Category: invasive plants

Urban centers as plant invasion “hotspots” – do global data fail to focus on most important?

Leucanthemum vulgare (ox-eye daisy); ranked by EICAT as “major impact”; photo via picryl

Because urban centers are “hotspots” of species introductions and reservoirs supporting their spread into areas less altered by human activity, a global group of scientists (Richardson et al. 2025) sought to determine whether the same plant species naturalize in urban areas around the world and – if so – where most of those plant species originate.

They chose to pursue this question because urban areas share many interacting environmental and biotic features that they thought might partially overcome the distinct biomes of the continents. These shared features include the prominence of impervious surfaces; increased habitat heterogeneity; eutrophication; fragmentation of any remaining semi-natural habitats; complex human-influenced disturbance regimes; diverse opportunities for dispersal; novel biotic assemblages and interactions; and human facilitation of non-native species’ colonization and local species’ extinction. In addition to the similarities of the receiving ecosystems, these commonalities are facilitated by shared introduction pathways – although Richardson et al. to not pursue this aspect.

The scientists consulted global invasive plant databases to compile a list of 7,792 plant species recorded as naturalized in one or more of 553 urban centers on all six continents (all except Antarctica). Just over 300 species (4%) were reported on all six continents. They call them the “omnipresent” taxa. Further refinement resulted in a list of 96 species that are particularly widespread, defined as being present in more than half of the urban centers of Oceania, North and South America, and Europe. These 96 species are present in a lower proportion of cities in Asia and Africa. Richardson et al. proposed that these species be folded into a new ecological category, the “urban florome”.

I wonder whether this set of species tells us more about biases in the data than the actual “urban florome”. First, 87% of the 96 “most widespread” species (n= 84) are annual or perennial herbs. Only seven tree, six vine, and six shrub or subshrub species were included among the 96 species. In other words, global lists of invasive species are heavily slanted toward species that thrive in disturbance. Is this surprising? As another study (Kinlock et al. 2025) notes, disturbance is ubiquitous!

Second, only a third of the “urban florome” species have been formally evaluated using the Environmental Impact Classification for Alien Taxa (EICAT) system. Of these 32 species, only six were categorized as having a “major” or “massive” impact. Richardson et al. (2025) conclude that many of the species on the most widespread list are human commensals that have few or negligible known impacts.

Still, this finding might underestimate their impacts. First, as noted, two-thirds have not been evaluated. Second, impacts important in urban systems might not be those that increase a species’ rank based on impacts to natural systems (Richardson et al.). Those with substantial nuisance value in the urban setting still require management. Of course, some of the species have severe impacts in both natural and urban ecosystems. For example, Ailanthus altissima causes major infrastructural damage and pollen allergies, while Robinia pseudoacacia alters soil fertility. Both reduce species richness.

Ailanthus

I note that these examples are both trees – which constitute only 7 of the 96 species. Fridley et al. report that trees and shrubs have severe impacts in closed forest systems. I suggest that since many of the urban areas in temperate, subtropical, and tropical regions are probably located in formerly forested areas, remnant (semi-)natural stands and even recreational parks have probably been invaded by these high-impact species. Surely that is more important – at least as regards the level/intensity of the non-native plant species’ impact on biodiversity – than the annual weeds growing along highway verges.

Richardson et al. fear that many cities also have substantial invasion debt. The note specifically that due to the heat island effect, species that can now survive only in cities are likely to spread into surrounding rural and natural areas as temps increase. Thus, these species amplify the urban source effect of plant invasions.

Generalities

Richardson et al. call attention to certain parts of the world acting as ‘factories’ for the evolution of plant species that are well equipped to become invasive when intro to new regions. They name Australian woody flora — although only one species, Melia azedarach, is included among the 96 most widespread species. They also name African grasses and Europe (no taxa specified).

Richardson et al. say that while non-native species in urban areas have usually been described as “passengers” taking advantage of anthropomorphic environmental change, bioinvasions are increasingly recognized as drivers of secondary changes that alter the capacity of these ecosystems to deliver key ecosystem services, or even create disservices. These modifications occur in urban as well as more natural environments.

Regional Differences

Richardson et al. developed lists of the most widespread naturalized urban species for each continent (‘continental lists’). Eighty-seven percent of the 96 “most widespread” species are present in cities of North America, 80% in cities of Oceania, and 34% in European cities. Only 17% of the “widespread” species are present in cities of South America, 13% in cities of Africa or Asia.

While there is considerable overlap regarding species found on several continents, Europe’s urban florome differed significantly from those of the other continents.

The principal source region for these naturalizing species was temperate Asia (145 records); followed by Europe (128 records) and Africa (121 records). Lower numbers came from tropical Asia (95 records); South America (54) records; North America (53 records); and Oceania (8 records). Europe has received 50% of its widespread urban invasive species equally from temperate Asia and North America. Africa has received 75% of its widespread urban species from the two Americas equally.

According to these data, Oceania has been a significant contributor only to South America. I am surprised given the publicized problems caused by Australian Acacia and Hakea in South Africa. I guess these trees are more invasive in the vicinity of urban areas rather than in the cities themselves.

Richardson et al. note a highly skewed relationship between North and South America: while 15.4% of species naturalized in South American cities come from North America, only 2.7% of naturalized species in North American cities are from South America.

Lepidium didymum – brassica from South America introduced widely, including throughout California; photo by Miguel A.C. via Pl@ntnet

Richardson et al. found a distinct division between the “Old” and “New” Worlds (defined by whether the soil was historically cultivated by plough vs. hoe). The latter has more naturalized species (9,905 taxa vs 7,923 taxa), although the “Old World” covers a larger area. Citing di Castri (1989), they suggest that the much longer history of intense human-mediated disturbances in Europe might have allowed its flora to adapt to coexist w/ humans. I wonder, however, whether it is just too difficult to distinguish introductions that occurred millennia ago.

Richardson et al. also found an “echo” from European colonization — strengthened by activities of acclimatization societies. The result is that the continents with longer histories of European colonization, i.e., South and North America and Oceania, have more widespread naturalized plant species than do Africa and Asia.

SOURCES

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

Kinlock, N.L., D.W. Adams, W. Dawson, F. Essl, J. Kartesz, H. Kreft, M. Nishino, Jan Pergl, P. Pyšek, P. Weigelt and M. van Kleunen.  Naturalization of ornamental plants in the United States depends on cultivation and historical land cover context. Ecography 2025: e07748 doi: 10.1002/ecog.077

Richardson, D.M., L.B. Trotta, M.F.J. Aronson, B. Baiser, M.W. Cadotte, M. Carboni, L. Celesti-Grapow, S. Knapp, I. Kühn, A.C. Lacerda de Matos, Z. Lososová, D. Li, F.A. Montaño-Centellas, L.J. Potgieter, R.D. Zenni, P. Pyšek. 2025. Here, There and Everywhere: Widespread Alien Plants in the World’s Urban Ecosystems. Global Ecology and Biogeography, 2025; 34:e70159  https://doi.org/10.1111/geb.70159

Posted by Faith Campbell

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

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

Or

https://fadingforests.org

The invasive risk of Eucalypts

Eucalyptus grandis (in Australia); photo by Poyt448 Peter Woodard via Wikimedia

Deus et al. 2025 (full citation at the end of this blog) have published a review of current knowledge on the invasiveness of trees in the Eucalyptus genus. They report that eucalypt plantations cover more than 30 million ha globally; they could not determine the actual extent more precisely. The area is expanding at an estimated 4% per year. Eucalypts are so popular as timber trees because of their fast growth, ease of management, wood quality and environmental tolerance.

Until recently, trees in the Eucalyptus genus were thought to pose a low invasion risk. This was because these trees have limited seed dispersal, high juvenile mortality, and were expected to lack compatible ectomycorrhizal fungi in novel environments. However, several risk assessments and reports of ongoing invasions in some locations have raised questions. So Deus et al. undertook a literature survey to try to resolve the issue.

One of the risk assessments concerns the United States; see Gordon et al. (2012). This study – completed a dozen years before Deus et al. undertook their literature survey – cited several other sources documenting harmful invasiveness of nearly a dozen species, including Eucalyptus globulus, E. camaldulensis, E. grandis, and  E. tereticornis.

Deus et al. found that the limitations listed above actually can be overcome, so they do not prevent invasions:

  • seeds can disperse farther than 100 meters from parent plants;
  • high recruitment densities can compensate for the high juvenile mortality; and
  • ectomychorrhizal fungi can be found in the root systems of introduced eucalypt plants.

In fact, several Eucalyptus species meet criteria defining invasiveness in the Australian Weed Risk Assessment system. Still, Deus et al. found that existing studies cover too few plantations and species to allow an in-depth comprehensive understanding of eucalypts’ invasion ecology.

One reason that eucalypt trees’ invasiveness remains unresolved is that the countries which have established most large Eucalyptus plantations (Brazil, India and China) have not conducted many studies. Instead, most studies have been done in Iberia and South Africa, which together host less than six percent by area of the world’s estate of eucalypt plantations.

Deus et al. say that several possible reasons have been proposed to explain why Eucalyptus is considered to pose an invasion risk by scientists in Iberia and South Africa, but not in Brazil. 

  1. The few studies in Brazil were conducted in intensively managed plantations with very short rotations, which are probably less prone to invasion than plantations managed at low intensity levels.
  2. The Brazilian plantations were established 40 to 50 years ago, whereas those in Iberia were introduced ~ 200 years ago.
  3. Iberia experiences recurrent forest fires.
  4. In Brazil, leaf eating ants attack the trees; this might reduce trees’ vigor.
  5. In Brazil, native forests dominate the environs.

Deus et al. say that these hypotheses have never been tested.

Since studies have been conducted in only a few countries, they have evaluated only a few of the species used in plantations. At least 372eucalypt species have been introduced outside their native range; nine species are planted widely. Yet most of the studies reviewed by Deus et al. covered just two species, Eucalyptus globulus (46% of the studies), and E. camaldulensis (33% of the studies). Still, these two widely cultivated species received the highest invasiveness ranking of all species reviewed (65 and 72, respectively). According to Deus et al., these scores are higher than the average score for 32 species of Acacia – a genus considered to be one of the most invasive tree genera in the world.

