Radial growth responses to drought converge across species with increased drought severity in Eastern US hardwood forests

Abstract:

While ongoing climate change affects a number of meteorological drivers relevant to plant functioning, an increase in the frequency and severity of drought events may ultimately have the largest impact on ecosystem carbon cycling. Much of what we know about tree response to drought comes from the relatively dry western U.S., where droughts are prolonged and often lead to tree mortality. Less attention has been focused on the humid eastern US, where drought-induced mortality is rarer but drought-related declines in carbon uptake and growth are nonetheless profound. Using a well replicated network of tree rings, we determine if a water use strategy (isohydric/anisohydric) explains species-specific response to drought. We find mixed results showing that some species that are extremely isohydric (e.g. L. tulipifera) are very responsive to drought. However, extremely anisohydric species such as oaks are also sensitive to extreme drought. We find the isohydric/anisohydric framework better explains the magnitude of growth reduction (i.e. effect size) of species-specific responses to mild droughts. Our findings suggest a convergence in effect size during extreme drought conditions across species. However, during times of mild drought, species that have shallow rooting depth that exhibit an isohydric behavior of water-use have greater growth reductions compared to anisohydric behaving species with deeper roots that have smaller growth reductions. Thus, understanding the mechanistic determinants of different water-use strategies advances our understanding of how changing eastern forest demographics interact with and respond to drought, allowing a better understanding of how eastern forest may respond to future climate.