Category: link to climate change

blogs that relate P&P or invasive plants to climate change; or discuss assisted migration; CO2 storage; etc.

Planting Trees to Sequester Carbon – Beware the Wrong Places!

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

or

www.fadingforests.org

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

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

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

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

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

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

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

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

Why we need to restructure the behavior of governments and societies

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

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

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

Critically important information that is often unspoken:

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

More Key Findings

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

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

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

Where the Report Is Weak: Interim Steps

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

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

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

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

SOURCES

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

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

Posted by Faith Campbell

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

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

or

www.fadingforests.org

Birds v. mosquitoes: hope in Hawai`i

‘i‘iwi (Drepanis coccinea) – formerly very common from low to high elevations; photo by James Petruzzii_U

The endangered honeycreepers (birds) of Hawaiian forests are receiving the attention they deserve – and desperately need. There is good news! Promising and significant efforts are under way, matched to a recent strategic plan.  However, it is too early to know their results.

Nearly two and a half years ago, I blogged about efforts by a multi-agency consortium (“Birds, Not Mosquitoes” ). It was working to suppress populations of non-native mosquitoes, which vector two lethal diseases: avian malaria (Plasmodium relictum) and avian pox virus (Avipoxvirus). A single bite from an infected mosquito is enough to weaken and kill birds of some species, e.g., the ‘i‘iwi.

The threats from these diseases – and their spread to higher elevations as mosquitoes respond to climate change – pile on top of – other forms of habitat loss and inroads by other invasive species. All of the 17 species of honeycreeper that have persisted until now are listed as endangered or threatened under the federal Endangered Species Act. Four are in danger of extinction within as little as 1 – 2 years. These are ‘Akeke`e (Loxops caeruleirostris), ‘Akikiki (Oreomsytis bairdi)), Kiwikiu (Maui parrotbill, (Pseudonestor xanthophrys), and `Akohekohe (Palmeria dolei).

Akikiki; photo by Carter Atkinson, USGS

All these bird species are endemic to the Hawaiian archipelago — found nowhere else on Earth. They are already remnants. Nearly 80 bird species have gone extinct since people first colonized the Hawaiian Islands 1,500 years ago. Eight of these extinctions were recognized in October 2021.  Extinction of the final cohort would compromise the integrity of unique ecosystems as well as the Islands’ natural and cultural heritage.

I rejoice to report that the federal government has responded to the crisis. In late 2022 several Interior Department agencies adopted a multiagency Strategy for Preventing the Extinction of Hawaiian Forest Birds. The strategy specifies responsibilities for the key components of the program. These include: a) planning and implementing landscape-level mosquito control using Incompatible Insect Technique (IIT); b) translocating birds to higher elevation sites on other Hawaiian islands; c)  establishing captive populations of at-risk birds; and d) developing next-generation tools that increase the scope or efficacy of these actions. All these activities are being developed and conducted through intensive consultation with Native Hawaiians.

On August 8, 2023, the Secretary of Interior announced the allocation of $15,511,066 for conservation and recovery efforts for Hawaiian forest birds. About $14 million of the total was from the Bipartisan Infrastructure Law (Public Law 117-58). The funds are being channelled primarily through the U.S. Fish and Wildlife Service (FWS) ($7.5 million) and the National Park Service (NPS) ($6 million). Other sources of funding are the “State of the Birds” Program (FWS – $963,786); the national-level competitive Natural Resource grants program (NPS – $450,000); and the Biological Threats Program of the U.S. Geological Survey (USGS – $100,000).

What Is Under Way

I do worry continuing these efforts will be harder once their funding is subject to annual appropriations. However, they are a good start!

Steps have been taken on each of the four key component of the Strategy for Preventing the Extinction of Hawaiian Forest Birds:  

a) Planning and implementing landscape-level mosquito control using Incompatible Insect Technique (IIT – see below) to reduce the mosquito vector of avian malaria.

  • The Consortium has obtained all necessary state permits, regulatory approval of the approach by the U.S. Environmental Protection Agency, and done required consultations under the Endangered Species Act.
  • The Department of the Interior has funded a public-private partnership between the National parks and The Nature Conservancy (TNC) to develop, test, and carry out the first deployments of IIT. These occurred in May 2023 at high-elevation sites on the island of Maui. The next releases are planned for Kaua`i.
  • Consortium participants are carrying out the consultations and scientific preparations need to support the next deployment on the Big Island.

b) Translocating birds to higher elevation sites on the one island where they exist – Hawai`i.