Other, potentially invasive species, have not received adequate attention. Deus et al. note that E. tereticornis, which is widely planted in China, India and other regions of Southern Asia, has an invasiveness score of 66, placing it second highest in the evaluation. However, only 12 of 140 articles analyzed by Deus et al. addressed this species.

These eucalypts’ high scores result from their potential to hybridize, to naturalize outside their natural habitat, and from high flammability. Other contributing factors are high seed production and ability to resprout after cutting or fire.

The analysis determined that the major drivers for Eucalyptus invasions are soil disturbance, availability of moisture (essential for seedling establishment), and fire. Recruitment density increases with harvesting and tree age; it decreases when the understory is managed. This partially explains why the abandonment of plantations might promote invasions by eucalypts.

Deus et al. fear that there might be a large “invasion debt” in the regions where few studies have been conducted. Assessments for California and Iberian Peninsula indicate that the best areas for cultivation – under either current conditions or expected new environments linked to climate change – are also those most prone to invasion. A further complication is that in some regions it might be difficult to distinguish plants escaping from small plantations from the plantations themselves. They suggest ways to overcome this difficulty: 1) surveys of recruitment along roadside, where trees would not have been planted; 2) genetic analysis of seedlings and possible parents

Another weakness is that that none of the studies considers changes in fire regime, which probably increases the areas prone to invasion.

Deus et al. think it is unlikely that eucalypt invasions will turn out to be as damaging as those of acacias or pines, but that further invasions involving more species and more regions are very likely.

Deus et al. call for considering eucalypt species’ potential invasiveness when developing strategies for the sustainable management of these plantations, including how to manage those that are no longer economically viable.

Status in the United States

The risk in the United States was evaluated by Gordon et al. in 2012. At the time, there were proposals to plant 5,000 to 10,000 ha/year in the Southeast over the next decade.

Gordon et al. adapted the Australian weed risk assessment system to evaluate 38 Eucalyptus taxa then being tested and cultivated in U.S. for pulp, biofuel, and other purposes. Their analysis concluded that 15 of these taxa posed a low risk; 14 taxa posed a high risk; and 9 taxa could not be ranked without further information. The four taxa cultivated most extensively –  E. globulus, E. camaldulensis, E. grandis, and E. tereticornis – all had high risk outcomes, as did several other taxa. Gordon et al. thought that these differences reflected both new data and differences in how the assessors reacted to insufficient data.

Gordon et al. warned that novel genotypes with unknown invasiveness were being propagated in the search for increased cold tolerance. This meant that the taxa they had assessed might not indicate of the actual long-term invasion risks associated from this genus. A major source of uncertainty is the long lag time in appearance of evidence of a tree species’ invasiveness. Only one study (as of 2012) had quantified lag time for introduced tree species; it found an average of 170 years from the time introduction to identification of the taxon as invasive. Propagule pressure also influences the lag time and the probability of invasion.

Since the bulk of expanded cultivation was expected to be in the southeast, Gordon et al. recommended that a regional assessment be conducted to more precisely specify the effects of possible differences in phenology, age at reproductive maturity, seed viability, and cold tolerance. 

Gordon et al. suggested several actions to reduce the invasion risk. First, selection and breeding strategies could aim to minimize relevant traits – especially eliminating seed production. Second, plantations could be so managed by avoiding cultivation near waterways, harvesting stems before seeds can mature, and restricting the extent of cultivation of any one taxon. More broadly, a fund could be established to cover control costs; growers would contribute the money.

What has happened in the dozen years since the analysis was published? My Google search led to publications from 2013 and earlier. I hope this indicates that no one has funded major expansions. Dr. Gordon reports that most Eucalyptus pulp is imported. ArborGen continues to breed Eucalyptus in Brazil – as I noted earlier, scientists there are not pursuing studies of possible invasiveness of eucalypts.

Still, the regional risk assessment has not been conducted. Worse, Dr. Gordon reports that the Florida Department of Agriculture and Consumer Services has exempted several species [E. amplifolia, E. benthamii, E. dorrigoensis, E. dunnii, E. grandis, E. gunni, E. nitens, E. smithii, and E. urograndis (E. grandis  E. urophylla)] from a requirement that growers obtain Non-Native Species Planting Permits. So if the market does take off, there will be no regulation by the state.

At the end of December 2025, Dr. Gordon received information from Florida Division of Plant Industry that no one has applied for a permit to grow Eucalyptus in the state other than under USDA research auspices. So my worst fears have not (yet) come to pass.

I note that in 2022, Potter, Riitters, & Guo ranked Eucalyptus grandis & E. globulus as potentially highly invasive. Their criterion was that at least 75% of stems detected by USFS Forest Inventory and Analysis (FIA) surveys were saplings or seedlings.

SOURCES

Deus, E., D.M. Richardson, F.X. Catry, F.C. Rego, J. Gaspar, M. Nereu, M. Larcombe, B. Potts, J.S. Silva. 2025. Invasion ecology of eucalypts: a review. Biol. Invasions (2025) 27:239  https://doi.org/10/1007/s10530-025-03695-1

Gordon, D.R.,S.L. Flory,.L. Cooper, and S.K. Morris. 2012. Assessing the Invasion Risk of Eucalyptus in the United States Using the Australian Weed Risk Assessment. International Journal of Forestry Research Volume 2012, Article ID 203768, 7 pages doi:10.1155/2012/203768

Potter K.M., Riitters, K.H. & Guo, Q. 2022. NIS tree regeneration indicates regional & national risks from current invasions. Frontiers in Forests & Global Change  

doi: 10.3389/ffgc.2022.966407

Posted by Faith Campbell

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

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

Or

https://fadingforests.org

Welcome high-level attention to bioinvasions – although key issues remain unresolved

Japanese knotweed (Reynoutria japonica) – one of the worst invaders around globe. Photo by Will Parson, Chesapeake Bay Program via Flickr

On 23 October, Science published a five-page, data-packed analysis of bioinvasion impacts on terrestrial ecosystems!!! 

Thakur, Gu, van Kleunen, and Zhou (full citation at end of this blog) analyzed 775 studies with the goal of improving understanding of factors contributing to invasions’ impacts – as distinct from “invasibility” (ability to establish). This knowledge is essential to assessing the risk posed by introduced species and setting priorities for management. They analyzed five ecological contexts—diversity of native species and introduced species in the recipient systems, latitude, invader residence time, and invader traits.

They concluded that ecological factors commonly used to explain invasion success do not consistently translate into strong predictors of invasion impacts. Impacts vary in response to the context of the invasion.

[In January 2026, the authors announced changes in details of the article due to some errors in the database and their understanding of it. (Science 8 Jan 2026 Vol. 391 Issue 6781) They conclude that the corrected analysis did not alter the trends described or the overall conclusions.]

limber pine (Pinus flexilis) – one of the species killed by Cronartium ribicoli; photo by F.T. Campbell

Among the studies available for analysis, reports on plants dominated: 605 focused on plant invasions, 114 on animal invasions, and only 56 on microbial invasions. Among the animals were one study of Adelges tsugae (hemlock woolly adelgid), two studies of Agrilus planipennis (emerald ash borer) and one study each of Lymantria dispar (spongy moth), and Ips pini (North American pine engraver). Studies also addressed earthworms, ants, rats, and feral hogs. Microorganisms included Cronartium ribicoli (white pine blister rust) and several Phytophthora species, including P. agathidicida (kauri dieback), P. alni (affects alders), and P. ramorum (sudden oak death).

Thakur et al. note the skewed taxonomic coverage and say that the low number and narrow taxonomic/ecological variety in the animals and microorganisms probably limit their ability to reach robust conclusions about the impacts of such invasions.

The most consistent negative impact they found is reductions in native plant diversity. While this is not surprising given the studies analyzed, I think it is still important since it counters the widespread sense that plant invasions are somehow less deserving of a robust response.

The authors also detected some broader ecosystem impacts of plant invasions. Plant invasions increased soil organic carbon; soil nitrogen (ammonium and nitrate), and available phosphorus; soil moisture, litter biomass; and emissions of carbon dioxide (CO2), nitrous oxide (N2O), and methane (CH4). The changes in biogeochemical properties might reinforce impacts on native plant communities. The reported increase in greenhouse gas emissions might reflect a bias in the studies so Thakur et al. call for more research to solidify this finding.

High native plant species richness had only a weak overall effect on ecosystem-level impacts. While plant invasions often resulted in higher overall plant species richness, when considering only native community responses, the gain in species numbers did not necessarily indicate conservation benefits. Native plants’ biomass increased after invasion. This might reflect short-term increases in productivity in response to altered resource conditions or structural facilitation, rather than a long-term reversal of competitive exclusion. Finally, the longer the invasive [plant] species had been present, the greater the negative effects on native diversity. However, soil abiotic property impacts weakened over time. In fact, the initial increase in soil organic carbon and total nitrogen disappeared after 6 to 10 years. This development might reflect fertilization of ecosystems by long-established nitrogen-fixing invaders such as non-native legumes.

Traits of non-native plant species related to growth and resource acquisition were overall weak predictors of ecosystem impacts. Thakur et al. consider that this finding reflects the relatively narrow range of specific leaf area exhibited by the plant species studied most commonly.

Consequently, Thakur et al. urge managers to focus on containment and impact mitigation, and to prioritize persistent losses of native plant diversity. When considering abiotic responses that might lessen over time, managers should apply “adaptive monitoring” (which is not defined).

Thakur et al. had greater difficulty determining the impacts of animal and microorganism invasions because of the smaller number of studies. They could not determine the effect of native species richness. The observed decline in soil organic carbon they thought was attributable to the large proportion of studies (9 out of 114) that focused on introduced earthworms. Earthworms reduce organic matter by consuming litter. Mammals were also found to reduce soil organic carbon. Introduced insects had no significant ecosystem effects on soil organic carbon. Non-native animals also increased soil emissions of carbon dioxide and nitrous oxide. The microorganisms included in reviewed studies decreased soil ammonium and increased nitrate, consistent with elevated nitrification. While data on body size of invasive animals were sparse, the authors could determine that larger-bodied species tended to increase soil nitrate while reducing effects on total soil N.

