Center for Invasive Species Prevention

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

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

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

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

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

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

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

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

Naming the invaders

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

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

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

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

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

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

Poland et al. looked at the full taxonomic range of possible bioinvaders in forest and grassland ecosystems. The Caribbean islands receive very brief coverage in the chapter on the Southeast (see Regional Summary Appendices). This chapter contains a statement that I consider unfortunate: “Introduction of species has enriched the flora and fauna of Puerto Rico and the Virgin Islands.” The chapter’s authors assert that many of the naturalized species are restoring forest conditions on formerly agricultural lands. They say that these islands’ experience demonstrates that introduced and native species can cohabitate and complement one another. I ask – but in what kind of forest? These forests, are novel communities that bear little relationship to pre-colonial biodiversity of the islands. Was not this chapter the right place to note that loss? Forests are more than CO₂sinks.

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

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

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

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

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

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

SOURCES

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

Poland, T.M., Patel-Weynand, T., Finch, D., Miniat, C. F., and Lopez, V. (Eds) (2019), Invasive Species in Forests and Grasslands of the United States: A Comprehensive Science Synthesis for the United States Forest Sector. Especially the Appendix on the Southeast and Caribbean. Springer Verlag. Available gratis at https://link.springer.com/book/10.1007/978-3-030-45367-1

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

Posted by Faith Campbell

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

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

www.fadingforests.org

Sobering News: Invasive Grasses, Trees, and Killer Pests in Hawai`i

At CISP, our hearts go out to all those affected by the terrible August fires on Maui. May the departed rest in peace. May the living find comfort and all that is needed for recovery.

Fire and Invasive Grasses

A fire in non-native grasses on Maui in 2009; photo by Forrest and Kim Starr

Major U.S. and international media continue to detail the fires’ devastation, especially in Lahaina. As time has passed, more news has highlighted the role that the widespread presence of introduced, fire-prone grasses played in the rapid growth and spread of Maui’s fires.

For example, The Washington Post devoted seven paragraphs in one story to the issue of grasses. The story quotes several experts: Alison Nugent, an associate atmospheric scientist at the University of Hawaii’s Water Resources Research Center; Jeff Masters, a meteorologist for Yale Climate Connections; and Clay Trauernicht, a fire researcher at the University of Hawaii.

These and others have been widely quoted in the many recent articles. I am glad that they – and the media – are making clear that climate change is not the sole factor causing damaging wildfires. It is clear that Maui’s recent weather patterns – including the high-velocity winds and drought – have been within the range of normal climate patterns. Fluctuations in the Pacific’s weather have also been normal, especially under the influence of the current El Niño.

The dangers caused by Hawai’i’s fire-prone grasses are also clear – and have been for years. Experts have identified policy weaknesses at the county and state level. Also, they have specified changes to land management that could better prevent or mitigate wildfires. There has been far too little action.

On the other hand, there are hopeful signs.

endangered ‘akikiki photo by Carter Atkinson, USGS

The Hawai’i Wildfire Management Organization, a nonprofit, is educating and engaging communities state-wide. Elizabeth Pickett, a Co-Executive Director, presented an overview of wildfire at the Hawai’i Invasive Species Awareness Month in February 2023. The Big Island Invasive Species Committee has successfully eradicated two species of pampas grass on Hawai’i Island – after 13 years’ work. A native species has been planted where pampas formerly grew.

Another Post article reported on efforts by staff and fire departments to protect the Maui Bird Conservation Center, which houses critically endangered Hawaiian birds found nowhere else on Earth, including some currently extinct in the wild. As I have blogged previously, the palila, kiwikiu, ‘akikiki, ‘alalā [Hawaiian crow; extinct in the wild] and other birds are dying from avian malaria, carried by nonnative mosquitoes. The Center on Maui and another on the Big Island are run by the San Diego Zoo Wildlife Alliance. Conservationists have completed field trials of a proposed mosquito suppression process for Maui and are seeking public comments for a similar program on Kaua’i. These programs represent groundbreaking and long-awaited progress on countering a principal threat to the survival of Hawai`i’s unique avifauna. Loss of the Center and its birds would have devastated post-suppression efforts to rebuild and restore bird populations in the wild.

The Post carried a second story about the effort to protect Hawai`i’s endangered birds – a full page of print, even longer – with many photos, on the web. The article mentions the “Birds, Not Mosquitoes” program and varying views about it. I rejoice that the dire situation for the Islands’ biodiversity is getting attention in the Nation’s capital. Again, see my earlier blog.

Plant Invasions in Hawaiian Forests

A team of scientists from the USDA Forest Service and Natural Resources Conservation Service, plus the Hawaii Division of Forestry and Wildlife, has carried out a new assessment of the extent of invasive plant species in forests on the Hawaiian Islands (Potter et al. 2023; full citation at end of blog).

The results of their analysis are – in their words – “sobering”. They portend “a more dire future for Hawai`i`s native forests.”

First, regarding the recent fires, Potter et al. found significantly higher cover by invasive grasses on Forest and Inventory Analysis (FIA) plots on Hawai‘i and Maui than on O‘ahu, Kaua‘i, and Lana‘i. Grass invasions were particularly high on the eastern coast of Maui – near Lahaina. Even so, the authors say their study’s methods resulted in a gross underestimate of areas invaded by fire-prone grasses. That is, most of Hawai’i’s xerophytic dry forests were converted to grasslands before the FIA program began. Therefore these grasslands are not included in FIA surveys.

*Psidium cattleyanum*; photo by Forrest and Kim Starr

The extent of current invasions in wetter forests is already significant – but trends point to an even more worrying future.

Naturalized non-native plant taxa constitute half of the Hawaiian flora.
56% of Hawaii’s 553,000 ha of forest land contained non-native tree species; about 39% of these forest lands are dominated by non-native tree species. Invasive plant species of particular concern were found in the understory of 27% of surveyed forest plots.
Across all islands, six of the ten most abundant species are non-native: Psidium cattleyanum, Schinus terebinthifolius, Leucaena leucocepahala, Ardisia elliptica, Psidium guajava, and Acacia confusa.
While less than one-third (29%) of large trees across the Islands are non-native, this proportion increases to about two-thirds of saplings (63%) and seedlings (66%). Potter et al. focus on the likelihood that plant succession will result in transformation of these forests’ canopies from native tree species to non-native species.
75% of forests in lower-elevation areas of all islands are already dominated by non-native tree species. “Only” 31% of higher-elevation forests are so dominated. These montane forests have been viewed as refugia for native species, but all are invaded to some extent – and likely to become more degraded.
Potter et al. say the high elevation forests might be more resistant to domination by non-natives. Such a result would be counter to well-documented experience, though. Even the authors report that the montane rainforests and mesophytic forests of O‘ahu and Kaua‘i are heavily invaded by non-native tree species. Such species constitute 86% or more of large trees, saplings, and seedlings in mesophytic forests; 45% of large trees and 66% of seedlings in their montane rainforests.
The most abundant tree species in Hawai`i is the invasive species Psidium cattleyanum (strawberry guava). It was recorded on 88, or37%, of 238 FIA plots. There are nearly twice as many P. cattleyanum saplings as Hawai`i’s most widespread native species, ‘ohi’a lehua (Metrosideros polymorpha).
Widescale replacement of native trees by non-native species is likely. Several factors favor these changes: 1) tree disease – rapid ‘ohi’a death has had drastic impacts on ‘ohi’a populations on several islands; 2) invasions by forbs and grasses; 3) soil damage and other disturbances caused by invasive ungulates; and 4) climate change. If succession conforms to these trends, non-native tree species could eventually constitute 75% or more of the forest tree stems and basal area on all islands and across forest types and elevations.

Loss of Hawai’i’s native tree species would be disastrous for biodiversity at the global level. More than 95% of native Hawaiian tree species are endemic, occurring nowhere else in the world.

The authors analyzed plant presence data from 238 FIA plots. Plots spanned the state’s various climates, soils, elevations, gradients, ownership, and management. However, access issues precluded inclusion of forests from several islands: Moloka‘i, Kaho’olawe, and Ni‘ihau. I know that Moloka‘i, at least, has a protected forest reserve (a Nature Conservancy property) at the island’s highest elevations.

Protecting Native Trees

Federal, state, and private landowners have carried out numerous actions to protect native forests. These efforts might be having some success. For example, forests on public lands, in conservation reserves, or in areas fenced to exclude ungulates were less impacted by non-native plants than unfenced plots, on average. However, the authors could not determine how much of this difference was the result of management or because protections were established in forests with the lowest presence of IAS species. Fencing did not prevent invasions by forbs and grasses – possibly because they are so widespread that seed sources are everywhere.

Hawaii’s two National parks (Hawai`i Volcanoes and Haleakala) have made major efforts to control invasive plants. Hawai`i Volcanoes, on the Big Island, began its efforts in the 1980s; Haleakala (on Maui) more recently. This might be one explanation for the fact that a smaller proportion of the forests on these two islands have been invaded. These efforts have not fully protected the parks, however. Low elevation native rainforests now have a high presence of non-native shrubs. Such forests on Hawai`i Island also have significant invasions by non-native woody vines, forbs and grasses.

More discouraging, intensive efforts have not returned lowland wet forest stands to a native-dominated state. Native tree species are not regenerating—even where there is plentiful seed from native canopy trees and managers have repeatedly removed competing non-native understory plants.

Potter et al. conclude that other approaches will be needed. They suggest deliberate planting of native and non-invasive non-native species or creation of small artificial gaps that might facilitate recovery of native tree species. In montane forests on Hawai`i and Maui, where native tree seedlings account for more than 70% of all tree seedlings, they propose enhancing early detection/rapid response efforts targetting invasive forbs. This would include both National parks.Certainly Haleakala National Park has this priority in mind. It launched a serious effort to try to eradicate Miconia calvescens when this tree first was detected.

Lloyd Loope, much-mourned scientist with US Geological Survey, attacking *Miconia* on Maui

Potter et al. note the challenge of managing remnant xerophytic dry forests, where natural regeneration of native plants has been strongly limited by invasive grasses; loss of native pollinators and seed dispersers; and the increasing frequency and intensity of droughts. They note that expanded management efforts must be implemented for decades, or longer, to be successful.

Native Trees at Risk to Nonnative Insects

Beyond the scope of the Potter et al. study is the fact that at least two dry forest endemic trees have faced their own threats from non-native insects.

The Erythrina gall wasp, Quadrastichus erythrinae, appeared in Hawai`i in 2005; it originates in east Africa. It attacks the endemic tree, wiliwili, Erythrina sandwicensis. I believe a biocontrol agent, Eurytoma erythrinae, first released in 2008, has effectively protected the wiliwili tree, lessening this threat.

The Myoporum thrips, Klambothrips myopori, from Tasmania, was detected on the Big Island in 2009. It threatens a second native tree. Naio, (Myoporum sandwicense), grows in dry forests, lowlands, upland shrublands, and mesic and wet forest habitats from sea level to 3000 m. The loss of this species would be both a signifcant loss of native biodiversity and a structural loss to native forest habitats. The thrips continues to spread; a decade after the first detection, it was found on the leeward (dry) side of Hawai`i Island with rising levels of infestation and tree dieback.

*Rhus sandwicensis* on Maui; photo by Forrest and Kim Starr

Two native shrubs, Hawaiian sumac Rhus sandwicensis and Dodonea viscosa, might be at risk from a biocontrol agent in the future. APHIS has approved a biocontrol for the highly invasive Brazilian pepper, Schinus terebinthifolia. Brazilian pepper is the second-most abundant non-native tree species in the State. It was found on 28 of 238 (12%) FIA plots. However, the APHIS-approved biocontrol agent is a thrips—Pseudophilothrips ichini. It is known to attack both of these two native Hawaiian shrubs. The APHIS approval allowed release of the thrips only on the mainland US. However, many insects have been introduced unintentionally from the mainland to Hawai`i. Furthermore, Hawaiian authorities were reported to be considering deliberate introduction of P. ichini to control peppertree on the Islands.

In Conclusion

In conclusion, Potter et al. found that most Hawaiian forests are now hybrid communities of native and non-native species; indeed, a large fraction are novel forests dominated by non-native trees. Business-as-usual management will probably mean that the hybrid forests – and probably those in which the canopy is currently dominated by native species—will follow successional trajectories to novel, non-native- dominated woodlands. This likelihood results in a more dire future for native plants in Hawaiian forests than has been previously described.

Potter at al. hope that their findings can guide research and conservation on other islands, especially those in the Pacific. However, Pacific islands already have the most naturalized species globally for their size—despite what was originally considered their protective geographic isolation.

SOURCE

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 https://doi.org/10.1007/s10980-023-01662-6

Posted by Faith Campbell

www.fadingforests.org

Tree Regeneration Rates: A Tool for Prioritizing Tree Conservation Efforts

Ponderosa pine, Coconico National Forest; photograph by Brady Smith, USFS

Have you noticed, as I have, a spurt of interest in conservation of trees? I can rejoice that more people now focus on this!!!!

