Current and potential tree species assemblages across 185 urban areas in the eastern US: Ecological relationships and dissimilarities to species at their climate analogs at end of century

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Louis Iverson, Anantha Prasad, Matt Peters and Stephen N. Matthews, Northern Institute of Applied Climate Science, Northern Research Station, USDA Forest Service, Delaware, OH, Bryce T. Adams, Northern Research Station, USDA Forest Service, Delaware, OH, Leslie Brandt, Northern Institute of Applied Climate Science, USDA Forest Service, St. Paul, MN

Background/Question/Methods
Tree planting may greatly increase in coming years, and urban areas are likely recipients. To increase understanding of species currently and potentially in the future, we assessed relationships of tree species richness to multiple environmental variables in all large eastern United States urban areas (area>200 km2, n=185). We used buffered neighborhoods of >8000 km2 to accommodate sufficient forest inventory plots for analysis. Tree information was obtained via extraction from the new Version 4 of the Climate Change Tree Atlas (www.fs.fed.us/nrs/atlas), which provides information on current and future-projected habitat suitability and colonization likelihoods, and now location-based current and potential species summary tables for the urban areas as well as other place-based regions. Tree information includes the area-weighted importance values of all species having suitable habitat under current and potential future climates, a capability rating on the species’ capacity to cope or persist, and an assessment of the potential for the species to migrate, or be planted via assisted migration, over the next 100 years. Additionally, for three urban areas (Columbus OH, Lexington KY, Minneapolis MN), we generated climate analogs that identify current climates expected to most resemble the urban area’s future climate at 2100. An analysis of Bray-Curtis dissimilarity was conducted between 1) each urban area’s currently existing species, or 2) species with habitat expected in 2100, to the current species list at the location of the climate analog.
Results/Conclusions
Statistical relationships between tree richness and the corresponding ancillary data for the 185 buffered urban areas revealed that tree richness in the urban buffer correlated positively with percent forest (r=0.63), annual precipitation (r=0.59), and forest cover within the urban area itself (r=0.49), and negatively with cumulative moisture deficit (r=-0.35), population density (r=-0.27), and annual temperature (r=-0.20). Northerly urban areas with sufficient moisture have more potential to receive new species via assisted migration than southerly urban areas. The climate analogs for the three cities, under RCP 8.5 at 2100, were at least 1000 km to the southwest of their current location. Bray-Curtis dissimilarity values revealed that, for the three areas, large differences (dissimilarity >0.65) exist among species between the urban area’s current species assemblages as compared to the species now present at the climate analog. However, the dissimilarity decreases substantially when comparing potential suitable habitat (at 2100) of the urban area to species present now in the climate analog. Implications include serious consideration of large potential habitat changes, depending on future carbon emissions.