Applying the Results

Thakur et al. report that residence time outperformed other factors as a predictor of invasion impacts. The authors regret the scarcity of long-term studies, especially in the Global South, that could increase our understanding of whether these impacts persist or shift under sustained invasion pressure.

How can scientists apply this information in risk assessments evaluating not-yet introduced species or in deciding what is the appropriate intensity of immediate response to newly detected incursions. Should they give greater weight to others’ studies that focus on long-established invasions by the species in question? Otherwise, this finding seems to largely duplicate the long-established “invasion curve”.

I hope scientists will note that observational studies generally showed stronger impacts than experimental ones, particularly in the case of plant invasions. Perhaps this is true because observational studies better incorporate environmental heterogeneity and longer time spans.

Agrostis stolonifera – one of the plants invading on Prince Edward Island, an Antarctic region island under South African jurisdiction. Photo by Stefan Iefnaer via Wikimedia

Thakur et al. note that one factor they analyzed, “latitude”, incorporates several ecological and anthropogenic components relevant to invasion impacts. One element is the greater native bioidiversity in warmer, lower-latitude, regions. According to the “biotic resistance” hypothesis, greater diversity might make these systems more resistant to bioinvasion. However, the situation is complicated by the fact that temperate regions have also often experienced longstanding and intensive land-use modifications — which are believed to facilitate invasive species establishment and spread. I regret that the authors make no attempt to separate the effects of factors that are anthropogenic from those arising from immutable conditions, e.g., latitude, topography, weather patterns, etc.

Thakur et al. call for more studies that cover a wider geographic range. In addition, the studies should include more experimental designs and explore the relationship between invaders’ traits and impacts — especially regarding animals and microbes.

SOURCE

Thakur, M.P., Z. Gu, M. van Kleunen, X. Zhou. 2025.  Invasion impacts in terrestrial ecosystems: Global patterns and predictors. Science 23 October 2025

Posted by Faith Campbell

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

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

Or

https://fadingforests.org

Threats to Spring

Erythronium americanum dominating herb layer in woods owned by the Institute for Advanced Studies, Princeton, in the 1970s; photo by F.T. Campbell

I fell in love with spring ephemerals in the woods of the Institute for Advanced Studies in Princeton. While the degree I was pursuing had no relationship to birding in the swamp, I spent a lot of time enjoying the woods. At that time, more than 50 years ago, the herbaceous layer was dominated by spring beauties (Claytonia virginica), trout lilies (Erythronium americanum), and violets (Viola species).

Beyond the beauty that delights us (or at least, me!), spring ephemerals are important ecologically. They support specialist pollinators and reduce nutrient losses at a time of year when vegetation cover is low and leaching and runoff rates high.

In the decades since I left Princeton, scientists and nature lovers have observed declines in native understory plant communities. These are predicted to continue due to invasion by plants and worms, worm blogs herbivore pressure by deer, E NPS blog, Blossey blog land use changes, and climate change.

Where I live, in the suburbs of the District of Columbia, these forces are clear. The formerly glorious riparian forests where I walk are overrun by invasive plants. The herb layer is dominated by Japanese stiltgrass (Microstegium vimineum) and – increasingly — lesser celandine (Ficaria verna = Ranunculus ficaria). (I found it interesting that Ficaria began taking over floodplain forests only in the last decades of the 20th century, although it was introduced more than 100 years earlier.) Many invasive shrubs (Rosa multiflora, various Lonicera species. …) and vines (Ampelopsis sp, Orbiculatus, Lonicera japonica, Hedera helix …) compound the problem. While I am not sure whether most earthworms here are native or not, high deer populations certainly are a factor.

Ficaria invasion in Fairfax County, Virginia in 2023; photo by F.T. Campbell

So I rejoice that scientists are studying how one taxon of spring ephemerals, trout lilies – Erythronium species – are coping with individual and combined threats. Gutiérrez and Hovick (full citation at the end of this blog) investigated how two species of Erythronium performed in the absence of a leaf litter layer – with and without competition by Ficaria. They chose to manipulate leaf litter as a proxy for impacts from invasive earthworms and non-native shrubs, especially those with rapidly decomposing leaves.  They refer to others’ studies focused on different spring ephemerals.

Gutiérrez and Hovick found that the absence of leaf litter reduced asexual reproduction (corm biomass) in both Erythronium albidum and E. americanum species by 30%. That is, the absence of leaf litter alone reduced the native plants’ performance. This is alarming because persistent leaf litter has been reduced across much of the deciduous forests of eastern North America as a result of action by invasive earthworms and the rapid decomposition of the leaves of most invasive shrubs.

Trout lilies’ performance declined even more when litter absence was coupled with direct competition from Ficaria. Under those conditions, corm biomass declined by 50%. Impacts by lesser celandine occurred despite these plants’ being smaller than counterparts in nearby woodlands. The reduced size of Erythronium corms was sufficient, in their view, to reduce the likelihood that Erythronium would flower to nearly zero. This has clear implications for the long-term population viability of Erythronium andtheir specialist pollinators.

 Gutiérrez and Hovick conclude restoration of these floodplain forests’ herb layer must incorporate management strategies that not only reduce Ficaria’s presence but also restore leaf litter.

Erythronium albidum along Accotink Creek in Fairfax County, Virginia; photo by F.T. Campbell

Underlying Factors

Native spring ephemerals in eastern North America evolved to emerge through litter layers in early spring. The litter layers impose both costs and benefits. In response to shading by leaf litter, Erythronium produces larger petioles compared to same-sized leaves, thus reducing the proportion of resources allocated to building photosynthetic tissue. In these cases, the corms that both perpetuate the individual and carry out asexual reproduction are smaller.

On the other hand, leaf litter increases moisture retention and reduces frost damage by buffering soil temperatures. While these results were seen in their experiment, Gutiérrez and Hovick believe the benefits are greater in nature than demonstrated in the study using potted plants. Leaf litter also increases nutrient availability, directly by increasing supply and indirectly by facilitating fine root growth. In this context, they note that their experiment used litter composed of just two tree species — red oak (Quercus rubra) and red maple (Acer rubrum). This narrow sample probably failed to capture the varied properties of other tree species’ litter and associated microbial activity.

Erythronium americanum along Pohick Creek; photo by F.T. Campbelle

Plants in the Erythronium genus reproduce primarily asexually through producing runners that form corms. The parent corm and runners disintegrate before summer dormancy; the offspring corms persist. Some individuals do not reproduce asexually; they simply replenish their own corm.

The few previous studies give mixed results regarding lesser celandine’s impacts on co-occurring native herbaceous plants (see the summaries in Gutiérrez and Hovick). The authors do not explicitly say whether lesser celandine is usually associated with low litter levels, but that appears to be the implication. They do say that it is not clear whether lesser celandine drives leaf litter loss by altering soil physiochemistry and microbial activity. Or, rather, that it simply performs well when leaf litter is absent.

Where lesser celandine and Erythronium co-occur at high densities, the former’s biomass per square meter can be more than an order of magnitude higher than Erythronium. Gutiérrez and Hovick suggest that competition between the species is primarily belowground. They cite their finding that by the time Erythronium shoots matured, lesser celandine roots occupied most of the belowground pot volume. They expect belowground competition in forests to be even more pronounced because of accumulated lesser celandine root biomass.

Aboveground, the principal factor appears to be the necessity for trout lilies to grow longer petioles to raise their leaves above lesser celandine rosettes, perhaps starving leaf formation. Since leaves are the plant’s photosynthetic organ, this tradeoff could ultimately result in fewer resources returned to the corm for future growth and reproduction. Although Gutiérrez and Hovick also mention that lesser celandine competition might delay Erythronium emergence and flowering, they do not discuss that.

A factor not mentioned by Gutiérrez and Hovick is the probability that Ficaria verna is allelopathic. See the article by Kendra Cipollini listed as a source at the end if this blog.

one of the few floodplains in Fairfax County still dominated by native herbs – Pohick Creek in the Burke area of Fairfax County, Virginia. Note the prevalence of beech in the canopy and subcanopy! photo by F.T. Campbell

Details of Impaired Performance of Erythronium

At the time of senescence, Erythronium plants grown in pots with leaf litter were nearly twice as large as those grown in bare soil conditions. One-third of their offspring corms grew to be larger than the putative biomass threshold for flowering. Only 9% of corms of plants grown in bare soil and 2% (one individual) of those grown with lesser celandine did. As noted above, corms developed by Erythronium grown in the presence of Ficaria actually lost biomass. This is the basis for their conclusion that there would be almost no sexual reproduction the following year where litter was absent and lesser celandine present.

Gutiérrez and Hovick think the principle mechanisms by which leaf litter affects performance of Erythronium plants is by buffering temperature ranges and increasing moisture retention. Indeed, they found that daily temperature ranges and maxima of soil in pots with bare soil or lesser celandine plants were both higher than temperatures under leaf litter. Reducing temperature maxima could be especially important with the increasing frequency and intensity of late-spring heatwaves associated with climate change. Absence of leaf litter advanced trout lily’s shoot emergence, flower emergence, and petal opening by 14 or more days.

This change might expose the plants to increased risk of frost damage. These dynamics will be system-specific, especially with complications added by climate change. However, Therefore, Gutiérrez and Hovick encourage future research to explore species-specific litter effects on spring ephemerals.

Broader Implications

Their findings regarding these two species of spring ephemerals prompt Gutiérrez and Hovick to assert that negative impacts from invasive plant species might be especially underestimated in spring ephemeral communities due to the combination of their short period of annual aboveground activity and tendency towards long lives. Changes might be very subtle over short timeframes.

They add that it is important to learn the role different conditions might play in the futures of related species. The two species’ ranges largely overlap, but E. americanum extends into the extreme southeast and northeast, E. albidum into the prairie states. Although these species’ respond to loss of leaf litter and lesser celandine invasions in similar ways, the fact that E. albidum occurs in areas of higher soil moisture makes it more vulnerable to negative population-level impacts from lesser celandine invasions.

