota; Delaware to West Virginia, then north of the Ohio River to Missouri) are trying to understand how long-term, continuous stressors, like deer browsing and invasive plants and earthworms, – interact with short-term gap-forming events. They call the long-term stressors “press” disturbances to distinguish them from the short-term “pulse” disturbances (Reed, Bronson, et al.; full citation at end of this blog). Understanding the processes by which forests recover from disturbance is increasingly important. Climate change is expected to raise the frequency and intensity of catastrophic natural disturbances (Spicer and Reed, Royo et al.).

The scientists emphasize that the impacts of these stressors – and effective solutions — vary depending on context.

Invasive Earthworms

USDA APHIS is responsible for regulating introduction of new species. For earthworms, APHIS’ principal concern is clearly the possibility that imported worms or soil might transport pathogens. However, the agency’s website does mention worms’ ability to disrupt the soil and possibly cause undesirable impacts on plant growth and diversity. At the 2023 National Plant Board meeting in early August 2023, Gregg Goodman, Senior Agriculturalist in APHIS PPQ NPB website for agenda? discussed issues that he considers when evaluating whether to grant permits for importing earthworms. APHIS allows imports to be used for fish bait. Dr. Goodman explained that APHIS surveyed fishermen to determine where they dump unused bait. He found no damage to plants along streams, etc. where they are dumped. A state plant health official from a northern state and I objected that the ecosystem damage caused by earthworms is well documented and we doubted that dumping of bait is not a pathway for introducing worms into natural areas.

Reed, Bronson et al. found lower earthworm biomass and density in both deer exclosures and canopy gaps. They hypothesize that the new plant growth associated with canopy gaps attracts deer, resulting in increased browse pressure. That browse pressure then affects the plant community, succession and forest structure. The changed plant community affects soil properties that then affect soil-dwelling fauna like earthworms. They believe the higher worm densities in closed-canopy sites might be the result of nutrient-rich tree leaf litter which provides both shelter and food. Another factor might be lack of recent soil disturbances in closed canopy sites. 

While they say need more research is needed on the complex, combined effects of earthworms and deer, Reed, Bronson et al. still suggest that reducing deer populations or – where that is not possible – creating gaps might help manage earthworm invasions.

Deer Interactions

The long-term, chronic effect of excessive deer herbivory are well documented. See the many presentations at the recent Northern Hardwood research forum (USDA FS 2023b Proceedings). Most studies show that deer browsing overwhelms other disturbances, such as fire and canopy gaps that typically promote seedling diversity. However, recent results refine our understanding.

Samuel P. Reed and colleagues (Reed, Royo et al.) found that on the Allegheny Plateau of western Pennsylvania high deer densities at the time of stand initiation resulted in long-term reduced tree species diversity, density, and basal area. These responses were still detectable nearly four decades later. Stands are dominated by the unpalatable black cherry (Prunus serotina). The reduced stand density and the cherries’ narrower crowns lead to less above-ground biomass and reductions in above-ground carbon stocks. These scientists recommend that managers reduce deer populations to prevent changes in forest structure with probably long-term and important ramifications for many ecosystem functions.

Prunus serotina; photo by Awinch1001 via Flickr

Hovena et al. concurred with the importance of reducing deer densities, but suggested focussing on wet sites where, in their study, deer browsing had its greatest effects. On drier sites deer browsing had no effect on the diversity of woody plant seedlings.

Spicer et al. seek particularly to maintain a heterogeneous landscape to allow coexistence of both early- and late-successional species. In the Eastern Deciduous Forest biome, herbs, shrubs, and vines comprise 93% of the species richness of vascular plants

These authors found that the impact of deer browsing diverged depending on vegetation management actions. In wind-throw gaps where the plant community was retained, deer caused a 14% decline in shrub cover. In contrast, when scientists removed the extant vegetation at the beginning of recovery, deer exclusion caused a 67% increase in shrub cover. The authors speculate that vegetation removal stimulated abundant blackberry (Rubus species) regrowth. Where they had access (in gaps lacking exclosures), deer heavily browsed young Rubus stalks that sprouted after the competing vegetation was cut down. However, when the pre-established vegetation was not removed, older Rubus thickets might have protected other herbs and shrubs from browsing. Spicer et al. did not observe any major shifts in browse-tolerant species in deer-exclusion plots.

Invasive Shrubs

Hovena et al. found that in drier forest plots, the presence of non-native shrubs reduced native seedling abundance, richness, and diversity. Instead there were more seedlings of introduced species, including Lonicera maackii, L. morrowii, Ligustrum sp., and Rosa multiflora. They are concerned that replacement by invasive honeysuckles might be particularly strong in gaps resulting from death of ash trees caused by emerald ash borer. Woodlands could become dominated introduced shrubs, reducing diversity. Consequently, they recommend removing non-native shrubs in drier forests to promote seedling numbers and diversity.

