Cutting to the core: Measuring variable drought sensitivity in tree species across their climatic niches

Robert Heilmayr, Environmental Studies Program, University of California, Santa Barbara, Santa Barbara, CA, Joan Dudney, Department of Plant Sciences, University of California, Davis, Davis, CA and Fran Moore, Environmental Science and Policy, University of California at Davis, Davis, CA

Background/Question/Methods
Rising temperatures and changes in precipitation patterns threaten the world’s forests with heightened drought stress, which could decrease productivity and accelerate mortality. Many ecological models currently assume that drought impacts are most severe at the species’ dry range edge. However, several studies have identified species that exhibit the greatest resistance to drought at this dry range edge. Here we systematically explore how tree species’ sensitivity to drought varies across their climatic niche. We apply econometric models to >9,000,000 annual growth observations from 59,582 trees recorded in the International Tree-Ring Data Bank to assess sensitivity to annual variations in Potential Evapotranspiration (PET) and Climatic Water Deficit (CWD).
Results/Conclusions
We find that increases in CWD generally inhibit growth, but this relationship varies dramatically along a gradient of historic CWD. Specifically, trees located in historically wet regions of a species’ range exhibit the greatest sensitivity to droughts, whereas trees in historically arid regions often show minimal sensitivity to drought. Reduced sensitivity to CWD increases could insulate populations along a species’ dry range edge from climate change impacts. We test for this possibility by combining our estimates of CWD sensitivity with climate projections from the Coupled Model Intercomparison Project Phase 5 (CMIP5). We estimate that climate change will cause the greatest reductions in growth in the wetter portions of many species’ ranges. As a result, populations in the core and wet range edge may be unexpectedly vulnerable to drought-induced mortality resulting from climate change.