Plant Water Storage Optimality Across Hydroclimatic Landscapes

Abstract

A quantification of the mechanisms underlying plant water use strategies is central to understanding plant stress vulnerability, productivity and subsequent responses to hydroclimatic shifts. To explore such dynamics, we developed a dynamical model for the changes of internal plant water storage (PWS) and soil moisture given a set of coupled balance equations. This trade-off was explored through the analysis of long-term plant fluxes over a range of climate regimes, providing constraints on water availability and demand while incorporating plant physiological mechanisms into the model framework. In conjunction, plant productivity was considered, taken as the plant carbon dioxide assimilation with an additional maintenance cost subtracted to account for varying internal plant water capacities. To explore these trade-offs analytically and characterize the long-term response to changing rainfall frequencies, a conceptual model was developed by linearizing the coupled ordinary differential equations (ODEs) governing PWS and soil moisture dynamics. This conceptual model produced clear PWS optima that decreased nonlinearly with increasing rainfall frequency, as plants with a higher PWS capacity maintained a higher minimum internal water storage overall. The model was then extended to include nonlinear components, including stochastic rainfall forcing. Under constant meteorological conditions, due to the cost associated with plant size along with the timescale of intake and water release, we found that net carbon uptake does not necessarily increase with larger maximum PWS capacities but is sustained for longer periods during drought due to transpiration being facilitated by internal water stores. This PWS reduces stress for water storing plants in climate regimes with high-intensity, low-frequency precipitation. Increased rainfall frequency and a decrease in intensity greatly reduce the overall optimal PWS capacity. Thus, the extended model confirms that optimal PWS decreases nonlinearly as the rainfall becomes more frequent and less intense, as in the conceptual model. This suggests that water storage plays a less critical role in wet environments that may show an increase in wet days, but not necessarily an increase in water availability. We then analysed remote sensing data trends in seasonally dry ecosystems and compared them with the nonlinear model to identify physically based mechanisms governing plant water use, identifying plant functional traits potentially coordinated with PWS.