Sakhalin knotweed (Fallopia (Reynoutria) sachalinensis) – an invasive plant widespread in Europe; photo by Katrin Schneider [korina.info] via Wikimedia
There is growing evidence that invasive plants – as distinct from invasive species of animals, microbes, etc. – play a significant role in causing the loss of floristic uniqueness at the local or regional level. I provide full citations of all sources at the end of this blog.
Less Diversity. More Similarity
Several studies show that plant invasions have a bigger impact than extinction in the homogenization of Earth’s flora. A major driver is sheer numbers. Daruet al. point out that 10,138 plant species have become naturalized to a region outside their native ranges while only 1,065 species have gone extinct. Even under a scenario in which all species currently included in IUCN Red List as “threatened” become extinct, non-native plant species naturalizations are by far the stronger contributor to biotic reorganization.
Winter et al. report that in Europe since AD 1500, plant invasions have greatly exceeded extinctions, resulting in increased taxonomic diversity (i.e., species richness) on the Continent but increased taxonomic and phylogenetic similarity among European regions. In other words, floras of individual European countries became phylogenetically and taxonomically impoverished. This situation is likely to worsen in the future because introductions continue.
Winter et al. conclude, more broadly, that a focus on species richness can be misleading because it does not capture the important effects of taxonomic or phylogenetic distinctiveness.
Yanget al. (2021) considered the situation globally. They divided most of Earth’s ice-free land surface into 658 regions. They found that introduction of non-native plants has increased the taxonomic similarity between any two of these regions in 90.7% of the time. Introductions increased phylogenetic similarity in 77.2% of those pairs. Australasia illustrates the situation. The region has a large proportion of endemic species, reflecting its unique evolutionary history and exhibiting high floristic diversity. However, the region has also received large numbers of non-native plants from other regions. The result is that the Australasian flora has lost much of its original uniqueness.
rubbervine (Cryptostegia madagascariensis) – one of the worst invasive plants in Australia; photo by Tatters via Flickr
Introduced plant species rarely cause outright extinction of members of the native flora of the receiving ecosystem – at least at the scale of a continent. Winter et al. found that in Europe, extinction usually occurs to plant species with small numbers that are limited to localized habitats. Often, however, the same species continue to thrive elsewhere on the continent. The “losing” country finds its flora becoming more similar to that of other European countries. It loses some uniqueness because it lost one or more components of its flora. However, for Europe as a whole, there is no loss. The homogenization of the “losing” country’s flora is exacerbated by the fact that more than half of plant species listed as invading a particular European country are from other European regions. Winter et al. say a similar pattern has been found in North America.
The picture is more complex for small isolated ecosystems. Carvallo and Castro (2017), writing about isolated volcanic islands in the southeastern Pacific Ocean, introduction of large numbers of non-native plant species has not caused extinction of native plant species. It has, however, resulted in the homogenization of the islands’ floras.
These authors worry that this reduction in phylogenetic diversity could have detrimental impacts for ecosystem function and ecosystem services. (Interestingly, at the level of order or family rather than species or genus, the combined effects of species introductions and extinctions did not change the islands’ taxonomic diversity. They don’t explicitly say whether that fact might mitigate effects on ecosystem function.)
What is the situation in Hawai`i? The Islands are the “capital” of both extinction and invasion. I know the Hawaiian flora has very high levels of endemism and of endangerment. In addition, naturalized non-native plant taxa constitute up to 54% of the archipelago’s flora (Potter et al. 2023). However, it is probably extremely difficult to distinguish the impacts of introduced plants separate from the impacts of the many non-native animals, e.g., feral hogs.
Extinction by Introduction
It has been reported that invasive species have caused the extinction of at least seven species of plants on the Cape of Good Hope and endangered another 14% (Houreld 2024). Unfortunately, the report doesn’t specify whether the non-native species are plants or animals. Either way, this is a tragedy. I remind you that the Cape Floral Kingdom is Earth’s smallest Plant Kingdom in geographic size (78,555 km2), and extremely important in uniqueness. According to the article in The Washington Post, two-thirds of the 20,400 plant species growing in South Africa are endemic – found nowhere else on Earth.
Nearly a decade ago, Downey and Richardson objected to measuring the impact of introduced plant species by considering only total extinction of native plant species. They complain that this approach fails to recognize that plants experience a long decline before reaching extinction. They divide this decline into six “thresholds”. Downey and Richardson say there is abundant evidence of invasive plants driving native plants along this extinction trajectory. For example, increases in non-native plant cover or density that result in decreased native plant species diversity or richness equates, under their hierarchy, to the native species crossing from the first to second threshold. They note there are also examples of species causing “extinction debts”. That is, the extinction occurs long after the invader is introduced and initiates a native species’ decline. They call for conservationists to intervene earlier in that trajectory.
The Global Assessment on Biodiversity and Ecosystem Services was recently published by the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services. This report notes that there are at least 1,061 invasive plants on Earth. In terrestrial systems, invasive plants are the taxonomic group most frequently reported as having negative impacts, especially in cultivated areas, plus temperate and boreal forests. As I have noted above, non-native plant taxa constitute a particularly high proportion of the flora on islands. The assessment found that the number of non-native plants exceeds the total number of native plants on more than one quarter of the Earth’s islands. However, this report does not distinguish the number of species endangered by plant invasions from the number of species endangered by invasive species of all taxonomic groups.
Tiburon mariposa lily (Calochortus tiburnensis) – a federally Threatened species in California; photo by T.J Gehring via Flickr
None of the experts denies the impact of extinction on biodiversity. Extinction represents the loss of phylogenetically and taxonomically unique organisms. This loss is exacerbated if some taxonomic groups are at disproportionately higher risk of extinction. Introduced non-native species compensate for these losses only to a point (Daru et al.). In Europe, Winter et al. found that extinctions usually befall specialized endemic or rare species, often from species-poor families. On the other hand, successful invaders are often ecological generalists with large ranges, often belonging to species-rich families. This results in the pronounced decrease of phylogenetic and taxonomic ß-diversity within and between regions to which the rare species are unique.
All these experts agree that species declines — short of extinction — have severe impacts on ecosystem functions, and even evolution.
Yang et al. emphasize that the rapid and accelerating loss of regional biotic uniqueness changes biotic interactions and species assemblages, with probable impairment of key ecosystem functions. Daru et al. stress that biotic homogenization— declining ß-diversity—reduces trait and phylogenetic differences between regions. Conceding that the consequences of this global biotic reorganization on ecosystems are poorly understood, Daru et al. cite increasing evidence that biotic heterogeneity provides insurance for the maintenance of ecosystem functioning in a time of rapid global change. They assert that declining ß-diversity is a more characteristic feature of the Anthropocene than species loss.
Winter et al. also state that the phylogenetic structure of a species assemblage represents the evolutionary history of its members and reflects the diversity of genetic and thus morphologic, physiologic, and behavioral characteristics. High phylogenetic diversity might enable rapid adaptation to changing environmental conditions.
According to Daru et al., the loss of 14 billion years of evolutionary history has affected some regions particularly. The most disturbed biotas include those of California and Florida; Mesoamerica; the Amazon; the Himalaya-Hengduan region; Southeast Asia; and Southwest Australia. These are regions that experienced spectacular taxonomic radiation over time, and now have both high levels of threat and also species invasion.
Carvallo and Castro, focused on the Pacific islands, call for integrating the two parallel channels of conservation that currently operate separately: those focused on reversing plant extinctions and those focused on reducing invasions. They call for a biogeographical approach that addresses all causes of phylogenetic homogenization.
Tetragonia tetragonoides – the most widespread invasive plant on these Pacific islands; photo by Jake Osborn via Flickr
I believe all these experts, in all their papers, have made the case for such integration world-wide.
Invasive plants’ impact on non-plant species
While I have focused here – and in most of my blogs more broadly — on impacts on wild, native plant communities, it is clear that alterations to floristic communities influence other taxonomic groups. A couple of years ago I summarized findings by Douglas Tallamy and colleagues on what happens to insects – and their predators – when a landscape is dominated by introduced plant species.
In short, domination by non-native plants – whether invasive or just widely planted – suppresses the numbers and diversity of native lepidopteran caterpillars. One study cited in the blog found that 75% of all lepidopteran species were found exclusively on native plant species. Non-native plants in the same genus as native plants often support a similar but depauperate subset of the native lepidopteran community. Tallamy and colleagues conclude that a reduction in the abundance and diversity of insect herbivores will probably cause a concomitant reduction in the insect predators and parasitoids of those herbivores – although few studies have measured this impact beyond spiders, which are generalists. Tallamy focuses on birds.
In the same blog I reviewed publications by Lalk and colleagues, which examined interactions between invasive woody plants and arthropod communities more broadly. They decried the insufficient data about most of these interactions.
A few weeks ago I saw a report of an unexpected impact of invasive plants: roots of beach naupaka [beach cabbage or sea lettuce] (Scaevola sericea) are penetrating sea turtle nests so aggressively that they kill the unhatched turtles. Apparently this is happening at several sites in the Caribbean, where the plant is not native (Houreld 2024). I could find no scientific reports of this phenomenon. One reference noted that a related species (S. taccada) can form large, dense stands that might prevent adult sea turtles’ access to nesting areas (Swensen et al. 2024).
Sources:
Daru, B.H., T.J. Davies, C.G. Willis, E.K. Meineke, A. Ronk, M. Zobel, M. Pärtel, A. Antonelli, and C.C. Davis. 2021. Widespread homogenization of plant communities in the Anthropocene. NATURE COMMUNICATIONS (2021) 12:6983. https://doi.org/10.1038/s41467-021-27186-8
www.nature.com/naturecommunications
Downey, P.O. and D.M. Richardson. 2016. Alien plant invasions and native plant extinctions: a six-threshold framework. AoB Plants, 2016; 8: plw047 DOI: 10.1093/aobpla/plw047; open access, available at http://aobpla.oxfordjournals.org/
Houreld, K. 2024. “Parched Cape Town copes with climate change by cutting down trade.”. The Washington Post. February 29, 2024.
Potter, K.M., C.Giardina, R.F. Hughes, S. Cordell, O. Kuegler, A. Koch, and E. Yuen. 2023. How invaded are Hawaiian forests? Non-native understory tree dominance signals potential canopy replacement. Landsc Ecol https://doi.org/10.1007/s10980-023-01662-6
Yang, Q., P. Weigelt, T.S. Fristoe, Z. Zhang, H. Kreft, A. Stein, H. Seebens, W. Dawson, F. Essl, C. König, B. Lenzner, J. Pergl, R. Pouteau, P. Pyšek, M. Winter, A.L. Ebel, N. Fuentes, E.L.H. Giehl, J. Kartesz, P. Krestov, T. Kukk, M. Nishino, A. Kupriyanov, J.L. Villaseñor, J.J. Wieringa, A. Zeddam, E. Zykova. and M. van Kleunen. 2021. The global loss of floristic uniqueness. NATURE COMMUNICATIONS (2021) 12:7290.
