Shifts in climate niches for major tree genera in eastern US caused by historic land use and climate change

Background/Question/Methods

Niche model-based projections of the effects of 21^st-century climate change on tree species distributions usually rely on calibrations based on contemporary climate data and species distributions, from datasets such as the Forest Inventory Analysis (FIA).  However, climates have changed over the last century and the species composition and structure of forests are heavily affected by legacies of past land use.  This combination of historical climate change and land use likely has altered species-climate relationships, but the degree and direction of these effects are largely unknown.  Here the PalEON Settlement Vegetation Group presents new reconstructions of relative forest composition during early Euro-American settlement, for a domain stretching from Minnesota to Maine, based on Public Land Survey and Township Proprietor Surveys, and apply them in conjunction with PRISM historical climate data and contemporary FIA surveys to assess the stability of taxa-climate relationships for major tree genera over the past several centuries.

Results/Conclusions

These analyses show that historic land use substantially affected taxa-climate relationships, primarily because selective distribution of past land use caused large reductions of some tree taxa from former portions of their geographic and climatic range. Effects of 20^th century climate change on taxa-climate relationships are secondary but detectable.  For example, Quercus occupied a dryer portion of climate space in the upper Midwest than it does today, suggesting it may be more tolerant to future drying than indicated by contemporary species-climate relationships. Combining historic and modern datasets thus creates a fuller, but likely still incomplete, description of the realized climatic niches for these tree taxa.