Funding key agencies – Your help needed!

EMERGENCY:

The Trump Administration proposes (again!) to end all funding for USFS Research and State, Private, and Tribal Forestry programs. The budget document claims that these cuts are necessary “to ensure fiscal responsibility w/ taxpayer dollars & appropriate alignment of resources w/ USFS’s responsibility to appropriately steward National Forest System lands.” Ending the SP&T programs is justified as “better balance[ing] the appropriate roles of federal & State governments. … and [restoring] federalism …] The document claims that the federal component of Forest Health Management [currently receiving $16 million] duplicates programs managed by the National Forest System; yet the actions listed under this second budget category all relate to water management, not insects or pathogens. The document says states should manage pests on non-federal lands [currently receives $42 million]. I think this approach ignores the need for coordinated management for each of hundreds of pest species, from detection to eradication or development of host resistance. Eliminating the Research program will deprive all forest managers of a scientific foundation for management efforts.

The Trump Administration’s proposed budget would hold funding for key APHIS programs steady. This is great news compared to the extreme cuts proposed for the Forest Service. The budget document says that it is essential to continue APHIS programs success; any stoppages or reductions would potentially cause catastrophic consequences for environmental health. Contrary to this statement, holding funding steady actually results in cuts due to continuing introductions of new pests and inflation.

Item	2024 Actual	2025 Actual	2026 Estimated	2027 Estimated
Field Crop & Rangeland Ecosystems Pests (incl cogongrass)……….	12,000	12,000	11,000	9,026
Pest Detection ………………………………………………..	29,000	29,000	29,000	29,000
Plant Protection Methods Development ………………….	21,500	21,500	21,500	21,500
Specialty Crop Pests …………………………………………	215,000	215,000	214,000	217,339
Tree & Wood Pests …………………………………………..	59,000	59,000	58,650	58,650
Subtotal, Plant Health …………………………………….	387,500	387,500	385,150	386,515

USDA Forest Service

Two USFS programs w/ vital roles in protecting resilience of the Nation’s forests in the face of invasions by non-native pests and plants: R&D program and FHM within SPT division

The many economic & ecological benefits from our forests are under growing threats from a variety of disturbances, ranging from fires & hurricanes to non-native pests. ~ 60% of forests owned by non-feds; USFS must address threats to forests outside NFS to achieve its mission of sustaining “health, diversity, & productivity of the nation’s forests & grasslands to meet the needs of present & future generations.”

Research & Development

The Continuing Resolution for FY26 funded Research at $308 million for the year. Ask Congress to maintain this level. + increase research on invasive species from the current level of 1% to 5%.

The area of our forests & woodlands that is threatened by alien pests is similar to that attributed to fire or western bark beetles. More than 41% of forest biomass in the “lower 48” states is at risk to established non-native pests.[1] If able, add reference to pests on Hawai`i or Caribbean islands. Since additional introductions almost guaranteed, even greater proportion of US’ forest resources at risk in future. If possible, name example, e.g., Phytophthora austrocedri. Forest managers cannot counter these threats without understanding how these P&P kill trees & what actions are effective counter measures. This knowledge is obtained by research.

At least 53 tree species in forests across America are already under attack by non-native pests and pathogens. Yet as of FY23, Research stations spent just 1% of appropriation studying a few of the dozens of NIS pests. Funding for alien pests has decreased 70% since FY2010 even as new pests enter our forests. This inadequate research effort means USFS cannot develop effective programs to prevent, suppress, & eradicate the majority of alien pests. One crucial strategy suffers particularly = efforts to breed trees able to thrive despite NIS pests. R&D currently supports only a few such projects.

Forest Health Management: Supporting the Full Continuum of Pest Management

The Continuing Resolution for FY26 funded State, Private, and Tribal forests program at $310.6 million. I have not found specifics for the FHM program. This was an increase over the $281 million level in FY25.

Non-native pests and pathogens arrive as contaminants or hitchhikers on imported goods, especially on wood packaging and plants. These imports usually arrive in cities or suburbs, so the pests establish there first. They immediately cause enormous damage to urban forests, forcing local governments and property owners to absorb high tree removal costs. They then spread to rural forests, including National forests. Examples include hemlock woolly adelgid, emerald ash borer, invasive shot hole borers, goldspotted oak borer, sudden oak death, and beech leaf disease.

The most effective approach is to kill the pests where they first appear – usually in those urban or semi-rural forests. This response is led by FHM Cooperative Lands subprogram. We urge maintain funding for this subprogram at the FY26 level (possibly $42 million) so that the agency’s experts can continue to assist the states and other partners in countering these pests. As these pests spread to rural areas – including to National forests, National parks, and other public lands, responsibility for their management involves FHM Federal Lands subprogram. So much maintain funding for this subprogram at FY26 levels.

A recent analysis[2] determined that the natural resource values of 92 National parks are threatened by forest pests. Western parks are threatened primarily by outbreaks of the native mountain pine beetle (Dendroctonus ponderosae). Those in the East face threats from more than a dozen species of non-native pests, including hemlock woolly adelgid, emerald ash borer, spongy moth, laurel wilt, and – most recently – beech leaf disease.

Again, combatting these pests requires understanding their life histories & traits – understanding gained through the research program mentioned above.

Funding reductions over the past decade have already shrunk the number of FHM projects & areas treated each year. While 53 tree species are threatened, only four [eastern oaks, loblolly & ponderosa pines, & hemlocks] are targeted by 95% of projects. To counter the threats to 50 additional tree taxa, FHM needs additional resources.[3]

Investing in urban forestry is key to addressing both parties’ priorities & advancing flexible & cost-effective solutions to a wide range of issues impacting American communities, businesses, & families. The USFS SPT division’s Urban & Community Forestry Program efficiently distributes funds to shovel-ready projects for improving communities by maintaining a healthy tree canopy. Federal “seed” money provides resources necessary to initiate & stabilize these local programs.

Breeding Resistant Trees: Critical — & Underfunded

A surprisingly high proportion of the (inadequate) funding for breeding trees to mitigate the damage caused by non-native pests is from FHM or the NFS, rather than R&D. These programs should receive substantial increases. The model program is the Dorena Genetic Resource Center. It provides decades-long commitment, skilled staff, necessary facilities; these result in breeding successes, i.e., western white pines and Port-Orford cedar.

Invasive Plants

Invasions of forests by non-native plant species erode forest productivity & provision of the full range of ecosystem services, hinder forest uses, degrade biodiversity & habitat, and impose substantial financial costs. A recent analysis[4] documents that this threat is growing: the number of FIA inventory plots containing invasive plant species rose in 58.9% of surveyed counties. Furthermore, in 73.2% of the counties the plots experienced an increase in species richness of invading plants. Increases occurred in all regions, but were greater in the East: from 46% to 52.3%. In the Rocky Mountains, the proportion of invaded plots rose from 6% to 11%. In Hawai`i, this proportion grew from 70% to 83.2%. Again, USFS Research and FHM programs, working together, are key to making progress in countering these bioinvasions.

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

[2] Michalak, J.L., C.E. Littlefield, J.E. Gross, T.G. Mozelewski, J.J. Lawler. 2026. Relative Vulnerability of US National Parks to Cumulative & Transformational Climate Impacts. Conservation Letters, 2026 Vol 19, Issue 1; 19:e70020

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

[4] Potter, K.M., B.V. Iannone III, K.H. Riitters, Q. Guo, K. Pandit, C.M. Oswalt. 2026. US Forests are Increasingly Invaded by Problematic NIS Plants. Forest Ecology & Management 599 (2026) 123281

USDA Animal and Plant Health Inspection Service

APHIS is responsible for preventing intro and spread of pests and invasive plants that harm agric, including forests. APHIS policy guides port inspections carried out by the DHS CBP. APHIS inspects imported live plants.

Introductions of pests and pathogens have continued to occur. APHIS funding has remained steady – which means it is not growing to match the rising threat. At minimum, maintain current levels.

FY2025 enacted FY26 House FY26 Senate

APHIS total $1,148 $1,146 $1,168

Plant health subtotal $387.5 $388.6

Agric. quarantine $35.5 $35.5 $35.5

Field crop and rangeland $12 $11 $11.5

Pest detection $29 $28.5 $29

Methods development $21.5 $21.5 $21.5

Specialty crops $206.5 $216.3 $208.5

Tree and wood pests $59 $59 $58.6

Emergency preparedness and response* $44.5 $44.5 $44.3

* this fund is apparently for both animal and plant emergencies

Rationale

Already introduced pests threaten the many forest products and services benefitting all Americans. Just 15 of the worst pests threaten 41% of forest biomass in the “lower 48” states – comparable to fire.[1] A significant proportion of the resulting costs are imposed on municipal governments and homeowners. Fifteen years ago, it was estimated[2] that the municipal governments were spending more than $1B / year, primarily on removing and replacing trees on public property killed by these non-native pests. Homeowners faced costs of $1B plus loss of another $1.5B in property value. A more recent study estimated that cities will have to spend $30M per year to remove and replace ~ 1.4M street trees by 2050. Additional trees in parks and on homeowners’ properties also die.[3]

A new pattern has appeared in recent years: more newly-introduced pests are being detected in the Pacific Coast states rather than in the East and Midwest. Two southern California counties are projected to pay $150M – $1B[4] to remove and replace trees killed by invasive shot hole borers. The emerald ash borer threatens 9,000 ash on the streets of Portland, Oregon and millions more in parks and the forested wetlands of Willamette Valley, including in Ankeny National Wildlife Refuge. The Mediterranean oak borer has already killed thousands of oak trees in the San Francisco Bay area; it also threatens urban forests and valued oak savannahs in Oregon.