In contrast, in wetter forests basal area of non-native shrubs did not affect introduced seedling abundance. However, the shrubs’ size did promote greater proportions of Lonicera maackii and Ligustrum seedlings. They suggest this might be the outcome of either abundant seed sources or allelopathic properties of some invasive shrubs e.g., L. maackii. In such sites, seedling diversity is already limited to plants that tolerate waterlogging. A hopeful note is that one native shrub, Lindera benzoin, seems able to prevent establishment of L. maackii.

Lonicera maackii; photo by pverdonk via Flickr

Hovena et al. do worry that death of ash trees might lead to declining transpiration rates, raising water tables, and further reducing seedling species richness and diversity.

Impact of Salvage Logging and Vegetation Removal

Spicer et al. studied how anthropogenic stressors affect succession. These scientists took advantage of tornado-caused gaps to compare interactions with deer browsing, salvage logging, and mechanical removal of the understory.

Contrary to expectations, none of these anthropogenic disturbances delayed community recovery or reduced diversity in comparison to the natural disturbance (tornado blowdown). Instead, adding either salvage logging or mechanical removal of understory vegetation substantially enhanced herbaceous species richness and shrub cover.

However, each major plant growth form responded differently. First, none of the manipulations affected species diversity or abundance of tree seedlings and saplings. Second, salvage logging in the wind-throw gaps increased species richness of herbs by 30%. Shrub abundance was doubled and cover almost tripled, but species richness did not change. Third, removing competing understory vegetation caused an increase of 23% in mean herbaceous cover. I have already discussed the impact of excluding deer.

Spicer et al. greet these increases in species richness with enthusiasm; they recommend managing to create a patchwork of combined natural and anthropogenic disturbances to promote plant diversity. However, I have some questions about which species are being promoted.

This study identified a total of 264 vascular plant species: 40 trees, 190 herbs, 15 shrubs, 17 vines, and 2 of unknown growth form. Only about half of these, 123 species, grew in portions of the mature forest not affected by either the tornado or one of the anthropogenic manipulations.

Gaps contained more plant species – as is to be expected. Natural blowdown areas where no manipulation was carried out had 49 more species than the undisturbed forest community (172 species). Blowdown sites subjected to salvage logging added another 53 species for a total of 225 species, or 102 more than the undisturbed reference forest.

A total of 17 species occurred only once in the authors’ data [= unique species]. Eight of these species grew only in the undisturbed forest. Two grew only in the tornado-impacted plots. Spicer et al. do not elaborate on whether these species are officially rare in that part of Pennsylvania – although it seems they might be. I wish Spicer et al. had addressed whether these possibly rare species might be affected by the forest management they recommend, i.e., intentionally creating a patchwork of various disturbances.  An additional seven unique species were found in plots that had been subjected to an anthropogenic disturbance — either salvage logging or removal of remnant vegetation.

nodding trillium (Trillium cernuum); imperiled by restricted range or low populations; photo by Jason Ryndock, Pennsylvania Natural Heritage Program

In tornado-disturbed sites, one native species associated with areas where vegetation was left intact is one of the gorgeous wildflowers of eastern deciduous forests: a Trillium (species not indicated). The one native plant associated with plots from which vegetation was removed was a grass (unspecified).

Spicer et al. report that the proportion of the flora composed of non-native species was very similar between the salvage-logged area (7%) and the undisturbed reference forest (5%). Half of the non-native plant species (3.5% or 9 species) are listed as invasive in Pennsylvania (the article does not list them).

Spicer et al. say these non-native species are relatively uncommon and that they pose a minimal threat. They do concede that the invasive thorny shrub barberry (Berberis thunbergii) was more common in disturbed than intact areas. [I saw plenty of barberry along forest edges in Cook State Forest, which is only 100 miles away from the study site.] I think Spicer and others are too blasé since invasive plant populations can build up quickly when seed sources are present.

Spicer et al. raise two caveats. First, their results regarding the beneficial effects of salvage logging and vegetation manipulation probably will not apply to situations in which vast areas are logged.

More pertinent to us, they warn that their results would also not apply to forest areas in which propagules have been drastically depleted. This can result from previous human land-use or repeated catastrophic disturbances, such as canopy fires. Nor would their results apply to forests that are more threatened by invasive species. They note that a widespread and dense understory of multiple non-native species can create invasional meltdowns, resulting in a lasting depauperate state. This is especially the case when invaders at higher trophic levels, such as earthworms, are part of the mix.

Other Lessons

Reed, Bronson et al. conclude that forest canopies’ responses to disturbance are too variable to be measured by a single method. Evaluating proposals for management will require multiple measures. The overwhelming recommendation of presenters at the recent northern hardwoods research symposium (USDA FS 2023a Proceedings) was to adapt more flexible management strategies to promote forest sustainability and species diversity.

Hovena et al.’s principal finding is that interactions among site wetness, non-native shrubs and the total basal area of trees in the stand had the largest impacts on the species composition of seedlings. In Ohio, site wetness and chronic stressors like deer and introduced shrubs are acting together to shift seedling communities towards fewer native species. Of these three long-term “press” stresses, the interaction between introduced shrubs and soil wetness overshadowed even the impact of deer herbivory on seedling species richness and abundance. Surprisingly, site-specific characteristics – e.g., wetness, canopy tree competition, deer herbivory and introduced shrubs – were more influential than ash mortality in shaping woody seedling communities.