Winter, M., O. Schweiger, S. Klotz, W. Nentwig, P. Andriopoulos, M. Arianoutsou, C. Basnou, P. Delipetrou, V. Didz.iulis, M. Hejdah, P.E. Hulme, P.W. Lambdon, J. Pergl, P. Pys.ek, D.B. Roy, and I. Kuhn. 2009. Plant extinctions and intros lead to phylogenetic and taxonomic homogenization of the European flora PNAS Vol 106 # 51 December 2009
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
white flowered trillium (Trillium grandiflorum); via PICRYL One of the “charismatic wildflowers” mentioned by Blossey and colleagues
Bernd Blossey, Darragh Hare, and Don Waller have published a plea that America’s federal government take the lead in formulating a new national program on managing deer. Otherwise, they fear that deer populations will not be reduced to ecologically sustainable levels. I find their argument convincing and well-sourced. I agree that Americans need to figure out how to address this threat. (The full citation is at the end of this blog).
First, Blossey and colleagues describe the damage caused by overabundant deer:
severe declines in populations of many native forest herbs and shrubs, probably including disappearing wildflowers;
their replacement by non-native species that are less palatable;
poor regeneration of many canopy hardwood species;
decreased forest resilience, lowering forests’ ability to adapt to stressors, especially climate change;
decreased ability of forests to deliver benefits that are of increasing value to many people;
increased prevalence of wildlife and human diseases associated with the spread and size of growing tick populations; and
people – and deer — killed vehicle accidents on roads.
The widespread impacts of white-tailed deer (Odocoileus virginianus) in forests of the East are well-documented (see my previous blogs for a few examples; scroll below the “Archives” to find “Categories”). Blossey and colleagues note examples of similar impacts in the West, attributed to elk (Cervus elaphus) and black-tailed and mule deer (Odocoileus hemionus).
The authors review the decimation of deer populations in earlier centuries and the efforts of state wildlife agencies to rebuild their populations during the 20th Century. The problem, in their view, is that federal and — especially — state wildlife agencies have retained their traditional focus on managing wildlife for recreational hunters. However, recreational hunters make up a small and shrinking proportion of all Americans. Many more people now engage in “non-consumptive” enjoyment of wildlife.
lack of regeneration in Rock Creek Park, Washington D.C.; photo by Sam Sheline, NatureServe, via Flickr
State agencies’ narrow focus might partly arise from fragmented authorities. Agencies other than wildlife departments are responsible for addressing some repercussions of overabundant deer. These include threats to human health, loss of agricultural crops.
For several reasons, Blossey and colleagues call for federal leadership. They think that only a national strategy can address, in a holistic way, the interrelated deer, human health, forest, and biodiversity crises. The strategy’s goal should be to protect species that are in decline because of over-browsing by deer and to avoid further declines in environmental and human health.
The authors reason that states are tied to traditional constituencies. Also, they have difficulty acting across jurisdictional boundaries. Second, few state wildlife agencies have authority to protect plant and invertebrate species. Yet these are the taxa most directly affected by overabundant deer. Blossey and colleagues point out that, of the ~1,300 species listed under the federal Endangered Species Act, 942 are plants and 287 are invertebrates.
They point out that deer also suffer the effects of overpopulation. Chronic wasting disease is spreading. It causes a slow, agonizing death of affected animals. Another 2.1 million deer are killed each year in vehicle crashes. [According to the World Animal Foundation, the current number of deer killed in traffic crashes is 1.8 million — 300,000 fewer.) Again, these deaths are often gruesome. Finally, the principal population “control” now is death by starvation in winter. This, too, is cruel.
Blossey and colleagues say that return of large predators, even where feasible, will not result in sufficient reduction in deer populations. Nor will encouragement of greater hunting pressure on does.
They note that the federal government owns nearly 30% of the United States’ terrestrial surface area. Management is divided among many agencies – National Park Service, Fish and Wildlife Service, Bureau of Land Management, USDA Forest Service, Department of Defense, and many smaller agencies. Management approaches vary. However, it would be possible to bring them into agreement – although, in some cases, this would require new legislation.
Another issue requires resolution: federal agencies’ authority to manage wildlife on federal land.. The states have repeatedly claimed constitutional and legal authority to manage (vertebrate) wildlife on the federal lands within their borders. This assertion was countered years ago by Nie et al. (2017):
‘Federal land management agencies have an obligation, not just the discretion, to manage and conserve fish and wildlife on federal lands. … [M]ost states have not addressed the conservation obligations inherent in trust management; rather, states wish to use the notion of sovereign ownership as … a source of unilateral power but not of public responsibility. Furthermore, the states’ trust responsibilities for wildlife are subordinate to the federal government’s statutory and trust obligations over federal lands and their integral resources.’
Blossey and colleagues assert that managing wildlife (typically defined as mammals, birds, and fish) is much broader than establishing hunting seasons or methods. Furthermore, the concept of “public trust resources” means resources should be managed for all citizens, not just the fewer than 10% of US residents who hunt. A growing proportion of society expects this management to support healthy and diverse environments.
The authors stress that reducing deer overpopulations is necessary to meet numerous policy goals. These include fulfilling obligations under international treaties related to climate change, invasive species, and threatened species; restoring and conserving the nation’s forests to provide habitat; and adopting “nature-based” climate adaptations, such as carbon sequestration. They express the hope that recent presidential mandates to better quantify and value natural assets will increase awareness of the harm caused by deer overpopulation. Their proposed national strategy would develop goals and metrics to match specific environmental and human health outcomes.
Of course, management of deer must extend beyond federal property lines. This will require cooperation among states, Tribes, and private landowners.
The paper proposes the North American Waterfowl Management Plan as a model. Under this scheme the US Fish and Wildlife Service works with states, tribal governments, Mexico, and Canada to ensure accurate information on waterfowl populations a to calculate harvest levels. States implement their assigned quotas through their own regulations. Waterfowl hunters purchase Duck Stamps to fund the monitoring efforts. This program has worked well for most species covered by the program. Waterfowl are one of the few bird groups that have not declined dramatically.
Reducing deer populations will probably require lethal control. Studies indicate that at least 60% of does must be removed from a population to reduce herd sizes over time. Other means have been attempted at regional or larger landscape levels, such as sterilization, fertility control. These methods have failed even when paired with recreational hunting. Lethal approaches will probably distress many people. However, Blossey, Hare, and Waller believe the program would be supported if it is understood to be undertaken with the goal of improving the health of both humans and also the environment.
In the end, Blossey, Hare, and Waller say they are not willing to leave the killing to cars, disease, and starvation. They emphasize our moral responsibility to protect humans and the many other species that rely on diverse ecosystems. Our policies and choices created the problem, so we must try to correct it.
SOURCES
Blossey. B., D. Hare, and D.M. Waller, 2024. Where have all the flowers gone? A call for federal leadership in deer management in the US. Front. Conserv. Sci. 5:1382132. doi: 10.3389/fcosc.2024.1382132
Nie, M., C. Barns, J. Haber, J. Joly, K. Pitt and S. Zellmer. 2017. Fish and Wildlife Management on Federal Lands: Debunking State Supremacy. Environmental Law, Vol. 47, no. 4 (2017).
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
spotted lanternfly – target of many projects funded by USDA’s Plant Pest & Disease Management & Disaster Prevention Program; photo by Holly Raguza of Pennsylvania Department of Agriculture
I am belatedly reporting on the forest-pest projects funded by annual grants under the Plant Pest & Disease Management & Disaster Prevention Program ( of the Plant Protection Act). As usual, APHIS funded projects totaling $62.975 million in FY24. In total, 353 projects were funded. These projects represented 70% of the 504 project proposals submitted (the total funding sought was $106 M).
APHIS reserved $11 million for responding to P&P emergencies. I applaud this choice since the agency’s annual appropriation provides only a completely inadequate $1 million (or less) to cover emergencies.
APHIS notes that since initiation of the Plant Pest & Disease Management & Disaster Prevention Program in 2009, it has funded more than 5,500 projects with a total of nearly $870 million.
In FY24 the program funded 30 more projects than the 322 projects funded in FY23. blog 320 The FY24 allocation provides more than $1 million more for goal area 1S — Enhance Plant Pest/Disease Survey (from $14.4 million to $15.7 million). This was balanced by small decreases for the other goal areas: enhancing mitigation capabilities received $13.6 million; inspections at domestic sites important in invasive species’ spread received $6.3 million; pest identification and detection received $5.3 million; and outreach and education received $4.1 million. Projects safeguarding nursery production and those improving pest and disease analysis each received about $2 million.
By my calculation – subject to error! – about $7.5 million went to projects clearly dealing with forest pests [12% of total funding]. This is a welcome increase from FY23 – when funding of such projects reached about $6.5 million (a little over 10%). blog 320 Funding for tree pest projects might be higher. Some $1.9 million is allocated to surveys of grapevines and orchards — hosts of the spotted lanternfly (SLF). However, it is not clear whether these projects are focused on detecting and managing SLF; they might have a much broader goal. If we do include these projects, the total for tree-killing pests rises to $9.4 million — nearly 15% of the total.
Over both FY23 and FY24, the majority of funds went to similar topics: survey and management of sudden oak death in nurseries; surveys for bark beetles, Asian defoliators, and forest pests generally; and outreach programs targetting the spotted lanternfly. In FY24, just under $100,000 paid for efforts to develop tools for rapid detection of laurel wilt link to DMF in avocados – that is, in a crop rather than the natural environment.
No projects addressing tree or forest pests were funded in seven states or territories: Guam, Idaho, Nebraska, New Mexico, Rhode Island, South Dakota, and Utah. This was three fewer states than in FY23. In neither year do I know whether these states submitted proposals in this category that ended up not being funded.
In FY24, spotted lanternfly is by far the pest addressed by the most projects. As noted above, I can’t be precise about the number because of the lack of information about the 23 projects that fund pest surveys of grapes and/or tree crops that are SLF hosts. Eleven projects named SLF specifically. A final project (not included in above) is one funding registration of Verticillium nonalfalfae as a biocontrol for Ailanthus altissima – an invasive tree that is the preferred host of SLF.
The District of Columbia, Kansas, Missouri, and Oklahoma each had one tree pest project funded. In the cases of Kansas and Missouri, the single project was surveys for thousand cankers disease of walnut. Three other states — Iowa, Maryland, and Pennsylvania — also obtained funding to survey for TCD.
The single Oklahoma project concerned efforts to ensure that the sudden oak death pathogen(Phytopthora ramorum) is not present in nurseries. (An Oklahoma wholesaler was one of the hubs of this pathogen’s spread to 18 states in 2019). Eleven other states were also funded to survey their nurseries for P. ramorum: Alabama, Kentucky, Louisiana, Nevada, North Carolina, Ohio, Pennsylvania, South Carolina, Virginia, and West Virginia. P. ramorum is a “program pest” in 2024. That is, APHIS had designated it as a regulated pest for which the agency wished to fill knowledge gaps about its distribution. I note that last year APHIS published a risk assessment that downplayed the likelihood that P. ramorum would establish in the eastern states. Is APHIS seeking more information to test this conclusion?
In a separate case, Oregon received $76,000 to evaluating the threat to nurseries and forests arising from the presence in the state’s forests of two strains or lineages of P. ramorum that previously had not been extant in the environment of North America.
Another approximately 53 projects fund surveys for tree pests other than spotted lanternfly; these are often fairly general surveys, such as for woodborers or “Asian defoliators”. About ten projects fund management efforts – including evaluation of the efficacy of emerald ash borer biocontrol programs.