Additional introductions of highly damaging wood-borers are likely because we continue to receive inadequately treated crates, pallets, and other forms of packaging made of wood. For 20 years, all countries shipping goods to North America must treat their wooden packaging per prescribed protocols. To address this risk, we urge a modest $1M increase in APHIS’ “Tree and Wood Pest” account. We also suggest that the Subcommittee inquire of APHIS what steps it will take to improve compliance with the treatment requirement. You should focus your inquiry on China; wood packaging from this country is three times more likely to harbor a tree-killing pest than the global average.[5]

Other pests—especially plant diseases and sap sucking insects—enter on imported plants. Pathogens introduced recently via this pathway include rapid ohia death in Hawai`i (threatening the species that constitutes 80% of the Islands’ forest biomass) and beech leaf disease (thin a dozen years has spread across much of the East).

All assessments of APHIS’ plant import programs’ effectiveness use data from 2009; at that time, plant imports were more than 100 times more likely to transport pests than was wood packaging.[6] APHIS has amended its regulations several times since 2009. We urge the Subcommittee to call for APHIS to facilitate independent analysis of the efficacy of its current phytosanitary programs in order to understand whether the updated regulations have reduced the risk of additional introductions.

Again, pests introduced via this pathway proliferate and spread – often facilitated by movement of firewood, plants, and outdoor household goods. APHIS’ programs have suffered severe failures to prevent such spread, for example in the cases of the emerald ash borer and sudden oak death. We suggest that the Subcommittee inquire of APHIS what steps it will take to improve containment efforts regarding damaging plant pests, including through collaboration with its state partners.

We ask for small increases to the Pest Detection and Methods Development programs. The first enables prompt detection of newly introduced pests … which is critical to successful pest eradication or containment. The second empowers APHIS to improve essential detection and eradication tools.

The current emergency fund of is far below the level needed to respond when a new pest is discovered. We thank both the House and the Senate for clearly recognizing that these appropriations are inadequate by including in their bills language reiterating the Agriculture Secretary’s power to access funds from other Departmental programs (usually the Commodity Credit Corporation) to respond to emergencies.

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

[2] Aukema, J.E., B. Leung, K. Kovacs, C. Chivers, K. O. Britton, J. Englin, S.J. Frankel, R. G. Haight, T. P. Holmes, A. Liebhold, D.G. McCullough, B. Von Holle.^. 2011. Economic Impacts of Non-Native Forest Insects in the Continental United States PLoS One September 2011 (Volume 6 Issue 9)

[3] Hudgins, E.J., F.H. Koch, M.J. Ambrose, and B. Leung. 2022. Hotspots of pest-induced US urban tree death, 2020–2050. Journal of Applied Ecology

[4] Jetter, K. A. Hollander, B.E. Nobua-Behrmann, N. Love, S. Lynch, E. Teach, N. Van Dorne, J. Kabashima, and J. Thorne. 2022. Bioeconomic modeling of invasive species management in urban forests: final report.

[5] Haack RA, Hardin JA, Caton BP and Petrice TR (2022) Wood borer detection rates on wood packaging materials entering the United States during different phases of ISPM#15 implementation and regulatory changes. Front. For. Glob. Change 5:1069117. doi: 10.3389/ffgc.2022.1069117

[6] Liebhold, A.M., E.G. Brockerhoff, L.J. Garrett, J.L. Parke, and K.O. Britton. 2012. Live Plant Imports: the Major Pathway for Forest Insect and Pathogen Invasions of the US. www.frontiersinecology.org

Congressional Committees with Jurisdiction … & how to submit testimony

FUNDING APHIS

House Committee on Appropriations, Subcommittee on Agriculture, Rural Development, Food and Drug Administration, and Related Agencies

Chairman: Andy Harris (R-MD)

Members: Robert Aderholt, David Valadao, John Moolenaar, Dan Newhouse, Julia Letlow, Ben Cline, Ashley Hinson, Scott Franklin

Democrats à Sanford Bishop, Jr., Chellie Pingree, Lauren Underwood, Marie Gluesenkamp Perez, Marcy Kaptur, Debbie Wasserman Schultz

deadline: May 1; email to ag.approp@mail.house.gov

instructions: 5 pages, double-spaced in Times New Roman, 12 Point Font; single-sided; PDF attachment to your email. At top of 1^st page, clearly indicate your name, title, & institutional affiliation (if any); In 1^st paragraph, clearly state agency, program, & amount of funding in the request

MUST also send Truth in Testimony form here .

Senate Committee on Appropriations, Subcommittee on Agriculture, Rural Development, Food and Drug Administration, and Related Agencies

Chairman: John Hoeven (R-ND)

Members: Republicans à Mitch McConnell, Susan Collins, Jerry Morn, Cindy Hyde-Smith, Deb Fischer, Mike Rounds

Democrats à Jeanne Shaheen, Jeff Merkley, Tammy Baldwin, Martin Heinrich, Gary Peter, Kirsten Gillibrand, Jon Ossof

deadline: not clear; might be 22 May; email to agri@appro.senate.gov

instructions: 4 pages.. At top of 1^st page, clearly indicate your name, title, & institutional affiliation; state agency, program, & amount of funding in the request

FUNDING USFS

House Committee on Appropriations, Subcommittee on Interior, Environment and Related Agencies

Chairman: Mike Simpson (R-WY)

Members: Republicans à Mark Amodei, Guy Reschenthaler, Michael Cloud, Ryan Zinke, Jake Ellzey, Celeste Maloy

Democrats à Chellie Pingree (D-ME), Betty McCollum, Josh Harder, James E. Clyburn

deadline: 22 April; email to IN.Approp@mail.house.gov

instructions: 4 pages, single-spaced in 12 Point Font; single-sided; prefer PDF but other formats OK. At top of 1^st page, clearly indicate your name, title, & institutional affiliation (if any); In 1^st paragraph, clearly state agency, program, & amount of funding in the request

MUST also send Truth in Testimony form here .

Senate Committee on Appropriations, Subcommittee on Interior, Environment and Related Agencies

Chairman: Lisa Murkowski (R- AK)

Members: Republicans à Mitch McConnell, Shelly Moore Capito, John Hoeven, Deb Fischer, Mike Rounds

Democrats à Jeff Merkley, Chris van Hollen, Martin Heinrich, Tammy Baldwin, Kirsetn Gillibrand, Jon Ossof

deadline: unclear; possibly mid-June; email to int@appro.senate.gov

instructions: 4 pages, single-spaced in Microsoft Word or Word Perfect; do NOT send PDF. At top of 1^st page, clearly indicate your name, title, & institutional affiliation (if any); In 1^st paragraph, clearly state agency, program, & amount of funding in the request

USFS Reorganization — implications unclear

Salt Lake City; by invictus323 via Wikimedia

In a press release on 31 March, 2026, the USDA announced major changes to the USFS structure. Agency headquarters will be moved to Salt Lake City. They point out that nearly 90% of USFS land is west of the Mississippi … but promise to sustain engagement in the Southeast (America’s “wood basket) by creating a regional office there. Furthermore, they will change the current regional organization to a state-based one; they plan to create 15 state directorships. State directors will serve as national leaders with primary oversight of forest supervisors, operational priorities, & relationships with states, tribes, & other partners. Each state office will include a small leadership support team responsible for functions such as legislative affairs, communications, & intergovernmental coordination.

There will still be some “operational service centers” in other cities; that for research will be in Fort Collins. The goal is to unify research priorities, accelerate the application of science to management decisions, & reduce administrative duplication. Information on which facilities will be retained or closed is available at this webpage. (I could not open this site.)

No specific information is provided re: forest health management program.

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

https://fadingforests.org

USDA invasive species research forum 2026: invasive plants

Callery/Bradford pear invasion in northern Virginia; photo by F.T. Campbell

The US Department of Agriculture (USDA) and the North American Invasive Species Management Association (NAISMA) held the 34^th annual forum on invasive species research at the end of February 2026. The agenda is available here In this blog I summarize the presentations about invasive alien plants (IAS); a separate blog discusses findings on tree-killing pests. Formal proceedings will be available in some months.

The most important information from the meeting:

If NAISMA had not taken on the task of hosting the conference it would not have happened.
Government leaders allowed only 1 staffer per USDA Forest Service region to participate. Not allowed to come were people who had organized the whole meeting or individual sessions, and presenters discussing several topics, including preventing IAS plant spread, and progress on controlling cogongrass (major impediment to pine plantations, affecting harvests).

What do these decisions say about the genuineness of the USDA Secretary’s recent memorandum listing invasive species as one of four priority areas for the department’s research efforts?

A reminder to us all: Rebekah Wallace of the Center for Invasive Species & Ecosystem Health at the University of Georgia urged us all to provide citations for images used in informal materials – posters, presentations, outreach efforts, blogs, videos. Images grab attention, provide context for communication, and support data cited. Providing the citation increases our credibility and ensures that we avoid perpetuating misinformation!

Callery/Bradford pear in Kentucky; photo by Sherry Bailey via NARA archive

Two presentations focused on Callery / Bradford pear

Jess Hartshorn of ecoLogic described efforts to develop a remote sensing tool that will be as accurate as human surveyers — but faster. What scientists learned from this exercise will help build tools for other invasive plants. Hartshorn noted that while there are many no-cost sources of satellite imagery, no single source is sufficient. But integrating data from several programs, plus adding new criteria proved challenging. One setback was a surprise: the spectrum emitted from the tree’s most conspicuous feature, its early-season white blooms, is similar to that reflected from concrete! – with which the species is associated … The authors had to use data from several satellite systems to identify unique wavelengths from the leaves. Accuracy was lost when an individual pixil contain mixed “vegetation”.

Marcin Nowicki, of the University of Tennessee, explored the genetic changes that allowed a species that is rare in Asia to become a prolific continent-wide invader in North America. “Evolutionary overdrive” resulted from planting plants from several origins close together, thus promoting cross pollination. This led to exceptionally rapid diversification in nuclear and mitochondrial DNA. A bonus: once Sequencing the genomes of several cultivars have been sequenced, bans on sales of those hybrids that are most invasive can be enforced.