SOURCES

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

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

Reed, S.P, A.A. Royo, A.T. Fotis, K.S. Knight, C.E. Flower, and P.S. Curtis. 2022. The long-term impacts of deer herbivory in determining temperate forest stand and canopy structural complexity. Journal of Applied Ecology.  2022; 59:812-821

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

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

United States Department of Agriculture. Forest Service. 2023b. Future of America’s Forests and Rangelands. Forest Service 2020 Resources Planning Act Assessment. GTR-WO-102. July 2023 https://www.fs.usda.gov/research/treesearch/66413

Posted by Faith Campbell

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

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

or

www.fadingforests.org

Invasive Tree Species in the U.S. Caribbean: New Attention!

African Tulip Tree (Spathodea campanulata) on Puerto Rico; photo by Joe Schlabotnik via Flickr

While it is widely accepted that tropical island ecosystems are especially vulnerable to invasions, there has been little attention to terrestrial bioinvaders in the Caribbean; there has been more attention to marine bioinvaders such as lionfish. I am glad that is starting to change. Here I review a new study by Potter et al. (full citation at end of this blog), supplemented by information from other recent studies, especially Poland et al.

Potter et al. used USFS Forest Inventory and Analysis (FIA) survey data to examine regeneration rates by non-native tree species introduced to the continental United States, Hawai`i, and Puerto Rico. I rejoice that they have included these tropical islands, often left out of studies. They are part of the United States and are centers of plant endemism!

Potter et al. sought to learn which individual non-indigenous tree species are regenerating sufficiently to raise concern that they will cause significant ecological and economic damage in the future. That is, those they consider highly invasive. They defined such species as those for which at least 75% of stems of that species detected by FIA surveys are in their small tree categories – saplings or seedlings. They concluded that these species are successfully reproducing after reaching the canopy so they might be more likely to alter forest ecosystem functions and services. They labelled species exhibiting 60 – 75% of stems in the “small” categories as moderately invasive.

The authors recognize that many factors might affect tree species’ regeneration success, especially at the stand level. They assert that successful reproduction reflects a suite of factors such as propagule pressure, time since invasion, and ability of a species to adapt to different environments.

As I reported in an earlier blog, link 17% of the total flora of the islands of the Caribbean archipelago – including but not limited to Puerto Rico – are not native (Potter et al.). In Puerto Rico, two-thirds of forests comprise novel tree assemblages. The FIA records the presence of 57 non-native tree species on Puerto Rico. Potter et al. identified 17 non-native tree species as highly invasive, 16 as potentially highly invasive, and two as moderately invasive. That is, 33 of 57 nonnative tree species, or 58% of those species tallied by FIA surveyors, are actual or potential high-impact bioinvaders. While on the continent only seven non-native tree species occurred on at least 2% of FIA plots across the ecoregions in which they were inventoried, on Puerto Rico 21 species occurred on at least 2% of the FIA plots (38%). They could not assess the invasiveness of the eight species that occurred only as small stems on a couple of survey plots. These species might be in the early stages of widespread invasion, or they might never be able to reproduce & spread.

The high invasion density probably reflects Puerto Rico’s small size (5,325 mi² / 1,379,000 ha); 500 years of exposure to colonial settlement and global trade; and wide-scale abandonment of agricultural land since the middle of the 20th Century

Naming the invaders

The most widespread and common of the highly invasive non-native tree species are river tamarind (Leucaena leucocephala), on 12.6% of 294 forested plots; algarroba (Prosopis pallida) on 10.9%; and African tuliptree (Spathodea campanulata)on 6.1%. Potter et al. attribute the prevalence of some species largely to land-use history, i.e., reforestation of formerly agricultural lands. In addition, some of the moderately to highly invasive species currently provide timber and non-timber forest products, including S. campanulata, L. leucocephala, Syzgium jambos (rose apple) and Mangifera indica (mango).

Potter et al. contrast the threat posed by Spathodea campanulata with that posed by Syzgium jambo. The latteris shade tolerant and can form dense, monotypic stands under closed canopies. Because it can reproduce under its own canopy, it might be able to remain indefinitely in forests unless it is managed. In contrast S. campanulata commonly colonizes abandoned pastures. Since it is shade intolerant, it might decline in the future as other species overtop it. Meanwhile, they suggest, S. campanulata might provide habitat appropriate for the colonization of native tree species.

Second-growth forest in Caribbean National Forest “El Yunque”

Poland et al. say the threat from Syzgium jambos might be reduced by the accidentally introduced rust fungus Puccinia psidii (= Austropuccinia psidii), which has been killing rose apple in Puerto Rico. In Hawai`i, the same fungus has devastated rose apple in wetter areas.