Note about additional threats

Most of the photos of Erythronium americanum in this blog were taken along a particular creek in Fairfax County, Virginia. Ficaria has just begun to invade this area (see photo above); deer are plentiful. These plants face another bioinvasion: beech leaf disease has arrived. Widespread mortality of the predominantly beech understory will presumably open areas to more light, probably spread of the extant invasive plants.

beech in Fairfax County, Virginia with symptoms of beech leaf disease; photo by F.T. Campbell

SOURCES

Cipollini, K. and K.D. Schradin. 2011. Guilty in the Court of Public Opinion: Testing Presumptive Impacts and Allelopathic Potential of Ranunculus ficaria” 

Gutiérrez, R.G. and S.M. Hovick. 2025. Compounding negative effects of leaf litter absence and belowground competition from an invasive spring ephemeral on native spring ephemeral growth and reproduction. Biol Invasions (2025) 27:213 https://doi.org/10.1007/s10530-025-03668-4

Y’all Come! National Plant Board Will Meet in Virginia in July 2026

The National Plant Board (NPB) represents the state officials responsible for preventing the introduction, establishment, and spread of invasive species called “plant pests” – including insects and pathogens that attack our native flora and invasive plants. The NPB has just held its 2025 meeting, on which I report here.

Coming to the Mid-Atlantic: NPB 2026 Annual Meeting

The next annual meeting will be in Alexandria, Virginia at the end of July 2026.I have attended these annual meetings since 2006 and always find them worth my time. They provide a wonderful opportunity to interact with the state and federal officials responsible for managing invasive plants and plant pests, and to assess regulatory issues. Contact me for more information.

The agendas focus on practical topics, such as science and technology tools, changes in APHIS policies or practices, and progress in cooperation among relevant federal agencies (i.e., the U.S. Department of Agriculture and the Department of Homeland Security’s Bureau of Border Protection) and with the states. While agricultural pest issues are stressed, tree-killing pests also get attention. Sometimes invasive plants are also discussed. The Board’s state representatives seek ways to coordinate their efforts both at these meetings and throughout the year.

Issues in the host location are part of the focus. Next year, that will be the Mid-Atlantic. The meeting is being co-hosted by the departments of Agriculture of Virginia, Maryland, Washington, D.C., and Delaware.

I expect that there will be opportunities for presenting concerns of non-governmental organizations – at least through staffed display tables and possibly other activities. I hope the many conservation organizations that have a Washington, D.C., presence will consider participating.

In Honolulu: NPB 2025 Annual Meeting

NPB’s 2025 Annual Meeting in Honolulu focused to some extent on the unique aspects of agriculture and introduced pests on remote Pacific islands. (Guam was co-host.) This blog reports on current efforts by federal and state authorities to counter bioinvasions there and around the country.

I took advantage of the meeting to visit the “Big Island” of Hawai`i to see for myself the impact of rapid ‘ōhi‘a death and enjoy the native flora (for example, the hapu tree fern – below). I posted another blog reporting what I learned there.

native Hawaiian tree ferns & ʻōhiʻa; photo by F.T. Campbell

Federal

In an earlier blog, I outlined the Administration’s proposed cuts to staff of the U.S. Department of Agriculture (USDA) and contradictory actions by Congress in the annual appropriations bills.

As that blog makes clear, the work of USDA’s Animal and Plant Health Inspection Service (APHIS) is viewed much more positively by the Trump Administration than is the USDA Forest Service. While APHIS’ funding is much more secure, staff cuts and reorganization of the USDA still have caused setbacks. APHIS is expected to lose 15% of employees – 1,180 people. Four hundred APHIS employees accepted the Administration’s deferred resignation offer. These included the leadership of many programs – including the previous Deputy Administrator, Mark Davidson. Higher up, no one has been appointed to the position of Deputy Secretary for Marketing and Regulatory Affairs.

In his report to the meeting, APHIS Acting Deputy Administrator for Plant Protection and Quarantine Matthew Rhoads noted that the Administration’s Farm Security Plan, which emphasizes efforts to combat bioterrorism, includes APHIS’ safeguarding role. However, abrupt and incomplete leadership changes hamper efforts to replace those who have left and set agency priorities. While I am cheered by the reported priority for preventing pest introductions, I fear that the focus might be quite narrow, leaving out threats to natural resources such as native forest trees.

Rhoads announced that after years of effort, the Asian longhorned beetle has been declared eradicated on 12.3 square miles of the Massachusetts quarantine zone.

Much of the presentation by Matthew Rhoads and later ones by other APHIS staff updated attendees on progress on technologies important in pest detection and control, and specific projects being carried out jointly by APHIS and NPB members (that is, state regulatory officials chosen to represent the state phytosanitary agencies). I consider the collaborative projects — begun in February 2023 – to be very important. Twenty years ago, relations between APHIS and its state counterparts were characterized by an “us vs. them” attitude.

I will summarize progress on the projects of greatest interest to those of us focused on non-native insects and disease pathogens threatening tree species. Rhodes mentioned improvements in the plant pathogen diagnostic certification program and development of improved molecular diagnostics for 45 insects and plant pathogens, including several Phytophthora species.

Joint APHIS-NPB teams have completed many risk analyses: 18 datasheets, 20 assessments, and four pathway analyses. As usual, insects – especially beetles – are the most numerous taxa detected. Many were surprised that the majority of new detections occurred in the south. When he was asked about this, Rhoads speculated that this reflected the region’s more hospitable climate and Florida’s surveillance efforts. I noted that ports in the southeast – e.g., Savannah and Charleston – are receiving higher import volumes; and that there have been problems with dunnage in the port of Houston.  

Large container ship docked at Port of Savannah; photo by F.T. Campbell

Rhoads praised the federal-state strategic alliance’s project targetting illegal importation of plants purchased on-line. His example should concern us: importation of as many as 10,000 black pine seedlings to Georgia. The state stopped sale of these plants and APHIS’ investigatory unit began an investigation. This example illustrates the volume of plants that might be moving in this trade. Several states asked APHIS to offer more help in countering trafficking involving smaller numbers.  All agree that no one has yet figured out an effective way to control this pathway.

A second example of successful coordination between APHIS and the states was said to be the decision to not regulate Phytophthora austrocedri, a pathogen detected in several nurseries in Oregon in 2024. Possible hosts in the Pacific Northwest include the already-depleted Port Orford cedar, and here; Juniperus californica, J. grandis, J. occidentalis, and J. maritima. Federal and state plant health officials, in coordination with the nursery industry trade association (AmericanHort), reached this decision after determining that the pathogen has probably been present in Oregon for many years and been spread to other states on the large volumes of host plants shipped. Now it will be up to states and non-governmental conservation organizations to try to detect whether this pathogen has established and devise management strategies.

New Information (as of December 2025): someone has posted on the web a written explanation of this decision by APHIS to the National Plant Board. [Visit cdn.ymaws.com, search for “Phytopthora austro”]. APHIS estimated that delimitation surveys in just one nursery would cost more than $9 million. Because the pathogen cannot be detected by visual symptoms, even tracking spread requires expensive destructive sampling of large numbers of plants. Meanwhile, thousands of possibly infected plants have been shipped from at least two Oregon nurseries in recent years. APHIS concluded that a Federal survey program for P. austrocedri would not contribute to ultimately controlling the spread or eradication of this pathogen. The agency recommended instead that natural resource agencies adopt a “protective-style approach”, focused on actively managing highest-value natural sites.  

Are federal, state, and non-governmental managers of the many types of ecosystems inhabited by junipers and cypresses equipped to do this?

Ordinarily, the USFS Forest Health Protection program would be in a position to assist states which want to manage this pest (assuming its establishment). But considering the current uncertainty regarding USFS’ future, blog states cannot count on that help.

Sky Stevens (entomologist on the staff of USFS Forest Health Protection program) reported on the situation at the USFS. She noted that the Congressional appropriations bills continue funding for the agency’s research program and collaboration with non-federal entities managing forests. Still, the USFS lost 5,200 people through “voluntary” resignations and firings.

The program of greatest importance to us, Forest Health, was cut from 18 people to 8. Stevens replaced the long-time national entomologist. The comparable pathologist has retired. Stevens is struggling to make decisions regarding the pathology program, especially since diseases are inherently more difficult. While the USFS is doing lateral exchanges to fill high-need vacancies, FHP has not yet been asked what the program needs.

According to Stevens, in 2024 about 9 million acres were impacted by forest pests. The FHP program treated 1 million acres. As usual, the (European) spongy moth was the largest target based on acreage. Other non-native species targetted were emerald ash borer, goldspotted oak borer, sudden oak death, Asian longhorned beetle, hemlock woolly adelgid, and rapid ‘ōhi‘a death. See summaries of these pests’ impacts and status here.  

Continuation of these projects in 2025 often became trapped in the new Administration’s funding freezes; opportune times for effective actions were often missed. On-going projects include several targetting emerald ash borer and its hosts in Oregon and black ash swamps of the Midwest and Northeast; managing sudden oak death in Oregon and California; and delimitation surveys for rapid ‘ōhi‘a death.  The SOD program benefits from approximately $3 million earmarked by Congress (out of the total funding for the forest health program of $48 million).

Stevens noted that it is difficult to discuss the program’s future given the uncertainty. Program staff hope to continue issuing products that help people understand forest health in their region – not limited to federal lands.

I learned from the review of the following programs and technical tools that many were funded by the grant program under APHIS’ Plant Pest and Disease Management and Disaster Prevention program  (Plant Protection Act Section 7721). Clearly, America’s efforts to prevent and respond to invasions by plant pests (including invasive plants) would be far less robust without this grant program.

boxwood (box tree) garden at Gunston Hall – an 18th Century plantation near Alexandria, Virginia (site of the 2026 NPB meeting); Photo by Roger 4336 via Wikipedia

Wendy Jin, APHIS PPQ Associate Deputy Administrator, urged states to use pest forecast models developed under the SAFARIS program. These models incorporate information on weather; pest biology, environmental needs and impact; hosts; land cover; and relevant human activities. Fifty pests have been evaluated so far, apparently including Asian longhorned beetle, spongy moth, spotted lanternfly, and boxtree moth. (All but the last are described briefly under the “invasive species” tab here.) The goal is to provide managers information about the insect’s life stage at specific times in specific localities so that they can time their surveillance and management actions. However, I am somewhat worried because the models use current and historical weather data – which might not be pertinent as the climate warms. Worse, the modelers lack sufficiently detailed data to develop models for Alaska, Hawai`i, Puerto Rico, or Guam.