I have blogged previously about international and national efforts to determine not only native species deserving conservation priority – by the Morton Arboretum and IUCN but also species most threatened by non-native pests. I have also reported on growing attention to breeding tree resistance to non-native pests.

Some scientists are now focusing on species’ regeneration as a way to understand the probable future of both native and introduced species. I hope that scientists will integrate these new data with existing information on the impacts of invasive non-tree plants and tree-killing introduced pests. We need such a comprehensive picture. That will be a challenge!

Also, I hope attempts to set conservation priorities will influence decisions by governmental and non-governmental funders – and those who influence them! So far, I see little evidence that these key players are paying attention. Some Forest Service scientists and academics are pushing for expanded resistance-breeding efforts. Others are writing sophisticated analyses of non-native pests’ ecosystem impacts. But is the USDA leadership supporting stronger pest-prevention measures? Or funding for research on restoration of species? Are conservation NGOs addressing introduced forest pests?

Here, I summarize new work by Kevin Potter and his colleagues, published in two papers (full references at the end of this blog). After reading my summary, I’d like to know: What do you think? Do you agree with the focus on individual species’ regeneration to set conservation and control priorities? Do you agree with the priority species and geographic regions they suggest?? How should we resolve inconsistencies compared to the priorities suggested by the IUCN and Morton Arboretum? If you do agree, how would you suggest we move forward? If not, what approach do you think would be more useful?

A New Approach to Evaluating Species at Risk

Potter and Riitters (2022) point out that a species’ successful regeneration is key to its population’s future genetic diversity. That, in turn, determines the organisms’ ability to adapt to environmental stress and change. The latter includes, but is not limited to, climate change. Because trees are immobile and long-lived, their populations probably require substantially more genetic variation than those of other kinds of plants.

Potter and colleagues (both articles) used FIA survey data to examine regeneration rates by both tree species native to the continental United States (= CONUS) and non-native tree species introduced to CONUS, Hawai`i, or Puerto Rico. I rejoice that they have included these tropical islands, which are part of the United States and are centers of plant endemism. (Two other blogs provide details on their findings in Hawai`i and Puerto Rico.

Native Trees at Risk: Focus on Poor Regeneration

For CONUS, Potter and Riitters (2022) asked whether 280 native forest tree species are regenerating at sustainable levels, both across their full ranges and in regional portions of their ranges, defined by provisional seed zones (an area within which plant materials are assumed to be adapted). Tree species for which FIA surveys placed 75% of the stems in the sapling or seedling classes are determined to be regenerating at sustainable levels. Tree species exhibiting lower proportions of their stems in these “small tree” classes are said to be failing to regenerate adequately.

Potter and Riitters (2022) found that 46 of the 280 native tree species (16.4%) might be at risk of losing important levels of genetic variation (see the list of species in Table 2 of the article). These included high proportions of species evaluated in the following genera: two of three Platanus species; two of four Nyssa species; about 40% of Juniperus and Pinus; and five of 46 Quercus species (10.9%).

[Many areas of the eastern forest, especially in the Mid-Atlantic region, are reported by Stout, Hille, and Royo (2023) to be have insufficient advance regeneration to replace canopy trees.]

Some species appear to be headed toward outright extinction, not only loss of genetic diversity. These include four relatively rare species in California: Pinus muricata, Platanus racemosa, Pseudotsuga macrocarpa, and Sequioadendron giganteum. No seedlings or saplings are recorded on the plots on which they occurred. I note that Platanus racemosa in southern California is being attacked and killed by the Fusarium dieback vectored by the polygamous and Kuroshio shot hole borers.

*Platanus racemosa* riddled by invasive shot hole borer; photo by Beatriz Nobua-Behrmann, University of California Cooperative Extension

I find it alarming that a few of the possibly at-risk species have extremely wide distributions. These are Populus deltoides (eastern cottonwood), Platanus occidentalis (American sycamore), and ponderosa pine (Pinus ponderosa). Another group of species are classified as at potential risk in all their seed zones: Juniperus californica, Juniperus osteosperma, Pinus pungens, and Quercus lobata (valley oak). I note that valley oak is also under attack by the recently introduced Mediterranean oak borer. Its vulnerability is exacerbated by its relatively small range.

Potter and Riitters (2022) found distinct geographic hot spots: 15 at-risk species occur primarily in the Southeast and 14 species are in California; both represent nearly a third of the at-risk species.

In general, high rates of regeneration failure are seen in the West. Nine at-risk species (19.6% of the 46) grow in the Southwest, eight in Texas (17.4%), and four in the Rocky Mountains (8.7%). However, the Northeast and Midwest are not immune. Seven species from the former and six from the latter are also regenerating poorly. Considering pines alone, seven of 14 at-risk speciesare in the West and five in the Southeast.

Seed Zones: a Proxy for Local Genotypes

As I noted at the beginning, Potter and Riitters (2022) used USDA Forest Service provisional seed zones as a proxy for areas in which a species is presumably locally adapted. In addition to the 46 species considered failing to regenerate adequately throughout their entire ranges, Potter and Riitters (2022) determined that another 39 species are at potential risk of losing locally adapted genotypes. That is, their regeneration levels fell below the threshold in at least half of the seed zones in which they occurred. These potentially at-risk species are in the same taxonomic groups: 13 pines (33.3% of the 39 species in the category), six junipers (15.3%), and three oaks (7.7 %). These, too are concentrated in the Southeast and California: 40% are in the former — including both bald-cypress species — and 30.8% are in California. Another seven species (17.9% of the 39) are in Texas. The Midwest is home to seven species, the Northeast and Southwest each has five species (12.8%), and the Rocky Mountain region has three species (7.7%).

Bald-cypress; photo by Kej605 via WikiMedia

The seed zones with the largest numbers of species regenerating poorly are in the East, specifically the central Great Lakes region, western New York and Pennsylvania, along the Mid-Atlantic and New England coasts, and the coastal plain from southern South Carolina to eastern Texas. Potter and Riitters (2022) say these areas have such high numbers of at-risk species because they are home to so many tree species. I note [although Potter and Riitters (2022) do not] that these regions have also experienced severe levels of tree mortality due to the emerald ash borer (mature and young trees), beech leaf disease (primarily young trees), and laurel wilt disease (sub-canopy trees).

A different geographic pattern appears when considering the proportion — rather than the number — of species facing deficits in regeneration. In several Western regions, 60 – 100% of the tree species fell below the study’s threshold of 75% of recorded stems being in the sapling or seedling sizes. These seed zones are found particularly in parts of California, the Southwest, the Great Basin, and the Pacific Northwest. In none of the seed zones in the East are more than 50% of tree species in the category of potentially losing genetic variation. The implication is that while more species might be lost from parts of the East, the loss of fewer species in some Western seed zones could result in larger impacts on the composition, structure, and function of forest ecosystems there.

Potter and Riitters (2022) say that their approach has limitations because it relies on an assumption that a lack of smaller (i.e., younger) trees is an indication that a species has inadequate regeneration across all or part of its distribution and thus is vulnerable to losing genetic variation. They are not able to quantify directly the genetic variation within most forest tree species. In addition, the choice of 75% or fewer of all trees being seedlings or saplings threshold as the threshold is arbitrary. They believe these decisions are defensible.

Potter and Riitters (2022) hope that indicators of forest sustainability such as this can bridge the gap between scientists, forest managers, policy makers, and other stakeholders.

Further, the authors hope that this approach will help prioritize species most in need of: 1) monitoring for genetic diversity, 2) in situ conservation, and 3) ex situ propagule collections. In a future blog I will compare the species highlighted by Potter and Riitters (2022) to the earlier priority list developed by the IUCN and Morton Arboretum. Finally, the focus on regeneration levels could help scientists design representative sampling protocols for range-wide ex situ propagule collections for genetic diversity studies using molecular markers.

Applying This Analysis to Invasions by Non-native Trees

In a second study, Potter, Riitters, and Guo (full citation at end of this blog) flipped the focus: they used the same approach to quantify the degree of invasion by non-native trees in the U.S. I’ve blogged about this study, in general, here. Also see my separate blogs for its welcome application to Hawai`i and Puerto Rico.

Again, Potter, Riitters, and Guo hope their approach will assist in the crucial, difficult task of distinguishing between high-impact and less threatening non-native species. They warn, however, that the FIA survey procotol does not suit the needs of an early detection system.

Differentiating Invasive Tree Species’ Impacts

Potter, Riitters, and Guo note that thousands of non-native tree species have been planted around world to provide an extensive list of ecosystem services. Globally, 400 tree species have been recognized as naturalized (= consistently reproducing) or invasive (= spreading) in areas outside their native ranges. Contrary to some expectations, even relatively undisturbed forests are affected by invasive plants. In the continental United States, many fewer invasive plant species are trees than other forms/habits – shrubs, forbs, gramminoids. On the tropical islands, a much higher proportion of invasive plants are trees.

Lugo et al. (2022; full citation at end of this blog) find non-native tree species occupy a tiny fraction of the forest area of the continental United States [= CONUS], i.e., only 2.8% of the area, and only 0.4% of all tree species recorded in the FIA plots. However, these non-native tree species are widespread. They are found in 61% of forested ecosections in CONUS. Also, they are becoming more common in invaded sites. [Ecosections are divisions within 37 ecological provinces in the hierarchical framework developed by Cleland et al. (2007). There are 190 ecosections in U.S. forest biomes.]

Potter, Riitters, and Guo categorized those non-native tree species with at least 75% of stems detected by FIA surveys to be in sapling or seedling size as highly invasive. In other words, these species are successfully reproducing after reaching the canopy. So they might be more likely to alter forest functions and ecosystem services than those reproducing less robustly. They classified as species with 60 – 75% of recorded stems in these “small tree” categories as “moderately invasive.”

Potter, Riitters, and Guo suggest that control might more productively target the moderately invasive species in geographic regions where they have spread less so far – so presumably fewer seed-bearing mature specimens are present. They list as examples Picea abies, Pinus sylvestris, and Paulownia tomentosa.

In CONUS, FIA protocols specify reporting of 30 non-indigenous tree species.

Acer platanoides

Ailanthus altissima

Albizia julibrissin

Alnus glutinosa

Castanea mollissima

Casuarina lepidophloia

Cinnamomum camphora

Citrus sp.

Elaeagnus angustifolia

Eucalyptus globulus

Eucalyptus grandis

Ginko biloba

Melaleuca quinquenervia

Melia azedarach

Morus alba

Paulownia tomentosa

Picea abies

Pinus nigra

Pinus sylvestris

Populus alba

Prunus avium

Prunus persica

Salix alba

Salix sepulcralis

Sorbus aucuparia

Tamarix spp

Triadica sebifera

Ulmus pumila

Vernicia fordii

About half of these –16 species – qualified under the Potter, Riitters, and Guo criteria as highly invasive: Acer platanoides, Ailanthus altissima, Albizia julibrissin, Cinnamomum camphora, Elaegnus angustifolia, Melia azedarach, Melaleuca quinquenervia, Morus alba, Picea abies, Pinus nigra, Prunus avium, Salix alba, Salix sepulcralis, Triadica sebifera, Ulmus pumila, Vernicia fordii. An additional four taxa are ranked as potentially highly invasive: Tamarix; Eucalyptus grandis and E. globulus, Populus alba.

ring-billed gulls eating berries of Chinese tallowtree (*Triadica sebifera*); photo by TexasEagle via Flickr

I ask : Do YOU agree that these taxa are the most important to be tracking as potentially invasive in forests of the continental United States?

Potter, Riitters, and Guo distinguish between the most “common” and the most “widespread” invasive tree species – although they do not define the differences. Some of the most “common” or “widespread” species are not a surprise: Ailanthus altissima, Triadica sebifera (syn. Sapium sebiferum), and Acer platanoides. Ailanthus is categorized as highly invasive in 39 of 44 ecoregions in which it occurs. It is also notoriously difficult to manage. Triadica sebifera is classified as highly invasive in every one of the 20 ecoregions in which it occurs. It produces prolific seed crops that are widely dispersed by birds and water. It can invade both disturbed and undisturbed habitats. Some of the common or widespread species do surprise me: Ulmus pumila, Morus alba and Picea abies.

Most of the non-native tree species occur on only 2% of plots in the ecoregions in which they occur. However, some highly invasive trees exceed this level:

Triadica sebifera is detected on 8.6% of plots on average across 20 ecoregions;

Ulmus pumila is detected on 3.7% of plots across 39 ecoregions;

Elaeagnus angustifolia is detected on 3.3% of plots in 13 ecoregions;

Melaleuca quinquenervia is detected on 2.7% of plots in 4 ecoregions.

A. altissima is detected on only 2% of plots in the 44 ecoregions. This is surprising to me. I see it everywhere in the Mid-Atlantic – and elsewhere!