Potter et al. note that stands dominated by L. leucocephala and Prosopis pallida in the island’s dry forests are sometimes arrested by chronic disturbance – presumably fire. However, they do not report whether other species – native or introduced – tend to replace these two after disturbance. The authors also say that areas with highly eroded soils might persist in a degraded state without trees. The prospect of longlasting bare soil or trashy scrub is certainly is alarming.

Potter et al. warn that the FIA’s sampling protocol is not designed to detect species that are early in the invasion process. However, they do advise targetting eradication or control efforts on the eight species that occurred only as small stems on a couple of survey plots. While their invasiveness cannot yet be determined, these species might be more easily managed because presumably few trees have yet reached reproductive age. They single out Schinus terebinthifolius (Brazilian pepper), since it is already recognized as moderately invasive in Hawai`i. I add that this species is seriously invasive in nearby peninsular Florida and here! APHIS recently approved release of a biocontrol insect in Florida targetting Brazilian pepper. It might easily reach nearby Puerto Rico or other islands in the Caribbean. I am not aware of native plant species in the Caribbean region that might be damaged by the biocontrol agent. However, two native Hawaiian shrubs might be harmed if/when this thrips reaches the Hawaiian Islands. Contact me for specifics, or read the accompanying blog about Potter et al. findings in Hawai`i.

Poland et al. looked at the full taxonomic range of possible bioinvaders in forest and grassland ecosystems. The Caribbean islands receive very brief coverage in the chapter on the Southeast (see Regional Summary Appendices). This chapter contains a statement that I consider unfortunate: “Introduction of species has enriched the flora and fauna of Puerto Rico and the Virgin Islands.” The chapter’s authors assert that many of the naturalized species are restoring forest conditions on formerly agricultural lands. They say that these islands’ experience demonstrates that introduced and native species can cohabitate and complement one another. I ask – but in what kind of forest? These forests, are novel communities that bear little relationship to pre-colonial biodiversity of the islands. Was not this chapter the right place to note that loss? Forests are more than CO2 sinks.

I also regret that the chapter does not mention that the Continental United States can be the source of potentially invasive species (see several examples below).

Mealybug-infested cactus at Cabo Rojo National Wildlife Refuge, Puerto Rico. Photo by Yorelyz Rodríguez-Reyes

The chapter does concede that some introduced species are causing ecological damage now. See Table A8.1. Some of these troublesome introduced species are insects:

  • the South American Harrisia cactus mealybug (Hypogeococcus pungens) is killing columnar cacti in the islands’ dry forests. The chapter discusses impacts on several cactus species and control efforts, especially the search for biocontrol agents.
  • the agave snout weevil (Scyphophorus acupunctatus), native to the U.S. Southwest and Mexico , is threatening the endemic and endangered century plant (Agave eggersiana) in St. Croix & Puerto Rico.
  • Tabebuia thrips (Holopothrips tabebuia) is of unknown origin. It is widespread around mainland Puerto Rico. Its impacts so far are primarily esthetic, but it does apparently feed on both native and introduced tree species in the Tabebuia and Crescentia genera.

The Caribbean discussion also devotes welcome attention to belowground invaders, i.e., earthworms. At least one species has been found in relatively undisturbed cloud forests, so it is apparently widespread. Little is known about its impact; more generally, introduced earthworms can increase soil carbon dioxide (CO2) emissions as through speeded-up litter decomposition and soil respiration.

A factsheet issued by the British forestry research arm DEFRA reports that the pine tortoise scale Toumeyella parvicornis has caused the death of 95% of the native Caicos pine (Pinus caribaea var. bahamensis) forests in the Turks and Caicos Islands (a UK Overseas Territory). The scale is native to North America. It has recently been introduced to Italy as well as to Puerto Rico, and the Turks and Caicos Islands.

SOURCES

Lugo, A.E., J.E. Smith, K.M. Potter, H. Marcano Vega, C.M. Kurtz. 2022. The Contribution of Non-native Tree Species to the Structure & Composition of Forests in the Conterminous United States in Comparison with Tropical Islands in the Pacific & Caribbean. USFS International Institute of Tropical Forestry General Technical Report IITF-54.

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. Especially the Appendix on the Southeast and Caribbean. Springer Verlag. Available gratis at https://link.springer.com/book/10.1007/978-3-030-45367-1

Potter K.M., Riitters, K.H. & Guo. Q. 2022. Non-nativetree regeneration indicates regional & national risks from current invasions. Frontiers in Forests & Global Change Front. For. Glob. Change 5:966407. doi: 10.3389/ffgc.2022.966407

Posted by Faith Campbell

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

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

or

www.fadingforests.org

Global Overview of Bioinvasion in Forests

black locust – one of the most widespread invasive tree species on Earth; photo via Flickr

In recent years there has been an encouraging effort to examine bioinvasions writ large see earlier blogs re: costs of invasive species – here and here. One of these products is the Routledge Handbook of Biosecurity and Invasive Species (full citation at end of this blog). I have seen only the chapter on bioinvasion in forest ecosystems written by Sitzia et al. While they describe this situation around the globe, their examples are mostly from Europe.