Becky K. Kerns, USFS Pacific Northwest Research Station reported on disturbing increases in invasive plants in forests of the Pacific Northwest. In the past, higher elevations, low light levels, and cooler temperatures appeared to protect the region’s forests from invasion. However, annual grasses, especially cheat grass (Bromus tectrorum), are now being found at unprecedented levels in forest plots that have been burned, grazed, or logged, burned, and grazed. This includes plots subjected to prescribed burns. Kern thinks the plant invasions are due to increased light, ground disturbance, changed competitive interactions, and potentially higher propagule pressure. Pyrophytic shrubs also of increasing concern; Kerns mentioned Scotch broom (Cytisus scoparius) in Douglas-fir forests. [I am uncertain how novel this threat is because academic scientists issued warnings about Scotch and other brooms in the mid-1990s.] [run together w/ following] She is working with the staff of the National Invasive Species Council’s task force on fire and invasives to increase attention to emerging threats and to encourage managers to prioritize managing known pyrophytic species along with fire.

Wavyleaf basketgrass infestation in closed-canopy forest in Maryland; photo by Kerry Kyde, Maryland DNR via Bugwood

Two speakers addressed aspects of the invasion by wavyleaf basket grass (Oplismenus hirtellus subsp. undulatifolius).

Wavyleaf basket grass was first detected in 1996 in Maryland. It is now widespread in the Mid-Atlantic and expected to spread along the Appalachian Trail and to other recreation sites. Thirty percent of public land in the East is considered vulnerable.

Carrie Wu of the University of Richmond is exploring the grass’ association with changes in the soil microbial community. She tested associated soil microbial communities in 12 locations with three types of soil. She found decreased fungal diversity but not homogenization of the fungal community. She is now constructing an invasion history to see how fast the changes occur, confirm the invaded range, and predict high-risk sites.

Michael Fulcher, of the USDA Agriculture Research Service’s Foreign Disease-Weed Science Lab, is concerned about the microbes associated with invasive plant species. We don’t know whether some of these microbes might be beneficial, perhaps as biocontrol agents? Or might they cause disease in desired plant species. He phenotyped 319 isolates from healthy leaves. This study detected two known crop pathogens on healthy wavy leaf basket grass plus an unknown species in a genus that includes some known pathogens. In lab tests, this organism stunted growth of wheat and tall fescue embryos

Fulcher emphasizes that even asymptomatic non-native plants can transport possible pathogens. Scientists should try to detect and analyze these as quickly as possible. I note that Eliana Torres Bedoya reported last year that healthy woody plants can also transport disease-causing fungi.

Fulcher is looking for collaborators to help collect plant samples

Other invading plants

Craig Barrett of West Virginia University seeks to answer questions related to “invasiveness” traits and whether selective pressures enhance those traits in the invasive range. To explore these topics, Barrett is mapping the invasion history of the widespread invasive species Japanese stiltgrass (Microstegium vimineum). He has found evidence of the grass’ rapid adaptation after introduction, including greater diversity in invasive populations in the Northeast than those in the Southeast. Barrett thinks it most likely that a genetic bottleneck at introduction was followed by mixing that created novel genotypes that might bridge gene transfer between larger populations. There is evidence of phenological adaptation to local climates and a genetic basis for whether a plant supports awns – which react to changes in moisture by “walking” across soil and burying themselves.

Elizabeth Ward, at the Connecticut Agriculture Experiment Station, documented how invasive plant species utilize forest gaps created by the death of ash caused by emerald ash borer (EAB). The progress of the EAB infestation across Connecticut is well-documented, so scientists can track plant responses to stages of canopy mortality. She found:

Larger canopy gaps contained more invasive plants and fewer native tree seedlings / reduced regeneration.
Higher soil nitrogen availability is also linked to higher non-native plant cover (all species) – including non-native tree seedlings.
Higher carbon availability led to lower non-native plant cover, including that of non-native tree seedlings.

Ward advises active management of EAB-invaded forests to reduce plant invasions and promote tree regeneration.

Ward is now comparing sites with passive management vs. salvage harvests. Early results find no difference in invasive plant cover. However, harvested sites had higher abundance of ash regeneration and and diversity of native plant species.

Jeremy Anderson, at the University of Massachusetts, discussed difficulties that have slowed the search for a biocontrol agent to control invasive knotweeds. North American scientists are collaborating with counterparts in Europe. Because knotweeds are related to rhubarb, scientists must ensure that any agent is host specific.

knotweed infestation in Maryland; photo by Will Parson, Chesapeake Bay Program

Initial surveys 20 years ago identified 180 candidate insects. However, the only speciesfound suitable for in- depth evaluation failed to establish. Why? First, there was apparently a climate mismatch: the insect is from southern Japan but the plant is from the North. Then a second difficulty was discovered: the target weeds are hybrids, not a pure species. Scientists are now testing a microbe that might overwinter on pine needles, so they are comparing needle chemistries of Japanese red pine with those of North American pines to determine whether there is a risk. In answer to a question, Anderson said scientists do not know how the microbe will respond to the warmer, wetter climate expected in New England in the future.

Ashley Schulz, of Mississippi State University, is continuing her efforts to identify clues to which newly introduced species might be most damaging. In this case she is analyzing efficacy of biocontrol agents to understand which establish and have significant impacts. Species with traits similar to successful biocontrol agents might be more successful invaders.

Schulz analyzed information from 394 insects introduced to North America to control 153 plant species and 87 agents targeting 325 insect pests. The data recorded on each species: whether it established, level of impact, insect’s feeding guild, climate matching, host specialization, and evolutionary history. For the 87 entomophagous insects, she also recorded host feeding guild and host specialization. See other blog.

Phytophagous insect biocontrol agents were more likely to establish if the insect is a generalist newly associated with the target plant species. The biocontrol agent is more likely to have a greater impact when released in environments similar to the agent’s native range. The introduced biocontrol agent will have less impact if it feeds on plant parts that the plant can easily restore (foliage, fruit/seeds).

What does this indicate re: invasive species? Schulz concluded that among phytophagous insects, generalists might be more likely to find a suitable host and survive. The “Goldilocks” premise applies: the host is sufficiently similar to the invader’s native host that it is recognizable but sufficiently distantly related to lack defenses effective against the invader. Bioinvasive phytophagous insects will have a greater impact when introduced to a similar climate and feeds on plant structures that are not easily restored – i.e., stem, root.

For traits of entomophagous insect biocontrol agents see my other blog here.

Schulz recommends more analysis of what can be learned from experience with biocontrol agents. However, such studies are challenged by poor records, lack of empirical evidence and quantitative data, the lower number of biocontrol agents introduced recently, and funding shortages that preclude post-release monitoring.

Schulz also mentioned that she worries that a proposal to drop the word “harm” from definition of invasiveness could result in biocontrol agents being lumped with invasive species. This would further hamper implementation of biocontrol. She considered this loss to have particularly bad affects at a time when there are growing restrictions on pesticide use.

Posted by Faith Campbell

https://fadingforests.org

EEICAT: improved method for assessing bioinvasion impacts

As bioinvasions and their impacts continue to expand globally, managers and decision-makers charged with developing effective management and mitigation strategies urgently need tools that can assess and rank all impacts. These start with impacts on species’ populations … but go much farther, to the assemblage, ecosystem, and abiotic levels. Impacts at the “species and assemblage” level include species extinction (locally or more broadly), changes in species range, assemblage structure, successional patterns, and the soundscape. Impacts at the “ecosystem function” and “abiotic” levels include changes to primary production, food webs, water quality, and nutrient cycles. The analysis also addresses changes that do not affect native biota directly, although they present no examples.

For a decade, scientists studying bioinvasions have used the Environmental Impact Classification for Alien Taxa (EICAT) framework to standardize categorization of species-level impacts. One group that has not used this methodology is experts on tree pests. Why? Does the approach fail to describe the impacts of non-native arthropods and pathogens on tree species and forest ecosystems more broadly? Or is it simply because of academic silos?

Even more important: are the science and practical management of invasive species and forest pests losing valuable insights, resources, policy choices, … because of this schism? Would both groups gain from closer interactions?

In any case, the framework used by many scientists working on “invasive species” is undergoing a revision to better capture cascading and systemic effects from bioinvasion. A group of scientists has created the Extended EICAT (EEICAT) framework. (See the publication reference at the end of this blog to learn the process of development and details of the new system.) The proponents claim that the new system recognizes the functional interdependence of species in ecosystems, which means that alterations in species assemblages inevitably amplify throughout the system. E.g., alterations in physico-chemical characteristics or habitat structure. Impacts can even cross-ecosystem impacts between ecosystems that are often managed separately. An example is a change in the quality, magnitude, and novelty of resource flows between terrestrial and aquatic systems. To address these multifaceted effects, EEICAT integrates 19 impact types into the analysis. The intention is to improve communication about the complex ecological impacts caused by bioinvasions and facilitate prioritization of responses to competing bioinvasions.

While the various outcomes from bioinvasion can be positive or negative for nature and people, the EEICAT does not use value-laden distinctions. These determinations are left to stakeholders, managers, and community members, based on their own perspectives. Instead, it compiles and standardizes information about the measurable changes to species numbers (some decrease, others increase); to ecosystem processes (e.g., nutrient dynamics or hydrological regimes).

EEICAT incorporates the “reversibility concept”, which addresses the potential for a native sp (including individuals, pops, and assemblages), ecosystem function, or abiotic environmental to recover after removal of the bioinvader. The system developers distinguish “naturally reversible changes” and “naturally irreversible changes”. In the former case, the affected spp, ecosystem processes or abiotic conditions are thought likely to return to their original state within 10 years or three generations (whichever is longer) through natural processes or human-assisted actions that do not exceed what is already being done. This does not include reintroductions or restoration efforts that require new efforts. Instances of “naturally irreversible changes” are those in which the affected species, ecosystem functions, or abiotic conditions cannot return to their original state within that timeframe without significant additional human intervention, or even after intense human intervention. The system has reached a different, stable equilibrium. These “permanent” changes are the result of one or more species’ global extinction, or persistent environmental alterations, e.g., soil modification, altered hydrology, or irreversible changes in nutrient cycling.

