Growth-Mortality Coordination Differs Among Xerophytic Versus Mesophytic Tree Species During Severe Drought

Forest composition is changing, yet the consequences for terrestrial carbon cycling are unclear. In the eastern United States, water-demanding "mesophytic" tree species are replacing "xerophytic" oaks (Quercus spp.) and hickories (Carya spp.), raising concerns that forest productivity will become increasingly sensitive to more frequent and severe drought conditions predicted for the region. However, we have a limited understanding of the extent to which the mortality risk of xerophytes versus mesophytes is coordinated with their growth sensitivity during drought. Here, we evaluated growth and mortality dynamics for 20 abundant eastern United States tree species following a severe drought in the summer of 2012. We synthesized data from ~4500 forest inventory plots and used an approach that quantified relative drought responses between co-located trees to minimize impacts from environmental heterogeneity. We found that mesophytes were just as likely to perish as co-occurring xerophytes but were more sensitive to drought in terms of diminished growth. These findings suggest that xerophytic decline is likely to lead to reduced carbon uptake during drought and that management efforts to conserve oak-hickory stands will be decisive to sustain the carbon mitigation potential of these forests. However, we also found that growth-mortality relationships differed between functional groups. Among xerophytes, growth and survival during drought were decoupled. Among mesophytes, there was a high degree of coordination, where species that experienced greater mortality also experienced greater growth reductions. Therefore, mesophytes with high growth sensitivity to water deficits are likely to be the most vulnerable to drought-driven die-off events moving forward.