Last year I noted that two states – Mississippi and Nevada — had projects to survey the “palm commodity”. Hawai`i joined this group in FY24. The project descriptions don’t specify which pests are the targets. The South American palm weevil (Rhynchophorus palmarum) seems most probable; it is established in far southern California and neighboring Mexico. APHIS prepared a risk assessment on the species in 2012. link? In Hawa`ii, concern probably focuses on the coconut rhinoceros beetle (Oryctes rhinoceros). link? There are other threats to palms, e.g., the red palm weevil (Rhynochophorus ferrugineus), link? and a deadly Fusarium wilt. link?
native palms in the desert at Anza-Borrego, California; photo by F.T. Campbell
California has native palms (Washingtonia filifera); southern states from Texas to at least South Carolina have native palmettos. Of course, many species of palms are important ornamental plants in these states, and dates are raised commercially.
Another “program pest” that I have blogged about in the past is box tree moth. link to blog 287 In FY24 five projects addressed this pest, including surveys and efforts to develop better control tools.
beavertail cactus (Opuntia basilaris) in Anza-Borrego, California; photo by F.T. Campbell
I am pleased by continued funding of projects trying to utilize biocontrol agents to protect two groups of cactus severely threatened by non-native insects: lepidoptera that attack flat-padded prickly pear cacti (Opuntia spp.) link to DMF and the mealybug that attacks columnar cacti of Puerto Rico and the Virgin Islands. link to DMF
vulnerable cactus on St. John, US Virgin Islands; photo by F.T. Campbell
I applaud the decision to fund projects focused on determining the efficacy of biocontrol projects. As noted above, three projects are asking these questions in the case of the emerald ash borer. link to DMF Another project funds production, release, and efficacy evaluation of biocontrol agents targetting Brazilian peppertree in Florida & Texas.
I am also pleased that three projects assist Washington State in its efforts to eradicate the invasion by giant hornets from Asia. link to blogs & Hornet Herald – no detections in 2023 … A company in California also received funding to developing hornet detection tools.
Nineteen projects funded outreach efforts, including continued funding for the “Don’t Move Firewood” program. In addition to those focused on spotted lanternfly, such projects also included other firewood programs, Asian longhorned beetle awareness, and the nursery industry.
I note that while California received funding for 27 projects, none dealt with any of several deadly tree pests extant in the state – goldspotted oak borer, polyphagous and Kuroshio shot hole borers, Mediterranean oak borer, and the palm weevils. Nor did Hawai`i obtain funding to address rapid ohia death. Did no one submit proposals to address any of the many issues impeding management of these killers?
South American palm weevil; photo by Allan Hopkins via Flickr
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
Greater prairie chicken – denizen of the Tallgrass Prairie; NPS photo
In August 2022 I blogged about unwise planting of trees in New Zealand as a warning about rushing to ramp up tree planting as one solution to climate change.
New Zealand has adopted a major afforestation initiative (“One Billion Trees”). This program is ostensibly governed by a policy of “right tree, right place, right purpose”. However, Bellingham et al. (2022) [full citation at end of blog] say the program will probably increase the already extensive area of radiata pine plantations and thus the likelihood of exacerbated invasion. They say the species’ potential invasiveness and its effects in natural ecosystems need more thorough consideration given that the pines
have already invaded several grasslands and shrublands;
are altering primary succession;
are climatically suitable to three-quarters of New Zealand’s land
North American Tallgrass Prairie; photo by National Park Service
A new study by Moyano et al. [full citation at the end of the blog] tackles head-on the question of whether widespread planting of trees to counter climate change makes sense. They focus on plantings in naturally treeless ecosystems, i.e., grasslands, shrublands and wetlands. They find that:
relying on tree planting to significantly counter carbon change in the absence of reducing carbon emissions would require converting more than a third of Earth’s of global grasslands into tree plantations.
Reforestation of areas previously forested has the potential to produce a net increase in carbon sequestration more than twice as great as can be done by afforesting unforested areas.
Moyano et al. conclude that conservation and restoration of degraded forests should be prioritized over afforestation projects. This recommendation confirms points made in an earlier blog. Then I reported that Calders et al. (2022) said temperate forests account for ~14% of global forest carbon stocks in their biomass and soil. They worried that ash dieback link will kill enough large trees that European temperate deciduous forests will become a substantial carbon source, rather than sink, in the next decades. In my blog I pointed out that other tree taxa that also formerly grew large – elms, plane trees, and pines – have either already been decimated by non-native insects and pathogens, or face severe threats now.
Moyano et al. also point out that naturally treeless ecosystems are often at risk to a variety of threats, they provide numerous ecosystem services, and they should be conserved.
Loss of Biodiversity
Tree planting in naturally treeless areas changes ecosystems at the landscape scale. Moyano et al. say these changes inevitably degrade the natural biodiversity of the affected area. For example, grasslands provide habitats for numerous plant and animal species and deliver a wide range of ecosystem services, including provisioning of forage for livestock, wild food and medicinal herbs, + recreation and aesthetic value. Already 49% of Earth’s grassland area is degraded. Restoration of herbaceous plant diversity in old growth grasslands requires at least 100 years.
These obvious impacts are not the only losses caused by conversion of treeless areas to planted forests.
Ambiguous Carbon Sequestration Benefits
Grasslands store 34% of the terrestrial carbon stock primarily in the soil. Tree planting in grasslands can result in so much loss of carbon stocks in the soil that it completely offsets the increment in carbon sequestration in tree biomass. The underlying science is complicated so scientists cannot yet predict where afforestation will increase soil carbon and where it will reduce it. Important factors appear to be
Humid sites tend to lose less soil carbon loss than drier sites;
Soil carbon increases as the plantation ages;
Tree species: conifers either reduce soil carbon or have no effect; broadleaf species either increase soil carbon or have no effect.
Sites with higher initial soil carbon tend to lose more carbon during afforestation.
Afforestation has greater impacts on upper soil layers.
Moyano et al. assert that appropriate management of grasslands can provide low cost, high carbon gains: a potential net carbon sequestration of 0.35 Gt C/ year at a global level, which is comparable to the potential for carbon sequestration of afforestation in all suitable dryland regions (0.40 Gt C/year).
Changes in Albedo
Trees absorb more solar energy than snow, bare soil or other life forms (such as grasses) because they reflect less solar radiation (reduced albedo). Moyano et al. say the resulting warmer air above the trees might initially offset the cooling brought about by increased carbon sequestration in the growing trees’ wood. Only after decades does the increase in carbon sequestration compensate for the reduction in albedo and produce a cooling effect. Furthermore, they say, the eventual cooling effect that afforestation could create is slight, reducing the global temperature only 0.45°C by 2100 if afforestation was carried out across the total area actually covered by crops. As they note, replacing all crops by trees maintained to sequester carbon is highly unlikely.
Eucalyptus-pine plantation burned in Portugal; photo by Paolo Fernandez via Flickr
Increased fire severity
Planting trees in many treeless habitats – deserts, xeric shrublands, and temperate and tropical grasslands – increases fire intensity. This risk is exacerbated when managers choose to plant highly flammable taxa, e.g.,Eucalyptus.Already the fire risk is expected to increase due to climate change. These fires not only threaten nearby people’s well-being and infrastructure; they also release large portions of the carbon previously sequestered, thus undermining the purpose of the project. Moyano et al. note that the carbon stored in the soil of grasslands is better protected from fire.
Water supplies reduced
Afforestation changes the hydrological cycle because an increase in carbon assimilation requires an increase in evapotranspiration. The result at the local scale is decreased water yield and increased soil salinization and acidification. Water yield losses are greater when plantations are composed of broadleaf species. Moyano et al. point out that these water losses are more worrying in areas where water is naturally scarce, e.g., the American southwest, including southern California. On the other hand, increased evapotranspiration can enhance rain in neighboring areas through a redistribution of water at the regional scale and increased albedo through the formation of clouds.
Moyano et al. say planting trees also alters nutrient cycles. To my frustration, they don’t discuss this impact further.
Bioinvasion risk
Moyano et al. cite several experts as documenting a higher risk of bioinvasion associated with planting trees in naturally treeless systems. These invasions expose the wider landscapes to the impacts arising from tree plantations, e.g., increased plant biomass carbon sequestration, reduced soil carbon, reduced surface albedo, increased fuel loads and fuel connectivity, reduced water yield, and altered nutrient cycles. Even native ecosystems that are legally protected can be threatened. Thickets of invading trees can exacerbate some of the impacts listed above since the invading trees usually grow at higher densities. On a more positive side, invading stands of trees often are more variable in age; in this case, they can be more like a natural forest than are even-aged stands in plantations. Because of these complexities, the effect of tree invasions on ecosystem carbon storage becomes highly context dependent. This is rarely evaluated by scientists. See Lugo below.
Moyano et al. say woody plant invasions can exacerbate human health issues by providing habitat for wildlife hosts of important disease vectors, including mosquitoes and ticks. I ask whether plantations using unwisely chosen tree species might raise the same risks. They decry the minimal research conducted on this issue.
Assessing the tradeoffs
The goal is to remove CO2 from the atmosphere by fixing more carbon in plant biomass. Moyano et al. say careful consideration of projects’ potential impacts can minimize any negative consequences. An integrated strategy to address climate change should balance multiple ecological goals. Efforts to increase carbon storage should not compromise other key aspects of native ecosystems, such as biodiversity, nutrient and hydrological cycles, and fire regimes. First, they say, planners should avoid the obvious risks:
don’t plant fire-prone/flammable tree species; do adopt fuel- and fire-management plans.
don’t plant potentially invasive species.
don’t plant forests in vulnerable environments where negative impacts are likely.
In order to both minimize that certain risks will arise and ensure counter measures are implemented if they do, Moyano et al. suggest incorporating into carbon certification standards two requirements:
that soil carbon be measured throughout the whole soil depth.
that plantation owners be legally responsible for managing potential tree invasions.
The authors praise a new law in Chile, which prohibits planting monospecific tree plantations as a natural climate solution.
Furthermore, they advocate for regulators conducting risk analyses rather than accepting groundless assumptions about carbon storage and climate cooling effects.
Recognizing the uncertainty about some effects of introducing trees into naturally treeless areas, and interactions between these effects and the key role of the ecological context, Moyano et al. call for increased study of plant ecology. They specify research on the above-mentioned highly variable impacts on soil carbon as well as albedo.
Role of NIS trees in sequestering /storing carbon in U.S.
According to Lugo et al. (2022; full citation at the end of this blog), in the Continental United States, non-indigenous tree species contribute a tiny fraction of the forests’ carbon storage at the current time: about 0.05%. This is because non-native trees are widely scattered; while individuals can be found in more than 61% of forested ecosections on the continent, they actually occupy only 2.8% of the forested area.
However, non-native tree species are slowly increasing in both their area and their proportion of species in specific stands. Consequently, they are increasingly important in the forest’s carbon sink – that is, the amount of additional carbon sequestered between two points in time. In fact, non-native trees represent 0.5% of new carbon sequestered each year. This is ten times higher than their overall role in carbon storage. In other words, the invasive species play increasingly important ecosystem roles in the stands in which they occur.
neem tree – considered invasive in the Virgin Islands; photo by Miekks via Wikimedia
On the United States’ Caribbean and Pacific islands, non-native tree species are already much more common, so they are more important in carbon sequestration. On Puerto Rico, 22% of the tree species are non-native; link to blog 340 they accounted for 38% of the live aboveground tree carbon in forests. On the Hawaiian Islands, an estimated 29% of large trees and 63% of saplings or small trees are non-native. link to blog 339 Consequently, they store 39% of the mean plot area-weighted live aboveground tree carbon.
