Distributional changes in western US tree species are predicted by climatic drying, not temperature

Link To Share This Poster: https://cdmcd.co/v9DZjY

Michel Au, Erin McCann and Marko Spasojevic, Department of Evolution, Ecology, and Organismal Biology, University of California Riverside, Riverside, CA

Background/Question/Methods
Tree species are expected to shift their ranges to higher latitudes and elevations to track warming temperatures associated with anthropogenic climate change. However, many other climatic variables are exhibiting idiosyncrasies in their direction, rate, and magnitude of change. The velocity of climate change, which describes the direction and rate organisms must shift their distributions to track changing climatic conditions, provides a method to individually evaluate spatial shifts in climatic variables and compare those to species’ distributional shifts. Here, we used Forest Inventory Analysis data to develop separate species distribution models for 26 tree species in the Western United States. We then compared adult distributions to seedling distributions as seedlings may serve as a proxy for a species’ future distribution. Next, we calculated the velocity of 12 climatic variables from 2001 to 2018 to assess differences in conspecific seedling and adult distributions to evaluate species response to various climatic velocities.
Results/Conclusions
We found that there was significant variation in both the direction and rate of velocities across all 12 climatic variables. The annual average temperature velocity exhibited the expected trends with directional shifts towards higher latitudes. However, temperature dew point mean, vapor pressure deficit, actual evapotranspiration, potential evapotranspiration, soil moisture, and climate water deficit all exhibited negative velocities across the Western United States, which indicates that in those areas species will need to shift their distributions to lower latitudes or elevations to track the temporal shifts in climate. This suggests that while temperature is largely shifting in the anticipated way, many climatic variables are idiosyncratic. Critically, temperature velocities did not play a significant role in explaining species’ southern range contractions nor northern range expansions. Instead, the velocity of vapor pressure deficit was the most important climatic variable in explaining species southern range contractions and both climate water deficit and vapor pressure deficit explained species northern expansions. Overall, these results emphasize the importance of considering the variability in the direction and magnitude of multiple climatic variables as well as the importance of considering climatic variables other than temperature in understanding species distributional shifts in response to climate change.