[In USFS Region 9 (24 states in the Northeast and Midwest), FIA surveys in 2019 detected Ailanthus on only 3% of plots, Norway maple and Siberian elm each on only 1% of plots (Kurz 2023).]

Eastern U.S. forests are invaded at rates several times those in Western forests, both as a proportion of plots that are invaded and the diversity of plant growth forms. The probability of invasion is highest in Eastern forests that are relatively productive and located in fragmented landscapes that contain developed or agricultural land. Non-native invasive trees are most prevalent along the Gulf Coast and in Mid-Atlantic and Midwestern States. Highly invasive non-native trees are most diverse in the ecoregions of the Mid-Atlantic and Southeast. I note that these regions also rank high in numbers of native tree species determined by Potter et al.’s other study to be reproducing an unsustainable levels.

The study found that non-native trees are almost entirely absent from the Rocky Mountain States and Alaska. However, I have seen Ailanthus in riparian areas of Utah, Arizona, and New Mexico. While few non-native tree species are recorded from ecoregions along the Pacific Coast, those areas are heavily invaded by other types of plants. Lugo et al. say those shrubs and forbs are not interfering with forest regeneration. Do YOU agree?

BLM & USFS botanists removing Spanish broom from Rogue River Canyon; photo by Stacy Johnson, BLM

On tropical islands included in the study – Hawai`i and Puerto Rico – the situation is very different. Together, these islands’ tree canopy covers less than 0.5% that of the area in the lower 48. Hawai`i is recognized as a global hotspot of non-native species richness. Naturalized non-native plant taxa constitute about half of the Hawaiian flora. The US Forest Service tracks twice as many non-native tree species in Hawai`i (62) than over the entire continental U.S. plus Alaska.

Of these 62 species, Potter, Riitters, and Guo identified 26 tree species as either highly or moderately invasive, either already or potentially highly invasive, three as moderately invasive, seven as potentially moderately invasive. In general, the richness of non-native tree species is higher in lower-elevation ecoregions, especially the lowland/leeward dry and mesic forests on O’ahu and lowland wet and mesic forests of the Big Island. [The article makes a brief reference to the probable role of rapid ʻōhiʻa death opening the canopy of the mesic and wet forests, thereby facilitating plant invasions.] Most Hawaiian ecoregions, especially those on O’ahu and Hawai’i Island, had higher non-native tree species richness than even the most highly invaded ecoregions in the lower 48 states. Parts of O’ahu & Maui had the most non-native tree species classified as highly invasive.

The Caribbean archipelago – including but not limited to Puerto Rico – has a lower proportion of non-native plant species than Hawai’i — 17% of plant species are not native. However, their presence is even higher: two-thirds of Puerto Rico’s forests comprise novel tree assemblages. This is probably because Puerto Rico has half the land area of the Hawaiian archipelago and has been part of global trade networks for 500 years instead of 200. Potter and colleagues identified 17 non-native tree species as highly invasive, 16 as potentially highly invasive, and two as moderately invasive.

On the continent only seven of 30 non-native tree species occurr on at least 2% of FIA plots across the ecoregions in which they are inventoried. Hawai’i is stunningly different: 56 of 62 species occurr on at least 2% of plots across ecoregions on average; 24 species are present on at least 10% of plots on average. One species, Psidium cattleyanum, is present on nearly half of surveyed plots across 13 ecoregions! In Puerto Rico, 21 species occurred on at least 2% of the FIA plots.

Acacia confusa – highly invasive in dry forests of Hawai`i; photo by Forrest and Kim Starr

Potter, Riitters, and Guo could not assess the invasiveness of several species that occurred only as small stems in a couple of plots. There are 11 such species on Hawai`i, eight on Puerto Rico. These species might be in the early stages of widespread invasion, or they might never be able to reproduce and spread. Despite the uncertainty, the authors suggest that eradication or control efforts targetting these species might be more cost-effective since presumably few trees have reached reproductive age yet. In Puerto Rico, they single out Schinus terebinthifolius, since it is already recognized as moderately invasive in Hawai`i [I add – seriously invasive in nearby Florida!]. However, they also emphasize the threat from one of the widespread species, Syzgium jambos, because it is a shade-tolerant species that can form dense, monotypic stands under closed canopies

I have posted separate blogs providing more details on the invasive tree species in Hawai`i and Puerto Rico.

Limits of the FIA Dataset

As in the study of native species regeneration, Potter, Riitters, and Guo specify limits arising from use of the FIA dataset. Two seem particularly pertinent to evaluation of the situation on the tropical islands.

First, they cataloged only those non-native tree species chosen by the FIA program administrators to track in the three major regions. Again, I ask YOU whether you agree with the species being recorded. Should others species be included? Should some of these species be dropped?

Second, the survey protocol does not differentiate between sites with significantly different status and history. For example, non-native trees growing on abandoned agricultural sites are counted the same way as those growing in presumably old-growth forests. They conclude that including such sites might explain the records of Eucalyptus and pine species in surveys on the islands.

Finally, as noted in the other study, the program incorporates plots that contain at least 10% canopy cover by live trees or had such cover in the past. The inventory has not included urban parks – although in recent years an urban inventory protocol has been developed.

I remind you that Potter, Riitters, and Guo warned that the FIA inventory is not designed to detect newly introduced species that are early in the invasion process.

SOURCES

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

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

Potter, K.M and Riitters, K. 2022. A National Multi-Scale Assessment of Regeneration Deficit as an Indicator of Potential Risk of Forest Genetic Variation Loss. Forests 2022, 13, 19. https://doi.org/10.3390/f13010019.

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

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

Posted by Faith Campbell

www.fadingforests.org

FY24 Appropriations for Key Programs: current status

In March I asked your help in asking Congress to fund USDA programs that protect forests from non-native insects and pathogens. The Congress has now taken major steps to specify funding for Fiscal Year 2024 – which begins on 1 October. Both the House and Senate Appropriations committees have adopted their bills. They differ substantially. When Congress returns from its August recess in September, it will face many difficulties in negotiating the final spending levels – not just the different funding levels but also attached “riders” dealing with social and political issues, most of which have nothing to do with invasive species. [Some of the riders to seek to restrict application of the Endangered Species Act to several species, e.g., sage grouse and grey wolf.]

USDA APHIS

As you know, USDA’s Animal and Plant Health Inspection Service (APHIS) is responsible for preventing introduction of pests that harm agriculture, including forests; and for immediate efforts to eradicate or contain those pests that do enter. While most port inspections are carried out by the Department of Homeland Security Bureau of Customs and Border Protection, APHIS sets the policy guidance. APHIS also inspects imports of living plants. In the table below, I provide information on funding for key APHIS programs in FY23, the Administration’s request for FY24, the funding level the Center for Invasive Species thought necessary, and the House and Senate funding levels.

The earlier blog link has additional information: the FY22 funding levels and a fairly long justification for funding these APHIS programs. I never posted a blog discussing USFS funding due to my trip to Europe.

Appropriations for APHIS programs (in $ millions)

Program	FY 2023	FY 2024 Pres.’ request	CISP ask	House bill	Senate bill
Tree & Wood Pest	$63	$64	$65	$55.6	$62.6
Specialty Crops	$216	$222	$222	$224.5	$217.9
Pest Detection	$29	$30	$30	?	?
Methods Development	$23	$23	$25	?	$21.8
Emerg. Preparedness	$44	$45.2	NA	$44.6	$48
Contingency fund	$514	$543		$514	$514

I have not seen a report from the House Committee so I don’t know whether that body prioritized any invasive species issues.

The Senate report included this statement re: Sudden oak death

“The European strain 1 [EU1] and the North American strain 1 [NA1] of the sudden oak death pathogen are major threats to western Douglas-fir/tanoak forests, resulting in quarantine restrictions that threaten U.S. forests and export markets for log shipments and lily bulbs. The Committee recommendation includes no less than the fiscal year 2023 funding level to improve understanding of EU1 and NA1 strains of the sudden oak death pathogen and treatment methods to inform control and management techniques in wildlands.”

Appropriations for USFS programs (in $ millions)

Program	FY 2023	FY 2024 Pres.’ request	CISP ask	House bill	Senate bill
Forest Health Protection
Federal Lands			$32		$17
Coop Lands			$51		$33
Research & Development		$349.1	$349.1	$275	$307.3
Forest Inventory		$30.2	$30.2	$32.2	$32.2
Work on 10 invasive spp	$.5	$4.4	$8.5*	0	0

* CISP ask was intended as first step to increasing funding for invasive species to 5% of R&D funds. See the March blog for an explanation.

SOD-killed tanoaks in southern Oregon; Oregon Department of Agriculture photo

Senate report = Sudden Oak Death. “Since 2001, USFS has been treating SOD infestations on public lands in Oregon and California, in cooperation with Bureau of Land Managemebt. The Committee expects USFS continue these efforts; it provides $3M for SOD treatments and partnerships with States and private landowners.”

Urban & Community Forestry.—”The bill provides $40 M for urban and community forestry. The Committee recognizes the critical need to restore and improve urban forests due to dominance of exotic invasive woody species. USFS should prioritize regional multi-organizational collaborations to support conservation efforts that help trees adapt to and offset climate change, which model best practices for effective urban and community forestry grants. The Committee also expects the program to prioritize tree-planting in socially disadvantaged and historically underserved communities with low canopy coverage, including Tribal communities.”

Summary of Justifications: The Costs of Introduced Pests

Introduced pests threaten many forest products and ecosystem services benefitting all Americans. Already, the 15 most damaging non-native pests threaten at least 41% of forest biomass in the “lower 48” states. In total, these 15 species have caused an additional annual conversion of live biomass to dead wood at a rate similar in magnitude to that attributed to fire (5.53 TgC per year for pests versus 5.4 to 14.2 TgC per year for fire). Fei et al.; full citation at end of blog.

These pests also impose significant costs that are borne principally by municipal governments and homeowners. As more pests have been accidentally introduced over time, these costs have risen.

Pathways of Introduction

The many tree-killing wood-boring pests arrive in inadequately treated crates, pallets, and other forms of packaging made of wood. The March blog presents 2023 data on import volumes and the findings of Haack et al. 2022. The point is, ISPM#15 has fallen short. [See blogs under “wood packaging” category on this site]

APHIS’ Tree and Wood Pests account supports eradication and control efforts targeting principally the Asian longhorned beetle and spongy (= gypsy) moth. Eradicating the ALB normally receives about two-thirds of the funds. The programs in Massachusetts, New York, Ohio, and South Carolina must continue until eradication succeeds. The emerald ash borer continues to spread since APHIS dropped regulations attempting to halt this. EAB was detected in Oregon in 2022; and on Colorado’s western slope in 2023.

Other pests—especially plant diseases like sudden oak death and sap sucking insects like hemlock woolly adelgid—come on imported plants. I noted that no studies have examined the risk of pests arriving on the ~5 billion plants we Americans now import annually (see March blog). The information gap is particularly alarming regarding pathogens. Evidence of failures:

the recent detection of two strains of Phytopthora ramorum that were formerly limited to Europe in forests of Oregon and California;
introduction of rapid ‘Ōhi‘a death in Hawai`i;
and the apparent introduction of the causal agent(s) of beech leaf disease.

APHIS manages damaging pests introduced on imported plants or other items through its Specialty Crops program. The principal example is its efforts to prevent spread of the SOD pathogen through the interstate trade in nursery plants. I am pleased that the Senate report calls on APHIS to focus on that pathogen’s growing genetic diversity.

Beech leaf disease has spread >700 miles since its first detection just 11 years ago

The Administration did not persuade the Congress to fund a $1 million emergency fund for APHIS – although they did fund both “emergency” and “contingency” programs. I am not certain about the difference.

Furthermore, both chambers of Congress included in their legislation – not in the report – language instructing the Secretary of Agriculture to use his authority to obtain emergency funds from other USDA agencies to address animal or plant health emergencies:

“Provided further, That in addition, in emergencies which threaten any segment of the agricultural production industry of the United States, the Secretary may transfer from other appropriations or funds available to the agencies or corporations of the Department such sums as may be deemed necessary, to be available only in such emergencies for the arrest and eradication of contagious or infectious disease or pests of animals, poultry, or plants, and for expenses in accordance with sections 10411 and 10417 of the Animal Health Protection Act (7 U.S.C. 8310 and 8316) and sections 431 and 442 of the Plant Protection 15 Act (7 U.S.C. 7751 and 7772), and any unexpended bal1ances of funds transferred for such emergency purposes in the preceding fiscal year shall be merged with such transferred amounts.”

The Congress has included this or similar language in appropriations reports for almost two decades, but it has not succeeded in freeing up many funds for countering plant pests. Perhaps placing the language in the legislation rather than the report will help … we will have to see.

In the meantime, I have been working with others to amend the Plant Protection Act to ensure that the emergencies so referenced include threats to forests. See §2 in H.R. 3174 link (Balint, Vermont) and S. 1238 (Welch, Vermont).