The proponents assert that EEICAT allows multiple impacts reported in a single study to be classified independently at each impact level. Furthermore, the EEICAT analysis does not require extensive research on the assessed species or understanding of the mechanisms through which the invasive species affects native species or the environment. EEICAT framework is applicable to any amount of info available in each study. It also explicitly assesses the adequacy / reliability of evidence [data, methods, approach] used in studies of bioinvasions that are included in the analysis.

EEICAT framework enables researchers to evaluate how “ecosystem engineer” species influence key ecological functions by explicitly accounting for changes to ecosystem processes, e.g., nutrient dynamics or hydrological regimes. For example introduced bivalves increase water clarity in certain systems, triggering cascading effects on biodiversity and ecosystem functions.

The EEICAT framework also allows separation of the mechanisms of impact vs. attribution of impact. For example, when a non-native plant species alters nutrient availability, thereby changing the microbial community, EEICAT assigns separate impact categories to the two impacts.

Regarding cross-ecosystem effects, the proponents cite rats on islands. Their predation suppresses seabird pops; reduced guano alters the nutrient dynamics of adjacent coral reef ecosystems. Thus assign impact categories not only to the changes in nutrients, but also to ecological functioning. This provides a more comprehensive view of interconnected effects.

Proponents of the proposed new framework assert that the fundamental distinction between EEICAT and the earlier EICAT is that the earlier assessment is “species-based”, whereas the new one is “impact-based”. It is broader because it focuses on specific combinations of invading species plus the affected systems. It is better able, they assert, to account for contrasting impacts in different invasions.

EEICAT can be applied to any invasion event (i.e., a specific combination of invasive species, recipient system, and context). It broadens the range of evidence that can be integrated into the assessment. Decision-makers benefit from access to more information. The information can also be provided in more easily understood form through two visualization tools:

An “invasive species profile” aggregates all recorded impacts caused by a single invading species. This facilitates clear communication of the bioinvasion’s impact severity to managers and stakeholders, plus how those impacts vary by context.
An “invaded ecosystem profile” compiles impacts from different species to a site or location. This is particularly useful for synthetic analyses (e.g., meta-analyses), evidence syntheses, and manager assessments.

Resulting profiles can help stakeholders prioritize species or ecosystems for responses.

https://www.dontmovefirewood.org/pest_pathogen/phytophthora-root-rot-html/to are ants. No disease agent is discussed or even named. This gap is surprising given the devastating and geographically extensive impacts of e.g., avian malaria, chitrid fungi (Batrachochytrium dendrobatidis and Batrachochytrium salamandrivorans) on amphibians, and Phytophthora cinnamomi on the flora of western Australia.

One example in Table 3 pertains to native Hawaiian forests. The underlying study analyzed changes in ecosystem functions caused by the invasive nitrogen-fixing tree Falcataria moluccana. The EEICAT proponents say their analysis of this study would supports more informed decisions in conservation planning and ecosystem management. Indeed, the principal author of the underlying study has recently published a suggested method to manage the Falcataria moluccana invasions by replacing these trees with either native species or valued crops under an agroforestry program. Neither of the articles mentions that exactly this same area (the Puna District on the “Big Island) has suffered widespread death of the native tree ʻōhiʻa lehua (Metrosideros polymorpha) as a result of the invasive disease rapid ʻōhiʻa death (ROD). The more recent article does address the fact that native plant species are extremely rare in this region.

Would integrating studies of tree-killing arthropods and pathogens into the EEICAT system provide benefits? First, let’s consider analytical methodology. Many analyses of forest pests’ impacts already discuss at least some of the wider ecological (and economic) outcomes. (To explor this, visit www.dontmovefirewood.org and read some of the species profiles under the “invasive species” tab.) Would comparing these findings to an EEICAT analysis confirm the proposed methodology? Or would it instead suggest needed adaptations? In either case, the results should improve scientists’ work.

Second, would the science and practice of managing invasive species be strengthened by bridging the differences in methods and terminology between those focused on plants and vertebrates and those focused on tree-killing invertebrates and microbes? Would greater unity result in more attention to bioinvaders from policy-makers and/or conservation practitioners and advocates? Especially since (nearly) all the major forest pest invasions would qualify as “naturally irreversible changes” or even “permanent”: the affected species, ecosystem processes or abiotic conditions are thought unlikely to return to their original state within 10 years or 3 generations (whichever is longer) in the absence of intense human-assisted actions. If joining forces might bring about greater societal efforts, is the EEICAT methodology a promising tool to achieve this goal?

Finally, would applying the EEICAT system improve the analyses of tree-pest impacts? Would this approach result in incorporation of types of effects that would otherwise be missed – either often or in specific cases? Are there relationships among forest species, or between species and ecological functions, that might be discovered? Might preparation of “invaded ecosystem profiles” that include bioinvaders from earthworms to canopy foliage feeders provide an informative perspectives that is now lacking?

SOURCE

Carneiro, L., Pincheira-Donoso, D., Leroy, B., Bertolino, S., Camacho-Cervantes, M., Cuthbert, R.N., et al. (2026) Expanding invasive species impact assessments to the ecosystem level with EEICAT. PLoS Biol 24(3): e3003665. https://doi.org/10.1371/journal.pbio.3003665

Posted by Faith Campbell

Or https://fadingforests.org/

A “fix” for some invaded Hawaiian ecosystems?

*Falcataria moluccana* tree; photo by Forest & Kim Starr via Flickr

Nitrogen-fixing tree species have been recognized as damaging to invaded ecosystems for decades. These trees increase soil N availability through increased N content in litterfall. The elevated soil N availability might persist long after the mature individuals responsible for creating such litterfall have ceased to exist. When this happens, some plant species able to exploit increases in nutrients and light, e.g., non-native grasses and forbs, might quickly dominate post-control succession.

In Hawai`i one of the worst nitrogen-fixing tree species is albizia (Falcataria falcata) [formerly Falcataria moluccana, Paraserianthes falcataria, or Albizia falcataria]. This fast-growing species has aggressively invaded across the archipelago, transforming composition, structure, and function of remnant lowland wet forests. There are an estimated four million F. falcata trees across the Hawaiian islands; 720,000 large trees (i.e., > 25 cm DBH). The trees spread rapidly once established because the small seeds remain attached to the lighweight pods, which can be blown for long distances in wind storms (J.B. Friday, University of Hawaii, pers. comm.).

Stands with contiguous overstory F. falcata canopies reduce light availability to 20% of ambient levels; adding in understory vegetation further reduces light to ~5% of ambient levels. Albizia’s abundant and persistent seedbank promotes its return to dominance after mature individuals controlled.

understory of an albizia-invaded area; invasive plants: forbs along roadside; *Miconia calvescens* in the shade. Photo by F.T. Campbell

Beyond the conservation threats, albizia also poses a threat to residential communities & agricultural lands. The trees are some of the fastest growing species in the world, easily growing 5 m in height annually over the first few years and reaching up to 40 m. When their brittle branches fall they crush structures and entire trees can topple during windstorms. The damage is exacerbated by trees’ widespread presence. When Tropical Storm Iselle hit Hawai‘i island in 2014, over 10,000 people were stuck in their subdivisions or on their farms because fallen albizia had blocked all their access roads (Friday, pers. comm.).

Until recently control efforts have relied largely on clearing the land using large machinery (e.g., bulldozers). This is expensive and – worse – not very effective because the magnitude of disturbance to the soil disturbance often leads to explosive germination of the trees’ seeds.

There has been success recently through application of a target-specific herbicide (aminopyralid) at low doses (Leary et al. 2014). Hughes et al. (2025) found that herbicide-killed F. falcata quickly lost their leaves. This litterfall increased litter inputs of N and P that translated to increased soil nutrient availability that is exploited by extant understory vegetation (non-native grasses and forbs). These plants formed a continuous layer that severely limited germination of F. falcata seeds. In their study plots the number of saplings per ha after three years was only 18, despite the presence of perhaps 8 million seeds!

As an early successional pioneer species, F. falcata requires high light conditions to germinate, persist, & grow. The rapid growth & thorough occupation of the understory by other species prevents the species’ re-establishment. However, these aggressive non-native plants also prevent restoration of native Hawaiian species. There is little to no regeneration of native plants under albizia, either on stands that established on abandoned agricultural or ranch lands or under trees that spread into native forests.

Hughes et al. (2025) suggest manipulating the succession trajectory by planting desired species – either native species or species that have cultural importance to native Hawaiians – under albizia stands before herbicide treatment. If the land is to be restored to agricultural use, mechanical clearing would be used rather than herbicide used as felling the brittle dead trees is hazardous to equipment operators, and standing dead trees would pose a risk to farmers. In a forest setting, understory planting before herbicide treatment of the canopy-forming F. falcata stands would allow desired species to take maximum advantage of the increased resources (i.e., light and nutrients) (Friday pers. comm.).

Even after invasive N-fixing trees have been physically removed, the soil legacy effects of transformed microbial communities, depleted native seedbanks, increased available soil N, and dominance by undesirable weed species are daunting barriers to restoration of native species. With intensive management, though, these lands can be restored to agricultural production. Dozens of acres of papaya farms have been established on areas in the Puna district of Hawai‘i island on lands formerly occupied by albizia (Friday, pers. comm.).

In this case, re-establishment by native species is not expected due to their scarcity in study areas. These areas had experienced significant disturbance (i.e., fire, and/or conversion to agriculture) before albiziast and establishment. Instead, the proposal’s objective is primarily to understand whether, how, and to what extent F. falcata stands could be eliminated from areas in a manner that constrains the species’ seedling recruitment and subsequent re-establishment leading to overstory dominance once again (Friday, pers. comm.).

Hughes et al. (2025) emphasize the need for long-term follow-up to ensure that F. falcata does not re-establish later on. The species’seeds retain 70 – 90% viability following 18 months in storage; possibly some much longer. Also, a few saplings did still establish. The non-native grass invasion might lead to declines in soil N availability that provide opportunities for secondary invasion by N₂-fixing treesin light gaps. Dr. Friday reports that practitioners revisit treated areas to kill these seedling while they are still 10 – 20 feet tall.