Similar to other overviews, this article re-states the widely-accepted attribution of rising numbers of species introductions to globalization, especially trade. In so doing, Sitzia et al. assert that the solution is not to curtail trade and movement of people, but to improve scientific knowledge with the goal of strengthening biosecurity and control programs. As readers of this blog know, I have long advocated more aggressive application of stronger restrictions on the most high-risk pathways. Still, I applaud efforts to apply science to risk assessment.

Sitzia et al. attempt to provide a global perspective. They remind readers that all major forest ecosystems of Earth are undergoing significant change as a result of conversion to different land-uses; invasion by a wide range of non-native introduced species—including plants, insects, and mammals; and climate change. These change agents act individually and synergistically. Sitzia et al. give greater emphasis than other writers to managing the tree component of forests. They explain this focus by asserting that forest management could be either the major disturbance favoring spread of non-native species or, conversely, the only way to prevent further invasions. They explore these relationships with the goal of improving conservation of forest habitats.

Japanese stiltgrass invasion; photo by mightyjoepye via Flickr

Sitzia et al. focus first on plant invasions. They contend that – contrary to some expectations – plants can invade even dense forests despite competition for resources. They cite a recent assessment by Rejmánek & Richardson that identified 434 tree species that are invasive around Earth. Many of these species are from Asia, South America, Europe, and Australia. These non-native trees can drive not only changes in composition but also in conservation trajectories in natural forests. However, the example they cite, Japanese stilt grass (Microstegium vimineum) in the United States, is not a tree! Sitzia et al. note that in other cases it is difficult to separate the impacts of management decisions, native competitive species, and non-native species.

Sitzia et al. note that plant invasions might have a wide array of ecological impacts on forests. They attempt to distinguish between

  • “drivers” of environmental change – including those with such powerful effects that they call them “transformers”;  
  • “passengers” whose invasions are facilitated by other changes in ecosystem properties; and
  • “backseat drivers” that benefit from changes to ecosystem processes or properties and cause additional changes to native plant communities.

An example of the last is black locust (Robinia pseudoacacia). This North American tree has naturalized on all continents. It is a good example of the management complexities raised by conflicting views of an invasive species’ value, since it is used for timber, firewood, and honey production.

Sitzia et al. then consider invasions by plant pathogens. They say that these invasions are one of the main causes of decline or extirpations in tree populations. I applaud their explicit recognition that even when a host is not driven to extinction, the strong and sudden reduction in tree numbers produces significant changes in the impacted ecosystems.

American chestnut – not extinct but ecological role gone; photo by F.T. Campbell

Sitzia et al. contend that social and economic factors determine the likelihood of a species’ transportation and introduction. Specifically, global trade in plants for planting is widely recognized as being responsible for the majority of introductions. Introductions via this pathway are difficult to regulate because of the economic importance (and political clout) of the ornamental plants industry, large volumes of plants traded, rapid changes in varieties available, and multiple origins of trade. As noted above, the authors seek to resolve these challenges by improving the scientific knowledge guiding biosecurity and control programs. In the case of plant pathogens, they suggest adopting innovative molecular techniques to improve interception efficiency, esp. in the case of latent fungi in asymptomatic plants.

The likelihood that a pathogen transported to a new region will establish is determined by biogeographic and ecological factors. Like other recent studies, Sitzia et al. attempt to identify important factors. They name a large and confusing combination of pathogen- and host-specific traits and ecosystem conditions. These include the fungus’ virulence, host specificity, and modes of action, reproduction, and dispersal, as well as the host’s abundance, demography, and phytosociology. A key attribute is the non-native fungus’ ability to exploit micro-organism-insect interactions in the introduced range. (A separate study by Raffa et al. listed Dutch elm disease as an example of this phenomenon.)  I find it interesting that they also say that pathogens that attack both ornamental and forest trees spread faster. They do not discuss why this might be so. I suggest a possible explanation: the ornamental hosts are probably shipped over wide areas by the plant trade.

surviving elms in an urban environment; photo by F.T. Campbell

Sitzia et al. devote considerable attention to bioinvasions that involve symbiotic relationships between bark and ambrosia beetles and their associated fungi. These beetles are highly invasive and present high ecological risk in forest ecosystems. Since ambrosia beetle larvae feed on symbiotic fungi carried on and farmed by the adults inside the host trees, they are often polyphagous. Bark beetles feed on the tree host’s tissues directly, so they tend to develop in a more restricted number of hosts. Both can be transported in almost all kinds of wood products, where they are protected from environmental extremes and detection by inspectors. Sitzia et al. specify the usual suspects: wood packaging and plants for planting, as ideal pathways. These invasions threaten indigenous species by shifting the distribution and abundance of certain plants, altering habitats, and changing food supplies. The resulting damage to native forests induces severe alterations of the landscape and causes economic losses in tree plantations and managed forests. The latter losses are primarily in the high costs of eradication efforts – and their frequent failure.