SOURCES CITED

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

Haack R.A., J.A. Hardin, B.P. Caton and T.R. Petrice .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

Posted by Faith Campbell

www.fadingforests.org

Countering Non-Compliant wood packaging – is new study the 1st step?

infested wood bearing ISPM#15 mark; photo by Oregon Department of Agriculture

SWPM has been recognized as a major pathway for introduction of tree-killing pests since the Asian longhorned beetle was detected in New York and Chicago in late 1990s. As of 2014, 58 new species of non-native wood- or bark-boring insects had been detected – many probably introduced via wood packaging [Leung et al. 2014]. Other examples include the emerald ash borer, redbay ambrosia beetle, and, possibly, the invasive shot hole borers.

In response to recognition of the pest risk associated with wood packaging, countries adopted ISPM#15. This process was reviewed in the two articles by Haack et al. and my recent blog. I provided the broader context of the World Trade Organization (WTO) in my Fading Forests II report.

I have blogged often about the continuing poor compliance with wood packaging regulations, especially by China; and USDA APHIS’ insufficient efforts to fix the problems. The DHS Bureau of Customs and Border Protection (CBP) has tried much harder. See particularly my blog about Bob Haack’s re-evaluation of the pest approach risk in wood packaging. Given the high volumes of imports, pests infesting even a small proportion of incoming shipments can result in tens of thousands of pest-infested containers entering the U.S. or Canada each year. For an explanation of these calculations, see the “background” section of this blog.

Since 2010, CBP has discovered actionable pests in more than 700 shipments each year (pers. comm.). [APHIS reports half as many detections – 300 wood boring and bark beetles (Greenwood et al. citing APHIS report from 2021). Perhaps the difference arises from some of the actionable pests not being wood-borers, e.g., snails.] The persistence of pest presence has disappointed CBP staffers, because the agency has taken several actions intended to discourage violations. These include imposing fines and revoking the violators’ participation in the U.S. Trade Partnership Against Terrorism (C-TPAT) program. Greenwood et al. describe these consequences of non-compliance, as well as the expense of re-exporting the goods and associated wood packaging, as “significant”. Regardless of how significant they might be, so far these consequences have not reduced non-compliances substantially.

The fact is, countries cannot rely on the presence of the ISPM#15 mark or stamp to indicate that the wood packaging is pest-free. In both the United States and Europe, more than 90% of the SWPM found to be infested has born the ISPM#15 stamp (pers. comm.; Eyre et al. 2018). All the pest-infected shipments imported after 2006 discussed in the Haack et al. 2022 study were in wood packaging bearing the ISPM#15 mark. While many of the problems arise on shipments from Asia, findings occur sporadically with countries all across the globe- and notably, U.S. importers have also found serious problems with dunnage from Europe.

But that is the purpose of the standard!

Two outstanding questions that need answers

Continuing poor compliance with regulations by China. This is despite the fact that the U.S. and Canada have required treatment of wood packaging from China since December 1998 – nearly 24 years. Haack et al. found that the proportion of Chinese consignments with infested wood is five times greater than expected based on their proportion of the dataset. The rate of wood packaging from China that is infested has remained relatively steady: the Chinese infestation rate was 1.26% during 2003–2004, and ranged from 0.58 to 1.11% during the next three periods.

Why are the responsible agencies in the United States not taking more aggressive action to correct this long-standing problem? This is a matter of political will.

Despite the ISPM#15 mark being unreliable for more than a decade, countries have not carried out research to determine the root causes. Even now (i.e., Haack et al. 2022; Greenwood et al.) no one can say what proportion of these ISPM-marked but pest-infested pieces of wood results from the treatment not being effective in killing all pests; what proportion results from inadequate application of treatments that are per se effective; and what proportion from fraud (deliberate claims to have applied a treatment that was not done)?

Admittedly, answering these questions will not be easy. First, there is no independent test for whether treatments have been applied; the treatments do not alter the wood’s properties in measurable ways. Scientists need experiments to test the real-world efficacy of treatments in the specific contexts of solid wood packaging.

Second, each country is responsible for its own compliance. Countries differ in their capacity and political will to address this issue. However, success of ISPM#15 depends on determining the cause of continuing pest presence in wood marked as treated, and taking appropriate action to solve the underlying problem.

Greenwood et al. attempt to make progress toward carrying out this necessary task by describing the many steps in the wood packaging supply chain, associated opportunities for pests to infest the wood at each step, and actions exporters and importers can take to try to minimize the risk.

Again, as I discussed in the earlier blog, Haack et al. (2022) found several disturbing situations:

While the pest approach rate has fallen since U.S. implementation of ISPM#15, the extent of the decline has progressively decreased as time passes. The reduction during 2005–2006 was 61%; during 2007–2009, 47%; during 2010-2020 only 36%.
The 2010 – 2020 pest approach rate was calculated at 0.22%. This is more than double the rate based on 2009 data (0.1%, as stated in Haack et al. 2014). While we cannot directly compare these two data points (the two studies used different methods, as discussed in the blog), the bottom line is that the approach rate remains too high. Our forests continue to be exposed to the risk of introduction of highly damaging wood-boring pests. Furthermore, since the number of countries sending us infested wood packaging has increased, those potential pests include insects from a greater variety of countries (biomes).
The two most commonly intercepted families of wood borers are Cerambycidae and Scolytinae (Haack et al. 2022). These families include the Asian longhorned beetle, , redbay ambrosia beetle, and invasive shot hole borers. The 2009 amendment requiring debarking has not apparently resulted in substantial decreases in pest presence, although the proportion of pests that are true bark beetles has declined – from 100% of Scolytinae identified to genus or species detected before 2009 to only 23% in 2010–2020 period.

Michigan’s champion green ash killed by emerald ash borer

Haack et al. (2022) Recommendations

Haack et al. (2022) call for several improvements. Several pertain to how data are collected. Recording the number of infested pieces of wood instead of reporting only consignments would help clarify whether the numbers of insects reaching our borders has fallen, risen, or remained steady. Recording the presence of bark – and the size of any bark remnants – would help clarify whether pests are re-infesting treated wood.

They also note opportunities to improve ISPM#15 implementation and enforcement through training. However, compliance issues persist despite past educational efforts by APHIS and the IPPC.

The Wood Packaging Supply Chain Offers many Opportunities for Pests to Infest the Wood

Greenwood et al. describe each step in fabricating wood packaging material and the opportunities each step presents for unwanted organisms to enter that supply chain. They note that ensuring that these organisms are not then transported on wood packaging being used to carry goods requires that the pests be removed; rendered infertile, inactive, unable to complete development or reproduce; or killed.

The first step in fabricating wood packaging is to harvest trees. Those trees probably harbor various insects, fungi, nematodes, and other organisms that use trees as a resource — for food, shelter, or as a substrate for oviposition. Greenwood et al. mention that the multiplicity of organisms’ life histories pose different challenges for detection and management depending on size, type of tissue utilized, and other factors. The likelihood that a pest or pathogen will be present on or in tree tissues depends on several biotic and abiotic factors, including a species’ proclivity to experience periodic or episodic outbreaks; blow-down events (e.g., hurricanes, windstorms); and harvesting practices. Some of these factors can be controlled by people harvesting the wood.

One of the most frequent opportunities for pest infestation, escape, or cross-contamination is when the wood is stored in the environment. Such storage events happen after the tree is felled — at either the harvest site or processing facility; after the pallet or crate is built – either empty or after the goods have been packed; at the port of export before embarkation; at the importing port before inspection or onward transport; at distribution centers; at retailers; at “pallet graveyards” while awaiting repair or recycling. Retailers and customers have few resources for responsible handling of SWPM – and few incentives to be careful.

a “pallet graveyard”; photo by Adnan Prasad, then with Davey Tree

The risk is exacerbated if storage takes place near woodlands. photo from Savannah At ports and distribution centers, the presence of SWPM from many origins adds to the risk of cross-contamination. Enclosing the SWPM in containers does not completely eliminate the risk since organisms might enter through cracks or air vents. Greenwood et al. suggest management tactics to prevent or reduce pest interaction with the wood during these periods.

container storage near a treed area – Port of Savannah; photo by F.T. Campbell

One of the ISPM#15 requirements intended to minimize the pest risk is debarking the wood. This process removes most organisms that live in and just under the bark. However, debarked wood usually retains some patches of bark because trees are not perfectly round cylinders. Therefore ISPM#15 specifies that remaining bark must be less than 3 cm wide or, if the piece is longer than 3 cm, less than 50 cm² in area.

Greenwood et al. state that after debarking and treatments per ISPM#15, the risk that a pest will be present on the SWPM has been significantly reduced. However, other challenges appear as the newly-minted packaging is put into use – primarily through the possibility of contamination during storage – as described above. There are also risks associated with inadequate or insufficient treatment or fraud.

Once loaded onto a ship, containers and any SWPM, including dunnage, are very difficult to inspect. That means that the loading process presents that last opportunity for inspection and mitigation of contaminating pests. Greenwood et al. note that it is the shipper’s responsibility to ensure containers are “clean, free of cargo residues, noxious materials, plants, plant products and visible pests” before being loaded on the ship. However, the International Maritime Organization (IMO) provides only recommendations, not mandates. Australia has adopted more stringent requirements.

Arrival at the importing country’s port presents the first opportunity for non-indigenous organisms to escape and the first domestic opportunity for the receiving country to inspect the shipment. While U.S. and Canadian customs agencies have authority to board ships before they dock to inspect them, Mexican agencies do not. The most extensive pre-docking requirements are aimed at preventing arrival of moths in the Lymantria genus from Asia.

dunnage in Houston; photo by S. Useman, CBP

Greenwood et al. note that dunnage presents unique risks. After it is removed from ships during the unloading process it is often stored at the port. As noted above, storage in the open allows pests to escape to nearby trees or to cross-contaminate other SWPM. Ports struggle to manage these piles. In 2016 the U.S. revised its regulations to allow for the more rapid destruction of illegally deposited dunnage via incineration at the port. Since 2008 Canada has considered all shipborne dunnage to be non-compliant – regardless of whether it bears the ISPM#15 stamp. In the largest Mexican ports, dunnage is fumigated and destroyed. However, the dunnage might be stored in the open for considerable periods before being destroyed.

Worse, it is often impossible to assign chain of custody information and responsibility for either disposition of non-compliant dunnage or penalties for non-compliance. Dunnage or blocking pieces might be added immediately before shipping by entities other than the owners or brokers for the commodities being shipped. I have already noted that it is nearly impossible to inspect dunnage in a ship’s hold.

Unfortunately, studies have not clarified the level of infestation of dunnage in comparison to other wood packaging types made from multiple pieces of milled wood, such as pallets or spools.

Greenwood et al. describe the different fates of pallets, dunnage, crates, spools, and other types of SWPM. Wood pallets are frequently recycled or remanufactured in the U.S., although there are no data on the proportion of the recovery market that is composed of pallets initially manufactured overseas. In the U.S., most repairs are done with components from reclaimed pallets so they probably conform to ISPM#15 repair guidelines. However, contamination could happen while the pallets are in storage awaiting reuse. As SWPM ages, different types of pests might be attracted.

SWPM deemed not suitable for reuse is either destroyed in controlled settings (i.e., solid waste facilities, wood processing facilities, or landfills), used in recycling or downcycling markets, or reclaimed. It might be chipped and sold as mulch, soil amendment, or animal bedding; or it might enter the commercial fiber market and be manufactured into other wood products (e.g., paper, chipboard, fuel pellets). These dispositions present very low pest risk, due to the final dimensions of the wood products being too small to sustain pest development in most cases. However, some microorganisms and very minute arthropods might persist even on chipped or shredded material. There is little data on the final disposition of SWPM globally.

Greenwood et al. reiterate that the presence of hitchhiking or contaminating pests does not imply failure of ISPM#15 treatments, which do not target such organisms. Such pests can also be present on non-wood packaging material such as plastic and metal. Countries vary in their concern about these hitchhiking pests, which include dry wood borers and brown marmorated stinkbug (Halyomorpha halys). Since these pests are not addressed by ISPM#15, countries can implement their own management strategies to counter contaminating pests on all SWPM, containers, and conveyances. Indeed, Pennsylvania regulates the movement of SWPM and other high risk articles to prevent the spread of the non-specific hitchhiking pest, spotted lanternfly, Lycorma delicatula.

They also note that reuse, disposal, and recycling of packaging made from metal, plastic, or even paper requires very different processes and facilities than those used for wood.