Conclusions

Hughes et al. (2025) assert that management of this large, fast-growing, & disruptive invasive tree is possible by exploiting its weakness of shade intolerance. Dr. Friday agrees that fast-growing timber species, e.g., Eucalyptus, could outcompete regenerating albizia. However, will there be a market for locally grown timber? Dr. Friday doubts the possibility of agro-forestry plantings of smaller or slower-growing species because of the danger that the overtopping dead F. falcate would fall on and crush agricultural workers or structures.

The fall hazard would presumably apply in other parts of the Pacific & elsewhere where F. faclata poses the same invasiveness problems.

ʻōhiʻa trees killed by ROD in the Puna District of Hawai`i Island; photo by F.T. Campbell

Hughes et al. (2025) do not mention that the native tree that was probably most widespread before the disturbances is ʻōhiʻa lehua (Metrosideros polymorpha). In precisely the same lowland region of the Big Island where they conducted their study, ʻōhiʻa has been killed by a newly introduced disease, rapid ʻōhiʻa rust (ROD). This new invader greatly complicates any effort aimed at restoring native plant species.

healthy ʻōhiʻa in Hawaii Volcanoes National Park; photo by F.T. Campbell

SOURCES

Hughes, R.F., C. Morrison, E. Bufil, J. Leary. 2025. Ecosystem response to management of an invasive N-fixing tree in Hawai`i. Trees, Forests and People 21 (2025) 100932

Leary, J., J. B. Friday, S. Kaye, and F. Hughes. 2014. Proper technique of injecting albizia (Falcataria moluccana L.) with the herbicide Milestone ® (active ingredient aminopyralid).

Dr. Friday provided the following more local references:

https://plantpono.org/high-risk-plants/falcataria-moluccana-albizia

https://dlnr.hawaii.gov/hisc/info/biocontrol/latest-biocontrol/falcataria-molucca

Posted by Faith Campbell

Or https://fadingforests.org/

Plant invasions grow everywhere

invasion of Chinese privet (*Ligustrum sinense*)

A decade ago I posted a blog reporting that 39% of forests surveyed under the Forest Inventory and Analysis (FIA) system were invaded by one or more invasive plants (Oswald et al. 2015). By regions, Hawai`i had the highest invasion intensity – 70%. The second highest density was in the eastern forests – 46%. Forests in the West ranked third, with 11% of plots containing at least one of the monitored invasive plant species. Finally, forests in Alaska and the Intermountain regions both had 6% of plots invaded.

I rejoice that US Forest Service scientists have continued to analyze their data on plant invasions. Analysis of the most recent data shows alarming increases in invasions everywhere since 2015. However, the scientists could not determine a nation-wide percentage because many areas in the West had not yet been surveyed anew. They did determine that the number of inventory plots containing invasive plant species rose in 58.9% of surveyed counties. Furthermore, in 73.2% of the counties the plots experienced an increase in species richness of invading plant species. While increases were observed in all regions, they were greater in the East than in the West — and in the USFS Southern region compared to the Northern region. Specifically, the proportion of forest plots in the East (USFS Southern and Northern regions) invaded has risen from 46% to 52.8%. In the Rocky Mountains they rose from 6% to 11%. In Hawai`i plots having invasive plants grew from 70% to 83.2%. Surveys in the Pacific Coast states have not yet been completed so this region is not included in the analysis (Potter et al. 2026). It is not clear to me how the current boundaries of the western regions – which are based on Bailey’s ecosystem boundaries relate to the 2015 boundaries, which were based on USFS official regions. Hawai`i is clearly the same.

Porter et al. (2026) concluded that in the forests of the East plant invasions are so extensive that elimination of their impacts is practically impossible. Their spread to new areas is unhindered now and, I would add, is likely to remain so without heroic counter measures.

Forests in the East have a greater mean richness of invasive plant species than do western forests. In particular, there is a profusion of shrubs and vines as well as trees. The West has a greater diversity of invasive forbs. The diversity of invasive grasses is high in both regions.

kudzu (*Pueraria montana*) spreading from edge into forest in Virginia; photo by F.T. Campbell

Potter at al. (2026) worry that the apparently lower level of plant invasions in the West might be an artifact of a higher proportion of plant species being at an earlier stage of invasion. That is, the species have not yet established sufficiently widely to be classified as invasive.

thicket of guava (*Psidium cattleianum* ) replacing `ohi`a killed by ROD; Hawai`i Island; photo by F.T. Campbell

Of course, the situation in Hawai`i is much worse. Another, more detailed, discussion of invasive plant species in Hawai`i pointed out that relying on data reflecting canopy-level trees obscures the real picture. While “only” 29% of large trees across the Islands are non-native, about two-thirds of saplings and seedlings are. Potter et al. (2023) expected that plant succession will result in non-native tree species taking over the canopy. This likelihood exists regardless of the impact of rapid ‘ohi’a death since ‘ohi’a lehua (Metrosideros polymorpha) is not reproducing even when seed sources are plentiful and people remove invasive forbs and grasses Potter et al. (2023).

The nation-wide analysis of Potter et al. (2026) does not include forests on U.S. Caribbean islands, i.e., Puerto Rico and the Virgin Islands. See here for a description of this situation. In summary, 33 of 57 (58%) of non-native tree species tallied by FIA surveyors are actual or potential high-impact bioinvaders. Furthermore, 21 (38%) of the non-native species occurred on at least 2% of the FIA plots – far above the seven species fitting this description in the continental U.S.

As these sources, and those with a broader perspective, demonstrate that we should not ignore invasions of our forests by non-native plants. These species erode forest productivity and provision of the full range of ecosystem services, hinder shifting (?) forest uses, and degrade biodiversity and habitat.

These invasions also impose extensive financial costs from lost or damaged resources (Potter et al. ( 2022). Potter et al. (2026) note that these negative outcomes depend on interactions between the traits of the non-native plants and the biomes being invaded. These impacts are greatly exacerbated in Hawai`i because more than 95% of native species on the Islands are endemic. This includes 67% of the large trees still present in the forests. As Potter et al. (2023) point out, extirpation of any of these species is a global loss.

ʻōhiʻa lehua (*Metrosideros polymorpha*); photo by F.T. Campbell

Data issues

Potter et al. (2026) note that in the Northern region only about 20% of plots were surveyed for invasive plants. They state that these difference in sampling intensity does not affect statistical analyses across broad scales.

The regional lists of invasive plants were developed by experts. They include those species thought at the time to be most damaging. Of course, there are other non-native plant species that might be present – and some might prove to be invasive over time (Potter et al. 2026). I have been unable to determine whether the regional lists are updated periodically. Because of this structure of the FIA system, these surveys can assess only spread of already-established species. It is not suitable for early detection of new species entering the forest.

For all these reasons, the analyses in Porter et al. (2026) probably underestimate the total abundance of non-native plant species in U.S. forests. Indeed, the time lag between introduction or even identification of invasive species and their eventual ecological and economic impact obscures their full impact. This ever-increasing invasion debt probably contributes to decisions not to implement effective countermeasures.

Recommendations

How do we set priorities for responding to nearly unmanageable situations? We sharpen our focus on the most damaging pathways of introduction, the most vulnerable regions, and the most at-risk species.

The high-risk pathways are imports of plants for planting and wood – including but not limited to crates, pallets, and other forms of packaging.

Vulnerable regions start with the Hawaiian Islands, Puerto Rico, and the Virgin Islands; and include many biodiversity-rich areas on the continent. We should enhance monitoring of these vulnerable regions by federal, state, and tribal agencies, conservation organizations, citizen scientists, and others. Surveys must report all non-native plant present, not just those already known to be invasive. These data will improve detection of new species and better inform us about factors affecting species’ spread.

Also, I support Potter et al.’s (2026) emphasis on the wildland-urban interface as an area of high human-environment conflict.These include, but are not limited to, plant invasions. The authors point out that we need new policy, management, and scientific tools to address threats in these vulnerable and too-often ignored social and ecological zones.

This increase in available information must be paired with management of the factors that facilitate invasion. Some of these are associated with ecosystems. But the key target must be plant species being brought into the region by people for various purposes. This is often for ornamental horticulture.

lesser celandine (*Ficaria verna*) dominating herb layer in a Virginia forest; photo by F.T. Campbell

We must ask state legislatures and Congress to empower regulatory agencies – e.g., their state departments of agriculture and USDA’s Animal and Plant Health Inspection Service – to be far more more assertive and pro-active. For example, they must give higher priority to the full range of ecological and economic impacts of invading plants, not just damage to agriculture.

Evans et al. (2024) urged prioritizing for state regulation those species in the ornamental trade that are projected to remain or become abundant under evolving climate conditions. Beaury et al. (2023) called for regulating the nursery trade at the national level – reflecting the scope of sales.

SOURCES

Beaury, E.M., J.M. Allen, A.E. Evans, M.E. Fertakos, W.G. Pfadenhauer, B.A. Bradley. 2023. Horticulture could facilitate invasive plant range infilling and range expansion with climate change. BioScience 2023 0 1-8 https://doi.org/10.1093/biosci/biad069

Evans, A.E., C.S. Jarnevich, E.M. Beaury, P.S. Engelstad, N.B. Teich, J.M. LaRoe, B.A. Bradley. 2024. Shifting hotspots: Climate change projected to drive contractions and expansions of invasive plant abundance habitats. Diversity and Distributions 2024;30:4154

Potter, K.M., C. Giardina, R.F. Hughes, S. Cordell, O. Kuegler, A. Koch, E. Yuen. 2023. How invaded are Hawaiian forests? Non-native understory tree dominance signals potential canopy replacement. Landsc Ecol 2023 https://doi.org/10.1007/s10980-023-01662-6

Potter, K.M., B.V. Iannone III, K.H. Riitters, Q. Guo, K. Pandit, C.M. Oswalt. 2026. US Forests are Increasingly Invaded by Problematic Non-Native Plants. Forest Ecology and Management 599 (2026) 123281

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

Potter, K.M., K.H. Riitters, B.V. Iannone, III, Q. Guo and S. Fei. 2024. Forest plant invasions in eastern US: evidence of invasion debt in the wildland‑urban interface. Landsc Ecol (2024) 39:207 https://doi.org/10.1007/s10980-024-01985-y

Posted by Faith Campbell

https://fadingforests.org

Invasive plants threaten integrity of eastern U.S. forests

garlic mustard (*Alliaria petiolata*); photo by Katja Schulz via Wikimedia

I welcome a recent series of studies documenting the extent of plant invasions in forests of the eastern United States and the socio-economic conditions that contribute to a state of affairs increasingly recognized as a crisis. I wish, however, that the authors had devoted more attention to the role of deliberate planting of non-native species and the resulting propagule pressure.

