Paper Reviewed
Hanberry, B.B. and Hansen, M.H. 2015. Latitudinal range shifts of tree species in the United States across multi-decadal time scales. Basic and Applied Ecology 16: 231-238.

Introducing their study of this contentious subject, Hanberry and Hansen (2015) say "it is expected that some species will shift poleward in range due to climate change." And, therefore, they developed a method to search for evidence of this phenomenon using continental USDA Forest Service data obtained over an interval of approximately 28 years. More specifically, as they describe it, they "analyzed compositional differences in ecological subsections within latitudinal bands ... and considered differences in bands at the outer northern and southern range edges combined with increased area to indicate range expansion." And what did they thereby learn?

The two researchers report that out of a pool of 74 relatively common species, they "detected 12 species that had significant increases and expansion in the northern outer band," as might have been expected; but they say that 13 species "had significant increases and expansion in the southern outer band," while one additional species actually "increased and expanded in both northern and southern bands." And even more telling, they report that "seven species with significant increases in the northern outer band in the contiguous United States had distributions that extended within Canada," so that these U.S. observed increases, as they put it, "do not represent increases in the northernmost latitudinal band for these species."

Faced with these findings, Hanberry and Hansen were forced to conclude that "observed patterns did not match the expected outcome of general poleward shifts under climate change," which real-world fact leaves one with the option of concluding that either (1) CO2-induced global warming does not lead to poleward tree species expansion, or that (2) there was not any significant global warming over the period of their study, or that (3) both of these possibilities are correct.