Greenwood Recommendations

Greenwood et al. advocate additional research on several questions:

to test whether currently accepted ISPM#15 treatments are sufficiently effective within the newly proposed metrics found in Ormsby 2022.
to determine the risk profile and enforcement of dunnage, especially whether organisms in dunnage are more likely to survive treatment (dunnage pieces are often much larger than any component piece of a pallet or crate).
to develop new treatments – including to counter re-infestation later in the supply chain. Scientists will probably have to replace Probit9 as a standard because it is not practical to exposing tens of thousands of wood-infesting insects to the new treatment. This is also discussed in Ormsby 2022.
to develop ways to test whether treatments have been applied – needed to verify whether fraud has occurred.
social and economic motivations around compliance

Most of these studies will require international cooperation.

Other steps are also need. As U.S. importers of break-bulk cargo have found out, procuring apparently compliant SWPM does not protect them from legal, financial, and logistical consequences if that SWPM turns out to be non-compliant or otherwise infested with live actionable pests. Some importers have begun exploring options toward additional private inspection at the exporting port, beyond solely requiring the use of ISPM#15 compliant materials. Greenwood et al. suggest the possibility of third-party certification. They also supported calls for officials to release of information about which foreign facilities have a history of selling SWPM subsequently found to be non-compliant. This information would empower importers to procure pest-free SWPM – thus harnessing market incentives to improve compliance.

Managing all this + pest risks? Photo by Port Authority of Long Beach

Greenwood et al. say that reducing external contamination on conveyances – ships, airplanes, trucks, and trains – is challenging. It would require the cooperation of multiple entities who manage yards, equipment, and facilities. Improved management must make sense to people who have severe constraints on time, staffing, space, and safety protocols. Persuading them to act will probably depend on improved information (research) on the cost effectiveness of various strategies and real-world incidence of contamination in different storage scenarios (beyond Lymantria complex), plus development of new surveillance tools.

Greenwood et al. suggest that conducting a HACCP assessment of the supply chain could help identify how a systems approach might better mitigate pest risks of SWPM. They think systems approaches might be especially promising for reducing risks of contaminating organisms. NAPPO recently adopted a standard for designing and implementing systems approaches for wood commodities.

Finally, I remind you of my recommendations for immediate policy actions to hold foreign suppliers responsible for non-compliant wood packaging:

U.S. and Canada should refuse to accept wood packaging from foreign suppliers that have a record of repeated violations – whatever the apparent cause of the non-compliance. They should institute severe penalties to deter foreign suppliers from taking devious steps to escape being associated with their violation record.
I also support the suggestion (above) that phytosanitary agencies inform importers on which foreign treatment facilities have a record of poor compliance or suspected fraud – so the importers can avoid purchasing SWPM from them.
U.S. and Canada should encourage importers to switch to materials that won’t transport wood-borers. Cardboard and manufactured wood packaging (e.g. oriented strand board and compressed wood block) are wood fiber products that have near zero risk of wood-borer infestation. Plastic is also one such material. I note that Earth is drowning under discarded plastic.

APHIS and CFIA have the authority to take these action under the “emergency action” provision (Sec. 5.7) of the World Trade Organization’s Agreement on the Application of Sanitary and Phytosanitary Standards (WTO SPS Agreement). (For a discussion of the SPS Agreement, go to Fading Forests II, here.)

Longer-term Actions

APHIS and CFIA should exercise their right to set a higher “level of protection” to minimize introductions of pest that threaten our forests (described inter alia here.) They should prepare a risk assessment to justify adopting more restrictive regulations that would prohibit use of packaging made from solid wood – at least from the countries with records of high levels of non-compliance.

The studies needed to determine the cause of the continuing issue of the wood treatment mark’s unreliability, and appropriate actions to fix the problem, should be conducted with other countries. Appropriate entities would be the International Plant Protection Convention (IPPC) and International Forest Quarantine Research Group (IFQRG). However, if attempting such collaboration causes delays, APHIS and CFIA should begin unilaterally.

Meanwhile, what can we do?

Urge Congress to conduct oversight on APHIS’ failure to protect America’s natural resources from continuing introductions of nonnative insects and diseases. Note that the Mediterranean oak borer has apparently been introduced several times in recent years – despite ISPM#15.
Raise the issue with local, state, and federal candidates for office;
Urge Congress to include provisions of H.R. 3174 / S. 1238 in the 2023 Farm Bill;
Ask any associations of which you are a member to join in communicating these concerns to Congressional representatives and senators. These include:
- if you work for a federal or state agency – raise to leadership; they can act directly or through National Plant Board, National Association of State Departments of Agriculture, National Association of State Foresters, National Governors Association, National Association of Counties …
- scientific membership societies – e.g., Society of American Foresters, Entomological Society of America, Phytopathological Society;
- individual conservation organizations, either with state chapters or at the national level;
- woodland owners’ organizations, e.g., National Woodland Owners Association, National Alliance of Forest Owners, and their state chapters
- urban tree advocates
- International Forest Quarantine Research Group
Write letters to the editors of your local newspaper or TV news station.

SOURCES

Eyre, D., R. Macarthur, R.A. Haack, Y. Lu, and H. Krehan. 2018. Variation in Inspection Efficacy by Member States of Wood Packaging Material Entering the European Union. Journal of Economic Entomology, XX(X), 2018, 1–9 doi: 10.1093/jee/tox357

Greenwood, L.F., D.R. Coyle, M.E. Guerrero, G. Hernández, C.J. K. MacQuarrie, O. Trejo, M.K. Noseworthy. 2023. Exploring pest mitigation research and management associated with the global wood packaging supply chain: What and where are the weak links? Biol Invasions https://doi.org/10.1007/s10530-023-03058-8

Haack, R.A., K.O. Britton, E.G. Brockerhoff, J.F. Cavey, L.J. Garrett, et al. 2014. Effectiveness of the International Phytosanitary Standard ISPM No. 15 on Reducing Wood Borer Infestation Rates in Wood Packaging Material Entering the United States. PLoS ONE 9(5): e96611. doi:10.1371/journal.pone.0096611

Haack R.A., J.A. Hardin, B.P. Caton and T.R. Petrice. 2022. Wood borer detection rates on wood packaging materials entering the United States during different phases of ISPM#15 implementation and regulatory changes. Frontiers in Forests and Global Change 5:1069117. doi: 10.3389/ffgc.2022.1069117

Leung, B., M.R. Springborn, J.A. Turner, and E.G. Brockerhoff. 2014. Pathway-level risk analysis: the net present value of an invasive species policy in the US. Front Ecol Environ. 2014. doi:10.1890/130311

Posted by Faith Campbell

www.fadingforests.org

Introduced pests linked (again) to introduced plants; Prevention needs to recognize this nexus

I have blogged many times about the risk of pest introductions on imports of live plants [= “plants for planting” in USDA’s terms]. Last October I reviewed 14-year old data indicating that nearly 70% of 455 damaging tree pests introduced to the continental U.S. had probably been introduced via plant imports. These included 95% of sap feeding and 89% of foliage feeding insects and about half of the pathogens. The approach rate of pests on imported plants was apparently 12% (Liebhold et al. 2012) — more than 100 times higher than the 0.1% approach rate found by Haack et al. (2014) for wood packaging.

First, those analyses focus almost exclusively on insects (MacLachlan et al. 2022 focused on a single insect order, the Hemiptera!), despite the many pathogens probably introduced by the plant trade in recent decades. Examples I cited included several Phytophthoras, rapid ohia death, beech leaf disease, and boxwood blight. There have been repeated detections of the Ralstonia solanacearum Race 3 biovar 2.

SOD- infected rhododendrons; photo by Jennifer Parke, Oregon State University

Second, most studies analyzing the pest risk associated with plant imports use port inspection data – which are not reliable indicators of the pest approach rate – as explained by Liebhold et al. 2012 and Haack et al. 2014 (as it pertains to wood packaging).

Third, many of the studies are based on data from a decade or longer in the past. This means the studies do not address whether APHIS’ recent changes in its approach – including adoption of NAPPRA – have resulted in reduced introductions.

A complication is that, since insects are difficult to detect, those associated with the high volumes of plants imported in recent years might not be detected for years or decades after their introduction.

I have called for APHIS to update the Liebhold et al. 2012 study to determine the approach rate for all types of organisms that threaten North American tree species. Any such study should include trees on Hawai`i, Guam, Puerto Rico, and other U.S possessions and territories. These islands are nearly always excluded from analyses of imported pests. I concede that there are probably scientific and data-management challenges but these islands are immensely important from a biodiversity point of view, and they are parts of the United States!

eastern hemlocks killed by hemlock woolly adelgid; Linville Gorge; photo by Steven Norman, USFS

MacLachlan et al. (2022) estimated that new establishments – of insects in the order Hemiptera – per unit of additional plant imports have shrunk substantially. They attribute this decline to a combination of increased imports and the presence of a growing number of insect species introduced in the past. They found that introductions to the Asian Palearctic and Neotropic regions have been reduced by depletion of species pools. Other factors are thought to explain the substantial decline in establishment likelihood for the other regions. However, lag times in detecting insect introductions complicate this assessment.

However, despite that significant decrease in risk per unit of imports, MacLachlan et al. (2022) found that the number of establishments has remained relatively constant over the past century because of substantial increases in overall import levels and diversification of the origins of imports across regions, which exposed the U.S. to new source species pools.

MacLachlan et al. (2022) suggested that APHIS should target biosecurity resources to the specific commodity-country pairs associated with a higher relative risk of introducing additional insect species.

Recent studies are taking a welcome new stance: looking at links between introductions of non-native plant and insect species. I first raised this approach a year ago. Studies by teams led by Doug Tallany and Sara Lalk [Lalk et al.; articles by Tallamy] agree that:

Non-native plants – some of which are invasive – are altering ecosystems across broad swaths of North America and the impacts are insufficiently understood.
The invasive plant problem will get worse because non-native species continue to be imported, planted … and to invade.
Plant-insect interactions are the foundation of food webs – they transfer energy captured by plants through photosynthesis to other trophic levels, plus play a major role as pollinators. Consequently, changes to a region’s flora will have repercussions throughout ecosystems.

Dr. Tallamy studies the response of herbivorous insects to non-native woody plants – not just invasive plants, but also non-native plants deliberately planted as crops or ornamentals, or in forestry. Introduced plants have completely transformed the composition of plant communities in both natural and human-dominated ecosystems world-wide. The impacts can be significant: Burghardt et al. found that 75% of North American lepidopteran species and 93% of specialist species were found exclusively on native plant species.

monarch butterfly on milkweed; photograph by Jim Hudgins, USFWS

Lalk and colleagues studied the relationships between individual species of invasive woody plants and the full range of arthropod feeding guilds – pollinators, herbivores, twig and stem borers, leaf litter and soil organisms. They decry the absence of data on the complex interactions between invasive woody plants and arthropod communities at a time when invasive shrubs and trees are so widespread and causing considerable ecological damage. (See the blog for their specific research recommendations.)

Nor is the impact of non-native plants on insect fauna limited to North America. Outhwaite et al. found that the combination of climate warming and intensive agriculture is associated with reductions of almost 50% in the abundance and 27% in the number of species within insect assemblages relative to levels in less-disturbed habitats with lower rates of historical climate warming. These patterns were particularly clear in the tropics (perhaps partially because of the longer history of intensive agriculture in temperate zones). They found that high availability of nearby natural habitat (that is, native plants) can mitigate these reductions — but only in low-intensity agricultural systems.

Recognizing that plant diversity drives global patterns of insect invasion, Liebhold et al. (2023) compared various factors associated with numbers of invasive insect species in 44 land areas.They determined that the numbers of established non-native insect species are primarily driven by diversity of plants – both native and non-indigenous. Other factors, e.g., land area, latitude, climate, and insularity, strongly affect plant diversity; thus they influence insect diversity as a secondary impact. When I blogged about this study, I noted that the article appeared more than four years earlier, but has apparently had little influence on either policy formulation governing plant introductions or pest risk analysis applied to insects or pathogens that might be introduced. I suggested that we need a separate analysis of whether fungi, oomycetes, nematodes, and other pathogens show the same association with plant diversity in the receiving environment.

Studies of plant-insect relationships continue to be published. I welcome this!

Bonnamour et al. (2023) builds on the earlier studies. They also found that the presence of non-native plant species was a better predictor of insect invasions than such more widely discussed socioeconomic variables as trade volumes generally or even trade in plant products. However, detection of the associated insect invasions occurs years after detection of the plant invasions. Indeed, numbers of established non-native insect species corresponded more closely to plant introduction volumes in 1900 than current or recent import volumes.

Bonnamour et al. note that while the insect taxa that respond most directly to the non-native plant diversity are those that rely on those plants as hosts, pollinators, and plant visitors, over time those non-native herbaceous insects support introduced predators and parasites also.

Because of the “invasion debt” associated with that lag, Bonnamour et al. estimate that newly detected insect invasions will increase by 35% worldwide as a result of only recent plant introductions. They differentiate this “invasion debt” from “future invasions”, meaning the actual introduction of additional species resulting from future trade activities.