  • Initial planning has begun to guide translocation of the endangered Kiwikiu (Maui parrotbill) and Akohekohe to higher-elevation, mosquito-free, habitats on the Big Island.

c) Establishing captive populations of the most at-risk species

  • To facilitate captive breeding of the four most endangered species, the two existing aviaries in Hawai`i need to be expanded. Space must be provided for at least 80 more birds. A contract has been signed for construction of this new aviary space.

d) Developing next-generation tools that increase the scope or efficacy of these actions.

  • Lab capacity has been expanded to monitor the effectiveness of IIT, as well as for developing next-generation mosquito control tools.
those who decide funding work here … & they work for us!!!!

The Incompatible Insect Technique (IIT) explained

The incompatible insect technique has been used successfully elsewhere to combat mosquitoes that transmit human diseases. Many insect taxa – including mosquitoes – harbor a naturally-occurring bacteria (Wolbachia). This bacterium has more than one strain or type. When a male mosquito with one type of Wolbachia mates with a female mosquito bearing a different, incompatible type, resulting eggs do not hatch. The IIT project releases male mosquitoes that have an incompatible strain of the bacterium than do local females. (Male mosquitoes do not bite animals seeking a blood meal, so releasing them does not increase the threat to either birds or people.) Implementation requires repeat treatment of sites at a cost of more than $1 million per site per year. It is hoped that this cost will fall with more experience.

Funding for the Strategy’s Four Components

As I noted above, much of the funding for these efforts has come from the Bipartisan Infrastructure Law (Public Law 117-58). Grants under this one-time statute are intended to cover project costs for perhaps five years. Other sources of funds are Congressional appropriations to Interior Department agencies under programs which presumably will continue to be funded in future years. These include the “State of the Birds” program; Endangered Species Act (ESA) implementation, especially its §6 Cooperative Endangered Species Conservation Fund; and State Wildlife Grants administered by the U.S. Fish and wildlife Service. However, funding under these programs is never guaranteed and competition is fierce. I hope participants – and the rest of us! – can be effective in lobbying for future funds required to save Hawaii’s birds from extinction.

a) Deploying IIT

Over Fiscal Years 2017 – 2021 (ending September 2021), Interior Department agencies supported the IIT program by:

  • Providing $948,000  to the State of Hawai`i from “State of the Birds”, State Wildlife Grants, and Endangered Species Act (ESA) §6;
  • The U.S. Fish and Wildlife Service  provided $545,000 plus staff time’ 
  • National Park Service  provided $1.2 million for IIT preparations at Haleakala National Park and surrounding state and Nature Conservancy lands
  • U.S. Geological Survey provided about $7.05 million in research on Hawaiian forest birds, invasive mosquitoes, and avian malaria.

The State of Hawai’i allocated $503,000 and employee staff time.

In addition,

  • the National Fish and Wildlife Fund provided a total of $627,000 in grants to TNC and American Bird Conservancy for Wolbachia IIT.
  • TNC committed to supporting some of the initial costs to deploy Wolbachia IIT for the first site in Hawai`i through a contractor (see below)
  • American Bird Conservancy provided funding for coordination and public outreach.

In FY2022 (which ended in September 2022),

  • NPS provided $6 million for on-the-ground work on Maui, also development and initial production of Wolbachia IIT.
  • Interior Department Office of Native Hawaiian Relations provided in-kind services to engage with Native communities’ members

b) Moving endangered birds to mosquito-free areas at high elevations on the Big Island

This is planned to begin by 2030. Interior committed unspecified funds to planning and consultation with Native Hawaiians.

c) Rearing captive birds

 FWS supports operation of the two existing aviaries through two funding channels: $700,000 annually provided directly to the aviaries, plus another $500,000 per year through ESA §6through the State of Hawai`i. The San Diego Zoo – which operates the aviaries — provides $600,000 – $800,000 per year in the form of in-kind services, staffing, veterinarians, and administrative support. Interior’s Office of Native Hawaiian Relations provided in-kind services to support to engagement with Native Hawaiian community members

d) Regarding exploration of “next-generation” mosquito control tools

The FWS provided $60,000 to a scientific laboratory to study precision-guided Sterile Insect Technique (pgSIT) tools to protect bird species threatened by avian malaria.

