| Description |
Drought induced tree mortality is a global phenomenon that is on the rise due to increasing severity and duration of drought events in many regions. Despite the ambiguities surrounding the specific physiological mechanisms underlying an individual case of tree death, plant hydraulics provide a reliable indicator of mortality risk across a diverse variety of tree species and forest types. In particular significant and chronic disruption in water-transport capacity of the xylem network caused by cavitation has been consistently associated with heightened mortality risk. Additionally, the hydraulic function of trees defines a space in which the coupled fluxes of water exiting and carbon dioxide entering the leaf through stomatal pores can occur. Constraining models of forest-level photosynthesis and water use, by incorporating plant hydraulics, holds promise as a means to more accurately simulate the responses of stomata to environmental variability. Current land surface models do not accurately simulate the response of forests to drought stress, leading to unrealistic predictions of forest productivity and mortality risk during drought. This uncertainty has profound implications for our ability to forecast future bio-geochemical cycling as well as the productivity and mortality risk of forest ecosystems that provide necessary resources for human society and provide refuge for global biodiversity. This dissertation describes work aimed at increasing our ability to predict the dynamics of tree gas exchange and the mortality risk for forests under drought stress. The included chapters describe 1) the development of a model and hydraulic theory for predicting tree-water use and mortality risk, 2) a controlled test of the theory using a manipulative field experiment, 3) validation of an extended version of the original model that incorporated the dynamics of photosynthesis in a controlled drought experiment, and 4) application of the updated model to predict the water use and mortality risk of naturally growing forest stands. Together these works have helped advance the field of plant hydraulics with applications to bridge the divide between the scales of tree and regional scale ecophysiology to forecast the mortality risk of forests. |