SOURCES
Bellingham, P.J., E.A. Arnst, B.D. Clarkson, T.R. Etherington, L.J. Forester, W.B. Shaw, R. Sprague, S.K. Wiser, and D.A. Peltzer. 2022. The right tree in the right place? A major economic tree species poses major ecological threats. Biol Invasions Vol.: (0123456789) https://doi.org/10.1007/s10530-022-02892-6
Calders, K., H. Verbeeck, A. Burt, N. Origo, J. Nightingale, Y. Malhi, P. Wilkes, P. Raumonen, R.G.H. Bunce, M. Disney. Laser scanning reveals potential underestimation of biomass carbon in temperate forest. Ecol Solut Evid. 2022;3:e12197. wileyonlinelibrary.com/journal/eso3
Lugo, A.E., J.E. Smith, K.M. Potter, H. Marcano Vega, and C.M. Kurtz. 2022. The Contribution of NIS Tree Species to the Structure and Composition of Forests in the Conterminous US in Comparison with Tropical Islands in the Pacific and Caribbean. USFS International Institute of Tropical Forestr. January 2022. General Technical Report IITF-54 https://doi.org/10.2737/IITF-GTR-54
Moyano, J., R.D. Dimarco, J. Paritsis, T. Peterson, D.A. Peltzer, K.M. Crawford, M.A. McCary,| K.T. Davis, A. Pauchard, and M.A. Nuñez. 2024. Unintended consequences of planting native and NIS trees in treeless ecosystems to mitigate climate change. Journal of Ecology. 2024;00:1-12
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
invasion of wild/black mustard Brassica nigra; photo by carlbegge via Flickr
A California state legislator has proposed a bill to expand state efforts to counter invasive species. Should we support it – and others like it in other states?
The bill is Assembly Bill 2827 introduced by Assembly Member (and former Majority Leader) Eloise Reyes of the 50th Assembly District. She represents urban parts of southwestern San Bernardino County, including the cities of Rialto, Colton, and Fontana.
According to media reports, Reyes was prompted to act by the current outbreak of exotic fruit flies, which as of some months ago resulted in detections in 15 California counties.
The bill is much broader than agricultural pests, however. It would find and declare that it is a primary goal of the state to prevent the introduction, and suppress the spread, of invasive species within its borders. I applaud the language of the “findings” section:
(a) Invasive species have the potential to cause extensive damage to California’s natural and working landscapes, native species, agriculture, the public, and economy.
(b) Invasive species can threaten native flora and fauna, disrupt ecosystems, damage critical infrastructure, and result in further loss of biodiversity.
Paragraph (c) cites rising threats associated with increased movement of goods, international travel, and climate change — all said to create conditions that may enhance the survival, reproduction, and spread of these invasive species, posing additional threats to the state.
(d) It is in the best interest of the state to adopt a proactive and coordinated approach to prevent the introduction and spread of invasive species.
California sycamore attacked by invasive shot hole borer; photo by Beatriz Nobua-Behrmann
The bill calls for
The state agencies, in collaboration with relevant stakeholders, to develop and implement pertinent strategies to protect the state’s agriculture, environment, and natural resources.
The state to invest in research, outreach, and education programs to raise awareness and promote responsible practices among residents, industries, and visitors.
State agencies to coordinate efforts with federal, local, and tribal authorities.
However, the bill falls short when it comes to action. Having declared that countering bioinvasion is “a primary goal of the state”, and mandated the above efforts, the bill says only that the California Department of Food and Agriculture (which has responsibility for plant pests) is to allocate funds, if available, to implement and enforce this article. Under this provision, significant action is likely to depend on holding agencies accountable and providing increased funding.
removing coast live oak killed by goldspotted oak borer; photo by F.T. Campbell
Would this proposed legislation make a practical difference? I have often complained that CDFA has not taken action to protect the state’s wonderful flora. For example, CDFA does not regulate firewood to prevent movement of pests within the State. It has not regulated numerous invasive plants or several wood-boring insects. These include the goldspotted oak borer; the polyphagous and Kuroshio shothole borers; and the Mediterranean oak borer.
On the other hand, CDFA is quick to act against pests that might enter the state from elsewhere in the country, e.g., spongy moth (European or Asian), emerald ash borer and spotted lanternfly.
I hope Californians and the several non-governmental organizations focused on invasive species will lobby the legislature to adopt Assembly Bill 2827. I hope further that they will try to identify and secure a source of funds to support the mandated action by CDFA and other agencies responsible for managing the fauna, flora, and other taxa to which invasive species belong.
I applaud Ms. Reyes’ initiative. I hope legislators in other states will consider proposing similar bills.
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
Whitebark pine – a foundational & keystone species in high elevation mountains of the West; photo by Walter Siegmund
A British scientist has proposed a new way to conduct early pest detection surveillance that she thinks will better serve resource managers: prioritize ecosystems which would suffer the greatest alteration if a non-native plant pest decimated one or more plant species. She says scientists should focus on foundational species and maintaining habitat resilience.
Dr. Ruth J. Mitchell leads the Biodiversity and Ecosystems Group within the Ecological Sciences Department at the James Hutton Institute in Aberdeen, Scotland. The Institute works on issues relevant to sustainable management of natural resources. I provide a full citation of her article at the end of the blog.
Dr. Mitchell’s focus is on protecting biological diversity. She worries that introduced plant pests can drive large-scale declines in native plant species. She mentions several examples, including chestnut blight and ash decline. Those declines, in turn, can cause a range of cascading effects on associated species that use the host plant for feeding, breeding and shelter, and on ecosystem functioning. To be prepared to counter this level of risk, managers of natural habitats need to know which habitats and plants are at greatest risk in order to prioritize surveillance of the most likely human actions and sites; and allocate resources to address the most damaging invasions.
Her proposal: prioritize host plant species or habitats which ecological theory indicates an invasion would have the greatest ecological impact. In other words, focus on “foundational species” — plant species that drive key ecosystem functions; or low (plant) diversity habitats — based on the assumption that diverse communities are more stable and resilient than less diverse communities.
Mitchell notes that ecological theory posits that if a foundation species is lost or declines, its disappearance will have a greater effect on the ecosystem than if non-foundation species are impacted. She believes that although there is no list of foundation species, scientific staff can develop appropriate lists for their site. For her study, she made the simplistic assumption that those species that occur at high abundance are most likely to be foundation species. Regarding the second, habitat-resilience criterion, Mitchell assumed that a pest which eliminates a plant species in a low-diversity habitat is likely to have a greater ecological impact on that habitat’s functioning than would extinction of a species in a high-diversity habitat, which is likely to have redundancies.
Mitchell asserts that these approaches to surveillance take account of an invasion’s impacts on broader associated species and ecosystem functions – on biodiversity broadly. These suggested methods have other advantages, too. They avoid the bias in existing lists of pests, which consist predominantly plants of commercial importance; and they don’t need to be updated frequently.
Mitchell identifies four ways to prioritize surveillance efforts based on the potential host rather than the potential pest. The surveillance monitoring might target:
(1) Plant genera known to host the pests (including pathogens) most likely to establish (Host-pest);
(2) Habitats harboring hosts for the greatest number of pests most likely to establish (Habitat-pest);
(3) Plants classed as foundation species (Foundation-species);
(4) Habitats with low plant species diversity and hence low resilience (Habitat-resilience).
Mitchell analyzed the damage that 91 pest species might cause to plant species which occur at 25% or higher cover in 12 broad habitat types in the United Kingdom. As a case study, she also looked at 22 vegetation communities within one of those habitat types (heathland). (See the article for a discussion of how she derived her list of 91 pests, their hosts, and the entity responsible for designating the habitat types.)
For both hosts and habitats, Mitchell compared results of two approaches: (a) assessment based on lists of known known pests; and (b) assessment based on potential ecological impact. Surveillance based on known risks i.e. lists of plant pests(i.e., the Hosts-pest and Habitat-pest methods) assumes that scientists have a complete list of pests, their risk of establishment, and their impacts. We know that is not the case. As an illustration, Mitchell’s review of the literature identified 142 insects or pathogens hosted by plant genera present on British moorlands that are not listed as pests by the appropriate British authority, the UK Plant Health Risk Register (PHRR).
To conduct a “Foundation-species” surveillance program, one must first identify foundational plant species. Mitchell defined those as species that constitute more than 75% cover in any plant community. (While this is admittedly an oversimplification, Mitchell says that the loss or severe decline of such abundant species will have a major impact on community composition.) One then prioritizes surveillance of these species – regardless of whether they are at risk from a known pest. This method emphasizes attention to potential impact to the habitat or plant community. Furthermore, this approach accommodates detection of the ‘known unknown’ pests.
To conduct a “Habitat-resilience” surveillance program, one must first identify the number of species in each habitat or vegetation community that occur at more than 25% cover. One then prioritizes surveillance of those habitats with the lowest average species diversity.
Differences in results
When basing the analysis on lists of known pests threatening all 12 habitat types, two genera stood out as at particular risk: Prunus and Solanum. Each consists of hosts supporting more than 20 of the 91 pests. Another 17 genera comprised hosts of six or more pests. Many of these genera include species that are important in ornamental horticulture or production forestry. Mitchell considers this a flaw. She points out that different genera ranked highest under this system when the focus narrowed to heathland communities. In heathlands, the genera comprising hosts of the most pests were Calluna, Erica, Festuca and Vaccinium.
American elm – a deserving priority for pest surveillance! Photo by F.T. Campbell
I note that from my perspective – concern about pests that kill native trees – several of the 17 genera included in the “known pests” analysis do raise alarm: Acer, Salix, Ulmus, Fraxinus, Pinus, Quercus, Betula, Viburnum, and Juniperus.
Mitchell then tested the results of focusing on habitat types where the highest number of pests were likely to become established. This method gave highest priority to woodlands – because plants in this habitat type can host 87 of the 91 pests. The second priority should be open habitats (defined as disturbed habitats, arable weed communities, weedy pastures, paths, verges, wasteland and urban habitats). Plants in the “open habitat” type can host 54 pests. (While Mitchell did not specify whether she excluded non-native plant species from her calculations, she does write generally about impacts on native flora – so I believe she did.)
Looking specifically at the 22 heathland vegetation communities, Mitchell identified four communities as able to host the greatest number of pests so deserving surveillance priority.
When she focused on “foundation species”, Mitchell found a range of plant species that occur at 75% or greater cover in each habitat. Again, the highest number (71 species) occur in woodlands; the lowest (11 species) grow in Calcicolous grasslands. In the 22 heathland plant communities, the number of plant species meeting this criterion numbered fewer than five in each. Two communities have no “foundation” species for surveillance since no vascular plant species that occur at 75% cover. In both the habitat and community cases, the surveillance priority of managers of each habitat type would concentrate on the species that fit this criterion for the appropriate biome.
