Data sharing for conservation: A standardized checklist of US native tree species and threat assessments to prioritize and coordinate action

Societal Impact Statement Understanding the current state of trees within the United States is imperative for protecting those species, their habitats, and the countless communities they support, as well as the ecosystem services they provide. We present an updated checklist of all tree species native to the contiguous United States, their state distribution, extinction risk, and most common threats. Knowledge of national threat "hotspots" and conservation priorities facilitates efficient conservation efforts and the allocation of resources to safeguard the 11-16% of US tree species that are threatened. These results lay the groundwork for tree and ecosystem conservation efforts in the United States that contribute to achieving critical international conservation goals, including the United Nations Decade for Ecosystem Restoration and the Global Tree Assessment. The Global Tree Assessment aims to complete threat assessments for all the world's similar to 60,000 tree species, but most species native to the continental United States had either never been assessed or were outdated on the two most widely used threat assessment platforms in the United States, International Union for the Conservation of Nature (IUCN) Red List and NatureServe. There was also no coordinated mechanism for sharing data between these platforms, resulting in missing, duplicated, or outdated information. We (1) created an updated checklist of all tree species native to the contiguous United States based on the standardized Global Tree Assessment tree definition, (2) created over 700 new or updated IUCN Red List assessments and NatureServe Global Ranks, and (3) developed a replicable assessment data sharing process. We present an updated checklist of native US trees that includes 881 species from 269 genera, with Quercus and Crataegus as the most species-rich tree genera. We present the first country-wide analysis of tree extinction risk, patterns of geographic and taxonomic diversity, and leading threats. An estimated 11-16% of US tree species are threatened with extinction, with the most common threat being invasive and problematic pests and diseases. We introduce a "crosswalk" process for efficient, large-scale data sharing between the IUCN Red List and NatureServe, using IUCN Red List Species Information Service (SIS) Connect, which can be applied to other taxonomic groups in North America. The checklist, threat assessments, and crosswalk methodology represent a significant advancement in prioritizing conservation action for at-risk tree species and restoration of forests in the United States, supporting the global goals of the United Nations Decade for Ecosystem Restoration and the Global Tree Assessment effort.