Because forest tree species face the risk of extinction from threats such as climate change and insect and disease infestation, prioritizing species and forests for conservation is an important management goal. Key criteria for such efforts should include a species’ rarity of occurrence (because of its increased risk of its loss) and evolutionary distinctiveness (because distinct species may possess rare or unique traits and ecosystem functions).
We ranked 369 forest tree species of the mainland United States based on rarity, evolutionary distinctiveness (ED), and an index combining both. Rarity was determined based on how often each species occurred on ~130,000 Forest Inventory and Analysis (FIA) plots across the country. ED was the degree to which a species is related to the other North American tree species, based on a phylogenetic tree encompassing the 369 species. The rarity and ED scores were combined to generate a Rarity and Evolutionary Distinctiveness Index (REDI) score for each species, with each species given a REDI rank.
Species REDI scores were combined with tree importance values from each FIA plot to generate a plot-level measure of REDI forest conservation value. This information was interpolated nationally to generate a map of forest REDI conservation value.
Tree species that occur on very few, if any, FIA plots had the highest rarity scores. Among the most evolutionarily distinct species were several relatively phylogenetically isolated gymnosperms and angiosperms. The tree species with the highest REDI values included rare conifers such as giant redwood (Sequoiadendron giganteum) and Florida yew (Taxus brevifolia) and angiosperms that included three mangrove species. Mapped forest-level forest tree rarity was the highest in California, the Southwestern United States, southern Louisiana, Florida, northwest Washington, and northern Minnesota. Mapped forest evolutionary distinctiveness was highest along the coast of the Pacific Northwest and northern California, northern Idaho, the Piedmont of the Southeast, southern Louisiana, northern New York, and northern Minnesota. REDI conservation values were highest along the Pacific Coast and in Florida, northern Minnesota, and Louisiana.
The focus of this effort is to prioritize species and forests for conservation based on two aspects of rarity: rarity of occurrence and evolutionary distinctiveness. The specific objective is to safeguard unique evolutionary lineages and to create conducive conditions for continued evolution. This assessment tool should be valuable for scientists and managers attempting to determine which species and forests to target for monitoring efforts and for pro-active gene conservation and management activities.