Funding for the portions of these programs dependent upon annual appropriations is uncertain. Current signs are promising: House and Senate bills to fund for the current year (Fiscal Year 2024) – which began in October 2023! – both support at least some aspects of the program. According to American Bird Conservancy, the Senate appropriations bill has allocated $2.5 million to parts of the program. According to the Committee report, the House appropriations bill allots $4.7 million to the State of the Birds program to respond to urgent needs of critically endangered birds. The report goes on to direct the FWS to “incorporate adaptation actions into new and revised recovery plans and recovery implementation strategies, such as with the mosquito vector of avian pox & malaria in the revised Hawaiian Forest Birds recovery plan. …” Per the report, the Appropriations Committee “continues to encourage the [NPS] to respond to the urgent landscape-scale needs of critically endangered forest birds with habitats in national parks.” The report then specifies species threatened by non-native mosquitoes carrying avian malaria and other pathogens. Finally, the report allocates $500,000 to the U.S. Geological Survey for research on the Hawaiian forest birds.

Meanwhile, the American Bird Conservancy is preparing to advocate for $20 million for FY25 through “State of the Birds” Activities and associated NPS and USGS programs. The details of this amount have not yet been laid out.

CISP will support this request and urges you to do so also. We will suggests ways to help when we know more.

Posted by Faith Campbell

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

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

or

www.fadingforests.org

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

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

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

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

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

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

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

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

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

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

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

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

1. Integrate Climate and Biodiversity Efforts

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

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

2. Enhance Ecosystem Integrity

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

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

3. Equitably transforming the way we live

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

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

4. Prop the Window Open

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

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

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

SOURCES

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

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

Posted by Faith Campbell

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

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

or

www.fadingforests.org

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

lesser celandine invade bottomlands of Delaware Water Gap National Recreation Area

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

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

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

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

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

SOURCES

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

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

Posted by Faith Campbell

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

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

or

www.fadingforests.org

The White Mountain forest in New Hampshire: 80 years of change, with more ahead

hemlock woolly adelgid

I have been disappointed  that a research symposium focused on the northern hardwood forest workshop gave little attention to non-native pests (see citation at end of this blog). A new study based in the Bartlett Experimental Forest in the White Mountains of New Hampshire is more balanced. Ducey et al. (full citation at the end of this blog) analyzed changes in the forest’s species composition and tree size over the past 80 years.

They found that trees of nearly all species are growing into larger sizes as the forest continues to age since the last widespread clearing at the end of the 19th Century. The same aging is causing a rapid decline in two shade-intolerant species – paper birch (Betula papyrifera) and aspen (Populus tremuloides and P. grandidentata) – which had grown quickly once the cleared areas were abandoned. The mid-shade -tolerant species yellow birch (Betula alleghaniensis) also is declining. Together, the birch and aspen species have declined from a quarter to a third of basal area in 1931 to 10 – 12% in 2015.

Some developments are unexpected. Red maple (Acer rubrum) increased in abundance until the early 1990s, but that growth then levelled off. Sugar maple (Acer saccharum) has declined in abundance except where the forest is managed to retain it.

There is little evidence of tree species migrating upward on slopes in response to changes in the local climate.  Major weather events – a hurricane in 1938 and an ice storm in 1998 — caused significant tree mortality across Bartlett Experimental Forest, but not a dramatic change in forest composition.

Eastern hemlock (Tsuga canadensis) is replacing the disappearing birch and aspen on low elevation sites. Hemlock has increased its proportion of basal area from 8 – 10% to a quarter or more. Despite aggressive management aimed at reducing the tree’s presence, American beech (Fagus grandifolia) is on track to dominate large areas of the Bartlett Experimental Forest. Given the tree-killing pests already present in the region, large increases in eastern hemlock, American beech, and red spruce (Picea rubens) are worrying.

