Sensing and modeling plant ecohydrology for understanding tree and ecosystem responses to water stress

Vegetation provides a critical nexus between the subsurface, biosphere, and atmosphere through the mediation of the exchange of water, carbon, and energy. Plants respond dynamically to local microclimates at both short and long timescales via mechanisms ranging from physiological behaviors, such as stomatal closure, to acclimation and adaptation. These responses influence land-atmosphere fluxes directly and are therefore crucial to understanding and predicting Earth system responses to a changing climate. As our community progresses towards increasingly physically-realistic models of vegetation responses to the environment, we face several new challenges such as understanding how whole-plant hydraulic strategies can best be represented using limited parameters, connecting non-linearly related observations such as water content and water potential within different organs, and representing responses to simultaneous stressor such as high temperatures and low water supply or high salinities and high evaporative demands. We use vignettes from two long-term tree and ecosystem level studies to demonstrate both progress and pitfalls towards overcoming each of these hurdles in terms of observational understanding of plant function and individual tree-based simulations of new observational data and the accompanying uncertainties. As we progress towards further incorporation of such vegetation ecohydrology modules within climate and weather models, it is becoming increasingly critical to represent well, and with limited parameters the manners in which plants manifest stress responses across time scales in order to better predict the complex feedbacks to carbon, water, and energy fluxes that subsequently develop.