Dr. Carrie Harmon (Deputy Director, National Plant Diagnostic Network) described the resources available for states use from two diagnostics tools. Both were developed under grants which are now expiring. Therefore updates and further development will depend on renewal of the grants.  The National Plant Diagnostic Network (NPDN) provides accurate data and alerts about appearances of plant diseases. APHIS is said to be collaborating closely to ensure as much data as possible is shared. A separate body, the Diagnostic Assay Validation Network, is validating diagnostic assays.

A few years ago the NPB and APHIS formalized their new level of collaboration as the “Strategic Alliance, Strategic Initiative”. The Plant Board surveyed its members to gauge their feelings about several issues: 1) data-sharing issues that impede decision-making; 2) ways to strengthen coordination when dealing with on-line sales of plants or other vectors of plant pests (see the pine-Georgia example above); and 3) what structures and practices could make resolving these problems easier. 

One of the resulting initiatives is an analysis of implementation of the Federal Noxious Weed program in the absence of a line-item appropriation. However, the President’s “Department of Government Efficiency” (DOGE) prompted resignations and firings, including this project’s APHIS liaison. Without a replacement, it is unclear how the analysis can proceed.

Another speaker, representing Bob Baca, Assistant Director of APHIS Plant Protection and Quarantine, warned state officials about new pressure to phase out use of methyl bromide (MB) as a phytosanitary tool. Use of ozone-depleting chemicals – including MB – has been regulated since 1988 under the Montreal Protocol. Americans use more MB for this purpose than any other country. Already manufacturers are ending its production. After mentioning substitutes under development, the speaker urged state departments of Agriculture to meet with growers and develop a nation-wide plan to weather this impending change. She noted that APHIS has no authority to require companies to produce substitutes. 

The NPB leadership discussed turnover in the organization (several states are represented by officials new to their jobs); advocacy to APHIS for even better coordination and recognition of states’ need to act quickly; and efforts to expand its collaboration with other entities. A series of presentations tallied lessons learned during specific plant pest crises. These included the role of the public in pest detection; mobilizing initial responses to a new pest; and building higher-ups’ and legislators’ support for funding a “rapid response” capability before arrival of a new damaging pest.

In a separate blog I reviewed topics discussed that pertain particularly to Pacific island plant health issues.

Posted by Faith Campbell

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

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

Or

https://fadingforests.org

Status of Hawaiian species threatened by bioinvasion

stand of Miconia under albizia overstory on Big Island, Hawai`i; photo by F.T. Campbell

As I will describe in another blog, participants in the annual meeting of the National Plant Board link in Honolulu learned the basics about the uniqueness of agriculture and native species on remote Pacific islands. I want to complement this information by reminding you about other Hawaiian and Guamaian species at risk – although did not learn anything new.

As Martin and Andreozzi pointed out, the Pacific islands import nearly all their food and other consumables. Considerable interest in some quarters in Hawai`i to increase agricultural production. However, large swaths of land in the low-elevation area surrounding Pahoa on the Big Island is completely dominated by the albizia (Falcataria Molucca) [see photo above]. J.B. Friday says it is cost-prohibitive to remove these trees in order to restore agriculture in the area. Local people are concerned because in storms the trees fall onto houses and roads, causing considerable damage.

I saw numerous clumps of the notorious invasive plant Miconia calvescens. Dr. Friday told me that conservationists now focus on keeping this plant out of key areas, not trying to eradicate it completely.

area being restored by volunteers; photo by F.T. Campbell

Local people trying to restore disease-damaged forests by planting other native plants and hand-clearing invasive plants. Some of the ohia seedlings infected by Austropuccinia psidii.

ohia seedling with symptoms of ohia rust (Austropuccinia psdii); detected by J.B. Friday; photo by F.T. Campbell

Dr. Friday showed me many areas where ʻōhiʻa trees have been killed by rapid ʻōhiʻa death. Since this mortality occurred a decade or more ago, other plants have grown up. Pic  In many if not most cases, this jungle includes dense growths of guava Latin the most widespread invasive tree on the islands (Potter). ‘Ōhi‘a trees continue to thrive in Hawai`i Volcanoes National Park – also on the Big Island – because the NPS makes considerable efforts to protect them from wounding by feral pigs. Demonstrates importance of fencing and mammal eradication in efforts to protect this tree species.

healthy ʻōhiʻa tree on cinder cone created by eruption of Kilauea Iki in 1959; photo by F.T. Campbell

I also saw healthy koa (Acacia koa) in the park, especially at sites along the road to the trail climbing Mauna Loa.

Regarding the wiliwili tree, I was told that it remains extremely scarce on Oahu.

wiliwili tree in flower; photo by Forrest Starr

I heard nothing about the status of naio – another shrub native to the Big Island – but on the dry western side of the island.

I rejoice that scientists are making progress in protecting and restoring Hawaii’s endemic bird species. Specifically, they are at the early stages of controlling mosquitoes that transmit fatal diseases. All 17 species of endemic honeycreepers that have persisted through the 250 years since Europeans first landed on the Islands are now listed as endangered or threatened under the federal Endangered Spp Act. The “Birds, not Mosquitoes” project has developed lab-reared male mosquitoes that, when they mate with wild female, the resulting eggs are sterile. (Male mosquitoes don’t bite, so increasing their number does not affect either animals or people.) Over time, the invasive mosquito population will be reduced, giving vulnerable native bird populations the chance to recover. Scientists began releasing these modified mosquitoes in remote forests on Maui and Kaua‘i in November 2023. In spring 2025, they began testing releases using drones. Use of drones instead of helicopters reduces the danger associated with flying close to complicated mountain rides in regions with variable weather.   This project should be able to continue; the Senate Appropriations Committee report for FY26 allocates $5,250,000 for this project.

American Bird Conservancy is sponsoring a webinar about this program. It will be Wednesday, August 27, 2025 4:00 PM – 5:00 PM ET. Sign up for the webinar here

thicket of guava on the Big Island, Hawai`i; photo by F.T. Campbell

Finally, scientists are releasing a biocontrol agent targetting strawberry guava, Psidium cattleyanum, the most widespread invasive tree on the Islands (Potter et al. 2023). Distribution involves an interesting process. A stand of guava is cut down to stimulate rapid growth. The leaf-galling insect Tectococcus ovatus reproduces prolifically on the new foliage. Twigs bearing the eggs of these insects are collected and tied into small bundles. The bundles are then dropped from helicopters into the canopies of dense guava stands, where they establish and feed – damaging the unwanted host.  

brown tree snake; photo via Wikimedia

Guam

Guam’s endemic birds have famously been extinguished by the non-native brown tree snake. Dr. Aaron Collins, State Director, Guam and Western Pacific, USDA APHIS Wildlife Services, informed participants at the National Plant Board meeting about the extensive efforts to suppress snake populations in military housing on the island, reduce damage to the electric grid, and prevent snakes from hitchhiking to other environments, especially Hawai`i and the U.S. mainland.

The program began more than 30 years ago, in 1993. The program now employs 80 FTEs and has a budget of $4 million per year. It was initiated because live and dead snakes had been found in shipments and planes that landed in Hawai`i and the U.S. mainland. Avoiding the snake’s establishment on Hawai`i is estimated to save $500 million per year. The program is a coordinated effort by USDA, U.S. Fish and Wildlife Service, and the Department of Defense. Probably this estimate helped advocates reverse a decision by the “Department of Government Efficiency” to defund the program.

The program enjoys some advantages over vertebrate eradication programs on the mainland. For example, since Guam has no native snakes, it can use poison, e.g., in mouse-baited traps that can be dropped from planes. A recent innovation is auto-resetting traps baited with mammals; they can electrocute numerous snakes per night.

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

Posted by Faith Campbell

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

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

Or

https://fadingforests.org

Tree deaths in a National Park – what I saw

In June I visited Shenandoah National Park (SHNP) (above) for the first time in years. The Park’s forests are mostly mature secondary forests, having recovered over the 90 years since establishment from earlier logging and clearing for small-scale farms and pasture.

While I loved the forest and the vistas, I was aware of which species are missing …

Five years ago I blogged about a study by Anderson-Teixeira et al. (full citation at the end of this blog) that reported on the changes in the forests of SHNP and the neighboring Smithsonian Conservation Biology Institute (SCBI). This is important because, as Fei et al. (2019) (full citation at the end of this blog) documented, nine of the 15 most damaging introduced forest pests grow in eastern forests. In fact, the greatest increase in biomass loss has occurred in Eastern forests. Seven are found specifically in SHNP (Potter et al. 2019; full citation at the end of the blog).

Anderson-Teixeira et al. report that non-native forest pests caused a loss of about a quarter of ecosystem above-ground biomass between 1991 and  2013 across 66 sites. These invasions occurred after the worst impacts of chestnut blight, which entered the country ~120 years ago – before “modern” phytosanitary programs were instituted. Still, total above-ground biomass has largely recovered through germination and growth by trees in other genera. Greatest increases have been by tulip poplar (Liriodendron); oaks (Quercus); ash (Fraxinus) – but see below; birch (Betula); and maples (Acer). And while several taxa were lost from monitoring plots in SHNP and SCBI, a-diversity also remained steady.

So what does that look like on the ground?

American chestnut used to dominate many Eastern forests, composing more than one-third of the pollen assemblage in some stands (Fei et al.) According to Anderson-Teixeira et al., chestnut trees larger than 10 cm DBH disappeared by 1910, killed by chestnut blight. In past decades I frequently saw chestnut root sprouts when hiking. The National Park Service now urges visitors to hike to low elevation sections of the South River Trail to see such sprouts.

