The role of plant water storage and hydraulic strategies in relation to soil moisture availability

Plants rely on water storage capacity to increase accessibility of water for transpiration, reduce competition for water with neighboring plants, and buffer water supply during dry periods. The resulting benefits, typically a decrease in plant water stress and increase in productivity, are highly climate dependent and vary with soil moisture, vapor pressure deficit, and solar radiation. This paper analyzes the effects of plant water storage capacity on the relationship between soil moisture and carbon assimilation in woody plants. A resistance-capacitance model is used to examine the role of plant water storage at various soil moisture levels. Hydraulic traits are co-varied according to empirical relationships, and effects of sapwood volume and wood density on carbon assimilation are explored. The time scales of plant water storage and withdrawal are analyzed as a function of plant hydraulic capacitance, water storage capacity, and resistance to transport between water storage tissue and xylem. The effects of plant water storage on carbon assimilation are found to depend strongly on soil moisture levels. The theoretically optimal sapwood volume lies near naturally occurring ranges and increases with increasing soil moisture. The theoretically optimal wood density also lies within expected ranges and decreases with increasing soil moisture. A large portion of sapwood volume appears to be justified by its role in buffering diurnal variability in evaporative demand. The outlined coordination between soil moisture and optimal hydraulic traits is consistent with observed increases in sapwood capacitance and decreases in wood density across increasing rainfall gradients. This coordination provides support for the drought-tolerance vs. drought-avoidance hypothesis.