I summarize here findings of two studies written by largely the same scientists and relying on the same underlying data: surveys of forest plots conducted under the Forest Inventory and Analysis (FIA) program. In this blog, if focus on the extent of invasive plant presence in the forests of the eastern United States. In an accompanying blog I will summarize the status of plant invasions in forests nation-wide.

As I have noted in earlier blogs, link a decade ago one or more invasive plant species had already invaded 46% of FIA plots in the eastern U.S. (Oswald et al. 2015). This situation has worsened. Updated data show that 52.8% of these plots contain invasive plants. In the USFS Southern Region, invasive plants have been documented on 55.3 million ha. In the Northern Region, they are found on 36.9 million ha. (Only ~20% of FIA plots in the Northern Region were surveyed for invasive plants.) In some counties of the 37 states constituting these two USFS regions, 80% of inventoried forest plots contain invasive plants. Areas with lower levels of invasion are found in parts of New England, the Great Lakes states, southern Appalachians, southeastern coastal plain, and western Texas and Oklahoma (Potter et al. 2026). Spread of these bioinvaders is largely unchecked – either throughout the East or “just” in the South. In any case, the extent and intensity of these invasions are so great that their complete removal – or elimination of their impacts – is “practically impossible” (Potter et al., 2024; Potter et al. 2026). [It is not clear whether the scientists mean “nearly” or “in practice”. Or that this difference is important.]

[In comparison, in the West less than 30% of FIA plots are invaded, on average. In Hawai`i, more than70% are (Potter et al. 2026).]

The scientists analyzing the FIA data warn that the extent and impact of plant invasions in eastern forests is undoubtedly worse than these data indicate. The records include only some of the non-native plant species present — those considered to be the worst invaders at the time regional lists were compiled – apparently in the first years of the 21^st Century (Potter et al. 2026).

Japanese honeysuckle (*Lonicera japonica*) photo by Chuck Bargeron

The scientists emphasize the role of disturbance in promoting plant invasions. They cite various studies as well as the FIA data to document that forest edges facilitate non-native plant establishment and spread into forests. They stress various aspects of suburban development, including roads and other transportation corridors. It follows that invasion rates are highest in the “wildland-urban interface (WUI).” [The wildlife-urban interface is the zone of transition between unoccupied land and human development; the zone where structures meet or intermix with undeveloped land and its vegetation.] They worry that the WUI is growing faster than any other land use type in the country – and especially rapidly in the East. As a result, the scientists expect more and worse invasions in the future (Potter et al., 2024 and Potter et al. 2026).

I appreciate that they highlight the uniqueness of WUI ecosystems. Housing development in the WUI has numerous effects on natural ecosystems, including habitat modification and fragmentation followed by diffusion of the direct and indirect effects of anthropogenic activities into neighboring ecosystems at different scales. As regards specifically non-native plants, this transmission occurs through a combination of (1) human-driven disturbances to native ecosystems that promote plant invasion and (2) providing a source of non-native plant propagules in their yards and gardens. These plants can then spread into and establish in nearby ecosystems (in this case, forests). [I note that tree-killing arthropods and pathogens also can be introduced in the WUI.] (Scroll below “Archives” to “Categories”, click on “forest pests” and “wood packaging”.)

They also found that plant invasions are more strongly related to older, than more recent, land-cover changes. Survey plots that have been located in the WUI since 1990 or earlier had on average 2.6% more invasive plant cover and 0.33 more invasive plant species than those that were classified as being in the WUI in 2000 or 2010. Their explanation is that the WUI forests experienced decreased spatial integrity, increased forest-developed area edges, and falling proportions of forest in the surrounding landscapes. In addition, the human population in the vicinity might have grown. All these factors that would increase forest fragmentation and the plots’ susceptibility to invasion.

The other side of the coin is propagule pressure. Both Potter et al (2024) and Potter et al. (2026) note that the flora of residential landscapes – rural as well as suburban – is typically dominated by non-native plant species. Still, I think these studies downplay the impact of this ubiquity of non-native plants in all anthropogenic landscapes.

In discussing the higher invasion rates found in survey plots located in WUIs dating from the 1990s they made no mention of human activities that promote plant invasions. There are several. Plants growing in those older yards had one or two more decades to flower – and for their fruits and seeds to be transported into the forest by birds, wind, or water. Residents might have decided to beautify their neighborhood by planting shrubs or flowers in the woods. Maybe they succumbed to the temptation to dump yard waste in the woods – thinking it would be absorbed by “nature”. Since plant invasions take time to unfold, these additional years of human-mediated exposure are highly relevant. Another factor is that people who choose to live in wooded surroundings probably choose horticultural plants that thrive under such conditions – exactly those best able to establish beyond the property line.

Another opportunity to discuss these factors came from the discovery that plant invasion rates are higher in association with “interface” rather than “intermix” WUI forests. [“WUI interface forests” are those where settled areas abut wildlands. In “WUI intermix forests” the structures are scattered.] They speculate about reasons. Potter, et al. (2024) mention that invasions originating from older housing developments have had more time to establish (or at least to be detected) given the well-known lag associated with plant invasions.

I wish they had focused more on the probable difference in suburban development across time. While I was growing up in expanding suburbs in the 1950s, I observed that the earlier housing developments were either built on land that had been cleared to support agriculture or the builders cleared the forest to make construction easier and cheaper. More recently, wealthier buyers have sought residences on more wooded sites – so creating an “intermix” WUI. Potter et al. (2024) speculate that locations in the “interface” WUI are closer to high-density urbanization so have higher exposure to non-native plants. They do not discuss whether the “interface” WUIs are older, thus giving associated plantings longer years to proceed through the stages of bioinvasion.

burning bush (*Euonymus alatus*) invading a forest in Virginia; photo by F.T. Campbell

The Role of Deliberate Planting?

I recognize that these authors analyzed mountains of data. However, I wish they had incorporated the findings of numerous scientists who have analyzed the role of deliberate planting – especially ornamental horticulture – in facilitating introduction and spread of invasive plants. (Scroll below “Archives” to “Categories” and click on “invasive plants”. Also See Reichard and White 2001 and Mack 2000).

As I hope USFS scientists are aware, recent studies confirm the continuing role of ornamental horticulture in plant invasions. Kinlock et al. (2025) blog 440 found that more than 1,600 plant species sold by nursery and seed catalogs over 200 years had “naturalized” somewhere in the continental 48 states. They do not discuss what proportion of these species are truly damaging invaders. Fertakos and Bradley (2024) found that species were likely to establish if they were introduced to as few as eight locations. Beaury et al. (2024) found that half of 89 plant species recognized as invasive are sold in the same locations where they are invasive. Another 25 species are sold by one or more nurseries located in an area that is currently unsuitable for those species, but that will become more suitable for invasion as temperatures warm.

Potter et al. (2026) acknowledge that the ornamental plant trade is likely to continue introducing new plant species into U.S. forests. However, they recommend only updating the lists of invasive plants to be included in future surveys. Apparently these lists have not been updated since 2004.

Potter et al. (2024) go farther, urging efforts to encourage homeowners to plant more native and environmentally friendly private landscapes. They note that such advocacy is complicated by the fact that non-native – even invasive – species provide valued ecosystem and cultural services.

I add that the nursery industry and their customers enjoy enormous lobbying clout.

Many associations – native plant societies, regional or state invasive plant councils, etc. – are pursuing this approach. To research these efforts, visit the websites for the state native plant societies and the Southeast Exotic Pest Plant Council, Mid-Atlantic Invasive Plant Council, and Midwest Invasive Plant Network. These voluntary efforts have yielded some success. But they have not resulted in adequate protection for our ecosystems. Dr. Douglas Tallamy points out that even non-invasive, non-native plants disrupt food webs.

The insufficient attention to the role of the plant trade in articles intended to be comprehensive has crucially important impacts. As both Potter, et al. (2024) and Potter et al. (2026) affirm, determining which factors are most important in facilitating plant invasions of eastern American forests is the necessary foundation for identifying and implementing the most efficient and effective counter measures.

These scientists are employees of the U.S. Department of Agriculture. If departmental leadership interpret their studies as justifying inaction on regulating plant sales, USDA’s regulatory agencies will not respond. And we will continue failing to curtail introduction and spread of damaging plant invasions.

I agree with the authors on the need for enhanced monitoring and management of WUI zones in the East to detect new species or new locations of invasion and the need to develop better tools for these purposes. However, I ask all stakeholders to follow Evans et al. (2024), who urge prioritizing for state regulation those species in the ornamental trade that are projected to remain or become abundant under evolving climate conditions. Or, more aggressively, follow Beaury et al. (2023)’s call for regulating the nursery trade in a manner consistent with the scope of the horticultural trade at the national level. That would require legislation, since the Federal Noxious Weed Act does not currently address long-established, widespread species. Beaury et al. (2023) also note that existing state restrictions are outdated, tend to include only a few weeds that plague agriculture rather than those that invade natural systems, and are irregularly enforced.

orchids in Everglades National Park; photo by F.T. Campbell

I conclude by agreeing with the scientists that managing the disturbance component of plant invasions points to protecting particularly forests of high conservation value. They suggest adoption of land-use planning rules aimed at this goal. However, as they point out, such action will be extremely unlikely given the magnitude of predicted land-use changes in the country and powerful demographic factors driving them. I would add other barriers: the lobbying clout of the real estate industry and homeowners plus the local nature of zoning decisions.