Finally, Mitchell identified those habitats or communities with the lowest species richness / fewest species as being at greatest risk of unravelling if they lose one or more species to an introduced pest. The data indicated these to be the Salt Marsh and Swamps and tall-herb fens systems. At the other end of the spectrum, Mesotrophic grasslands and Woodlands have the lowest priority for surveillance because they are species-rich. Of course, communities within a habitat type vary greatly in their species richness and associated resilience. For example, the one heathland community which has only two species occurring at 25% or greater cover has a higher priority than the communities with more such species.
heath – Erica carnea ; photo by H. Zell
Mitchell asserts that prioritizing plant species or habitats for surveillance based on potential ecological impact rather than risk (known pests) provides a less biased process and allows for the detection of the known unknowns pests. The resulting set of priority surveillance targets differs significantly from the set developed by reliance on pest lists. For example, looking at heathland communities, the Host-pest and Foundation-species methodologies share only three of 24 host genera. The differences arise from the PHRR’s bias oflisting predominantly species relevant to agriculture, horticulture, or forestry. None of these genera is listed under the Foundation-species methodology.
Since trade in plants for planting is the main pathway of introduction of non-native pests, Mitchell concedes that plant species in natural habitats that are closely related to species of commercial importance might be more threatened than other species. However, such an approach takes no account of the potential for a pest to jump hosts.
Prioritization based on potential ecological impact rather than known risk has many advantages. The Foundation-species method prioritizes those plant species whose decline would have the greatest impact on wider biological diversity, ecosystem function and service delivery. That is, it incorporates consideration of the wider risks to the whole ecosystem rather than just the risk to a specific plant species. The Habitat-resilience method similarly takes account of the wider ecosystem level impacts, targeting those habitats or communities that might recover less quickly
On a practical level, these approach do not require surveyors (who might be citizen scientists or land manager) to identify specific pests. Instead, the surveyors report signs of unhealthy-looking plants to the relevant authorities, who then identify the cause.
These methods address a universal problem for plant health: the many pests that are previously unknown before their emergence in new regions and on naïve hosts. Mitchell briefly mentions scientists’ continue struggle to identify traits that can forecast potential pest impacts. [See my blogs re: studies by Mech, Schulz, Raffa]
redbay tree killed by laurel wilt disease – a pathogen unknown until it was introduced to southeastern U.S. Photo by F.T. Campbell
Mitchell suggests several ways to adapt these approaches to other countries or improve their targetting. First, scientists can link various pest/host databases (e.g., EPPO or CABI databases) to landcover or biome data and national or regional vegetation classification systems to make the system appropriate for their country or region. Incorporating attention to dirty equipment and movement of soil &/or plants is fitting at sites undergoing habitat restoration.
It is possible to refine the “foundation species” approach by applying a trait-based approach. She names two examples.
Finally, the Habitat-resilience method could be enhanced by integrating metrics of plant phylogenetic and functional diversity to the idea functional redundancy.
Mitchell stresses the need to unite efforts by many agencies and stakeholders within each country, as well as across political boundaries. She asserts that such collaborative efforts are more efficient / less costly, so lessening the restrictions imposed by resource limits. She also advocates reliance on citizen science and “passive surveillance” or chance observations by professionals agents, land-users and owners. These steps can facilitate large-scale surveillance that would otherwise be financially infeasible.
Mitchell highlights the difficulties imposed by the division of responsibilities. Usually the National Plant Protection Organization (NPPO) is responsible for early detection surveillance. The agency’s goal is to detect pests sufficiently early to facilitate eradication – or at least effective control. Its program is linked to regulatory requirements under the international plant health system. link to blogs & FF reports While the NPPO’s responsibilities include both cultivated and uncultivated (wild) plants, in many countries the NPPO prioritizes plants with commercial value. (This is certainly true in the United States – see my previous blogs & the Fading Forest reports – links provided below; and apparently the United Kingdom [Dr. Mitchell’s article] and Australia.) Protecting plant health in habitats is usually the task of conservation organizations. Mitchell calls for unifying these programs. CISP is advocating draft legislation that aims to fix this gap in the U.S. link to Welsh bill
What do you think? Is this approach as promising as Dr. Mitchell believes? Is it feasible?
I certainly concur that pest-based surveillance ignores the various categories of “unknown” pests and focus on commercially important species to the detriment of ecologically important ones. However, can such a system provide “early detection” of introduced pests? We have learned that insects and pathogens causing noticeable damage in natural environments have probably been present in a country or region for years – or decades. Perhaps these ecosystem-based criteria should be applied as guidance for selecting species to be monitored in “sentinel plant” programs. The plantings would be established in situations likely to receive pests early in their invasion process, e.g., warehouse districts (for pests in wood packaging) and ornamental nurseries that import growing stock.
Mitchell says the same issues pertain with regard to wildlife disease. See her article for sources.
SOURCE
Mitchell, R.J. 2024. A host-based approach for the prioritization of surveillance of plant pests and pathogens in wild flora and natural habitats in the UK. Biol Invasions (2024) 26:1125–1137 https://doi.org/10.1007/s10530-023-03233-x
Posted by Faith Campbell
We welcome comments that supplement or correct factual information, suggest new approaches, or promote thoughtful consideration. We post comments that disagree with us — but not those we judge to be not civil or inflammatory.
For a detailed discussion of the policies and practices that have allowed these pests to enter and spread – and that do not promote effective restoration strategies – review the Fading Forests report at http://treeimprovement.utk.edu/FadingForests.htm
U.S. Department of Agriculture headquarters; lets lobby these people! photo by Wikimedia
Twenty-three scientists based around the world published a Letter to the Editor titled “Overwhelming evidence galvanizes a global consensus on the need for action against Invasive Alien Species” It appears in the most recent edition of Biological Invasions (2024) 26:621–626.
The authors’ purpose is to draw attention to the release of a new assessment by the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services’ (IPBES).
The report was issued in September 2023. It is described as the most comprehensive global synthesis of the current knowledge on the bioinvasion process and the impacts of invasive alien species (952 pages!). Its preparation took nearly a decade. Most important, it represents the first consensus among governments and scientists worldwide on the magnitude and extent of the threats that bioinvasions pose to nature, people, and the economy.
The proposed solutions are astoundingly broad and ambitious: transformation of how governments and societies perform. I don’t disagree! However, we need interim steps – “bites of the elephant.” In my view, the report falls short on providing these.
Our challenge: join others in bringing this analysis to decision-makers’ attention. Can we pull out information that will help persuade U.S. decision-makers – governmental and non-governmental – that the threat is both urgent and solvable? How do we more effectively advocate for the aggressive, science-based action that we all know is needed?
(I hope that the fact that the report was prepared under the auspices of the Convention on Biodiversity, to which the U.S. is not a party, does not intensify the challenge for us.)
Why we need to restructure the behavior of governments and societies
Bioinvasions are facilitated by policies, decision-making structures, institutions, and technologies that are almost always focused on achieving other goals. Species transport and introduction are driven by policies aimed at promoting economic growth – especially trade. Later stages of invasions, i.e., establishment and some spread, are accelerated by certain uses of land and sea plus climate change. For example, activities that fragment habitats or cause widespread habitat disturbance provide ready places for bioinvasions. Rarely are those who gain by such policies held accountable for the harms they produce via bioinvasions.
To address these unintended consequences, the IPBES report calls for “integrated governance.” Its authors want coordination of all policies and agencies that touch on the indirect drivers, e.g., conservation; trade; economic development; transport; and human, animal, and plant health. Policy instruments need to reinforce – rather than conflict with — strategic invasive species management across sectors and scales. This involves international agreements, national regulations, all governmental sectors, as well as industry, the scientific community, and ordinary people – including local communities and Indigenous Peoples.
The report also calls for establishment of open and inter-operable information systems. This improved access to information is critical for setting priorities; evaluating and improving regulations’ effectiveness; and reducing costs by avoiding duplication of efforts.
Critically important information that is often unspoken:
Indirect causes underlying the usual list of human activities that directly promote bioinvasions are the rapid rise of human population and even more rapid rise in consumption and global trade.
Biosecurity measures at international borders have not kept pace with the growing volume, diversity, and geographic origins of goods in trade.
Continuation of current patterns is expected to result in one-third more invasive species globally by 2050. However, this is an underestimate because today’s harms reflect the consequences of past actions – often from decades ago. Drivers of invasions are expected to grow in both volume and impact.
We can prevent and control invasive alien species – but that success depends on the availability of adequate, sustained resources, plus capacity building; scientific cooperation and transfer of technology; appropriate biosecurity legislation and enforcement; and engaging the full range of stakeholders. These require political will.
A major impact of bioinvasion is increased biotic homogenization (loss of biological communities’ uniqueness). This concerns us because we are losing the biotic heterogeneity that provides insurance for the maintenance of ecosystem functioning in the face of ongoing global change.
The IPBES study asserts that successfully addressing bioinvasions can also strengthen the effectiveness of policies designed to respond to other drivers, especially programs addressing conservation of biological diversity, ensuring food security, sustaining economic growth, and slowing climate change. All these challenges interact. The authors affirm that evidence-based policy planning can reflect the interconnectedness of the drivers so that efforts to solve one problem do not exacerbate the magnitude of others and might even have multiple benefits.
More Key Findings
Overall, 9% (3,500) of an estimated 37,000 alien species established in novel environments are invasive (those for which scientists have evidence of negative impacts). Proportions of invasives is high among many taxonomic groups: 22% of all 1,852 alien invertebrates; 14% of all 461 alien vertebrates; 11% of all 141 alien microbes; and 6% of all 1,061 alien plants. (The discussion of probable undercounts relates to aquatic systems and certain geographic regions. However, I believe these data are all undermined by gaps in studies.)
Invasive alien species – solely or in combination with other drivers – have contributed to 60% of recorded global extinctions. Invasive species are the only driver in 16% of global animal and plant extinctions. Some invasive species have broader impacts, affecting not just individual species but also communities or whole ecosystems. Sometimes these create complexoutcomes that push the system across a threshold beyond which ecosystem restoration is not possible. (No tree pests are listed among the examples.)
dead whitebark pine in Glacier National Park; photo by National Park Service
The benefits that some non-native – even invasive – species provide to some groups of people do not mitigate or undo their negative impacts broadly, including to the global commons. The report authors note that beneficiaries usually differ from those people or sectors that bear the costs. The authors cite many resulting inequities.
There are insufficient studies of, or data from, aquatic systems, and from Africa; Latin America and the Caribbean; and parts of Asia.
The number of alien species is rising globally at unprecedented and increasing rates. There are insufficient data specifically on invasive species, but they, too, are thought to be rising at similar rates.
Horticulure is a major pathway for introducing 46% of invasive alien plant species worldwide.
Regarding invasive species’ greater impact on islands,the IPBES report mentions brown tree snakes on Guam and black rats on the Galapagos Islands. It also notes that on more than a quarter of the world’s islands, the number of alien plants exceeds the total number of native ones. See my blogs on non-native plants on Hawai`i and Puerto Rico. In addition, I have posted several blogs regarding disease threats to rare bird species in Hawai`. The IPBES report does not mention these.
Where the Report Is Weak: Interim Steps
The report endorses adoption of regulated species (“black”) lists.
The report emphasizes risk analysis of species. Unfortunately IPBES’ analysis was completed before publication of the critique of risk analysis methods by Raffa et al. ( (2023) (see references). However, we must take the latter into consideration when deciding what to advocate as U.S. policy.
