Latitudinal range shifts of tree species in the United States across multi-decadal time scales

It is expected that some species will shift poleward in range due to climate change. However, despite numerous publications that project species distributions under climate change scenarios, there is less evidence for current latitudinal shifts by tree species, in part because ranges are dynamic and complex to measure. We developed a method to compare tree ranges using continental USDA Forest Service Forest Inventory and Analysis tree surveys during an interval of approximately 28 years. We analyzed compositional differences in ecological subsections within latitudinal bands using repeated measures ANOVA and considered differences in bands at the outer northern and southern range edges combined with increased area to indicate range expansion. Out of a pool of 74 relatively common species, we detected 12 species that had significant increases and expansion in the northern outer band, 13 species that had significant increases and expansion in the southern outer band, and one species that increased and expanded in both northern and southern bands. However, seven species with significant increases in the northern outer band in the contiguous United States had distributions that extended within Canada so that shifts do not represent increases in the northernmost latitudinal band for these species. Uniform poleward migration was not detectable and the number of species shifting southward suggests influence of other driving factors than climate change, including assisted migration of tree species through landscaping and a cascade of range shifts along ecotonal boundaries due to fire exclusion and land use.