Drought-induced forest mortality has major consequences for biodiversity and ecosystem services and could result in a substantial positive carbon-climate feedback by releasing large amounts of carbon into the atmosphere within a relatively short period, which would cause further climate change. High mortality rates in large trees are particularly concerning because large trees serve as habitat for multitudes of tropical fauna species and comprise a disproportionately large amount of woody carbon in forests. Critically, the physiological mechanisms underlying size dependent mortality are not fully understood. We used a tree-level photosynthesis model dependent on leaf area, functional xylem, soil water, atmospheric vapor pressure deficit, and atmospheric CO2, and optimality theory to understand the mechanisms governing the success and failure of tree recovery from drought-induced catastrophic xylem embolism. We simulated tree recovery from different degrees of xylem embolism for different size classes. We further examined how species-specific physiological traits and environmental conditions impacted post-drought tree recovery.
Results/Conclusions
Through a combination of meta-analyses, tree physiological observations, and an optimization-based model of tree carbon allocation constrained by plant hydraulics, we tested the degree to which plant carbon allocation strategy and environment influenced tree productivity and mortality risk. We found that optimal carbon allocation in response to environmental conditions explained observed patterns of size-dependent tree drought mortality and variability in healthy tree allocation strategy across environmental gradients. We further found that species-specific trait strategies and atmospheric CO2 fertilization may moderate negative impacts of increasing drought on forest productivity under mean future climate conditions. Our results demonstrate that tree carbon allocation strategy has enormous leverage on the terrestrial carbon cycle because it mediates both forest productivity and resilience to climate extremes, and that plasticity in tree carbon allocation in Earth system models may be necessary to accurately predict changes in the terrestrial carbon cycle with changing water availability.