A better understanding of plant stomatal strategies holds strong promise for improving predictions of vegetation responses to drought because stomata are the primary mechanism through which plants mitigate water stress. It has been assumed that plants regulate stomata to maintain a constant marginal water use efficiency and forego carbon gain when water is scarce. However, recent hypotheses pose that plants maximize carbon assimilation while also accounting for the risk of hydraulic damage via cavitation and hydraulic failure. This “gain-risk” framework incorporates competition in stomatal regulation because it takes into account neighboring plants can steal unused water. Determining which hypotheses better represents stomatal behavior is critical for predicting plant responses to different biotic and abiotic environmental conditions. This study utilizes stomatal models representing both water use efficiency and carbon-maximization frameworks and empirical data from three species in a potted growth chamber experiment to investigate the effects of drought and competition on seedling stomatal strategy.
Results/Conclusions
We found that drought and competition responses in the empirical data were best explained by the carbon-maximization hypothesis and that both drought and competition affected stomatal strategy. Interestingly, stomatal responses differed substantially by species with seedlings employing a riskier strategy when competing against higher water user and behaving more conservatively when competing against a lower water user. Lower water users in general, gymnosperms, had less stomatal sensitivity to decreasing leaf water potential than moderate to high water users. Repeated water stress also resulted in legacy effects on plant stomatal behavior, increasing stomatal sensitivity and conservative behavior even when returned to well-watered conditions. These results indicate that stomatal strategies are dynamic and change with climate and competition stressors, thus incorporating stomatal behavioral changes in response to water limitation may be an important step to improving carbon cycle projections in coupled climate-Earth system models.