Eastern hemlock creates important wildlife habitat for deer and more than 100 other vertebrate species in New England. However, hemlock woolly adelgid (HWA) has been present in New Hampshire since 2000. It is now within 15-20 km of Bartlett Experimental Forest. There is some hope that the region’s cold temperatures might limit HWA’s spread and impacts, but Ducey et al. expect major change when the adelgid arrives.  

beech saplings; photo by FT Campbell

Ducey et al. cite a separate study demonstrating that mortality caused by beech bark disease (BBD) can be sufficient to upset carbon storage in old-growth forests. On the Bartlett Forest, nearly 90% of beech trees had become diseased by 1950.

Ducey et al. express concern about the possible impact of beech leaf disease (BLD), as well.

BLD has not yet been detected in the White Mountains or New Hampshire, but is in so New England and coastal Maine. Much remains unknown about the disease, including how it spreads and its long-term impacts.

Ducey et al. do not raise pest concerns about red spruce or balsam fir (Abies balsamea), which co-dominate the Bartlett Forest at higher elevations (above 500 m). This silence is disturbing since red spruce can be killed by the brown spruce longhorned beetle, a European woodborer established in Nova Scotia and threatening to spread south. Balsam firs suffer some mortality from feeding by the balsam woolly adelgid, a Eurasian sap-sucker which has been in New England for more than a century.

brown spruce longhorned beetle

White ash (Fraxinus americana) is present as a minor component of the Bartlett Forest. Because it is considered to be a valuable timber species, management has resulted in a modest increase in abundance of ash. Ducey et al. expect dramatic reduction — or even elimination of the species — when the emerald ash borer (EAB) arrives. EAB has been detected within ~ 15 km from Bartlett Experimental Forest.

Ducey et al. conclude that silvicultural management applied at the scope and intensity of that in the Bartlett Experimental Forest has moderated some changes. That is, it is maintaining sugar maple and suppressing the increase of beech. Its effect is secondary, however to overall forest development as the forest ages.

SOURCES

Ducey, M.J, O.L., Yamasaki, M. Belair, E.P., Leak, W.B. 2023.  Eight decades of compositional change in a managed northern hardwood landscape. Forest Ecosystems 10 (2023) 100121

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

Posted by Faith Campbell

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

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

or

www.fadingforests.org

USFS Lays Out Incomplete Picture of the Future

tanoak trees in southern Oregon killed by sudden oak death; photo by Oregon Department of Forestry; this pathogen is not mentioned by USFS RPA report

In August the USDA Forest Service published the agency’s 2020 assessment of the future of America’s forests under the auspices of the Resources Planning Act. [See United States Department of Agriculture Forest Service Future of America’s Forests and Rangelands, full citation at the end of the blog.] To my amazement, this report is the first in the series (which are published every ten years) to address disturbance agents, specifically invasive species. In 2023! Worse, I think its coverage of the threat does not reflect the true state of affairs – as documented by Forest Service scientists among others.

This is most unfortunate because policy-makers presumably rely on this report when considering which threats to focus on.

Here I discuss some of the USFS RPA report and what other authors say about the same topics.

The RPA Report’s Principle Foci: Extent of the Forest and Carbon Sequestration

The USFS RPA report informs us that America’s forested area will probably decrease 1- 2% over the next 50 years (from 635.3 million acres to between 619 and 627 million acres), due largely to conversion to other uses. This decline in extent, plus trees’ aging and increases in disturbance will result in a slow-down in carbon sequestration by forests. In fact, if demand for wood products is high, or land conversion to other uses proceeds apace, U.S. forest ecosystems are projected to become a net source of atmospheric CO2 by 2070.

Eastern forests sequester the majority of U.S. forest carbon stocks. These forests are expected to continue aging – thereby increasing their carbon storage. Yet we know that these forests have suffered the greatest impact from non-native pests.

I don’t understand why the USFS RPA report does not explicitly address the implications of non-native pests. In 2019, Songlin Fei and three USFS research scientists did address this topic. Fei et al. estimated that tree mortality due to the 15 most damaging introduced pest species have resulted in releases of an additional 5.53 terragrams of carbon per year. Fei and colleagues conceded this is probably an underestimate. They say that annual levels of biomass loss are virtually certain to increase because current pests are still spreading to new host ranges (as demonstrated by detection of the emerald ash borer in Oregon). Also, infestations in already-invaded ranges will intensify, and additional pests will be introduced (for example, beech leaf disease).