In the 1980’s, groves of eastern hemlocks occupied about 9,800 acres in SHNP, primarily in shaded valleys and along streams. Invasion by the hemlock woolly adelgid killed 95% of these hemlocks. Anderson-Teixeira et al. document the species’ disappearance from their study plots by 2007. Park staff treated more than 20,000 hemlocks using injections of imidacloprid. In 2015, the Park began partnering with Virginia Polytechnic Institute and State University in releasing predatory biocontrol beetles (Laricobius spp.)  While the beetles have shown promising establishment and spread, it is now recognized that additional biocontrol agents will be needed to suppress the adelgid. The Park plans to allow releases of predatory silver flies (Leucotaraxis spp.) in remaining hemlock sites and will begin to phase out the imidacloprid treatments.

I remember the hemlocks! But this year, at least in the creek valleys where I hiked, I saw almost no remnants – not even fallen logs.

fallen hemlock; all photos by F.T. Campbell in Shenandoah NP in June 2025

And I remember the flowering dogwoods. They are almost gone now from the Appalachian chain, killed by dogwood anthracnose. Their status in SHNP is unclear. Anderson-Teixeira et al. report flowering dogwoods only from the Smithsonian property. There, they declined by almost 90% from the study plots from 2008 to 2019. The Park’s list of tree and shrub species reports that flowering dogwood is still “abundant”; my visit was too late in the season to observe how visible flowering dogwoods still are. Certainly the species survives the disease better in open settings, e.g., meadows and roadsides. I don’t know how the three other native Cornus species were affected.

Dead ash are still visible. Ash trees made up about 5% of the Park’s forest cover. Anderson-Teixeira et al. report that ash aboveground biomass was increasing in SHNP and stable on the SBCI property before arrival of the emerald ash borer (EAB). EAB-caused mortality was first detected in 2016. In just three years — by 2019 – 28% of green, white, and black ash had died; this meant a loss of 30% of ashes’ aboveground biomass. Ninety-five percent of remaining live trees were described as “unhealthy’’. In an effort to retain ash trees for visitor enjoyment, reduce threats to visitors from hazard trees, and to preserve a portion of the park’s ash tree communities until host-specific biological controls become available, SHNP staff – supported by specially trained volunteers and interns, Virginia Department of Forestry and Fairfax County – began treating high-value ash with emamectin benzoate. They began at Loft Mountain Campground, a location (elevation 3,300 feet) where ash trees make up most of the forest. Three hundred forty three trees were treated there — exceeding expectations for what could be accomplished in a single year. The park hopes to treat an additional 200-400 trees. They will target ash trees around campgrounds, picnic areas, overlooks and other areas frequently used by visitors. These efforts were supported by the Shenandoah National Park Trust and here.

I saw many dead oaks – probably the result primarily of repeated attacks by the spongy moth link beginning in 1982. Oak-dominated study plots in SHNP lost on average 25% of individuals and 15% of above-ground biomass. After 1995, when spraying of Bacillus thuringiensis var. curstaki improved control efforts (at the expense of native moths), oak aboveground biomass increased gradually, driven by individual tree growth rather than recruitment. Oak abundance continues to decline due to oak decline and absence of management actions to promote regeneration (Anderson-Teixeira et al.).  These authors do not mention oak wilt although a decade-old map shows the disease to be present just to the west of the Blue Ridge (visible here).

Fortunately Shenandoah National Park has relatively few American beech, so it will be less affected by beech leaf disease (BLD). The Blue Ridge is also far from large waterbodies — which promote the disease. However, I did see some beech sprouting in creek valleys – probably in gaps opened when the hemlocks died. These valleys with higher humidity are the type of ecosystem most conducive to the disease! Anderson-Teixeira et al. note that they did not analyze the impact of beech bark disease – which was the disease of concern before arrival of BLD and continues to be present.

They also did not evaluate the impacts of balsam woolly adelgid, described as having decimated high-elevation populations of firs (Abies balsamea); white pine blister rust on eastern white pine; or EAB on fringetree (Chionanthus virginicus) in SCBI. Nor did they document the impact of thousand cankers disease (TCD) on walnuts or butternuts. This concerns me because they report that the disease “appears to be affecting Juglans spp. in our plots.” Furthermore, butternut (J. cinera) had been ‘‘common’’ in 1939, but had disappeared from SHNP by 1987. On the Smithsonian property, the four individuals found originally had declined by half – to two living individuals. Butternut has suffered high levels of mortality throughout its range from butternut canker.

The understory tree redbud (Cercis canadensis) also declined precipitously – by almost76% from 1995 to 2018 in SCBI plots. While Anderson-Teixeira et al. do not speculate why, a few years ago a wider decline was reported.

Of course, Shenandoah also has been invaded by non-native plants! So I saw some plants that should not be there. At least the mid- and high-elevations that I visited appear to be much less abundant in the Park than in coastal and piedmont regions of Virgina. Ailanthus is listed as “common” in the Park. I didn’t see Japanese stiltgrass but it is clearly present at lower elevations. I was particularly disturbed to see oriental bittersweet along trails located in all three sections of the Park.

The Blue Ridge PRISM is targeting 12 species: autumn olive, garlic mustard, Japanese honeysuckle, Japanese stiltgrass, kudzu, mile-a-minute, multiflora rose, oriental bittersweet, porcelainberry, privet, tree of heaven, and wavyleaf grass. 

SOURCES

Anderson-Teixeira, K.J., V. Herrmann, W.B. Cass, A.B. Williams, S.J. Paull, E.B. Gonzalez-Akre, R. Helcoski, A.J. Tepley, N.A. Bourg, C.T. Cosma, A.E. Ferson, C. Kittle, V. Meakem, I.R. McGregor, M. N. Prestipino, M.K. Scott, A.R. Terrell, A. Alonso, F. Dallmeier, & W.J. McShea. 2021. Long-Term Impacts of Invasive Insects & Pathogens on Composition, Biomass, & Diversity of Forests in Virginia’s Blue Ridge Mountains. Ecosystems

Fei, S., R.S. Morin, C.M. Oswalt, & A.M. Liebhold. 2019. Biomass losses resulting from insect & disease invasions in United States forests. Proceedings of the National academy of Sciences.

Potter, K.M., M.E. Escanferla, R.M. Jetton, G. Man, & B.S. Crane. 2019. Prioritizing the conservation needs of United States tree spp: Evaluating vulnerability to forest insect & disease threats. Global Ecology & Conservation.

Posted by Faith Campbell

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

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

Or

https://fadingforests.org

Invasive species cost more than extreme weather attributable to climate change; 17 times more than previously estimated!

ash tree killed by emerald ash borer; photo courtesy of (then) Mayor of Ann Arbor John Hieftje

Since the 1990s, scientists have been trying to the determine costs imposed by invasive species. They hope that measuring monetary costs will motivate political decision-makers to take more assertive actions to counter this ecological treat. As Daigne et al. (2021) point out, too few countries are implementing effective control and mitigation strategies. They say this inaction stems, largely, from undervaluing bioinvasions’ impacts by the general public, stakeholders and decision-makers.

A major step in this effort was creation of the InvaCost database. The goal was to provide a reliable, comprehensive, standardized and easily updatable synthesis of bioinvasions’ monetary costs worldwide.

Several publications based on this database appeared. I have blogged about studies published in 2021 or 2022: a) the costs of bioinvasions generally (Cuthbert et al. 2022);  b) the costs imposed by invasive species in protected areas (Moodley et al. 2022; c) a focus on the “worst” 100 invasives (as determined by the IUCN) (Ahmed et al. 2022); and d) assessing costs associated with various pathways of introduction (Turbelin et al. 2022).

The InvaCost database, as applied in these studies, demonstrated that bioinvasions impose tremendous costs –a minimum of US $1.288 trillion for the period 1970 – 2017. These costs increased on average three times per decade (Daigne et al. 2022).

Still, everyone has recognized that InvaCost data have significant limitations. First, three-quarters of the records in the original database came from North America, Oceania and Europe; and referred to animal taxa, even though plants are a major group of invaders. Also, a large proportion of total invasion costs – for all taxa – probably is undetected. Finally, the many non-market values of species and ecosystems are extremely difficult to calculate (Daigne et al. 2022).

As a result of these deficiencies, the earlier studies discussed in the blogs referenced above substantially underestimated the true costs associated with bioinvasion (Cuthbert et al. 2022).

Now a new study, led by Ismael Soto, finds that the underestimate is huge. Global costs associated with a subset of 162 species (17% of all the species in the InvaCost database) is nearly 17 times higher than reported in the InvaCost database.

Soto et al. (2025) applied species distribution models and macroeconomic data to interpolate these 162 species’ probable impacts in 172 countries  

Japanese knotweed – one of the invasive plants proving very costly in Europe, according to I. Soto

The newly identified costs were greatest in Europe; second place fell to North America. This is because both higher damage costs and management expenditures are linked to higher gross domestic product and extent of agricultural area, in addition to environmental suitability. Analysis of monetary costs per unit area revealed that ‘cost hot spots’ are predominantly located in densely populated urban areas and locations hosting key industries. These tend to be in coastal zones, i.e., Europe, the east coast of China, and the east and west coasts of the US.

cypress aphid Cinara cupressi – a threat to both native & plantation trees in Africa; photo by Blackman & Eastop via Wikimedia

The authors found that the greatest increase in estimated costs for countries in Africa and Asia. These countries had not previously recorded any economic costs arising from invasions by these 162 species. I have blogged about forest pest threats in Africa.

The authors also significantly increased estimated costs linked to invasive plants. Daigne et al. found that invasive insects caused ~90% of reported costs in the InvaCost database as of 2022. Vertebrates ranked second, plants third. In contrast, Soto et al. determined that invasive plants had the highest average estimated damage costs (US $42.10 billion) and management expenditures ($0.81 billion).

Substantial total costs were also reported for arthropods, mammals and birds. Reported damage and management costs were much lower for molluscs, fish, reptiles and amphibians. Daigne et al. suggest this might be due to their lower (observable) damage to human infrastructure, research biases leading to fewer studies, or disparities resulting from the filtering process used in their own study.

Williams et al. (2023) focus on insects, which cause damage primarily to agriculture, human health, and forestry. Insects constitute the highest number of species introduced as ‘Contaminants’ (n = 74) and ‘Stowaways’ (n = 43). They also impose the highest costs among species using these two pathways.