The model developed by Bonnamour et al. points to the highest numbers of newly introduced insect species occurring in areas with less capacity to deal with bioinvasions. Thus, the Afrotropics are anticipated to receive 869 new insect species, or a 10-fold increase over the number currently known to be established in the region. The Neotropics are projected to be invaded by 809 insect species, also a 10-fold increase. The Indomalayan region will probably detect 776 new insect species, a startling 20-fold increase. In reality, the “invasion debt” might not be quite this severe, since – as Bonnamour et al. note several times – the low numbers of introduced insects currently reported for these tropical regions probably partially reflect limited sampling. They note that already a high proportion of insect species intercepted by biosecurity services on imports arriving from Africa and South America are not yet recorded as established in the exporting regions.

Although both the European Palearctic and Australasia have already received many non-native insect species, their “invasion debt” is relatively high: 417 species for Europe, 317 species for Australasia.

The Neotropics are expected to be the greatest source of insect invasions in the future (904 exported species), followed by the European Palearctic (732 species).

Bonnamour et al. did not include non-native plant species used in agriculture, forestry, or ornamental horticulture. As noted above, these widespread deliberate plantings also affect insect fauna and higher trophic layers.

The greatest number of recorded insect introductions so far are in the Nearctic, Oceania (primarily Hawaii), Europe, and Australasia. While this imbalance is probably caused in part by the significantly limited sampling of non-native insect species in the Asian Palearctic and tropics, it is also true that these regions have received the majority of plant introductions through 1900. This factor has changed in the century since then; many non-native plant species have been recorded in the Afrotropics, Oceania, and Asia.

Eucalyptus plantation in Kwa-Zulu-Natal, South Africa; Kwa-Zulu-Natal Dept. of Transportation

Bonnamour et al. offer several potential explanations for the lag in detecting introduced insects compared to detecting introduced plants. First, it might be necessary for non-native host plants to reach a threshold of abundance before the associated insects are able to establish and spread. Second, reaching that threshold might require repeated introductions of the insect’s host plant species. Third, since only some of the imported plants are transporting insects, repeated imports of host plants might be necessary for the insect to achieve sufficient numbers to establish. Fourth, while their analysis included all non-native insect species, only some insect feeding guilds – herbivores and pollinators – are probably directly facilitated by introduced host plants. Fifth, plant species’ presence tends to be more quickly recorded than insects’ presence. Indeed, MacLaughlin et al. reported a median delay of 80 years between establishment and discovery of plant-feeding Hemiptera. This suggests that the actual time lag between plant and insect establishments might be shorter than the period discussed in Bonnamour et al.

Many insects from the European Palearctic have been introduced to the Nearctic; fewer insects have been introduced in the opposite direction. There is no consensus on the explanation. Thirty years ago Mattson et al. argued that there might be fewer niches for non-native insects in Europe due to the lower host plant diversity in this region caused by the Pleistocene/Holocene glaciations. On the other hand, more plant species from the European Palearctic to the Nearctic than the opposite.

Bonnamour et al. call for further research on:

1) time lags at the scale of individual insect species with their host plants.

2) effects of non-native plants used in agriculture, forestry, or ornamental horticulture.

3) whether time lags between plant and insect invasions vary among taxonomic groups, feeding guilds, or among regions.

4) effect of non-native plant abundance, rather than just species richness, on non-native insect establishment.

Recommendations

Writers about interactions of non-native plant species and insect introductions make a common plea: limit the introduction and spread of non-native plants in order to prevent future invasions of both plants and insects. Bonnamour et al. suggest including the risk of insect introductions in plant invasion risk screening tools. Earlier, the Tallamy and Lalk teams called for ending widespread planting of non-native plants.

Will policy-makers accept this advice?

I believe that these same interaction of plant host and “pest” introductions presumably applies to pathogens, too. I reiterate my frequent complaint that regulators have not responded to two or more decades of criticism of the failures of the international phytosanitary system re: insect and pathogen introductions via the international nursery trade. Examples include Brasier 2008; Liebhold el. al. 2012; Santini et al. 2013; Roy et al. 2014; Eschen et al. 2015; Jung et al. 2015; Meurisse et al. 2019; O’Hanlon et al. 2021.

As I have said earlier, I appreciate that some scientists are trying to reduce scientific uncertainty about the invasive potential of pathogens native to regions other than North America; I refer here to Jiri Hulcr (see Li et al.), Mech, and Schultz. Many more such studies are needed, addressing potential impacts on a wider variety of North American host trees and shrubs.

The late (& very much lamented!) Gary Lovett of the Cary Institute had advocated halting imports of plants that are congenerics of important North American tree species, in order to minimize the risk that pests that damage those genera will be introduced.

In January I suggested that at the global level we need:

National agricultural agencies, stakeholders, FAO & International Plant Protection Convention (IPPC) should consider amending the IPPC requirement that scientists identify a disease’s causal agents before regulating it. Experience shows that this policy virtually guarantees that pathogens will continue to enter, establish, & damage natural and agricultural environments.
National governments & FAO / IPPC should fund greatly expanded research to identify microbes resident in regions that are important sources of origin for traded plants, vulnerability of hosts in importing countries, and new technologies for detecting pathogens (e.g., molecular tools, volatile organic compounds [VOCs]).
Researchers & agencies should expand international “sentinel plants” networks; incorporate data from forestry plantations, urban plantings, etc. of non-native trees.
NPPOs should adopt regulations that apply the “systems approach” or HACCP programs outlined in ISPM#36. I had discussed these approaches in my Fading Forests III report – link at end of this blog.)

I suggested further that Americans need to

Evaluate the efficacy of current regulations – that is, implementing NAPPRA & Q-37 revision. This evaluation should be based on AQIM data, not port interception data. It should include arthropods, fungal pathogens, oomycetes, bacteria, viruses, nematodes. It should include threats to U.S. tropical islands (Hawai`i, Puerto Rico, Guam, etc.) which are centers of plant endemism.
Apply existing programs (e.g., NAPPRA, Clean Stock Network, post-entry quarantine) to strictly regulate trade in plant taxa most likely to transport pests that threaten our native plants; e.g., plants belonging to genera shared between North American trees & plants on other continents.
Recognize that plant nurseries are incubators for microbial growth, hybridization, and evolution; require nurseries to adopt sanitary operation procedures regardless of whether they sell in inter-state or intra-state commerce

SOURCES

Bonnamour, A., R.E. Blake, A.M. Liebhold, H.F. Nahrung, A. Roques, R.M. Turner, T. Yamanaka, and C. Bertelsmeier. 2023. Historical plant intros predict current insect invasions. PNAS 2023 Vol. 120 No. 24 e2221826120 https://doi.org/10.1073/pnas.2221826120

Burghardt, K. T., D. W. Tallamy, C. Philips, and K. J. Shropshire. 2010. Non-native plants reduce abundance, richness, and host specialization in lepidopteran communities. Ecosphere 1(5):art11. doi:10.1890/ES10-00032.

Lalk, S. J. Hartshorn, and D.R. Coyle. 2021. IAS Woody Plants and Their Effects on Arthropods in the US: Challenges and Opportunities. Annals of the Entomological Society of America, 114(2), 2021, 192–205 doi: 10.1093/aesa/saaa054

Li, Y., C. Bateman, J. Skelton, B. Wang, A. Black, Y-T. Huang, A. Gonzalez, M.A. Jusino, Z.J. Nolen, S. Freeman, Z. Mendel, C-Y. Chen, H-F. Li, M. Kolařík, M. Knížek, J-H. Park, W. Sittichaya, T-H.

Pham, S. Itoo, M. Torii, L. Gao, A.J. Johnson, M. Lur, J. Sun, Z. Zhang, D.C. Adams, J. Hulcr. 2022. Pre-invasion assessment of exotic bark beetle-vectored fungi to detect tree-killing pathogens. https://apsjournals.apsnet.org/doi/full/10.1094/PHYTO-01-21-0041-R

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

Liebhold, A.M., T. Yamanaka, A. Roques, S. August, S.L. Chown, E.G. Brockerhoff & P. Pyšek. 2018. Plant diversity drives global patterns of insect invasions. Sci Rep 8, 12095 (2018). https://doi.org/10.1038/s41598-018-30605-4

MacLachlan, M.J., A. M. Liebhold, T. Yamanaka, M. R. Springborn. 2022. Hidden patterns of insect establishment risk revealed from two centuries of alien species discoveries. Sci. Adv. 7, eabj1012 (2021).

Mattson, W. J., P. Niemela, I. Millers, and Y. Ingauazo. 1994. Immigrant phytophagous insects on woody plants in the United States and Canada: an annotated list. USDA For. Ser. Gen. Tech. Rep. NC-169, 27 pp.

Mech, A.M., K.A. Thomas, T.D. Marisco, D.A. Herms, C.R. Allen, M.P. Ayres, K.J.K. Gandhi, J. Gurevitch, N.P. Havill, R.A. Hufbauer, A.M. Liebhold, K.F. Raffa, A.N. Schulz, D.R. Uden, and P.C. Tobin. 2019. Evolutionary history predicts high-impact invasions by herbivorous insects. Ecol Evol. 2019 Nov; 9(21): 12216-12230.,

Outhwaite, C.L., P. McCann, and T. Newbold. 2022. Agriculture and climate change are shaping insect biodiversity worldwide. Nature 605 97-192 (2022) https://www.nature.com/articles/s41586-022-04644-x

Richard, M., D.W. Tallamy and A.B. Mitchell. 2019. Intro plants reduce species interactions. Biol Invasions https://doi.org/10.1007/s10530-018-1876-z

Schulz, A.N., A.M. Mech, M.P. Ayres, K. J. K. Gandhi, N.P. Havill, D.A. Herms, A.M. Hoover, R.A. Hufbauer, A.M. Liebhold, T.D. Marsico, K.F. Raffa, P.C. Tobin, D.R. Uden, K.A. Thomas. 2021. Predicting non-native insect impact: focusing on the trees to see the forest. Biological Invasions.

Tallamy, D.W., D.L. Narango and A.B. Mitchell. 2020. Ecological Entomology (2020), DOI: 10.1111/een.12973 Do NIS plants contribute to insect declines? Conservation Biology DOI: 10.1111/j.1523-1739.2009.01202.x

Uden, D.R, A.M. Mech, N.P. Havill, A.N. Schulz, M.P. Ayres, D.A. Herms, A.M. Hoover, K.J. K. Gandhi, R.A. Hufbauer, A.M. Liebhold, T.D. M., K.F. Raffa, K.A. Thomas, P.C. Tobin, C.R. Allen. 2023. Phylogenetic risk assessment is robust for forecasting the impact of European insects on North American conifers. Ecological Applications. 2023; 33:e2761.

Posted by Faith Campbell

www.fadingforests.org

Invasives in Italy & Croatia – what I saw

plane trees in Parco della Montagnola, Bologna, Italy

The CISP blog has had a pause while I travelled in Italy, Slovenia, and Croatia. My purpose was tourism … but I could not forget my interest in invasive species! Most prominent was that global traveller, Ailanthus altissima. I saw lots of the trees in both urban and rural settings. I am unclear about whether some other plant taxa are native v. non-native, e.g., ivy and several species of pines widespread along the Dinaric mountains. Scotch broom Cytisus scoparius was widespread along the Dalmatian coast – but it is native there so not a concern. (Should have been a warning that it would invade other areas with Mediterranean climates!) We were told that the city of Split is even named for the shrub – a shortened/corrupted version of the Greek name for the plant.

Regarding tree-killing pests, I was pleased to see lots of young olive trees along the Dalmatian coast – apparently the Xylella fastidiosa bacterial pathogen that is killing trees in Italy, France, Spain and Portugal is not yet a problem there.

I also saw many of the columnar cedars that I understand are under attack by fungi in the genus Seiridium in Italy. I also saw apparently healthy boxwoods at our hotel near Dubrovnik.

Some of the boxwoods growing in monastery cloisters in Dubrovnik looked as if they have been hit by box tree moth (Cydalima perspectalis) — which is no surprising since the pest was detected in northern Croatia a decade ago.

Finally, I saw apparently healthy plane trees growing in parks in Bologna; apparently this region has not yet been invaded by the canker stain disease caused by North American fungus Ceratocystis platani. – reported to have caused high levels of mortality in Italy, France, and especially in Greece. (See photo at top of blog) [See Tsopelas, Santini, Wingfield and de Beer. 2017. Canker stain: a Lethal Disease Destroying Iconic Plane Trees. Plant Disease. 2017. 101:645-658. American Phytopathological Society]

I am concerned that the forests at Plitvice National Park are composed of European beech, which is vulnerable to beech leaf disease if it arrives there.

Posted by Faith Campbell

www.fadingforests.org

Support House & Senate bills to Enhance Response to Forest Pests

white ash: a species that might be restored under the programs envisioned in the proposed bills

Bills have been introduced into both the House and Senate to enhance USDA APHIS and Forest Service programs intended to curtail introduction and spread of non-native forest pests and disease and – especially – programs aimed at restoring pest-decimated trees to the forest.

