Research

Tree Conservation Biology

Checklist and Threat Assessments of Tree Species Native to the Contiguous United States

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Understanding the current state of trees within the U.S. is imperative to protecting those species, their habitats, and the countless communities they support. Until 2022, most species native to the contiguous U.S. were not assessed or were outdated in the two most widely used threat assessment platforms in the U.S.: International Union for Conservation of Nature (IUCN) Red List and NatureServe.

With the goal of cooperative action in mind, The Morton Arboretum coordinated an updated, standardized checklist and threat analysis of all native trees in the contiguous United States. These efforts were undertaken in partnership with Botanic Gardens Conservation International-US, NatureServe, United States Botanic Garden, and the USDA Forest Service.

The final report presents a summary of the analysis, including patterns in threats and conservation efforts for the most at-risk species. The authors report that approximately 11 to 16 percent of the 881 tree species in the U.S. are threatened with extinction, with the greatest threats being invasive pests and diseases.

The report, published in a special issue of Plants, People, Planet focused on the Global Trees Assessment, details the repeatable assessment process to further collaboration in global tree conservation.

Researchers encourage the conservation community to use these results to prioritize and advocate for strategic species and ecosystem conservation to ensure the future of trees.

The State of Trees in the U.S.

  • Based on the standardized Global Tree Assessment definition of a tree, the checklist of trees native to the contiguous United States contains 881 species, 294 of which are endemic to the contiguous United States. Approximately 11–16% of species are threatened with extinction.
  • The most common threats are invasive and problematic pests and diseases.
  • Two genera dominate the tree flora of the U.S.: oaks (Quercus) and hawthorns (Crataegus), with 85 and 84 native species, respectively. Oaks and hawthorns were also found to have the most threatened species, with 17 and 29 species, respectively.
  • Geographically, the distribution of native and endemic U.S. trees is primarily concentrated in the Southeastern United States, California, and Texas. Florida and Texas have the highest number of native tree species, with 338 and 320 respectively. Florida and California have the highest number of threatened tree species, with 45 and 44 respectively.

A Path Forward for U.S. Conservation

  • The checklist and synthesis includes more than 700 new or updated IUCN Red List assessments and NatureServe Global Ranks.
  • According to BGCI’s PlantSearch database, 95% (849) of native U.S. tree species are located in at least one ex-situ collection, such as a botanic garden, arboretum, or seed bank. However, 17 threatened tree species are not currently conserved in any botanical collections and thus have no insurance policy against extinction, highlighting a clear gap for botanic gardens and arboreta to fill.
  • The authors created a replicable data-sharing methodology to encourage threat assessment updates and spur coordinated action.

Collaborators

Amanda Treher Eberly

NatureServe

Anne Frances

USDA Agricultural Research Service

Diana Jerome

University of Edinburgh

Wesley Knapp

NatureServe

Abby Meyer

Botanic Gardens Conservation International US

Ray Mims

United States Botanic Garden

David Pivorunas

USDA Forest Service

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