I see this importance of eastern forests in sequestering carbon as one more reason to expand efforts to protect them from new pest introductions, and the spread of those already in the country, etc.

A second issue is the role of non-native tree species in supporting the structure and ecological functions of forests. Ariel Lugo and colleagues report that 18.8 million acres (7.6 million ha, or 2.8% of the forest area in the continental U.S.) is occupied by non-native tree species. (I know of no overall estimate for all invasive plants.) They found that non-native tree species constitute 12–23% (!) of the basal area of those forest stands in which they occur.

Norway maple (Acer platanoides); one of the most widespread invasive species in the East. Photo by Hermann Falkner via Flickr

Lugo and colleagues confine their analysis of ecosystem impacts to carbon sequestration. They found that the contribution of non-native trees to carbon storage is not significant at the national level. In the forests of the continental states (lower 48 states), these trees provide 10% of the total carbon storage in the forest plots where they occur. (While Lugo and colleagues state that the proportion of live tree biomass made up of non-native tree species varies greatly among ecological subregions, they do not provide examples of areas on the continent where their biomass – and contribution to carbon storage — is greater than this average.) In contrast, on Hawai`i, non-native tree species provide an estimated 29% of live tree carbon storage. On Puerto Rico, they provide an even higher proportion: 36%.

Brazilian pepper (Schinus terebinthifolius) – widespread invasive in Hawai`i and Florida; early stage invasive in Puerto Rico. Photo by Javier Alexandro via Flickr

In the future, non-native trees will play an even bigger role. Since tree invasions on the continent are expanding at ~500,000 acres (202,343 ha) per year, it is not surprising that non-native species’ saplings provide 19% of the total carbon storage for that size of trees in the lower 48 states (Lugo et al.).

Forming a More Complete Picture: Biodiversity, Disturbance, and Combining Data.

The USFS RPA report has a chapter on biodiversity. However, the chapter does not discuss historic or future diversity of tree species within biomes, nor the genetic diversity within tree species.

Treatment of Invasive Species

The USFS 2020 RPA report is the first to include a chapter on disturbance, including invasive species. I applaud its inclusion while wondering why they have included it only now? Why is the coverage so minimal? I think these lapses undercut the report’s purpose. The RPA is supposed to inform decision-makers and stakeholders about the status, trends, and projected future of renewable natural resources and related economic sectors for which USFS has management responsibilities. These include: forests, forest products, rangelands, water, biological diversity, and outdoor recreation. The report also has not met its claim to “capitalize on” areas where the USFS has research capacity. One excuse might be that several important publications have appeared after the cut-off date for the assessment (2020). Still, the report’s authors cite some of the evaluations that were in preparation as of 2020, e.g., Poland et al.

I suggest also that it would be helpful to integrate data from other agencies, especially the invasive species database compiled by the U.S. Geological Survey, into the RPA. For example, the USGS lists just over 4,000 non-native plant species in the continental U.S. (defined as the lower 48 plus Alaska). On Hawai`i, the USGS lists 530 non-native plant species as widespread. Caveat: many of the species included in these lists probably coexist with the native plants and make up minor components of the plant community.

Specifically: Invading Plants

The USFS RPA report gives much more attention to invasive plants than non-native insects and pathogens. The report relies on the findings of Oswalt et al., who based their data on forested plots sampled by the Forest Inventory and Analysis (FIA) program. (The RPA also reports on invasive plants detected on rangelands, primarily grasslands.) Oswalt et al. found that 39% of FIA plots nationwide contained at least one plant species that the FIA protocol considers to be invasive and monitors. The highest intensity of plant invasions is in Hawai`i – 70% of the plots are invaded. The second-greatest intensity is in the eastern forests: 46%. However, the map showing which plots were inventoried for invasive plants makes clear how incomplete these data are – a situation I had not realized previously.