The House bill is H.R. 3174; it was introduced by Reps. Becca Balint (VT).

The Senate bill is S. 1238; it was introduced by Senators Peter Welch (VT), Mike Braun (IN), and Maggie Hassen (NH). [Both senators Welch and Braun are on the Agriculture Committee – which will write the bill.]

CISP hopes that the contents of these two bills will be incorporated in the Farm Bill that Congress is expected to adopt this year or next. The proposals have the support of the Forests in the Farm Bill coalition. [Unfortunately, neither the “Consolidated Recommendations” nor “Summarized Recommendations appears to be posted on the internet at present.]

In the last Congress, a nearly identical bill introduced by then-Representative Peter Welch was endorsed by the organizations listed below. We hope they will endorse the new bills now! If you are a member of one of these organizations, please ask them to do so.

Organizations that endorsed the previous bill: Vermont Woodlands Association, American Forest Foundation, Center for Invasive Species Prevention, Reduce Risk from Invasive Species Coalition, National Woodland Owners Association (NWOA), National Association of State Foresters (NASF), The Society of American Foresters (SAF), the North American Invasive Species Management Association (NAISMA), the Ecological Society of America, Entomological Society of America, a broad group of university professors and scientists, The Nature Conservancy (TNC) Vermont, Audubon Vermont, the Massachusetts Forest Alliance, the New Hampshire Timberland Owners Association, the Maine Woodland Owners Association, and the Pennsylvania Forestry Association.

I seek your help in generating support for incorporating these proposals into the 2023 Farm Bill. Please urge your representative and senators to co-sponsor the bills or otherwise support that action.

beech in a breeding experiment at The Holden Arboretum; photo by Jennifer Koch

Key points of the two bills:

They strengthen APHIS’ access to emergency funds. APHIS has had the authority to access emergency funds from the Commodity Credit Corporation since 2000. However, the Office of Management and Budget has often blocked its requests. See § 2, of the bills, EMERGENCY AUTHORITY WITH RESPECT TO INVASIVE SPECIES.
It creates two separate but related grant programs.
- The first grant program – in § 3. FOREST RECLAMATION GRANTS – funds research addressing specific questions impeding the recovery of tree species that are native to the US and have suffered severe levels of mortality caused by non-native plant pests or noxious weeds.
- The second grant program – in § 4. FOREST RESTORATION IMPLEMENTATION GRANTS – funds implementation of projects to restore these pest-decimated tree species to the forest. These projects must be part of a forest restoration strategy that incorporates a majority of the following components:

(1) Collection and conservation of native tree genetic material.

(2) Production of propagules of the target tree species in numbers sufficient for landscape-scale restoration.

(3) Preparation of planting sites in the target tree species’ former habitats.

(4) Planting of native tree seedlings.

(5) Post-planting maintenance of native trees.

§ 5 states that the absence of a national policy on addressing nonnative forest pests has resulted in their receiving a low priority within all Federal agencies. It then mandates a study to analyze agencies’ available resources, raise the issue’s priority, and improve coordination among agencies. This study is to be carried out by an independent institution, for example the National Academy of Sciences. The authors are to consult with specialists in entomology, genetics, forest pathology, tree breeding, forest and urban ecology, and invasive species management.
Funding for all three action components – the emergency response and both grant programs – would come from the Commodity Credit Corporation, so it would not be subject to the vagaries of annual appropriations bills.

Forest Restoration Alliance volunteers potting hemlock seedlings; photo provided by Fred Hains

Entities which could apply for the research grants (§ 3 of the bills) include Federal agencies; State cooperative institutions; academic institutions offering degrees in the study of food, forestry, and agricultural sciences; and non-profit organizations exempt from taxes under §501(c)(3) of the tax code. Types of research funded could include:

‘‘(A) biocontrol of nonnative pests & diseases or noxious weeds severely damaging native tree species [the bill does not specify, but Project CAPTURE identifies many qualifying species; see also my earlier blog];

‘‘(B) exploration of genetic manipulation of the plant pests or noxious weeds;

‘‘(C) enhancement of pest-resistance mechanisms of hosts; and

‘‘(D) development of other strategies for restoring individual tree species.

The maximum amount of such grants is $400,000 per year.

Entities which could apply for the implementation grants (§ 4 of the bills) include a cooperating forestry school; a land-grant college or university; a State agricultural experimental station; a 501(c)(3) organization. Funding would begin at $3 million for FY 2023 and rise to $10 million for FY 2026.

The Secretary of Agriculture would be guided in implementing these programs by two committees. One – the committee of experts – would constitute representatives of the USFS, APHIS, ARS & State forestry agencies. The second – the advisory committee – would be composed of representatives of land-grant colleges and universities and affiliated State agriculture experiment stations, forest products industry, recreationists, and professional forester, conservation, and conservation scientist organizations.

Port-Orford cedar seedlings at USFS Dorena Center – a model for success! Photo provided by Richard Sniezko

Please contact your Member of Congress (Representative) and senators to urge them to support inclusion of these provisions in the Farm Bill. [Remember: they work for us!] Telling them of your support for these bills is especially important if your Representative or Senator is on the Agriculture Committee. I list those legislators here:

State	HOUSE AGRIC COMM	SENATE AGRIC COMM
AL	Barry Moore	Tommy Tuberville
AR	Rick Crawford	John Boozman
CA	Doug Lamalfa John Duarte Jim Costa Salud Carbajal
CO	Yadira Caraveo	Michael Bennet
CT	Jahana Hayes
FL	Kat Cammack Darren Soto
GA	Austin Scott David Scott Sanford Bishop	Raphael Warnock
HI	Jill Tokuda
IA	Randy Feenstra Zach Nunn	Joni Ernst Charles Grassley
IL	Mike Bost Mary Miller Nikki Budzinski Eric Sorensen Jonathan Jackson	Richard Durbin
IN	Jim Baird	Mike Braun
KS	Tracey Mann Sharice Davids	Roger Marshall
KY		Mitch McConnell
MA	Jim McGovern
ME	Chellie Pingree
MI	Elissa Slotkin	Debbie Stabenow
MN	Angie Craig	Amy Klobuchar Tina Smith
MO	Mark Alford
MS	Trent Kelly	Cindy Hyde-Smith
NC	David Rouzer Alma Adams
ND		John Hoeven
NE	Don Bacon	Deb Fischer
NJ		Cory Booker
NM	Gabe Vasquez	Ben Ray Lujan
NY	Marc Molinaro Nick Langworthy	Kirsten Gillibrand
OH	Max Miller Shontel Brown	Sherrod Brown
OK	Frank Lucas
OR	Lori Chavez-Deremer Andrea Salinas
PA	Glenn Thompson	John Fetterman

SD	Dusty Johnson	John Thune
TN	Scott Desjarlais Brad Finstad
TX	Ronny Jackson Monica de la Cruz Jasmine Crockett
VA	Abigail Spanberger
VT	Peter Welch
WA	Marie Gluesenkamp Perez
WI	Derrick van Orden

Posted by Faith Campbell

www.fadingforests.org

New Tool for Evaluating Insect Pests’ Possible Impacts: One Test Shows Great Potential for Identifying the Greatest Threats to our Forests

red spruce (*Picea rubens*) — the conifer at greatest risk; This grove is in Great Smoky Mountains National Park; photo by Famartin via Wikimedia Commons

Scientists have incorporated into the widely-used urban tree management tool, i-Tree, a tool to help predict the damage that an insect species little known in North America might cause to trees growing in a specific area if it is introduced. This tool is available to all here.

I rejoice that predictive tools are becoming widely available. The tool is obviously the result of a lot of work by participating scientists – who are listed below. I hope many of you will try it out! Perhaps you and your students can join efforts by the tool-development team, especially in analyzing insect species from Central America and Asia that have not yet arrived in North America? If you are interested in helping, contact Katheryn Thomas, Angela Mech, or Ashley Schulz; you can obtain their contact information by visiting their institution’s website. You might choose which insect species to evaluate by consulting your own or colleagues’ research, reviewing the refereed and grey literature, APHIS and CFIA interception databases, databases maintained by several countries, websites such as CABI, EPPO, etc.

The new tool might help create a more effective “early warning” system. Whether this happens depends on what others do now. Anyone – perhaps a staffer of a federal or state agency, or a city tree manager, or an academic – can apply the tool to meet his/her own objectives. If a more effective national or continental “early warning” system is to be created, someone needs to set up a process for conveying the findings to responsible federal or state/provincial agencies or even the scientific societies, e.g., Entomological Society (and, in the case of beetles transporting associated fungi, American Phytopathological Society). Perhaps the most challenging issue is to find an entity willing to receive these communications, review their accuracy, and – at a minimum – make the results accessible to phytosanitary agencies, interested public, etc. One possible entity is “PestLens”, a web-based early-warning system maintained by APHIS. The project’s objective is to provide early-warning information and facilitate a prompt, coordinated, and appropriate safeguarding response. PestLens posts alerts once a month. These are visible to anyone who subscribes. However, it remains unclear how often APHIS and state agencies act on the notices. The North American Plant Protection Organization (NAPPO) also hosts an alert system, but it records only official notices, leading to some absurdities. (E.g., NAPPO reported Mexico’s designation of the invasive shot hole borers as quarantine pests – without mentioning that they are well-established in California because neither APHIS nor California Department of Food and Agriculture has designated the insects as officially regulated.)

Those applying the tool need to have some knowledge and access to a range of scientific resources (including, in my view, people who can check the accuracy of the data entered into the system). Users must have appropriate skills to conduct some research into the insect and what it feeds on. Information required for the tool includes the following:

taxonomic information for the insect (Order, Family, Genus, Species)
the feeding guild of the insect (i.e., foliovore, gall, reproductive, root, sap, wood)
climate in the native range of the insect (i.e., Tropical, Dry, Temperate, Continental, Polar)
native range of the insect (i.e., Afrotropical, Australasian, Indomalayan, Neotropical, Oceanian, Palearctic Asia, Palearctic Europe)
the host trees of the insect in its native range (scientific name [Genus species]). The tool warns participants to include the full range of potential tree hosts – by listing either all or a representative sample. The tool will use this information to estimate the evolutionary distance between known native hosts and potential North American hosts using comprehensive phylogenetic tree of plants.

Clearly, those using the tool have their work cut out for them! The tool does provide definitions, descriptors, and drop-down lists for most of the factors, including insect orders and families, tree genera, geographic origins, and climate types. Users are now anticipated to be employees of federal and presumably state agencies; academics – even students!—and others who have the capacity to research what an insect feeds on in its native range.

This tool is intended to predict the probability that an insect species of concern – either newly detected in the country or thought likely to invade based on port detections or other reasons — will become a high impact invader. I rejoice that they are inclusive – the tool can test the vulnerability of 50+ conifer species and 360+ hardwood species native to North America. Assuming the assessor can enter accurate information for the categories outlined above, the tool can then provide a list of probabilities for each relevant North American host tree.

The tool is based on the findings of two studies, Mech et al. and Schulz et al. (full citations at the end of this blog). I discussed these studies in earlier blogs. They were also incorporated into the broader effort to identify predictive traits carried out by Raffa et al. (full citations at the end of this blog) and discussed in a separate blog. See the section titled “Potential” to see the exciting results of an application of the Mech et al. findings and methods.

To develop the tool, project scientists synthesized data on traits and factors representing four types of drivers: (1) insect traits, (2) tree traits (especially those associated with host defenses), (3) the relatedness between the insect’s native and North American tree hosts, and (4) the relatedness between the non-native insect and North American insects on the same tree. They tested key hypotheses, e.g., defense free space and enemy release. The team tested the tool with researchers from USDA APHIS and Canadian Food Inspection Agency (CFIA), Northeast Plant Diagnostic Network, and National Invasive Species Council.

Norway spruce (*Picea abies*) — host of 30 of the 62 insect species analyzed in Uden *et al*.; photo by Marzena via Pixabay

The research group hopes this tool will stimulate development of a global database of insects which will utilize the results of basic research on phytophagous insects and what they eat. Basic research on insects native to North America is also important and can benefit other countries that might want to develop a similar tool for their own phytosanitary needs.

The Tool’s Potential

Many of the scientists who developed the i-Tree tool have participated in an analysis of the threat to North American conifer species posed by insects native to Europe that have not yet been introduced to North America (Uden et al.). They applied the methodology from Mech et al., which is comparable to, although not identical to, the i-Tree system. They (1) created a list of 62 European insect species that appear to pose a risk to 47 species of North American conifers; (2) identified and compared the predicted likelihoods of high-impact invasion under each of four phylogenetic systems datasets; and (3) evaluated risk and vulnerability trends among insects & conifer hosts, respectively. In total they evaluated 2,914 insect–novel host pairs.