Forest insects and pathogens account for less than 1% of the records in the InvaCost database. I believe that this figure reflects significant under-reporting of these invasion events. Even at this paltry level of reported invasions, forest insects and pathogens were responsible for causing 25% of total annual costs ($43.4 billion) (Williams et al. 2023). This discrepancy illustrates the huge economic cost associated with widespread mortality of trees. Yet authorities in most countries continue to provide completely inadequate resources to counter this threat.

The authors of these publications examining economic losses associated with bioinvasion all note that ecological damage is additional. Soto et al. note that bioinvasions contribute to 60% of already recorded global extinctions. Interestingly, the species ranked third using the criterion of monetary damage is the cactus moth Cactoblastis cactorum. This insect threatens flat-padded Opuntia cacti across the United States and in the center of endemism, Mexico.

a flat-padded Opuntia — vulnerable to the cactus moth; photo by F.T. Campbell

Soto et al. found a lag of ~46 years between first (reported) detection of an introduced species and the peak of damage costs. They suggest that the rising monetary cost reflects the species becoming more abundant or occupying a larger area. The authors also say this finding demonstrates the value of implementing mitigation measures as soon as possible. Their finding thus validates others’ advocacy for investing in prevention and rapid response measures (see Cuthbert et al. and Daigne et al.). Soto et al. were cheered by the fact that spending on management measures – when it was reported – often followed soon after a species’ detection – or even before (e.g., prevention).

But Decision-Makers Usually Delay – Why?

Prevention is a hard sell. Decision-makers find it difficult to justify management expenditures before impacts become obvious. By that time, of course, management of the invasion is extremely difficult and expensive – if it is possible at all. Ahmed et al. found particularly effective wording to describe this problem: bioinvasion costs can be deceitfully slow to accrue, so policy makers don’t appreciate the urgency of taking action. Another contributing factor is that when efficient proactive management succeeds in preventing any impact, it paradoxically undermines evidence of the value of this action!

Programs to minimize the economic and ecological consequences of bioinvasion are severely obstructed – if not doomed! – by the following difficulties:

  • Resources are in short supply. Experts find that demands to address other threats to agriculture or natural systems outcompete appeals to ramp up invasive species efforts.
  • Prediction is uncertain. Cuthbert et al. found that none of the species with the highest pre-invasion investment was among the top 10 costliest invaders in terms of damages. Cuthbert et al. do not discuss whether this is evidence that the prevention efforts were effective? Or, alternatively, that prevention efforts target the wrong species.
  • Harm is in the eye of the beholder. Stakeholders’ perceptions of whether an introduced species causes a detrimental impact vary. For example, Moodley et al. point out that species imposing the highest economic costs might not be the ones causing the greatest ecological harm.
  • Externalities. Those harmed by a bioinvasion often are different from those that decide whether to act. Ahmed et al. argue that this creates a moral dilemma.

These decisions are political — influenced by citizens’ expressed wishes. Changing decision-makers’ perceptions of what is important is up to us!!! Start a parade!!!

SOURCES

Ahmed, D.A., E.J. Hudgins, R.N. Cuthbert, .M. Kourantidou, C. Diagne, P.J. Haubrock, B. Leung, C. Liu, B. Leroy, S. Petrovskii, A. Beidas, F. Courchamp. 2022. Managing biological invasions: the cost of inaction. Biol Invasions (2022) 24:1927–1946 https://doi.org/10.1007/s10530-022-02755-0

Cuthbert, R.N., C. Diagne, E.J. Hudgins, A. Turbelin, D.A. Ahmed, C. Albert, T.W. Bodey, E. Briski, F. Essl, P. J. Haubrock, R.E. Gozlan, N. Kirichenko, M. Kourantidou, A.M. Kramer, F. Courchamp. 2022. Bioinvasion costs reveal insufficient proactive management worldwide. Science of The Total Environment Volume 819, 1 May 2022, 153404

Diagne, C., B Leroy, A-C. Vaissière, R.E. Gozlan,  D. Roiz, I. Jaric,  J-M. Salles, C.A. Bradshaw, and F. Courchamp. 2021. High and rising econ costs of bioinvasions worldwide Published online: 31 March 2021

Moodley, D., E. Angulo, R.N. Cuthbert, B. Leung, A. Turbelin, A. Novoa, M. Kourantidou, G. Heringer, P.J. Haubrock, D. Renault, M. Robuchon, J. Fantle-Lepczyk, F. Courchamp, C. Diagne. 2022. Surprisingly high economic costs of bioinvasions in protected areas. Biol Invasions. https://doi.org/10.1007/s10530-022-02732-7

Soto, I., P. Courtois, A. Pili, E. Tordoni, E. Manfrini, E. Angulo, C. Bellard, E. Briski, M. Buric, R.N. Cuthbert, A. Kouba, M. Kourantidou, R.L. Macêdo, B. Leroy, P.J. Haubrock, F. Courchamp and B. Leung. 2025.  Using species ranges and macroeconomic data to fill gap in costs of biological invasions. Nat Ecol Evol doi: 10.1038/s41559-025-02697-5

Turbelin, A.J., C. Diagne, E.J. Hudgins, D. Moodley, M. Kourantidou, A. Novoa, P.J. Haubrock, C. Bernery, R.E. Gozlan, R.A. Francis, F. Courchamp. 2022. Introduction pathways of economically costly invasive alien spp. Biol Invasions (2022) 24:2061–2079 https://doi.org/10.1007/s10530-022-02796-5

Williams, G.M., M.D. Ginzel, Z. Ma, D.C. Adams, F.T. Campbell, G.M. Lovett, M. Belén Pildain, K.F. Raffa, K.J.K. Gandhi, A. Santini, R.A. Sniezko, M.J. Wingfield, and P. Bonello. 2023. The Global Forest Health Crisis: A Public Good Social Dilemma in Need of International Collective Action. Annual Review of Phytopathology Vol. 61, 2023

Posted by Faith Campbell

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

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

or

www.fadingforests.org

What will replace hemlocks? Intractions with other plants & introduced pathogens complicate the situation

eastern hemlocks in Cook Forest State Forest Pennsylvania; photo by F.T. Campbell

As Eastern hemlock (Tsuga canadensis) suffers high levels of mortality across nearly all its range due to hemlock woolly adelgid (HWA; Adelges tsugae),  scientists scramble to determine what the successor forests will look like. The transformation will be stark: from deeply shaded evergreen coniferous forest with a sparse understory to something very different. As this process takes place, most scientists expect cascading effects on not only terrestrial and aquatic wildlife but also onecosystem functions, including soils and nutrient and hydrologic cycles (Dharmadi et al. 2019 Plotkin et al. 2024).

New England

In southern New England, hemlock groves are being replaced by stands of deciduous hardwood forests dominated by black birch (Betula lenta). While birch are expected to continue to dominate, other species comprise at least one third of seedlings in the Harvard Forest experimental sites, primarily eastern white pine (Pinus strobus) and red maple (Acer rubrum). Plotkin et al. (2024) note that conversion of hemlock forests to pine forests would be a less dramatic ecosystem shift since both are evergreen conifers.

symptoms of beech leaf disease; photo by the Ohio State University

In both southern New England and farther north, in Vermont and New Hampshire, maples and American beech have increased in prominence. In the latter case, this is despite the prevalence of beech bark disease and managers’ efforts to suppress beech. I have noted that beech leaf disease now threatens to disrupt this process.

Landowners in the region often seek to get some financial return from their forests before a pest kills the trees. About a quarter of the almost 9,000 ha of hemlock stands in the southern Connecticut River Valley have been harvested as HWA spread into the area. To test the effect of pre-mortality logging of hemlock stands, Plotkin et al. tried to mimic HWA-caused mortality by girdling all the hemlocks in some plots in Harvard Forest. In other plots they harvested most hemlocks and some of the other tree species. The girdled plots had a dramatic increase in standing and downed deadwood and a longer period of elevated understory light levels than the logged plots. They note that standing snags and on-ground dead wood provide critical ecosystem functions. Many wildlife and microbial species depend on dead wood for nutrition and a variety of micro habitats. Plotkin et al. found that the slowly decomposing dead wood also stored a large amount of carbon: girdled plots stored 18% more above-ground carbon than logged sites, even after accounting for carbon stored in harvested wood products.

a beech snag with nesting cavities; photo by F.T. Campbell

The magnitude of these differences might be even larger than demonstrated in this experiment. In New England, hemlocks infested with HWA die over a decade, not the two years seen after girdling. The delayed mortality provides a longer window of opportunity for succeeding vegetation to adapt and preserve higher levels of biodiversity. Plotkin et al. (2024) suggest that logging pest-threatened hemlock forests might remove structural resources that would support forest response to ongoing climate stress and future disturbances.

Considering the disturbed plots’ invasibility by non-native plants, Plotkin et al. (2024) found that more non-native shrubs invaded the girdled plots than the logged plots. They suggest that birds that disperse the shrubs’ fleshy fruits were attracted by perch sites provided by the standing dead trees.

Southern Appalachians

In the Southern Appalachians, post-HWA forests will apparently be quite different. At the USDA Forest Service’ Coweeta Hydrologic Laboratory in the Nantahala Mountain Range of western North Carolina, eastern hemlock died much faster than in New England. Hemlocks comprised more than 40% of the basal area before arrival of HWA (detected in 2003). Within two years all hemlock trees were infested. Half were dead by 2010, 97% by 2014 (Dharmadi et al. 2919).

In some part of the southern Appalachian forests the shrub layer is dominated by Rhododendron maximum (rosebay rhododendron). This dense shrub layer is preventing recruitment of deciduous tree species that had been expected to replace the dead hemlocks. Tree seedlings died rather than grew into saplings. Scientists working in the Coweeta experimental forest attribute the seedlings’ demise to limited access to key resources, e.g., water, nutrients (especially inorganic nitrogen), and light (Dharmadi, Elliott and Miniat 2019).