Eucalyptus plantation in Kwa-Zulu-Natal, South Africa; photo by Kwa-Zulu-Natal Department of Transportation

Perhaps their greatest contribution is their warning about probable damage caused by invasive forest pests in tropical forests. (See an earlier blog about invasive pests in Africa.) Sitzia et al. believe that bark and ambrosia beetles introduced to tropical forests threaten to cause damage of the same magnitude as climate change and clear cutting, but there is little information about such introductions. Tropical forests are exposed to invading beetles in several ways:  

1) A long history of plant movement has occurred between tropical regions. Sitzia et al. contend that the same traits sought for commercial production contribute to risk of invasion.

2) Logging and conversion of tropical forests into plantation forestry and agriculture entails movement of potentially invasive plants to new areas. Canopies, understory plant communities, and soils are all disturbed. Seeds, insects, and pathogens can be introduced via contaminated equipment.

3) Less developed nations are often at a disadvantage in managing potential invasion. Resources may be fewer, competing priorities more compelling, or potential threats less obvious.

Sitzia et al. call for development of invasive species management strategies that are relevant to and realistic for less developed countries. These strategies must account for interactions between non-native species and other aspects of global environmental change. Professional foresters have a role here. One clear need is to set out practices for dealing with conflicts between actors driven by contrasting forestry and conservation interests. These approaches should incorporate the goals of shielding protected areas, habitat types and species from bioinvasion risk. Sitzia et al. also discuss how to address the fact that many widely used forestry trees are invasive. (See my earlier blog about pines planted in New Zealand.)

planted forest in Sardinia, Italy; photo by Torvlag via Flickr

In Europe, bark beetle invasions have damaged an estimated ~124 M m2 between 1958 and 2001. Sitzia et al. report that the introduction rate of non-native scolytins has increased sharply. As in the US, many are from Asia. They expect this trend to increase in the future, following rising global trade and climate change. Southern – Mediterranean – Europe is especially vulnerable. The region has great habitat diversity; a large number of potential host trees; and the climate is dry and warm with mild winters. The region has a legacy of widespread planting of non-native trees which are now important components of the region’s economy, history and culture. These include a significant number of tree species that are controversial because they are – or appear to be – invasive. Thus, new problems related to invasive plants are likely to emerge.

Noting that different species and invasion stages require different action, Sitzia et al. point to forest planning as an important tool. Again the discussion centers on Europe. Individual states set forest policies. Two complications are the facts that nearly half of European forests are privately owned; and stakeholders differ in their understanding of the concept of “sustainability”. Does it mean ‘sustainable yield’ of timber? Or providing multiple goods and services? Or sustaining evolution of forest ecosystems with restrictions on the use of non-native species? Resolving these issues requires engagement of all the stakeholders.

Sitzia et al. say there has recently been progress. The Council of Europe issued a voluntary Code of Conduct on Invasive Alien Trees in 2017 that provides guidelines on key pathways. A workshop in 2019 elaborated global guidelines for the sustainable use of non-native tree species, based on the Bern Convention Code of Conduct on Invasive Alien Trees. The workshop issued eight recommendations:

  • Use native trees, or non-invasive non-native trees;
  • Comply with international, national, and regional regulations concerning non-native trees;
  • Be aware of the risk of bioinvasion and consider global change trends;
  • Design and adopt tailored practices for plantation site selection and silvicultural management;
  • Promote and implement early detection and rapid response programs;
  • Design and adopt practices for invasive non-native tree control, habitat restoration, and for dealing with highly modified ecosystems;
  • Engage with stakeholders on the risks posed by invasive NIS trees, the impacts caused, and the options for management; and
  • Develop and support global networks, collaborative research, and information sharing on native and non-native trees.

SOURCE

Sitzia, T., T. Campagnaro, G. Brundu, M. Faccoli, A. Santini and B.L. Webber. 2021 Forest Ecosystems. in Barker, K. and R.A. Francis. Routledge Handbook of Biosecurity and Invasive Species. ISBN 9780367763213

Posted by Faith Campbell

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

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

or

www.fadingforests.org

Eradicating Invasive Species: You need “social license” to succeed

spread of non-native conifers in mountains of New Zealand; photos by Richard Bowman; New Zealand government website

As those of us who want to “do something” to counter bioinvasions struggle to mobilize both the  resources and the political will necessary, I rejoice that more studies are examining what factors affect “social license” [= public approva] for such programs. One such study was recently published in New Zealand — Mason et al. (full citation at the end of the blog). New Zealand enjoys a greater appreciation of the uniqueness of its biology and awareness of invasive species’ impacts than the United States. However, their findings might provide useful guidance in the US and elsewhere.

Mason et al. sought to understand motivations of, and constraints on, those local groups responsible for controlling the spread of non-native conifers into New Zealand’s remnant native ecosystems. Non-forest ecosystems across much of the country are at risk of rapidly transforming into exotic conifer forests. For these reasons, authorities are pressing for timely removal of existing seed sources, that is, mature non-native conifer trees of several species. The blog I posted earlier apparently describes effects of conifer invasions in lowland ecosystems, whereas the Programme described here is focused on high-elevation systems.