Fraser fir (*Abies fraseri*) in Great Smoky Mountains National Park; photo by James St. John via Flickr

Among their findings are the following:

Of the 2,914 pairs examined, 302 (10.4%) had a predicted risk of high impact. These pairs included 41 (66%) of the insect species and 20 (41.7%) of the conifer species. The proportion of potential invasions posing a significant risk is higher than those indicated by earlier studies.
The insect species posing a risk of high-impact invasion were spread among insect orders, with relatively high levels concentrated in Lepidoptera and Coleoptera, fewer in the Hymenoptera and Hemiptera.
Consistent with Mech et al., they found a “Goldilocks” period of evolutionary divergence of hosts exposing the North American tree species to the highest risk. Thus, if a North American conifer shared a common ancestor with the insect’s native European host ~2–10 million years ago, it was predicted to be more vulnerable to a high-impact invasion by a conifer specialist.
North American fir (Abies) and spruce (Picea) species are more vulnerable to the introduction of European conifer-specialist insects than are pines (Pinus). [Mech et al. found that trees with high shade tolerance and low drought tolerance are more vulnerable. These traits also fit fir and spruce; but not pine.] The most vulnerable tree species was red spruce (Picea rubens).

Uden et al. also say Fraser fir (Abies fraseri) and Carolina hemlock (Tsuga caroliniana) are highly vulnerable to European insect species. They identified 17 high-risk insect species for Fraser fir. Of course, both are already severely depleted by non-native insect pests (Balsam woolly adelgid and hemlock woolly adelgid, respectively). They have also been identified by the Potter et al. “Project CAPTURE” process as having high priorities for conservation efforts.

I worry that fir and spruce are less important as timber species than pines; I hope this does not result in agencies and important stakeholders assigning this risk finding a lower priority.

Uden et al. assert that their study shows that this system can identify vulnerable tree species in the absence of information about which particular insect might invade. This information helps managers focus biosecurity and management program programs on protecting the most vulnerable tree species. However, 57% of the North American conifers (27 species) were found to be vulnerable under at least one of the insect-host pairs. To further set priorities, they suggest combining predictions from this analysis with USFS Forest Inventory and Analysis (FIA) data to identify vulnerable biogeographic regions and vegetation communities. (Fraser fir and Carolina hemlock rank high under this process.) Scientists could also apply species importance indicators, such as the NatureServe Explorer plant community descriptions. They suggest linking these criteria to the USFS Early Detection Rapid Response surveillance program, link to website which currently targets specific insect species.

red pine (*Pinus resinosa*) – the pine species at greatest risk; photo by Charles Dawley via Flickr

Uden et al. also warn that their analysis focused on a narrow range of possible introduced species: insects from Europe that feed on conifers exclusively. They caution that no one should assume that tree species that have a low “vulnerability” rank in this study should be considered at low risk for all possible introduced insects. They suggest researchers should identify tree species from the wider Palearctic that are within the high-impact “Goldilocks” zone of divergence times in relation to specific North American tree species, and then identify the insects that feed on those Palearctic trees to determine the species that would have the highest predicted risk of causing a high impact on those North American conifers.

Of course, many North American tree species are not conifers! Applying the methods in Schulz et al. – now integrated into the i-Tree tool – would facilitate similar predictive findings for the angiosperms.

Participants

The importance of this project is seen in the impressive array of funders supporting it. They include:

U.S. Geological Survey John Wesley Powell Center for Analysis and Synthesis for a working group titled “Predicting the nest high-impact insect invasion: Elucidating traits and factors determining the risk of introduced herbivorous insects on North American native plants;”
USDA Forest Service National Urban and Community Forestry Advisory Council funded a working group titled “Forecasting high-impact insect invasions by integrating probability models with i-Tree from urban to continental scales”;
Nebraska Cooperative Fish and Wildlife Research Unit;
University of Washington;
USDA Forest Service Eastern Forest Environmental Threat Assessment;
National Science Foundation Long-Term Ecological Research program;
USDA Forest Service International Programs; and
USDA National Institute of Food and Agriculture (Hatch and McIntire-Stennis projects).

Scientists who created this tool:
Kathryn A. Thomas (USGS – Southwest Biological Research Center)
Travis D. Marsico (Arkansas State University)
Daniel A. Herms (The Davey Tree Expert Company)
Patrick C. Tobin (University of Washington)
Andrew Liebhold (U.S. Forest Service)
Nathan Havill (U.S. Forest Service)
Angela Mech (University of Maine)
Ashley Schulz (Mississippi State University)
Matthew Ayres (Dartmouth College)
Kamal Gandhi (University of Georgia)
Ruth A. Hufbauer (Colorado State University)

Kenneth Raffa (University of Wisconsin) Daniel

Uden (University of Nebraska-Lincoln)

Carissa Aoki (Maryland Institute College of Art)

Scott Maco (The Davey Tree Expert Company)

Angela Hoover (University of Arizona)

SOURCES

Mech, A.M., K.A. Thomas, T.D. Marsico, D.A. Herms, C.R. Allen, M.P. Ayres, K.J.K Gandhi, J. Gurevitch, N.P. Havill, R.A. Hufbauer, A.M. Liebhold, K.F. Raffa, A.N. Schulz, D.R. Uden, and P.C. Tobin. 2019. Evolutionary history predicts high-impact invasions by herbivorous insects. Ecol Evol. 2019. Nov; 9(21):12216-12230.

Potter, K.M., Escanferla, M.E., Jetton, R.M., Man, G., Crane, B.S. 2019. Prioritizing the conservation needs of United States tree species: Evaluating vulnerability to forest insect and disease threats. Global Ecology and Conservation (2019), doi: https://doi.org/10.1016/j.gecco.2019.e00622.

Raffa, K.F., E.G. Brockerhoff, J-C Gregoire, R.C. Hamelin, A.M. Liebhold, A. Santini, R.C. Venette, and M.J. Wingfield. 2023. Approaches to Forecasting Damage by Invasive Forest P&P: A Cross-Assessment. BioScience Vol. 73 No. 2: 85–111 https://doi.org/10.1093/biosci/biac108

Posted by Faith Campbell

www.fadingforests.org

Let’s Not Dismiss Conservation of Biodiversity While Seeking Carbon Storage in Forests

red deer on farm in New Zealand; photo by Bernard Spragg via Flickr

Among the non-native species damaging forest systems are mammals – introduced deer, goats and sheep, and swine, … These animals have the greatest impacts on island systems that are sufficiently isolated that they have no native terrestrial mammals, e.g., Hawai`i and New Zealand. Several New Zealanders have published a study of their impacts (Allen et al.; full citation at end of the blog). The focus of their analysis is the native forests’ ability to sequester carbon and thus mitigate climate change. The scientists are well aware, however, that forests provide many other ecosystem values and services, including biodiversity, water supply and quality, etc.

Introduced ungulates can have many direct effects: reduction and damage to understory biomass, depletion of seedling regeneration, exacerbated soil erosion, and local nutrient imbalances. Mammals’ browsing can modify the composition of plant communities by favoring abundance of unpalatable species. Changes also can alter ecosystem functions associated with nutrient cycling, e.g., by reducing nutrient returns to the soil and altering rates of litter decomposition

In these ways, introduced ungulates exert long-term impacts on forests’ capacity to store carbon.

Allen et al. aimed to determine the extent of these effects on forests’ capacity to store carbon, both above- and below-ground, and on forest structure and diversity. The authors compared data from 26 pairs of sites across New Zealand – half with ungulate exclosures and half adjacent unfenced control plots. The ungulate exclosures had all been established for at least 20 years. All the sites were in species-rich communities of conifers and broadleaved evergreen angiosperm trees. These forests (1) cover about one-third of the country’s remaining mature natural forest; (2) contain tree species of a wide range of palatability to ungulate herbivores; and (3) have been named a conservation priority for forest carbon management. The ungulates present on the plots were European red deer (Cervus elaphus), fallow deer (Dama dama), sika deer (Cervus nippon), and feral goats (Capra hircus).

They assert that New Zealand is a good place to do this type of study because ungulate introductions are relatively recent so their impacts are well documented.

Allen et al. found that managing invasive ungulates makes valuable contributions to conserving biodiversity but not to carbon sequestration. They found little difference in total ecosystem carbon between ungulate exclosures and unfenced control plots. Most of the difference they did find was explained by the biomass of the largest tree within each plot. As they point out, these large trees have been unaffected by invasive ungulates introduced during the last 20–50 years. However, they believe ungulate-caused changes in understory biomass, species composition, and functional diversity might result in major shifts in the diversity and composition of regenerating species. Hence, longer term consequences for both ecosystem processes and storage of forest carbon storage can be expected.

Indeed, excluding ungulates did increase the abundance and diversity of saplings and small trees. The basal area of the smallest class of tree size was 70% greater. Species richness of small trees and saplings was 44% and 68% higher, respectively. This difference had little impact on overall carbon storage, however, because the small trees and saplings store only about 5%. In contrast, the largest tree size class (dbh =/>30 cm), with their roots, contributed 44% of total ecosystem carbon in both exclosure and control plots. The largest effects of exclosures on carbon stocks were in early successional stands, e.g., those affected by such major disturbances as windthrow, volcanic activity, or landslides.

Climate change is expected to cause surprising interactions among forest productivity, herbivory, disturbance. Allen et al. suggest that authorities should focus on excluding ungulates on these highly productive regenerating forests rather than old-growth forests. I am disturbed by this suggestion. It exposes the most biologically diverse forests to continuing damage.

Data gaps

New Zealand has many long-lived, slow-growing tree species. Recruitment of understory trees is already low across both main islands. This situation has been attributed to ungulate browsing. Over centuries, this might result in shifts in the canopy composition. Allen et al. call for additional research to increase our understanding of how browsing and other short-and long-term drivers affect the regeneration of large trees. Also, data on soil CO₂ emissions needs better integration.

Australian brushtail possum; photo by Peter Firminger via Flickr

The study did not consider the impact of other introduced mammals, such as feral pigs (Sus scrofa), rodents, and Australian brushtail possum (Trichosurus vulpecula). The possum is known to damage New Zealand trees. The scientists did not explain this omission; I assume it might have been the result of either lack of resources to support a broader study or differences in management strategies – or both?

I note that the study also did not address the extent to which non-native pathogens threaten these large trees. In response to my query, Kara Allen said that their plots did not include many kauri (Agathis australis) trees, so the severe dieback disease caused by Phytophthora agathidicida did not affect their results. Naturally regenerating kauri is limited to a small area of warm temperate rainforests located at the top of the North Island. So kauri potentially play a relatively small role in terms of overall carbon stocks in New Zealand’s forests. On the other hand, Allen says that myrtle rust (Austropuccinia psidii) could have a major impact on New Zealand forests’ carbon storage. Trees in the host family, Myrtaceae, are ecologically important across both islands. Also, they comprise a large portion of overall forest carbon stocks (ranked in the top 5 largest families for above- and belowground biomass). An example is southern rata (Mterosideros umbellata), which are preferentially fed on by Australian brush possum.

Bernd Blossey, (free access!) who has long studied the role of high deer populations in North American forests, praises the study’s attempt to measure data, not just rely on models, and its inclusion of soil. However, he notes other limitations of the New Zealand study:

The small exclosures (20 x 20 m) are subject to edge effects. Some of Blossey’s exclosures occupy 2 hectares.
Twenty years is too short a time for analysis of such long-term processes as carbon sequestration and regeneration of slow-growing trees. Therefore, any results must be considered preliminary. Furthermore, no one recorded any differences in carbon sequestration of the paired plots at the time the exclosures were set up.
There’s no mention of possible impacts by introduced earthworms.

Dr. Blossey recognizes that the current study’s authors cannot re-do actions taken decades in the past. Still, the data gaps reduce the value of the findings.

I conclude that uncertainties continue due to: the long timelines of species’ regeneration and growth to full sizes; the requirement for large exclosures; the complexity of factors affecting carbon sequestration; and probably other influences.. Managers trying to maximize carbon sequestration are forced to act without truly knowing the best strategy or how their actions will affect the future.

For more about invasive mammals’ impacts in U.S. forests, also see the study by USFS scientists, Poland et al. (full citation listed in sources). One can enter “mammal” in the search box for the on-line PDF.

SOURCES

Allen, K., P.J. Bellingham, S.J. Richardson, R.B. Allen, L.E. Burrows, F.E. Carswell, S.W.Husheer, M.G. St. John, D.A. Peltzer, M. Whenua. 2023. Long-term exclusion of invasive ungulates alters tree recruitment and functional traits but not total forest carbon. Ecological Applications. 2023; e2836. https://onlinelibrary.wiley.com/r/eap

Poland, T.M., Patel-Weynand, T., Finch, D., Miniat, C. F., and Lopez, V. (Eds) (2019), Invasive Species in Forests and Grasslands of the United States: A Comprehensive Science Synthesis for the United States Forest Sector. Springer Verlag. The on-line version as at https://link.springer.com/book/10.1007/978-3-030-45367-1

Posted by Faith Campbell