SOURCES

Fertakos, M.E. and B.A. Bradley. 2024. Propagule pressure from historic U.S. plant sales explains establishment but not invasion. Ecology Letters 2024;27:e14494 doi: 10.1111/ele.14494

Kinlock, N.L., D.W. Adams, W. Dawson, F. Essl, J. Kartesz, H. Kreft, M. Nishino, Jan Pergl, P. Pyšek, P. Weigelt and M. van Kleunen. 2025. Naturalization of ornamental plants in the United States depends on cultivation and historical land cover context. Ecography 2025: e07748 doi: 10.1002/ecog.07748

Oswalt, C.M., S. Fei, Q. Guo, B.V. Iannone III, S.N. Oswalt, B.C. Pijanowski, K.M. Potter. 2016. A subcontinental view of forest plant invasions. NeoBiota. 24:49-54 http://www.srs.fs.usda.gov/pubs/48489

Potter, K.M., K.H. Riitters, B.V. Iannone III, Q. Guo and S. Fei. 2024. Forest plant invasions in the eastern United States: evidence of invasion debt in the wildland‑urban interface. Landsc Ecol (2024) 39:207 https://doi.org/10.1007/s10980-024-01985-y

Posted by Faith Campbell

https://fadingforests.org

Bird nesting habitats – why no mention of invasive species or deer?

ovenbird (*Seiurus aurocapilla*); photo by Rhododentrities via Wikimedia

Studies of forest ecosystems in eastern North America that claim to be comprehensive still too often make no reference to invasive species – pests, earthworms, or plants. I try here to bridge these gaps.

Akresh et al. (2023) conducted a meta-analysis of bird species’ use of forests as nesting habitat. They applied the Partners-in-Flight to evaluate the community-wide bird conservation values of unmanaged forests compared to various levels of tree removal by harvest. Because of the decline of many bird species that prefer shrubland or early-successional stands, their process gave highest ranks to management approaches that retained 40%–70% of the canopy trees.

Their study notes that habitats for shrubland birds comprise only about 6% of forests in the eastern U.S. They don’t provide data for southeastern Canada. But hasn’t this scarcity of open upland, non-wetland, habitats in this region been true for thousands of years?

The type of forest that undoubtedly has shrunk significantly in recent centuries is “virgin” (or old-growth or late-seral) forests. As Akresh et al. (2023) report, contemporary closed-canopy forests in eastern North America are predominantly structurally homogeneous, mid-seral, even-aged, stands that have regenerated on land previously cleared for either agriculture or timber. These forests are much younger from a forest developmental perspective than precolonial forests; they lack the latter’s range of tree fall gap sizes and multiple age-classes. The tiny fraction of eastern forests that are in the late-seral stage might have higher species richness and conservation value for birds, but since they are usually not under management, Akresh et al. (2023) did not include that question in their analysis.

Akresh et al. (2023) list the bird species whose density appears to be closely linked to various tree canopy densities. For example, ovenbirds and brown creepers promptly decline in abundance in response to any amount of tree harvesting. Two other species — wood thrush and cerulean warbler — have declined steeply range-wide in recent decades. Nesting densities of three of these four species (excluding the warbler) are significantly higher in areas harvested in ways that retain a greater percentage of trees. Densities of another five bird species (Acadian flycatcher, hermit thrush, black-throated green warbler, and red-breasted nuthatch) are also higher in areas with a greater proportion of trees retained.

Another nine species had a more complex relationship with tree densities but still had lower densities in stands with low tree retention. These were blue-gray gnatcatcher, blue-headed vireo, blackburnian warbler, black-throated blue warbler, eastern wood-pewee, least flycatcher, red-eyed vireo, scarlet tanager, and yellow-bellied sapsucker. They found little relationship between bird density and tree retention for five putative mature-forest species (American redstart, great-crested flycatcher, hooded warbler, veery, and yellow-rumped warbler).

scarlet tanager (*Piranga olivacea*); photographed in scrub at Edwin B. Forsythe (Brigantine) NWR by F.T. Campbell

Akresh et al. (2023) claim that silviculture approaches can be used to restore aspects of the structural and compositional conditions found in old-growth forests to second-growth systems, providing a potential pathway for rapidly increasing the conservation value of these areas for bird species. They advocate reducing canopies moderately via variable retention harvests, shelterwood establishment harvests, and irregular shelterwood systems. This strategy can increase understory vegetation density, which they assert can then increase foraging and nesting opportunities for both many mature-forest bird species and many shrubland birds.

I am skeptical; it is much easier to create openings in the canopy than to “create” large trees supporting cavities and associated fauna and flora utilized by some bird species. The authors do advise managers that late-seral, unharvested stands can provide important habitat for old-growth-dependent taxa and any intensive forestry should also take into account other factors.

old-growth hemlock stand in Cook Forest State Forest, Pennsylvania; photo by F.T. Campbell

In addition, often the understory vegetation that responds to the more open environment will be invasive non-native plants. Already about half of eastern U.S. forests have been invaded by non-native plants (Oswalt et al. 2016; Kurtz 2023). Many of these are shrubs: honeysuckles, privets, roses, buckthorn. Management of these plants is difficult – especially when opening the canopy to allow light to reach the forest floor. (at www.nivemnic.us, scroll down to “categories”, click on “invasive plants”.) So the question arises, do the non-native plant species adequately substitute for native shrubs in providing resources needed by those birds?

Maybe. Gleditsch and Carlo (2014) found that a shrub layer dominated by non-native honeysuckle shrubs (Lonicera species) does support nesting populations of several common species, especially catbird (Dumetella carolinensis), American robin (Turdus migratorius ), and northern cardinal (Cardinalis cardinalis). However, they did not consider the species of concern to Akresh et al. (2023) – the rare species that prefer open-canopy, early-successional communities. So they do not inform us whether these high-priority species can utilize shrublands dominated by non-native species. Gleditsch and Carlo (2014) apparently did not find nests of several species considered to be associated with mature forests. So, again, these forests’ value for conservation remains unclear. Gleditsch and Carlo (2014) do counter earlier fears that these non-native shrubs are “traps” for nesting passerine birds. (The concern was that the plants’ structure facilitated nest raiding by predators.) They say, instead, that these plants’ effects are species-specific, context-dependent, and often a mix of both positive and negative outcomes.

invasive shrub honeysuckle; photo by Kevin Casper via public.domain.pictures.net

Akresh et al. (2023) also do not address the impact of browsing by super-abundant deer. Others (at www.nivemnic.us, scroll down to “categories”, click on “deer”.) have demonstrated that interactions of deer predation with invasive plants is especially damaging to native flora. Considering forests from Virginia to Maine, Miller et al. (2023) advise opening the canopy or subcanopy of forests to promote tree regeneration where deer and invasive shrubs overlap only where deer are controlled.

I have seen no recent analyses of the impact of widespread pest-caused tree mortality beyond some early efforts focused on eastern hemlocks and on high-altitude whitebark pines.

SOURCES

Akresh, M.E., D.I. King, S.L. McInvale, J.L. Larkin, and A.W. D’Amato. 2023. “Effects of Forest Management on the Conservation of Bird Communities in E North America: A Meta-Analysis.” Ecosphere 14(1):e4315. https://doi.org/10.1002/ecs2.4315

Gleditsch, J.M. and T.A. Carlo. 2014. Living with Aliens: Effects of Invasive Honeysuckles on Avian Nesting. PLOS One. September 2014. Volume Nine Issue Nine. E107120

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

Posted by Faith Campbell

https://fadingforests.org

Will we act to minimize sales of invasive plants?

*Wisteria floribunda*; photo by Jack Stane via Wikimedia

For decades, it has been clear that deliberate introduction of plant species for cultivation plays a central role in the early stages of bioinvasion by plants (and associated insects, plant pathogens, earthworms … even vertebrates. Viz. coqui frogs in Hawai`i.)

Repeatedly over the two plus decades since Sarah Reichard demonstrated this role of ornamental horticulture (see Reichard and White 2001 and Mack 2000), new studies have provided corroborative details. Publications during the past two years show the risks we are still accepting in the United States. Will we act to protect our environment?

Fertakos and Bradley (2024) found that species were likely to establish if they were introduced to as few as eight locations. However, introduction history was not a strong predictor of an established species’ ultimate invasive success. They suggest that other characteristics, like plant traits and local-scale processes (e.g., interspecific interactions), may better predict whether a plant becomes invasive.

Kinlock et al. (2025) also found that plant species that were cultivated longer or were sold by more catalogs were more likely to have “naturalized”. This conclusion was based on analysis of the behavior of nearly 4,000 species sold in nursery and seed catalogs in the continental United States over 200 years. Nearly 41% of these species naturalized somewhere in the “lower 48” states. Unfortunately, they do not discuss what proportion of these species are truly damaging invaders.

Neither Fertakos and Bradley (2024) nor Kinlock et al. (2025) mention the concept of a lag between a species’ establishment and recognized symptoms of invasiveness. Has this concept been repudiated?

Evans et al. (2024) were focused on analyzing which regions of the eastern United States are likely to suffer the worst plant invasions under climate change. In this context, they worry that people will assist non-native plant species’ movement to newly suitable habitats. Evans et al. urge prioritizing for state regulation species in the ornamental trade that are projected to remain or become abundant under the new climate conditions. They say we Americans are poorly prepared to take this action, however, because plant sales are so poorly regulated and only 10% of land managers in eastern North America monitor for new invasive taxa. They say this is because the managing agencies lack of funding and personnel. After 2025’s losses of programs, appropriations, grants, and staff, this deficit is probably worse – not just for federal agencies but also the many state, local, and volunteer programs that have been supported by federal funding.