I appreciate that the USFS RPA report mentions that propagule pressure is an important factor in plant invasions. This aspect has often been left out in past analyses. I also appreciate the statement that international trade in plants for ornamental horticulture will probably lead to additional introductions in the future. Third, I concur with the report’s conclusions that once forest land is invaded, it is unlikely to become un-invaded. Invasive plant management in forests often results in one non-native species being replaced by another. In sum, the report envisions a future in which plant invasion rates are likely to increase on forest land.

If you wish to learn more about invasive plant presence and impacts, see the discussion of invasive plants in Poland et al., my blogs based on the work by Doug Tallamy, and several other of my blogs compiled under the category “invasive plants” on this website.

I believe all sources expect that the area invaded by non-native plant species, and the intensity of existing invasions, will increase in the future.

The USFS RPA links these invasions to expansion of the “wildland-urban interface” (“WUI”). These areas increased rapidly before 2010. At that time, they occupied 14% of forest land. The report published in 2023 did not assess their future expansion over the period 2020 to 2070. However, it did project increased fragmentation in many regions, especially in the RPA Western and Southeastern regions. Since “fragmentation” is very similar to wildland-urban interfaces, the report seems implicitly to project more widespread plant invasions in the future.

plant invasions facilitated by fragmentation; northern Virginia; photo by F.T. Campbell

Specifically: Insects and Pathogens

The USFS RPA report on insects and pathogens is brief and contains puzzling errors and gaps. It says that the tree canopy area affected by both native and non-native mortality-causing agents has been consistently large over the three most recent five-year FIA assessment periods. It notes that individual insects or diseases have extirpated entire tree species or genera and fundamentally altered forests across broad regions. Examples cited are chestnut blight and emerald ash borer.

The USFS RPA report warns that pest-related mortality might be underreported in the South, masked by more intense management cycles and higher rates of tree growth and decay. On the other hand, the report asserts that pest-related mortality is probably overrepresented in the Northern Region in the 2002 – 2006 period because surveyors drew polygons to encompass large areas affected by EAB and balsam woolly adelgid (Adelges piceae) infestations. The latter puzzles me; I think it is probably an error, and should have referred to hemlock woolly adegid, A. tsugae. Documented mortality has generally been much more widespread from insects than diseases, e.g., bark beetles, including several native ones, across all regions and over time, especially in the West – where the most significant morality agents are several native beetles. The USFS RPA report mentions that the Northern Region has been particularly affected by non-native pests, including EAB, HWA, BWA, beech bark disease, and oak wilt. It mentions that Hawai`i has also suffered substantial impacts from rapid ʻōhiʻa death.  

Defoliating insects have affected relatively consistent area over time. This area usually equaled or exceeded the area affected by the mortality agents. Principal non-native defoliators in the Northern Region have been the spongy moth (Lymantria dispar); larch casebearer (Coleophora laricella); and winter moth (Operophtera brumata). In the South they list the spongy moth.

More disturbing to me is the USFS RPA report’s conclusion that the future impact of forest insects is highly uncertain. The authorsblame the complexity of interactions among changing climate, those changes’ effects on insect and tree species’ distributions, and overall forest health. Also, they name uncertainty about which new non-native species will be introduced to the United States. I appreciate the report’s avoidance of blanket statements regarding the effects of climate change. However, other studies – e.g., Poland et al. – have incorporated these complexities while still offering conclusions about a number of currently established non-native pests. Finally, I am particularly dismayed that the USFS RPA does not provide analysis of any forest pathogens beyond the single mention of a few.

I am confused as to why the USFS RPA report makes no mention of Project CAPTURE (Conservation Assessment and Prioritization of Forest Trees Under Risk of Extirpation). This is a multi-partner effort to prioritize U.S. tree species for conservation actions based on invasive pests’ threats and the trees’ ability to adapt to them. Several USFS units participated, including the Southern Research Station, the Eastern Forest Environmental Threat Assessment Center, and the Forest Health Protection program. The findings were published in 2019. See here. Lead scientist Kevin Potter was one of the authors of the RPA’s chapter on disturbance.

redbay (Persea borbonia) trees in Georgia killed by laurel wilt; photo by Scott Cameron. Redbay is ranked by Project CAPTURE as 5th most severely at risk due to a non-native pest

“Project CAPTURE” provided useful summaries of non-native pests’ impacts, including the facts that

  •  54% of the tree species on the continent are infested by one or more non-native insect or pathogen;
  • nearly 70% of the host/agent combinations involve angiosperm (broadleaf) species, 30% gymnosperms (e.g., conifers). When considering only non-native pests, pests attacking angiosperms had greater average severity.
  • Disease impacts are more severe, on average, than insect pests. Wood-borers are more damaging than other types of insect pests.
  • Non-native agents have, on average, considerably more severe impacts than native pests.

