Cutting to the core: measuring variable drought sensitivity in tree species across their climatic niche

Abstract:

Many of the world’s forests face heightened drought stress under climate change, which could decrease productivity and accelerate mortality. Improved understanding of the magnitude and determinants of drought sensitivity will be critical to guide natural resource management. 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 each tree’s sensitivity to annual variations in Potential Evapotranspiration (PET) and Climatic Water Deficit (CWD). 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. While this effect was dominant for conifer species, our analyses suggests that drought sensitivity patterns vary by taxonomic group – angiosperms may be more drought sensitive at their dry range edges. We provide evidence suggesting that the elevated resistance to drought at dry range edges reflects local adaptations (e.g., evolutionary selection and density) rather than individual trees’ responses to past drought (e.g. phenotypic plasticity). 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, for multiple species, climate change will cause the greatest reductions in growth in the wetter portions of a species’ range. As a result, populations in the core and wet range edge may be unexpectedly vulnerable to drought-induced mortality resulting from climate change.