The report authors call for more countries to adopt national legislation or regulations specifically on preventing and controlling invasive species. (They note that 83% of countries lack such policies). They also list the many international agreements that touch on invasive species-relevant issues. However, Raffa et al. found that the number of such agreements to which a country is a party bears no relationship to the numbers of alien species detected at its border or established on its territory.
The challenge to risk assessment posed by multiple sources of uncertainty can be managed by recognizing, quantifying, and documenting the extent of that uncertainty.
Beech leaf disease – one of many non-native pests that were unknown before introduction to a naive ecosystem. Photo by Jennifer Koch, USDA Forest Service
I appreciate the report’s emphasis on the importance of public awareness and engagement, but I thought the discussion of effective campaigns lacked original ideas.
The report did not fulfill its own goal of fully exploring unappreciated impacts of policies in its discussion of habitat fragmentation. For example, the report notes that grazing by feral alien ungulates facilitates the spread of invasive alien plant species. However, it does not mention the similar impact by livestock grazing (Molvar, et al. 2024).
SOURCES
Molvar, E.M., R. Rosentreter, D. Mansfield, and G.M. Anderson. 2024. Cheat invasions: History, causes, consequences, and solutions. Hailey, Idaho: Western Watersheds Project, 128 pp.
Raffa, K.F., E.G. Brockerhoff, J-C. GRÉGOIRE, R.C. Hamelin, A.M. Liebhold, A. Santini, R.C. Venette, and M.J. Wingfield. 2023. Approaches to forecasting damage by invasive forest insects and pathogens: a cross-assessment. BioScience 85 Vol. 73 No. 2 (February 2023) https://academic.oup.com/bioscience
Posted by Faith Campbell
We welcome comments that supplement or correct factual information, suggest new approaches, or promote thoughtful consideration. We post comments that disagree with us — but not those we judge to be not civil or inflammatory.
For a detailed discussion of the policies and practices that have allowed these pests to enter and spread – and that do not promote effective restoration strategies – review the Fading Forests report at http://treeimprovement.utk.edu/FadingForests.htm
leaf symptoms of beech leaf disease; photo by Jennifer Koch, USFS
Recently I posted a blog on a paper by Paulo Vieira et al., reporting how the nematode Litylenchus crenatae subsp. mccannii (Lcm) [causal agent of beech leaf disease (BLD)] distorts the leaves of affected American beech trees (Fagus grandifolia). Now Leila Rose Fletcher and her colleagues have confirmed these structural changes and discussed how they might affect the anatomy and physiology of the leaves and harm the tree. For the full details, see this 2024 publication, cited at the end of this blog. Both articles contain stunning photographs of diseased leaf structures.
As Dr. Fletcher pointed out on a recent call involving most scientists and conservationists working on BLD, plant growth depends on the plant’s success in gaining more carbon (through photosynthesis) than it expends during growth, cell maintenance (respiration), and sugar storage. When plants open the stomata on their leaves to take in CO2 from the atmosphere, they lose water from the interior of their leaves.
Her team discovered that BLD-related leaf distortions reduced the tree’s carbon balance in two ways.
Two impacts of the nematode
First, alterations of the leaf structure reduce the tree’s photosynthetic rate (assimilation of carbon). The photosynthetic rate in affected leaves was 61% lower than in healthy leaves. The impact is heightened by the thinning of the beech tree’s canopy due to abortion of many leaf buds.
While veins in diseased leaves are also altered by BLD, the data in Fletcher et al. indicate that the main limitation on photosynthesis in symptomatic leaves is not from a decreased water supply, but from several limitations on stomatal exchange of CO2 with the atmosphere. First, stomata on symptomatic portions of diseased leaves are less dense, which means there are fewer openings through which CO2 can enter the leaf.
Second, the diseased leaves are thicker, meaning that once inside the leaf, CO2 molecules must travel farther from the stomatal pores to reach the photosynthetic cells. Fletcher et al. did not assess the possibility (raised in a separate study by Carta, et al.) that the stomata that are present are deformed, and that this might impact their function.
In addition, the deformed leaves demand more resources to grow and function. Production of the multiplicity of cells in affected portions of the leaf (these portions are 249% thicker than normal leaves) uses resources the tree would otherwise put into growth. In fact, the more severely symptomatic an individual leaf is, the more carbon the plant allocates to that leaf.Furthermore, the additional cell layers also appear to increase “operating costs” of these leaves, as seen in the higher respiration rate per unit leaf area. Finally, if the tree sheds deformed leaves and forms new ones, this further diverts resources.
Questions seeking answers
How is nematodes’ influence localized to domains bounded by second-order veins (large veins that branch off the central vein) – symptomatic and asymptomatic tissue in adjacent domains in the same leaf? Fletcher et al. propose that the presence of the nematode influences the physical or hormonal regulation of leaf development – but after the development of primary and secondary order veins (since they are not distorted). They place a high priority on investigating the tree’s hormonal signaling that might be disrupted by the nematode.
Given the carbon imbalance that the data in Fletcher et al. suggest might arise over time, will symptomatic trees face carbon shortages, and if so, will this eventually lead to mortality? Studies analyzing the non-structural carbohydrate (stored sugar) concentrations in symptomatic beech are urgently needed to explore this possibility.
What is the impact of beech trees’ suboptimal vigor – short of mortality – on composition of plant communities and animals reliant on beech leaves and beechnuts? One possible causal factor raised by Fletcher et al. is reduced development of symbiotic relationships with ectomycorrhizal fungi, which can also reduce production of beech nuts. Dr. Fletcher concedes that there have been no studies yet of these possible effects. I add that many animals also depend on tree cavities – which are also common in beech trees.
SOURCES
Carta, L.K., S. Li, J. Mowery. 2023. Chapter 8 – Beech leaf disease (BLD), Litylenchus crenatae and its potential microbial virulence factors. In F.O. Asiegbu & A. Kovalchuck (Eds.), Forest microbiology Vol. 3 (PP. 183-192) Academic Press. https://doi.org/10.1016/B978-0-443-18694-3.00018-3
Fletcher, L.R. A.M. Borsuk, A.C. Fanton, K.M. Johnson, J. Richburg, J. Zailaa, C.R. Brodersen. 2024. Anatomical & physiological consequences of beech leaf disease in Fagus grandifolia L. Forest Pathoklogy. 2024;54:e12842 https://doi.org/10.1111.efp.12842
Vieira P., M.R. Kantor, A. Jansen, Z.A. Handoo, J.D. Eisenback. (2023) Cellular insights of beech leaf disease reveal abnormal ectopic cell division of symptomatic interveinal leaf areas. PLoS ONE October 5, 2023. 18(10) https://doi.org/10.1371/pone.0292588
In the context of reading about forest succession (see previous blogs) I came upon a new publication by Akresh et al. (full citation at the end of this blog.) The article explores the impact of various silvicultural treatments on bird conservation in eastern North America. They note that forest managers are challenged to balance the opposing habitat needs of organisms that, on one hand, depend on structurally diverse old-growth forests and, on the other hand, those that inhabit more open areas or shrubs.
The authors conducted a meta-analysis of studies that examined birds’ responses to three silvicultural regimes: low-retention stands, shelterwoods, and high-retention stands. These terms were not defined in the article. According to Michigan State University extension, in shelterwood systems, all mature trees are harvested in a two- or three-stage process over several years. The other classes presumably reflect the proportion of trees remaining after the harvest.
Akresh et al. focussed on “community conservation scores,” not on protecting individual bird species. They followed the level of conservation concern for the two communities developed by the Partners-in-Flight program
Shrubland Birds
Akresh et al. note that a high proportion of open-canopy, shrubland bird species are declining range-wide; their habitat is already quite limited in eastern North America and continues to decline. Consequently, the Birds-in-Flight program gives them a high priority for conservation measures.
eastern bluebird – prefers open areas; photographer not named; Pickpic
The researchers found that clearcuts (presumably = low retention) and shelterwoods typically had the highest conservation scores because they provide habitat for the declining avian group, shrubland birds. More heavily harvested forests also support non-avian taxa such as pollinators and other arthropods, mammals, snakes, and vascular plants.
Forest Birds
Stands on which 40%–70% of tree were retained also have a high conservation score because they provide habitat for both shrubland and “mature forest” species. Only a few species, e.g., ovenbird and brown creeper, had lower densities in moderately harvested stands than in unharvested forests. The majority of “mature forest” species had relatively higher or equal densities in the sites on which 40%–70% of trees are retained than in unharvested stands. They suggest that several mature-forest bird species prefer the increased understory vegetation density found in these stands.
ovenbird; Wildreturn; Wikimedia Commons
Unharvested and lightly thinned stands, in which 70%–100% of trees remain, had the lowest conservation scores. The first explanation is that these forests don’t support shrubland bird species.
A second reason, Akresh et al. suggest, is that the second-growth forests now widespread in eastern North America are quite young (even if they have not been logged for at least 50 years). They are even-aged and lack the structural diversity of true old-growth forests. The authorsappear to place the greatest importance on the lack of dense understory vegetation, although otherkey elements of mature forests are also missing, e.g., large-diameter trees and snags, continuous canopy, and deep leaf litter. They concede that some bird species depend on forest characteristics that they did not examine. They did not provide examples of these other ecological attributes.
Akresh et al. note that their study concerns only bird species’ use of forests during the breeding season. Some species use other habitat types at other seasons. Furthermore, data were insufficient to analyze some species altogether. A more comprehensive analysis might have raised the conservation score of older forests. I would add that restoration of true old-growth forests depends on allowing some late-seral stands to continue aging.
Finally, fauna other than birds also depend on forest ecosystems and need to be considered when choosing management approaches. The authors mention salamanders and other amphibians, fungi, invertebrates, and lichens – some of which might be of conservation concern themselves.
Gaps in the forest: complicating factors
Akresh et al. mention deer browsing as an influence on understory conditions once, but do not explore this. I am surprised that they don’t expand this statement by a paragraph or two, given the role deer play in suppressing understory vegetation.
Nor do they mention possible impacts of invasions by non-native plants. As my earlier blogs have reported, plant invasions are common in many forested areas in eastern North America. These studies recommend great care in activities that open the forest canopy. Drs. Akresh and King have told me that they believe that forest managers in this region are well aware of invasive plant issues and already incorporate this concern into their management decisions. They referred me to two studies that indicate a very mixed picture of invasive plant impacts on birds (Labbe and King, 2020; Nelson et al, 2017. see full citations below).
multiflora rose – most common invasive plant on forest plots; photo by Famartin
Not Discussed: Insects as food sources
The studies analyzed by Akresh et al. explore levels of nesting success and bird species’ foraging on fruits of non-native shrubs. Others have focused on the reduced numbers of insects feeding on non-native plants; these insects are the principle food for many perching birds’ nestlings. Douglas Tallamy has documented lower numbers of a wide variety of birds which depend on the insect food supply.
SOURCES
Akresh, M.E., D.I. King, S.L. McInvale, J.L. Larkin, A.W. D’Amato. 2023. Effects of forest management on the conservation of bird communities in eastern North America: A meta-analysis. Ecosphere. 2023; 14:e4315. https://onlinelibrary.wiley.com/r/ecs2
Labbe, M.A. and D.I. King. 2020. Songbird Use of Native and Invasive Fruit in the Northeastern USA. Wildlife Society Bulletin. Volume 44, Issue 3. September 2020
Nelson, S.B, J.J. Coon, C.J. Duchardt, J.DL Fischer, A.J. Kranz, C.M. Parker, S.C. Schneider, T.M. Swartz, J.R. Miller. 2017. Patterns and mechanism of invasive plant impacts on North American birds: a systemic review. Biological Invasions. Volume 19, pp. 1547-1563.