Project CAPTURE also ranked priority tree species based on the threat from non-native pests  (Potter et al., 2019). Tree families at the highest risk to non-native pests are: a) Fagaceae (oaks, tanoaks, chestnuts, beech), b) Sapindaceae (soapberry family; includes maples, Aesculus (buckeye, horsechestnut); c) in some cases, Pinaceae (pines); d) Salicaceae (willows, poplars, aspens); e) Ulmaceae (elms) and f) Oleaceae (includes Fraxinus). I believe this information should have been included in the Resources Planning Act report in order to insure that decision-makers consider these threats in guiding USFS programs.

I also wish the USFS RPA had at least prominently referred readers to Poland et al. Among that study’s key points are:

  • Invasive (non-native) insects and diseases can reduce productivity of desired species, interactions at other trophic levels, and watershed hydrology. They also impose enormously high management costs.
  • Some non-native pests potentially threaten the survival of entire tree genera, not just individual species, e.g., emerald ash borer and Dutch elm disease.  I add white pine blister rust and laurel wilt.
  • Emerald ash borer and hemlock woolly adelgid are listed as among the most significant threats to forests in the Eastern US.
  • White pine blister rust and hemlock woolly adelgid are described as so profoundly affecting ecosystem structure and function as to cause an irreversible change of ecological state.
  • Restoration of severely impacted forests requires first, controlling the non-native pest, then identifying and enriching – through selection and breeding – levels of genetic resistance in native populations of the impacted host tree. Programs of varying length and success target five-needle pines killed by Cronartium ribicola; Port-Orford cedar killed by the oomycete Phytophthora lateralis; chestnut blight; Dutch elm disease; butternut canker (causal agent Ophiognomonia clavigignenti juglandacearum), emerald ash borer; and hemlock woolly adelgid.
  • Climate change will almost certainly lead to changes in the distribution of invasive species, as their populations respond to increased variability and longer-term changes in temperature, moisture, and biotic interactions. Predicting how particular species will respond is difficult but essential to developing effective prevention, control, and restoration strategies.

Poland et al. summarizes major bioinvaders in several regions. Each region except Hawai`i (!!) includes tree-killing insects or pathogens.

It is easier to understand the RPA report’s not mentioning priority-setting efforts by two other entities, the Morton Arboretum and International Union for the Conservation of Nature (IUCN). These studies were published in 2021 and their lead entities were not the Forest Service – although the USFS helped to fund the U.S. portion of the studies.

The Morton Arboretum led in the analysis of U.S. tree species. It published studies evaluating the status of tree species belonging to nine genera, considering all threats. The Morton study ranked as of conservation concern one third of native pine species; 31% of native oak species; significant proportion of species in the Lauraceae. The report on American beech — the only North American species in the genus Fagus – made no mention of beech leaf disease – despite it being a major concern in Ohio – only two states away from the location of the Morton Arboretum near Chicago.

valley oak (Quercus lobata) in Alameda Co, California; photo by Belinda Lo via Flickr

Most of the species listed by the Morton Arboretum are of conservation concern because of their small populations and restricted ranges. The report’s coverage of native pests is inconsistent, spotty, and sometimes focuses on odd examples.

Tree Species’ Regeneration

Too late for consideration by the authors of the USFS RPA report come new studies by Potter and Riitters that evaluate species at risk due to poor regeneration. This effort evaluated 280 forest tree species native to the continental United States – two-thirds of the species evaluated in the Kevin Potter’s earlier analysis of pest impacts.

The results of Potter and Riitters 2023 only partially matched those of the IUCN/Morton studies. The Morton study did not mention three genera with the highest proportions of poorly reproducing species according to Potter and Riitters: Platanus, Nyssa, and Juniperus. Potter, Morton, and the IUCN largely agree on the proportion of Pinus species at risk. Potter et al. 2023 found about 11% of oak species to be reproducing poorly, while Morton designated a third of 91 oak species to be of conservation concern.