Beaury et al. (2024) investigated whether plant species recognized as invasive are sold in the same locations as where they are invasive. They found that half of the 89 species named as invasive were sold by a nursery within 21km of an observed record of invasion. The authors say that data gaps mean that these findings underestimate the number of species sold near locations of documented invasions. They warn that at least 25 species are sold by one or more nurseries located in an area that is currently unsuitable for those species, but that will become more suitable for invasion as temperatures warm. Like Evans et al. (2024), they urge proactive regulation to limit these species’ spread.

burning bush *Euonymus*, Japanese honeysuckle, & English ivy invading a bottomland hardwood site in Fairfax County, Virgina; photo by F.T. Campbell

U.S. Regulatory response is completely inadequate

Beaury et al. (2023) call for regulating the nursery trade in a manner consistent with the scope of the horticultural trade – sales by both e-commerce and brick and mortar stores go to customers far outside a specific state’s jurisdiction. Despite the interstate nature of the trade, sales of horticultural plants are regulated primarily by state governments. Even when a state does restrict the sale of a specified list of invasive plants, the regulations are outdated, tend to include only a few weeds that plague agriculture rather than those that invade natural systems, or are irregularly enforced. The result is a checkerboard of places where a species may legally be offered for sale next to places where that sale is prohibited. Finally, the regulations are reactive; they rarely include plants in anticipation of their spread to new areas. Beaury et al. (2023) call this as a missed opportunity to reduce the likelihood of ornamental escapes.

Evans et al. (2024) also note that online plant sales are relatively unregulated, and state regulations are inconsistent.

Under the Constitution, the appropriate entity for regulating interstate commerce is the federal government. The U.S. Department of Agriculture’s Animal and Plant Health Inspection Service is responsible for populating and managing the federal noxious weed list. Unfortunately, APHIS lists only those taxa that qualify as quarantine pests under the definition of that term in the International Plant Protection Convention (IPPC) Glossary of Phytosanitary Terms. This means that the taxon is either not yet present in the United States or, if present, is not widely distributed and is being officially controlled. Under these criteria, the federal noxious weed list is required to exclude nearly all the invasive plant species sold by the nursery trade.

To counter this enormous regulatory failure, many associations – native plant societies, regional or state invasive plant councils, etc. – publish their own lists of invasive plants. They often encourage their members and the public to either avoid planting these species voluntarily or to plant predominantly native plants. Also, these stakeholders urge nurseries to halt sales of invasive species voluntarily. Dr. Douglas Tallamy points out that even non-invasive, non-native plants disrupt food webs.

These voluntary efforts have yielded some success. But they have not resulted in adequate protection for our ecosystems.

Will Americans choose to invigorate the regulatory system? At a minimum, can we urge neighboring states to adopt a regional approach? More difficult, but also more effective, would be to persuade Congress to strengthen APHIS’ invasive plant regulations to outlaw interstate sales of at least those species documented to be invasive.

*Cortadera selloana*; picture by Alex Borland via PublicDomainPictures.net

Do you have other suggestions?

SOURCES

Fertakos, M.E. and B.A. Bradley. 2024. Propagule pressure from historic U.S. plant sales explains establishment but not invasion. Public? doi: 10.1111/ele.14494.

Fridley, J.D., P.J. Bellingham, D. Closset-Kopp, C.C. Daehler, M.S. Dechoum, P.H. Martin, H.T. Murphy, J. Rojas- Sandoval, D. Tng. 2025. A general hypothesis of forest invasions by woody plants based on whole-plant carbon economics.

Posted by Faith Campbell

https://fadingforests.org

New thinking on how non-native plants invade forests

It used to be thought that closed-canopy forests are resistant to bioinvasion because of the low light availability and relatively infrequent disturbance. Yet many are badly invaded! (On this site, scroll down past the Archives, choose “invasive plants” category.)

Nor is it just temperate forests in North America. Subtropical and tropical forests have also been invaded, as have the temperate forests of South America and, to a lesser extent, Europe. Temperate forests in Asia are less invaded; boreal forests very little (Fridley et al. 2025; see full citation at the end of this blog).

Fridley et al. (2025) have proposed a conceptual model to explain how this happens: “superinvaders” – a special class of woody plants – that achieve competitive dominance across a wide range of forest conditions. The “superinvaders” pose especially grave threats to native biodiversity because they use life-history strategies unlike those of early successional native species.

The result is that existing invasion and succession theories poorly predict which forests are most invasible and by which species. This failure undermines pest risk analyses and early detection.

Fridley et al. have raised lots of interesting ideas – some of which cannot yet be demonstrated by observations.

Temperate forests of North and South America are increasingly dominated by non-native deciduous and semi-evergreen shrubs and trees that combine fast growth rate in high light and high survivorship in forest interiors. These traits enable them to outcompete the native species. Many invaders also produce many more seeds than co-occurring native species. Similar traits are found in the successful non-native plants in subtropical and tropical forests.

Fridley et al. stress that shade tolerance alone does not endow the invasive plants with sufficiently large advantages in their competition with native species. The forest “superinvader” phenotype must combine this ability to persist in shade with high maximum growth rate and high fecundity when conditions become favorable for reproduction.

They explain the invader’s competitive advantage as the result of their experiencing relatively fewer carbon costs because of enemy release in the novel environment, recent environmental changes that alleviate some stress formerly present in the novel environment, or phylogenetic constraints on the local flora that limit natives’ resource-use efficiency. The non-native plant species enjoy this advantage regardless of whether they also possess other competitive mechanisms, e.g., production of allelopathic compounds, denser growth or shading, greater apparent quantum yield. However, Fridley et al. concede that they lack sufficient evidence to incorporate these other competitive mechanisms into their model.

Since any reduction in carbon costs will enhance both shade tolerance and growth rate when light levels are high, these “superinvaders” can outcompete native species in either situation.

To support these concepts, Fridley et al. note that increased abundance of invaders following disturbance is more pronounced in forests than other habitats. They suggest this is because of the much greater magnitude of change in light levels in forests than in open habitats such as grasslands.

They propose that an analogous situation applies to the presence or absence of mutualist microbial associations, although existing studies are insufficient to reach conclusions about the role of carbon allocation to mycorrhizae in the “superinvader” phenotype. The extent to which these forest invasions alter ecosystem-level carbon dynamics, especially soil processes and litter decomposition is also largely unknown.

Fridley et al. emphasize the role of carbon costs in driving both growth rate and whole-plant light compensation point. This point is defined as the light level at which carbon gain through photosynthesis balances carbon losses from tissue respiration (maintenance and growth) and turnover (shedding and loss from disturbance and herbivory).

To survive in low-light conditions, plants must minimize tissue respiration and turnover. The traits that enable those behaviors have been thought to prevent rapid growth and competitive dominance in high-light conditions. However, the “superinvaders” defy this trade-off by growing faster than most co-occurring native species when light is abundant. Fridley et al. say this is because the plants’ reduced carbon costs enhance competitive advantage in both shade and adequate light conditions.

Fridley et al. name several reasons why a native plant’s carbon costs might exceed those of an introduced species. First on the list is either herbivory or investment in defensive traits. Native plants might be challenged by rising abundance or consumption rates of native or introduced herbivores, such as deer or seed predators, that avoid the introduced species.

A second factor is that the non-native species expends fewer resources to sustain adaptations that confer resistance to other stresses, such as drought or freezing. If a long-standing stress is weakened by global change processes (e.g., atmospheric CO₂ levels, growing season duration, precipitation levels and seasonality, suppression of fire, atmospheric nitrogen deposition), a non-native plant that lacks defenses against that now-weakened stress will have a lower carbon cost and therefore an advantage. In some cases, the non-native species might benefit directly from these changes, e.g., droughts.

In some regions phylogenetic constraints have limited evolution of adaptive solutions to various biotic and abiotic stresses. This is most obvious on tropical oceanic islands. Fridley et al. report that native trees in Hawaiian montane rain forests are less energy-efficient conducting photosynthesis than are the invaders. However, this phenomenon also occurs on continents. Two continents’ floras might experience different climatic histories even when at they are at similar latitudes. For example, Eurasian woody species leaf out earlier and senesce later than North American trees and shrubs – possibly as a result of more predictable spring and autumnal climate across Eurasia. They name as one example Norwegian maple (Acer platanoides) in North America.

The future is uncertain

Fridley et al. consider enemy release to be a key factor in these shrub invasions of closed-canopy forests. Therefore, if enemy release decays over time because the introduced plant species accumulate pests, or the forest environment shifts to favor more stress-tolerant phenotypes of some native species, then the dominance of superinvaders will decline. If, on the other hand, resource enrichment continues, e.g., nitrogen deposition and elevated CO₂, the impacts of woody invaders – present or newly introduced – might continue to rise. The likelihood that additional introductions of more resource-efficient species will continue to damage floras of oceanic islands.

Implications for risk assessments and management

Fridley et al. warn that habitat-matching criteria might be unreliable predictors of forest invasiveness. Among several examples of species that are invasive in interior forest systems in a novel region that do not exhibit this trait in their native range is red oak (Quercus rubra). It is locally dominant in both natural and managed forests in central Europe while in North America, red oak struggles to regenerate in closed-canopy forests. They suggest that Q. rubra in Europe has escaped seedling pathogens present in its native range in North America.

red oak sapling in swampy forest in Virginia; photo by F.T. Campbell

Fridley et al. call for research on traits they have identified as important but that are rarely measured in invasion studies. These include rates of tissue loss and respiratory processes above- and below- ground, plant carbon allocation to tissues and processes, and the whole-plant light compensation points of native and invasive plant species.

The Fridley et al. hypothesis has been supported explicitly by Kinlock et al. (2025). This article says that consistent findings have been reported by earlier small-scale studies in U.S. forests.

I ask for your input on how well the Fridley et al. hypothesis explains shrub and tree invasions in American forests – including those on tropical islands! Is it helpful? Is APHIS incorporating these ideas into plant risk assessments? –

Fridley et al. take pains to reiterate the long-accepted importance of ornamental horticulture in explaining invasive plants’ entry and establishment. They do so in the context of concurring that ruderal traits are not universally advantageous; traits’ benefits depend on the landscape into which the species was being introduced.

SOURCES

Posted by Faith Campbell

https://fadingforests.org