Allocation mediates plant drought stress and productivity in a changing climate

The extent to which future climate change will increase forest stress and the amount to which species and forest ecosystems can acclimate or adapt to increased stress is a major unknown. We used high resolution maps of hydraulic traits representing the diversity in tree drought tolerance across the United States combined with a hydraulically-enabled tree model to quantify the ability for within-species changes in allocation and between-species range shifts to mediate climate stress. We found that forests are likely to experience increases in chronic hydraulic stress with climate change, even with expected increases in atmospheric CO₂. Based on current species distributions, regional hydraulic trait diversity was sufficient to buffer against increased stress in 86% of forested areas. Importantly, changes in leaf allocation have the potential to substantially decrease stress, reducing the need for biogeographic shifts in species distributions. However, observed trait velocities are not keeping up with the rate required to ameliorate projected future stress.