The eradication effort in the study is the National Wilding Conifer Control Programme, establishedin 2016. A large increase in funding provided during the COVID-19 lockdown made it practical to try to eradicate seed sources from large swathes of vulnerable land. The Programme coordinates control efforts across the country, working across property and land-tenure boundaries. Landowners are expected to cover 20% of the cost of removing conifers from their land. Since removing all seed sources of high-risk conifer species from the landscape is key to achieving long-term goals, success is unlikely if significant seed sources are allowed to persist.

Mason et al. combined workshops, questionnaires, and site visits to gather data on particular aspects of this Programme. They found that social resistance, rather than lack of scientific knowledge, was often the main barrier to success in managing widespread invasive species. The authors do not address whether the fact that only 30 people provided information for their study might undermine the reliability of their findings.

map of conifer wilding sites; adapted from Wilding conifers – New Zealand history and research background, a presentation by Nick Ledgard at the “Managing wilding conifers in New Zealand – present and future” workshop (2003)

The authors suggest that the main benefit of scientific information might be to increase stakeholders’ support for management interventions — rather than to guide manager’ decisions about which strategies to pursue.  To support social license, invasive species research programs might need to focus not only on cost-effective control technologies and strategies, but – perhaps especially — the benefits (both tangible and intangible) of invasive species control for society.

Mason et al. found that people were motivated to combat conifer invasions by impacts with direct influence on humans or human activities (e.g., reduced water yield, damage to infrastructure from wildfires, reduced tourist activities due to landscape transformation) and also by impacts affect ecosystems (e.g.,  impacts on biodiversity, aquatic ecosystems and landscapes).  

People objected to control or eradication programs primarily because of social concerns. These included the unwillingness of landowners to participate and regulatory frameworks that had perverse incentives.

Mason et al. called for greater efforts by scientists to persuade stakeholders[p1]  to allow removal of “wilding” conifers from private land and development of more appropriate regulations. They found that forecasting models were particularly effective in persuading people to support these efforts. It seems to me that outreach teams might need “translators” to convert scientists’ findings to information that would be more useful by stakeholders.

The authors concede that the “wilding conifer” situation has unique attributes. First, invading conifers present a stark, easily seen difference between native and invaded ecosystems. Second, some – but not all—stakeholders appreciate the uniqueness of New Zealand’s biomes. Third, the impacts of conifer invasion are sufficiently well known that they can be described succinctly and accurately.

Do these unique attributes undercut the relevance of this study to North America? It is still true that ongoing support from local stakeholders (including landowners and community groups) influences the effectiveness or profitability of managing invasive species. .It is also true that groups’ varying values affect willingness to support the activities.

Mason et al. think through the issue of stakeholders’ conflicting perspectives on the value of particular invasive species and the values threatened by that invader. These can include ethical or safety concerns around management methods, particularly regarding toxins and genetic modification. Economoic costs are also a factor – especially if the landowner must pay all or some of them.

I find it interesting that the government simultaneously funded a 5-year research program to study various issues regarding the spread, ecosystem impacts, and control of wilding conifers. The result is the Mason et al. study discussed here. I wish the U.S. funded independent analyses of its invasive species programs!

Pinus contorta – the most rapidly growing Pinus introduced to New Zealand; photo by Walter Siegmund / Wikimedia

More Details, Policy Suggestions

Workshop attendees unanimously identified landscape impacts as a reason for controlling wilding conifers. This primarily concerned the loss of New Zealand’s visual heritage or cultural identity rather than loss of native species’ habitats. When the landowner was raised in Europe, these cultural or heritage values sometimes had the opposite effect, since they see conifer forests as important components of “natural” landscapes.

Currently, landowner funding and permission is required for conifer removal. Some individual landowners want to establish new forestry plantings. Some resist removal of existing forestry plantations (which provide income) and shelter belts (which provide shelter for livestock in high country landscapes). Some landowners were unwilling to pay their 20% of removal costs. Or they objected to certain conifer control methods—particularly helicopter spraying of herbicides. New Zealand’s regulatory process also requires years of negotiating to remove standing trees – further delaying any action. In theory, landowners who resist removal could be prosecuted under the Biosecurity Act. However, this approach has never been tried for removing wilding conifers.

Mason et al. suggested several changes in policy to overcome some of these barriers.

First, forestry consultants can “game” the wilding conifer “risk calculator” to obtain government approval to establish conifer plantations in high-risk environments. The authors suggest that authorities create a “liability calculator.” Under this system, landowners wishing to retain conifers on their land for whatever reason would be liable for any subsequent containment costs. However, developing such a tool requires more finely-scaled models of conifer spread.

Second, given the high costs of combatting invading conifers if seed sources are allowed to persist, they suggested that it might be more cost-effective for the control program to pay for plantation removal under New Zealand’s Emissions Trading Scheme.

