Drought is projected to increase in many tropical areas, while model predictions for forest gas exchange show considerable uncertainty. Here we present a dynamic optimization that derives the trajectory of stomatal conductance (gs) during drought from plant and soil water relations and the carbon economy of the plant hydraulic system. This optimization scheme is novel in accounting for the ability of capacitance (i.e., the release of water from plant storage tissue; C) to buffer water loss and maintain gs during drought. Representing both drought avoidance and tolerance mechanisms allowed us to apply this optimization across a range of hydraulic strategies, defined from a synthesis of physiology trait values from the literature, to evaluate shifts in the evolutionarily stable strategy (ESS), or competitively optimal set of trait values, under increased drought.
The ESS analysis indicated that, despite the beneficial role of capacitance in maintaining positive carbon balance during drought, species with drought tolerant traits (e.g., highly cavitation-resistant xylem) outcompeted those with drought avoidant traits (e.g. greater water storage, higher capacitance). The ESS value for the drought tolerance trait xylem water potential at 50% loss of conductivity (Y50) declined from -1.8 to -2.5 MPa, and ESS C declined from 230 to 150 kg MPa-1. This shift was greater in simulations with more consecutive days without rainfall, even if the number of total days per year without rain was held constant (ESS Y50 declined from -1.8 to -3 MPa, and C from 230 to 110 kg MPa-1). This analysis provides insight into the physiological drivers of species differences in stomatal regulation during drought, as well as a detailed trait-based framework to predict shifts in functional composition under global change.