I believe Potter and Riitters and the Morton study agree that the Southeast and California are geographic hot spots of tree species at risk.

Potter and Riiters found that several species with wide distributions might be at risk because they are reproducing at inadequate rates. Three of these exhibit poor reproduction across their full range: Populus deltoids (eastern cottonwood), Platanus occidentalis (American sycamore), and ponderosa pine(Pinus ponderosa). Four more species are reported to exhibit poor reproduction rates in all seed zones in which they grow (the difference from the former group is not explained). These are two Juniperus, Pinus pungens, and Quercus lobata. As I point out in my earlier blog, valley oak is also under attack by the Mediterranean oak borer.

SOURCES

Fei, S., R.S. Morin, C.M. Oswalt, and A.M. 2019. Biomass losses resulting from insect and disease invasions in United States forests. Proceedings of the National Academy of Sciences. Vol. 116, No. 35. August 27, 2019.

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

Poland, T.M., T. Patel-Weynand, D.M. Finch, C.F. Miniat, D.C. Hayes, V.M. Lopez, eds. 2021. Invasive Species in Forests and Rangelands of the United States: A Comprehensive Science Synthesis for the United States Forest Sector. Springer Verlag. Available gratis at https://link.springer.com/book/10.1007/978-3-030-45367-1

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

Potter, K.M. and Riitters, K. 2023. A National Multi-Scale Assessment of Regeneration Deficit as an Indicator of Potential Risk of Forest Genetic Variation Loss. Forests 2022, 13, 19. https://doi.org/10.3390/f13010019

United States Department of Agriculture Forest Service. 2023. Future of America’s Forests and Rangelands: The Forest Service 2020 Resource Planning Act Assessment. GTR-WO-102 July 2023

Posted by Faith Campbell

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

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

or

www.fadingforests.org

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

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

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

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

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

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

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

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

Naming the invaders

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

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

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

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

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

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

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

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

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

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

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

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

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

SOURCES

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

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

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

Posted by Faith Campbell

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

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

or

www.fadingforests.org

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

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

Fire and Invasive Grasses

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

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

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

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

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

On the other hand, there are hopeful signs.

endangered ‘akikiki photo by Carter Atkinson, USGS

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

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

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

Plant Invasions in Hawaiian Forests

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

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

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

Psidium cattleyanum; photo by Forrest and Kim Starr

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

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

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

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

Protecting Native Trees

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

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

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

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

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

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

Native Trees at Risk to Nonnative Insects

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

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

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

Rhus sandwicensis on Maui; photo by Forrest and Kim Starr

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

In Conclusion

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

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

SOURCE

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

Posted by Faith Campbell

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

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

or

www.fadingforests.org

Tree Regeneration Rates: A Tool for Prioritizing Tree Conservation Efforts

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

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

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

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

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

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

A New Approach to Evaluating Species at Risk

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

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

Native Trees at Risk: Focus on Poor Regeneration

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

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

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

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

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

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

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

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

Seed Zones: a Proxy for Local Genotypes

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

Bald-cypress; photo by Kej605 via WikiMedia

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

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

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

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

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

Applying This Analysis to Invasions by Non-native Trees

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

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

Differentiating Invasive Tree Species’ Impacts

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

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

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

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

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

 
Acer platanoides
Ailanthus altissima
Albizia julibrissin
Alnus glutinosa
Castanea mollissima
Casuarina lepidophloia
Cinnamomum camphora
Citrus sp.
Elaeagnus angustifolia
Eucalyptus globulus
Eucalyptus grandis
Ginko biloba
Melaleuca quinquenervia
Melia azedarach
Morus alba
Paulownia tomentosa
Picea abies
Pinus nigra
Pinus sylvestris
Populus alba
Prunus avium
Prunus persica
Salix alba
Salix sepulcralis
Sorbus aucuparia
Tamarix spp
Triadica sebifera
Ulmus pumila
Vernicia fordii

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Limits of the FIA Dataset

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

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

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

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

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

SOURCES

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

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

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

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

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

Posted by Faith Campbell

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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

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www.fadingforests.org