Given the overwhelmingly social and regulatory nature of barriers to success, the primary role for scientific information is providing assessments of outcomes in the absence of wilding conifer control. Preferred messages were return-on-investment estimates and forecasts of ecosystem impacts, particularly relating to biodiversity loss, water yield reduction, and wildfire hazard. Forecasts were key to demonstrating that management interventions reduced future control costs and avoided environmental impacts which large sections of the community value (i.e. biodiversity loss, reduction in water yield and agricultural productivity, increased wildfire risks). Practitioners felt that forecasting models might also channel research toward areas of high uncertainty. Mason et al. recognize the difficulties presented by inherent complexity of ecological systems. However, they think “good practice” guidelines on forecasting are emerging.

The authors find that information content and presentation need to be tailored to the various audiences – most of whom lack experience in interpreting data from environmental forecasting models. They suggest that outreach materials focus on clear illustration of the tangible and intangible benefits of wilding conifer management rather than detailed explorations of scenarios. Participants suggested ways to improve the web tool to make it more accessible to a non-expert audience. 

Mason et al. mention aspects that require balancing, but don’t suggest criteria for making these choices. They say it is important to include all relevant stakeholders in invasive species management governance bodies. The absence of stakeholders with positive attitudes to wilding conifer invasions led to unanticipated external social resistance to the Programme. They recognize that including stakeholders with conflicting interests might obstruct the decision-making process.  Also, in areas where there has been success in containing conifers’ spread, people can’t see invading trees, so they don’t recognize the problem. They also note that existing data do not adequately recognize risks of spread from deliberately planted seed sources such as shelter-belts, plantations and amenity plantings. The authors do not discuss how to integrate these data into analyses and public outreach.

Finally, Mason et al. recognize that many other factors strongly influence stakeholders’ willingness to support invasive species control programs, especially the level of trust and strength of relationships between bioinvasion program staff and stakeholders.

Also, they suggest topics for future research: assessing how well forecasting models are integrated with  communications with stakeholders; how qualitative and quantitative research methods in different fields might support one another; and empirical tests to measure the relative effects on social license of a) involving stakeholders in developing models, b) using forecasts to assess the consequences of different management decisions and, c) the usefulness of different methods for incorporating scientific information in stakeholder engagement.

SOURCE

Mason, N.W.H., Kirk, N.A., Price, R.J. et al. Science for social license to arrest an ecosystem-transforming invasion. Biol Invasions 25, 873–888 (2023). https://doi.org/10.1007/s10530-022-02953-w

see also https://www.doc.govt.nz/nature/pests-and-threats/weeds/common-weeds/wilding-conifers/

Posted by Faith Campbell

What do YOU think about the role “social license” plays in US invasive species programs? We welcome comments that supplement or correct factual information, suggest new approaches, or promote thoughtful consideration. We post comments that disagree with us — but not those we judge to be not civil or inflammatory.

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

or

www.fadingforests.org

 

Climate Change + CO2 Levels – Can Scientists Include the Complexity in their Analyses?

Spruce budworm (Choristoneura fumiferana); photo by Jerald E. Dewey, USFS; via Bugwood; populations of several forest birds, including Cape May, Tennessee and Bay-Breasted warblers, become more numerous during budworm outbreaks

Now that Drs. Ziska and Aucott have educated us about the strong impact atmospheric CO2 can have on both plants and phytopagous insects, I have asked the experts whether these interactions have been incorporated in the models scientists are using to forecast pest activity in American forests as the climate changes.

The answer is no.

bay-breasted warbler; photograph by Dave Inman at Presque Isle State Park, PA; via Flickr

Dr. Bethany A. Bradley, Co-Director, Northeast Climate Adaptation Science Center at the University of Massachusetts, says empirical models of species range shifts typically only use climate and sometimes other environmental factors (like soils or topography) as predictors of potential geography. Inclusion of demographic processes like how plant growth is affected by more or less water, CO2, competition with other plants etc. would require a lot of data. It is currently impossible since there are tens of thousands of plant species interacting in the forests of eastern North America – and perhaps these factors have been analysed for only a hundred of them.

Mike Aucott points to the same difficulty: inclusion of CO2 in models of the future populations of specific plants would be difficult since the impacts vary from species to species and are compounded by other factors such as soil nitrogen levels, moisture levels, temperature, presence of competing plants, etc.  

Regarding insects, Dr. Aucott thinks it is clear that some orders, such as Lepidoptera, don’t fare as well when feeding on plants grown under elevated CO2.  He is not aware of efforts to model impacts of high CO2 on specific insects or even orders or feeding guilds. 

juniper geometer (inchworm); Dr. Tallamy says inchworms are hairless & good tasting – so sought by birds

Dr. Ziska concurs about the difficulties. Dr. Ziska asks why there is so little funding to study these issues, especially given their probable impact on human food supplies and health – as described in his blog and an opinion piece published in Scientific American two years ago.

I hope that scientists, decision-makers, readers of this blog … maybe even the media! – take into consideration these complexities, even if they cannot be defined.

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 – [but do not address climate or CO2 aspects] review the Fading Forests report at http://treeimprovement.utk.edu/FadingForests.htm

or

www.fadingforests.org