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
TACF back-crossed American-Chinese chestnut; photo by F.T. Campbell
I have advocated for considerably expanding efforts to breed trees resistant to non-native pests (including pathogens) for a decade. Again and again, I and others have pointed out the dire consequences for our forests if we Americans do not rise to the challenge.
In 2014, Scott Schlarbaum – coauthor of Fading Forests III – American Forests: What Choice Will We Make? warned that without restoration becoming an integral part of a strategy addressing non-native plant pests, American ecosystems are doomed to continuing transformation. Once established, a non-native pest is never eliminated, but its impact can be reduced through a combination of measures – as long as support is made available. Scott advised initiating a germplasm conservation strategy when invasion is imminent or once the pest is likely to become a resident pest. (See Chapter 6).
I have posted seven blogs since August 2021 describing the current status of various efforts and urging the U.S. Government and conservation organizations to step up. [To view these blogs, go to www.nivemnic.us, scroll below Archives to “Categories” and click on “resistance breeding.”
More, and Recent, Voices: Implications of Not Acting
More recently, several USDA Forest Service (USFS) experts, including Richard Sniezko, C. Dana Nelson, and Jennifer Koch, have published articles making the same point. These scientists note that many of the decimated species were formerly among the most common trees in our forests. Therefore, the cumulative effect of their disappearance on forest species composition and function is multiplied.
One blog, posted in 2022, is particularly pertinent. It summarizes a special issue of the journal Plants, People, Planet devoted to resistance breeding. The opening essay, by R.J.A. Buggs, concisely reviews six major reasons why so many believe that resistance breeding is a failed strategy.
Port-Orford cedar – one of the trees for which resistance breeding has been successful; photo courtesy of Richard Sniezko, USFS
Others say there have been successes – all through application of classic tree improvement measures, not “genetic engineering.” Pike, Koch and Nelson (2021) list as successes Port-Orford-cedar (Chamaecyparis lawsoniana), the western five-needle pine species, koa (Acacia koa), and resistance to fusiform rust (Cronartium quercuum f. sp. fusiforme) in the commercially-important loblolly (Pinus taeda) and slash (P. elliottii) pines. They also cite encouraging progress by The American Chestnut Foundation (TACF) through backcross breeding of America and Asian chestnuts and a USFS/private foundation effort to expand the genetic base of American elms (Ulmus americana). I regret to say this, but some of these efforts seem to me to be still in experimental stages or — at best — early in widespread – ‘though still experimental — plantings.
Participants in a 2021 Purdue University workshop have again called for greatly expanding breeding. See the special issue of New Forests, Vol. 54 Issue 4. Once again, experts reiterate the urgency of acting, then outline the opportunities and challenges.
In one of the articles (Jacobs et al.) several people – including me! – note that several keystone tree species or genera in North America and Europe have been driven to functional extinction by non-native pests. By this we mean they are no longer sufficiently abundant and/or of adequate size to reproduce sexually or perform their ecological function. Examples include – on both continents – ashes (Fraxinus) and elms; and on North America – American chestnut (Castanea dentata), butternut (Juglans cinerea), and whitebark pine (Pinus albicaulis).If these threats are left unchecked, these at-risk tree species might develop truncated ranges, lose genetic diversity, and face becoming threatened, endangered, or extinct.
In another article, Nelson says the question that should be asked about applying genetic engineering (GE) techniques to tree breeding is whether we should let a species be reduced to a marginal role — or disappear — when GE provides a solution to saving and restoring the species. His case study is a detailed history of TACF’s development of a transgenic American chestnut (called “Darling 58”). He points out that decades of breeding efforts were based on the hope of developing blight resistance within the native gene pool or to obtain resistance from related species through hybridization. However, those efforts have not yet provided trees suitable for restoring the “king of the Appalachian forest” to native landscapes. Nelson wrote his description before TACF discovered flaws in the GE trees they had been working with and decided to pursue different GE “lines” (see below).
Barriers
The overall strategy is clear. Schlarbaum, Sniezko, and Dana Nelson all describe essentially the same steps, built on the same kinds of expertise and facilities.
Of course, each species will require years of input by a range of experts. These challenges are not trivial. However, the experts named above agree that the principal barrier is the absence of sustained, long-term commitment of resources and facilities. With sufficient resources, many of the scientific challenge can be overcome for at least some of the species at risk.
So, what are the scientific challenges? First, scientists must assess whether the tree species contains sufficient genetic variation in resistance. This involves locating candidate resistant trees; developing and applying short-term assay(s) to screen hundreds or thousands of candidate trees; and determining the levels of resistance present. Second, scientists must develop resistant planting stock for use in restoration. This stage includes scaling up the screening protocol; selecting the resistant candidates or progeny to be used; breeding to increase resistance; establishing seed orchards or other methods to deliver large numbers of resistant stock for planting; and additional field trials to further validate and delineate resistance. Sniezko and Koch (2017) and Sniezko and Nelson (2022) discuss the challenges and describe successes.
facilities at Dorena Genetic Resource Center; photos courtesy of Richard Sniezko, USFS
Complicating the restoration phase is the fact that the resistant tree must be able to thrive and compete in an ecosystem that has changed greatly from that in which it formerly resided. Causes of these changes include repercussions from the absence of the tree species – and possibly associated species; the possible presence of other biotic stresses (pests); and climate change. This is discussed by Nelson (2022). See also my blog.
Successfully completing these steps requires a long-term commitment, which includes significant funding and strong supportive infrastructure. Schlarbaum pointed out that the public and politicians don’t understand the complexity of the restoration challenge and the resources required. He documented the shrinking tree improvement infrastructure as of 2014. At that time, funding for all USFS regional breeding programs was just $6 million. State and land grant university breeding programs were fragmented and seriously underfunded. Only 28 states still had some type of tree improvement activity – and some of these programs were only seed orchards, not active breeding and testing programs. Members of university-industrial cooperatives focus on a small number of commercial species – which are not the species threatened by non-native pests. I believe these resources have shrunk even farther in the decade since 2014.
A separate source of funds for resistance breeding is the Forest Health Protection program, which is under the Deputy Chief for State, Private, and Tribal Forestry rather than the Deputy Chief for Research and Development. While nation-wide data on seed or scion collection or screening to identify and evaluate genetic resistance are poorly reported, Coleman et al. indicate that the USFS Dorena Genetic Resource Center screens unspecified “hundreds” of seed lots for resistance to pathogens annually. The Center also participates in seed, cone, and scion collections, especially of white pines vulnerable to white pine blister rust (WPBR). Supplemental Table S3 lists projects funded over the two decades analyzed by Coleman et al. (2011 – 2020). These included efforts to identify and evaluate possible genetic bases for resistance to, e.g., hemlock woolly adelgid, balsam woolly adelgid, laurel wilt, emerald ash borer, butternut canker, rapid ʻōhiʻa death; and gene conservation for eastern hemlock, ashes, chestnut, in addition to the five-needle pines. Currently, FHP allocates $1.2 million annually to support the group of activities called Genetic Conservation, Resistance and Restoration (R. Cooksey, pers. comm.).
American beech grafts to be tested for resistance to beech bark disease; at USFS center in Delaware, Ohio; photo courtesy of Jennifer Koch, USFS
USFS scientists involved in these projects describe challenges arising from efforts to cobble together funding from these many sources to support coherent programs. Overall funding levels still fall short of the need, and failure to obtain funding for one component of a program stymies the entire endeavor.
However, some developments are encouraging. The number of private foundations devoted to tree breeding has increased in the last decade. The American Chestnut Foundation (TACF) and American Chestnut Cooperators Foundation (ACCF) have been joined by the White Pine Ecosystem Foundation, the Great Lakes Basin Forest Health Collaborative, Forest Restoration Alliance, ‘Ohi‘a Disease Resistance Program … These organizations raise awareness, coordinate efforts by multiple parties, and provide opportunities for individuals to contribute funds and volunteer work.
In Hawai`i, disease resistance programs with both koa (Dudley et al.) and ʻōhiʻa ((Metrosideros polymorpha) (Luiz et al.) are active. Work with ash species to find and develop resistance to emerald ash borer is under way but limited due to lack of funds.
Finally, we can persuade Congress to incorporate the provisions of two bills, H.R. 3174 and S. 1238, into the next Farm Bill. The bills would, inter alia, create two grant program. One would fund research addressing specific questions impeding the recovery of native tree species that have suffered severe levels of mortality caused by non-native plant pests. The second would fund implementation of projects to restore these pest-decimated tree species to the forest.
Funded projects would be required to be part of a forest restoration strategy that incorporates a majority of the following components:
(1) Collection and conservation of native tree genetic material;
(2) Production of propagules of the target tree species in numbers sufficient for landscape-scale restoration;
(3) Preparation of planting sites in the target tree species’ former habitats;
Facilities needed to support successful breeding programs
Sniezko and Nelson identified these needs as follows:
(a) growing space (e.g., greenhouses);
(b) seed handling and cold storage capacity;
(c) inoculation infrastructure;
(d) field sites for testing;
(e) database capability for collecting, maintaining, and analyzing data;
(f) areas for seed orchard development;
(g) skilled personnel (tree breeders, data managers, technicians, administrative support personnel, and access to expertise in pathology and entomology).
There are very few facilities dedicated primarily to development of populations of trees with resistance to non-native pests; the most notable is the Dorena Genetic Resource Center. Even the existing programs require significant funding increases to accelerate current programs or expand to address additional species. Sniezko and Nelson stress further that a resistance breeding program has different objectives, magnitude and focus than most research projects. It is applied science, that is, an action-oriented effort that is solution-minded—countering the impact of a major disturbance caused by a pest (in our case, a non-native pest).
Schlarbaum provides a shorter but similar list of facilities needed:
production of propagules (seed or clones);
mass propagation in growing facilities, e.g., bare-root seedling nursery or greenhouses;
site preparation of former habitat and planting; and
post-planting maintenance.
Schlarbaum emphasized that each of these activities requires different skill sets, equipment, facilities, and infrastructure.
Genetic Engineering as a Specific Tool
There is considerable interest in the potential role of genetic engineering in pest resistance breeding. None of the successful programs world-wide has yet used genetic engineering (Sniezko and Koch 2017). While incorporating it into holistic breeding programs might result in greater efficiency for certain processes, it raises legal and social acceptability issues. Jacobs et al. discuss the type of education and outreach program needed to generate widespread public support this approach to tree species “rescues.” They call for USDA Forest Service to lead this education effort.
The focus of the 2021 workshop hosted by Purdue University was to explore the pros and cons of using biotechnology in restoring pest-threatened forest tree species. The special issue of New Forests contains several participants’ analyses.
The overall conclusions are that:
“Genetic engineering” – defined as “any technique that uses recombinant, synthesized, or amplified nucleic acids to modify a genome” – is only one type of biotechnology applicable to tree breeding. Other biotechnologies include tissue culture-based propagation, molecular-based genetic markers, gene cloning and sequencing, and genome mapping and sequencing.
These new technologies can increase the efficiency of more traditional breeding techniques, However, biotechnologies cannot substitute for holistic programs that incorporate all helpful methods. Careful consideration goes into selecting which techniques are appropriate for a particular host-pest system.