In the Coweeta Basin, hemlock loss is the most recent of a series of severe disturbances that have apparently led to a cascade of responses in the overstory, midstory, and soil that have promoted expansion of rhododendron. (The earlier disturbances were widespread logging in the 19th Century and the loss of American chestnut to chestnut blight in the first part of the 20th Century. Therefore, the response of future forests to changes in temperature and rainfall might now depend on these novel tree-shrub interactions .

R. maximum hampers succession by forming a dense subcanopy layer that greatly limits light reaching the forest floor and reduces soil moisture and temperature. These changes impede seed germination and seedling survival. In addition, rhododendron leaves that fall to the ground create a thick organic soil layer that decomposes very slowly. This affects soil chemistry, specifically availability of the key nutrient nitrogen.

The rhododendron shrubs in the region are younger than the deciduous trees now making up the canopy above them (Dharmadi, Elliott and Miniat 2019). The dense rhododendron stands resulted from the widespread mortality of American chestnut (Castanea dentata) in the early 20th century and of hemlock in the first years of the 21st Century. What’s more, even the mature deciduous trees appear to be suppressed by dense rhododendron stands. Canopy trees above rhododendrons are on average 6m shorter than those growing on sites without rhododendron thickets (Dharmadi, Elliott and Miniat 2019). In fact, by 2014, 10% of standing trees other than hemlocks had died. The tree suffering the highest level of mortality was flowering dogwood (Cornus florida). The authors do not mention a probable factor, the introduced disease dogwood anthracnose. Other species experiencing high levels of mortality are not, to my knowledge, under attack by non-native pests, so their demise seems more clearly linked to resource competition with rhododendron. These were striped maple (Acer pennsylvanicum), pitch pine (Pinus rigida), witch hazel (Hamamelis virginiana), and that staple of New England aftermath forests, black birch (Betula lenta).

Dharmadi, Elliott and Miniat (2019) suggested that managers should step in to increase recruitment in both understory and overstory layers. They proposed active management: removing rhododendrons and the soil organic layer. USFS scientists are applying these ideas experimentally at the Coweeta research station. I am unclear as to whether there is one study or more. In any case, rhododendronplants have been removed with the goal of restoring vegetation structure and composition – presumably both understory plant diversity and recruitment of tree species capable of growing into the canopy. In at least some cases, the rhododendron removal is followed by prescribed fire. One study is looking also at whether this action increased water yield.

Apparently this lack of tree regeneration is extensive – although published data are not easily accessible. Staff of the North Carolina Hemlock Restoration Initiative report they encounter similar issues (O.W. Hall, Hemlock Restoration Initiative, pers. comm.)

Several experiments have demonstrated that even in the southern Appalachians, where there are abundant moisture and rainfall, the trees and shrubs compete for water and other nutrients. However, Dharmadi et al. (2022) found that removal of the rhododendron shrub layer is unlikely to significantly alter streamflow, atr least during the growing season. In winter, when deciduous trees lack leaves, reduction in interception of precipitation might result in increased streamflow (Dharmadi et al. 2022). I ask whether increasing stream flow in winter is a goal? I thought the concern was stream flow levels in summer.

Nor is removal of the rhododendron shrub layer likely to alter stream chemistry during the growing season.

Removal of living Rhododendron and leaf litter apparently can help restore forest structure through improving tree seedling survival and recruitment as well as increasing growth of established trees.

Removing Privet

However, other management actions might bring about desired changes more effectively or broadly. Specifically Dharmadi and colleagues mentioned removal of privet (Ligustrum) – a very widespread invasive shrub in forests of the Southeast. (Fifteen years ago it was estimated that just one privet species, Chinese privet, occupied more than a million hectares in 12 southeastern states [Hanula 2009].)

Chinese privet

I ask also whether prescribed fire to remove the rhododendron-dominated soil organic layer is useful. Dharmadi and colleagues found that such fires reduced leaf litter temporarily, but annual leaf-fall replaced the litter layer the next year, so this management effort is unlikely to affect plot evapotranspiration rates.

Supporting Pollinators

Another study (Ulyshenet al. 2022) examined whether removing rosebay rhododendron would benefit bees and other pollinators. They found that removal of Rhododendron alone (without fire) did not dramatically improve pollinator habitat in the southern Appalachians. In fact, about a quarter of the bee species studied visited R. maximum flowers and might decline if the shrub’s population is reduced. Ulyshen and colleagues suggest that some factors that correlate with fire severity probably promotes growth of insect-pollinated plants. They suggest specifically the greater presence of downed woody debris, which provides nesting sites and other resources used by insects. They recommended creation of open areas to support wildflowers as a more effective way to benefit bees in this region. Again, rhododendron removal pales in effectiveness compared to eradication of privet.

SOURCES

Dharmadi, S.N., K.J. Elliott, C.F. Miniat. 2019. Lack of forest tree seedling recruitment and enhanced tree and shrub growth characterizes post-Tsuga canadensis mortality forests in the southern Appalachians. Forest Ecology and Management 440 (2019) 122–130.

Dharmadi, S.N., K.J. Elliott, C.F. Miniat. 2022.  Larger hardwood trees benefit from removing Rhododendron maximum following Tsuga canadensis mortality. Forest Ecology and Management

Hanula, J.L., S. Horn, and J.W. Taylor. 2009. Chinese Privet (Ligustrum sinense) Removal and its Effect on Native Plant Communities of Riparian Forests. Invasive Plant Science and Management 2009 2:292–300.

Plotkin, A.B., A.M. Ellison, D.A. Orwig, M.G. MacLean. 2024. Logging response alters trajectories of reorganization after loss of a foundation tree species. Ecological Applications. 2024;e2957.

Ulyshen, M., K. Elliott, J. Scott, S. Horn, P. Clinton, N. Liu, C.F. Miniat, P. Caldwell, C. Oishi,  J.  Knoepp, P. Bolstad. 2022. Effects of Rhododendron removal and prescribed fire on bees and plants in the southern Appalachians. Ecology and Evolution. 2022;12:e8677.

Posted by Faith Campbell

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

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

or

www.fadingforests.org

APHIS funding for pests that kill trees (& cacti)

emerald ash borer; some of PPA grants are funding evaluation of biocontrol efficacy

USDA APHIS has released information about its most recent annual allocation of funds under the Plant Pest and Disease Management & Disaster Prevention Program under §7721 of the Plant Protection Act. (Also see Fading Forests II and III; links provided at the end of this blog.) These funds support both critical needs and opportunities to strengthen the nation’s infrastructure for pest detection, surveillance, identification, and threat mitigation. Since 2009, this USDA program has provided nearly $940 million to more than 5,890 projects.

For FY25 APHIS allocated $62.725 million to fund 339 projects, about 58% of the proposals submitted. About $10 million has reserved for responding to pest and plant health emergencies throughout the year.

According to APHIS’ press release, the highest amount of funds (almost $16 million) is allocated to the category “Enhanced Plant Pest/Disease Survey.” Projects on “Enhanced Mitigation Capabilities” received $13.6 million. “Targetting Domestic Inspection Efforts to Vulnerable Points” received nearly $6 million. “Improving Pest Identification and Detection Technology” was funded at $5 million. Outreach & education received $4 million.  I am not sure why these do not total $63 million.

Funding for States and Specific Pests

Wood-boring insects received about $2.3 million. These included more than $869,800 to assess the efficacy of biocontrol for controlling emerald ash borer (EAB) Agrilus planipennis, $687,410 was provided for various detection projects, and $450,000 for outreach efforts related to various pests. Ohio State received $93,000 to optimize traps for the detection of non-native scolytines (bark beetles).

Biocontrol efficacy will also be assessed for hemlock woolly adelgid, invasive shot hole borers, cactus moth, and several invasive plants (including Brazilian pepper). (Contact me to obtain a copy of CISP’s comments on this biocontrol program.)

Opuntia basilaris in Anza Boreggo; one of flat-padded Opuntia vulnerable to the cactus moth; photo by F.T. Campbell

Funding for other pests exceeded $1 million for spotted lanternfly (nearly $1.4 million), Asian defoliators ($1.2 million) and box tree moth (just over $1 million).

$630,000 was provided for detection surveys and studies of the sudden oak death pathogen Phytophthora ramorum, especially how it infects nursery stock. Nursery surveys are funded in Alabama, Louisiana, North Carolina, Ohio, Oklahoma, Pennsylvania, South Carolina, Tennessee, Virginia, and West Virginia. Most of these states are in regions considered most at risk to SOD infection of wildland plants.    

sudden oak mortality of tanoak trees in southern Oregon; photo by Oregon Department of Forestry

Oregon received much-deserved $41,000 to evaluate the threat of the NA2 and EU2 lineages of P. ramorum to nurseries and forests Oregon also received $104,000 to respond to the detection of Phytophthora austrocedri in nurseries in the state. The Oregon outbreak has been traced to Ohio, but I see no record of funds to assist that state in determining how it was introduced.

Asian defoliator (e.g., Lymantrid moths) surveys have been funded for several years. This year’s projects are in Alaska, Arkansas, California, Kentucky, Maryland, Massachusetts, Mississippi, Montana, Nevada, North Carolina, Oregon, Tennessee, Texas, Washington, and West Virginia. While I agree that the introduction risk is not limited to coastal states with maritime ports, I don’t what criteria were applied in choosing the non-coastal states which are funded to search for these insects

Spotted lanternfly surveys (including technological improvements) or related outreach are funded in Alabama, Connecticut, Delaware, Kentucky, New Hampshire, New Jersey, North Carolina, Oregon, Pennsylvania, and Tennessee. California’s project is focused on postharvest treatments.

The Don’t Move Firewood project continues to be funded by APHIS. Several states also direct attention specifically to the firewood pathway: Kentucky, Maine, and Michigan.

I applaud the precautionary funding of the Agriculture Research Service to generate of high-quality genomic resources for managing the causal agent of Japanese oak wilt Dryadomyces quercivorous

Florida Department of Agriculture, North Carolina State University, and West Virginia University each received more than $100,000 to improve detection and management of invasive hornets.

Tennessee State University got $100,000 to continue efforts to detect and understand Vascular Streak Dieback in redbud Cercis canadensis.

Posted by Faith Campbell

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

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

or

www.fadingforests.org