Tree species distributions in the United States and Canada under climate from 20,000 years ago to year 2100

Abstract

Current locations of species may not be suited to future climate, resulting in ranges that shift to higher latitudes, with potential for range expansion or contraction in extent. To identify potential shifts in distributions of 119 tree species centered in the United States and Canada, I modeled species occurrences under current climate and then predicted to past (20, 10, and 6 thousand years ago) and future climates (six projections during 2071–2100 for North American warming of 4.3 °C to 8.8 °C), with a singular focus on the climate space of tree species, or bioclimatic envelope models. Mean accuracies of withheld samples for models, based on climate of 1900 to 1990, were 0.97, with an average of 6.2 variables per model; annual temperature and temperature of the warmest quarter generally were the most important variables. Based on model predictions under past and future climate, centers of species distributions shifted an average of 565 km latitudinally since climate 20,000 years ago and 440 km to 1065 km shifts were expected in response to end-of-century climates. Overall, 64 tree species may gain more area than lose area that matches current climate space under future climate, whereas 36 species may lose potential climate space, with relative stability for 19 species. Canada and the mountainous western U.S., and National Forests in the mountainous western U.S., were likely to gain climate space appropriate for these species, particularly species of the eastern U.S. Conversely, greatest number of species were predicted to be lost from most of the eastern half of the U.S.; National Forests in the southeastern U.S., particularly in Alabama, Mississippi, and Texas, may lose 25 to 50 of these tree species. Isolation of potential dynamics for tree species already native to northern North America demonstrated that a large pool of diverse species from the southeastern U.S. was predicted to have suitable climate space in Canada and the western U.S. It may be beneficial for managers to become familiar with tree species south or east of their regions, particularly climate-tolerant pine and oak species, to determine which ones are desirable to manage for the future.