Each tree species has unique needs regarding seed or scion collection; seedling propagation in nurseries; site preparation and planting techniques; and management of regeneration after its re-introduction into forests. Scientists don’t yet understand these various needs of many threatened species.
In the eastern U.S., the tree-breeding infrastructure is based in the Southeast and focused on a few pine species grown commercially. The facilities do not match the greatest need. That is, many of the at-risk species are hardwoods native to the Northeast.
Current resources are inadequate to support the sustained, long-term commitment of resources and facilities necessary to be successful.
Dana Nelson addressed the role of genetic engineering (GE) in detail. He emphasized repeatedly that GE is not a short-cut to tree improvement. Incorporating a GE component does not avoid the other steps. It can, though, provide new possibilities to address problems. Nelson says the crucial, initial question is – can GE solve the specific forest conservation or management problem more effectively and efficiently than existing methods? There are some important subtleties to consider. First, success does not require achieving immunity (100% resistance); the level of resistance needs to be only sufficient to allow the tree species to survive, reproduce and co-evolve with the pest. Second, “efficiency” is an important consideration. We cannot afford delay because during those years or decades the wild tree loses genetic variability as more trees die. Also, changes in the environment continues to change, and the decimated tree species is not adapting.
If genetic engineering promises to contribute meaningfully, then the breeders must answer several follow-up questions before proceeding to develop a specific plan. Nelson also stresses that the planned activities must be integrated with an ongoing tree breeding program to ensure project success.
Nelson provides a lengthy description of the process of integrating genetic engineering into tree breeding programs.
GE in Chestnut Breeding – Setback
The most prominent breeding effort incorporating genetic engineering in the U.S. has been The American Chestnut Foundation’s (TACF) program to restore American chestnut (Castanea dentata). For decades, TACF has pursued development of trees resistant to the fungus which causes chestnut blight (Cryphonectria parasitica). Over the past decade, hopes have centered on a genetically engineered line into which was inserted a gene from wheat (oxalate oxidase; OxO). The OxO gene detoxifies the oxalic acid produced by the chestnut blight fungus and thus prevents the cankers from killing the tree.
Years of tests have shown the gene to be effective and to cause no environmental harm. In 2023, when trees in outside test plots grew larger, scientists observed disappointing results. Trees’ blight tolerance varied greatly. Worse, resistant trees grew more slowly and exhibited lower overall fitness. [For a full discussion of the issues, visit TACF’s website] Prompted by these disappointments, scientists carried out further molecular analyses. They found that the OxO gene was on a different chromosome than expected.
TACF researchers now suspect that the trees’ variable performance stems primarily from the placement of the OxO gene and the fact that the gene is always “switched on”. That constant expression appears to result in high metabolic costs for the trees. Since all the genetic lines developed to date have this defect, TACF is no longer pursuing research efforts with any of the GE trees developed to date. The Foundation believes it would be irresponsible to continue efforts – by itself and by partners – focused on a genetic line that looks unable to compete successfully when introduced to the forest.
Instead, TACF has begun investigating other transgenic lines that use a “wound inducible” promoter that switches on the OxO gene only in cells where the plant is wounded. Researchers at both the State University of New York College of Environmental Science and Forestry (SUNY-ESF) and the University of Georgia are working with a variety of inducible promoters. TACF is also testing whether inducible OxO expression can be “stacked”onto genes for blight resistance present in the backcross hybrids. Finally, TACF and Virginia Tech are also exploring whether resistance can be enhanced by insertion of genes from Chinese chestnut directly into American chestnut using methods similar to OxO insertion.
It will be years before we know if these approaches provide sufficient levels of resistance. TACF will undertake more extensive testing for efficacy through the tree’s full life cycle – in the lab, greenhouse, and field – before submitting a new GE organism to regulators for review. Meanwhile, it will continue rigorous testing for plant health and environmental risks and will strengthen the cooperative structure to facilitate sharing of intellectual property and provide full transparency.
The Darling GE line was the most important transgenic hybrid chestnut line TACF had invested in. So this is a major setback – and comes when regulatory approval seemed near.
Let’s keep this in perspective, however. As a colleague has said, based on his years of teaching science to middle school students, “There are no failures in science, just reductions in the unknown; Edison failed a thousand times before getting the light bulb right, etc….” The technology is ready when it is ready. In addition, he praised TACF for choosing to explain its decision frankly: “nothing builds credibility like early failures openly admitted.”
Meanwhile, TACF continues to make gains in blight resistance with its traditional American chestnut backcross hybrid breeding program. They have established a genetically diverse, reproducing population of thousands of trees representing hundreds of breeding lines. These trees are planted in TACF’s expansive network of germplasm conservation orchards and regional breeding and backcross orchards. They have substantially increased resistance to both the blight and Phytophthora cinnanomi in these populations. The future inclusion of transgenic and/or gene-edited trees will further increase those gains.
Another Approach
Meantime, the American Chestnut Cooperators Foundation (ACCF), which breeds from persistent pure American chestnut, now has some trees that are nearly 50 years old. The program has bred five generations of pure American chestnuts that show durable blight resistance. Many trees are 60 feet tall or higher; they produce nuts. Vice President Jenny Abla (pers. comm.) reports that they show excellent canker response (swollen and superficial). The picture shows one of their most notable stands, which is in the Jefferson National Forest. Dr. Sniezko is exploring whether this program shows sufficient promise to justify increased support from the USFS.
ACCF chestnut trees; photo courtesy of Jenna Abla
Improving Coordination – will funds follow?
In July 2023, representatives from essentially all the forest tree resistance breeding programs in the U.S. met at Dorena Genetic Resource Center in Oregon to discuss their current successes and how to fast-track all programs. This is the first such meeting since 1982 (Richard Sniezko, pers. comm.). I encourage us all to study the report when it emerges and encourage USFS leadership to support the more unified enterprise.
Status of Efforts to Conserve Other Tree Species
The special issue of New Forests (Vol. 54 Issue 4) included several articles exploring the specifics of breeding elms, ashes, and ʻōhiʻa. These describe difficult challenges … and scientists determined to make progress on overcoming them.
“survival” American elm at Longwood Gardens; photo by F.T. Campbell
Elms (Ulmus spp.) (see article by Martin et al.)
Let’s not forget that elms were keystone species in Europe and North America until attacked by two epidemics of “Dutch” elm disease during the 20th Century. While hybrid elms are available for urban plantings, many consider them not appropriate for planting in natural forests because these genotypes are not native.
Martin et al. describe a bewildering conglomeration of complexities and possibilities arising from biotic and abiotic factors. Initiation and especially intensity of the disease in a particular tree depend on
the species or strain of the tree, vectoring beetle, and pathogen;
timing of the attack; and
adequacy of water supplies at that time.
Possible targets for manipulation include the pathogen, its beetle vector, and the tree’s response — either in its bark or xylem. Martin et al. suggest that a combination of resistance to the pathogen within the xylem, resistance to beetles’ feeding wounds, and lowering tree clues that attract the beetles could considerably enhance longer-term overall resistance in the field.
However, verifying which approaches produce the best result will be complicated by the trees’ sensitivity to environmental factors such as season and water supply. Apparent resistance might actually be tied to, for example, low water supplies during the spring when the attack occurred.
Restoration strategies, including resistance to pests, must accommodate the diverse ecological conditions in the species’ large range, the rapid evolution of the Ophiostoma pathogens; and other pests and pathogens that attack elms. Nor do scientists know appropriate planting strategies.
Martin et al. believe Dutch elm disease is unlikely to be spread by movement of living elm plants, although other pests could be (and have been).
ash trees to be tested for resistance to emerald ash borer; photo courtesy of Jennifer Koch, USFS
Ashes (Fraxinus spp.)
While a USFS team led by Jennifer Koch link are conducting much of the on-the-ground efforts to breed ash trees resistant to the emerald ash borer (EAB; Agrilus plannipennis), Stanley et al. note that scientists cannot simply cross most North American ash species with the Asian ash, F. mandshurica, because the two groups are sexually incompatible. Scientists have instead focused on trying to enhance the resistance to EAB that is apparently present in a small proportion of ash trees, called “lingering ash.” Scientists funded by USDA Forest Service have already devoted over 14 years to finding such lingering ash to be tested for resistance.
Testing these trees is not simple (see Stanley et al.). But scientists are overcoming some of the obstacles. They have shown that the capability of a few green ash (Fraxinus pennsylvanica) (less than 1%) to defend themselves from EAB attack is genetic. Genes determine the relative abundance of specific metabolites manufactured by the tree; high levels kill more beetle larvae. These trees’ tolerance is not immunity but it might be sufficient to allow the tree to survive and grow. The level of metabolites synthesized by succeeding generations of the tree can probably also be enhanced by breeding.
To restore ash it is necessary to propagate large numbers of clones and to root the resulting embryos. This has been challenging. Merkle et al. describe five years of efforts to develop techniques that allow in vitro propagation to speed up selection and breeding. These techniques will facilitate establishment of numerous groups of propagules with the genetic differences needed to accommodate the large geographic range of several ash trees. For example, the green ash range covers more than half the continental U.S. plus multiple Canadian provinces.
ʻōhiʻa on lava field, Hawaii Volcanoes National Park
‘Ōhi‘a (Metrosideros polymorpha)
‘Ohi‘a is the most widespread tree species on the Hawaiian Islands. It provides vitally important habitat for conservation of countless taxa of endemic birds, insects, and plants. It is also of great cultural importance for Native Hawaiians.
Luiz et al. review the tree species’ importance, the many threats to native Hawaiian forests, and a coalition’s efforts to counter the most recent – and alarming – threat, rapid ʻōhiʻa death (ROD).
Rapid ʻōhiʻa death is caused by two introduced species of in the genus Ceratocystis. C. lukuohia colonizes the tree’s sapwood and kills the tree quickly. This disease is present on two islands, Hawai`i and Kaua‘i. It has the potential to devastate ‘ohi‘a forests across the state. The other pathogen, C. huliohia, invades the phloem, cambium, and outer xylem, resulting in a well-defined area of necrotic tissue and slower mortality. This disease is on Hawai`i and Kaua‘i, plus Maui and O‘ahu. The two pathogens have different origins. C. lukuohia belongs to a genetic line that is based in Latin America, C. huliohia to a genetic line based in Asia and Australia.
Conservationists formed a coalition and developed a strategy to guide the process of identifying and developing disease resistance in M. polymorpha and, if possible, other Metrosideros species on the Islands. Luiz et al. describe the coalition’s many activities. The challenges are familiar ones:
obtaining sufficient facilities to screen large numbers of seedlings;
developing techniques for inoculation, propagation, and speeding up growth of seedlings;
improving techniques for detecting individual infected and healthy trees across difficult terrain;
testing trees native to all parts of the tree’s range, which is not large in area, but covers a great variety of elevations and climates); and
needing to develop trees resistant to both C. lukuohia and C. huliohia.
Luiz et al. reiterate the necessity to manage all threats to healthy ʻōhiʻa stands, for example, by
curtailing human spead of infected wood, using both quarantines and supportive public education;
testing repellants to reduce beetle attack.
reducing injuries to trees by fencing forests and removing feral ungulates. link to website?
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Posted by Faith Campbell
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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
