Weighing the options: a test of alternative stomatal optimisation models at high temperatures

Stomatal optimisation models centre upon a fundamental tradeoff for plants: opening stomata promotes carbon uptake, but closing stomata prevents water loss. However, stomatal opening can occur at high temperatures, causing evaporative cooling which limits thermal damage to leaves. Under hot, dry conditions, stomatal behaviour is therefore subject to another tradeoff: conservation of water vs the need to cool leaves below critical thermal thresholds. Here, we present a series of stomatal optimality models and test how different strategies weighing carbon gain against hydraulic, respiratory, and/or thermal costs influence predicted stomatal behaviour. We test these strategies against data from a whole-tree chamber experiment in which Eucalyptus parramattensis trees were subjected to a heatwave. Under heatwave conditions, models with a hydraulic cost predict decoupling between photosynthesis and transpiration better than a conventional model. Incorporation of thermal and respiratory costs does not reduce the discrepancy between observations and models, suggesting that plants do not optimise their stomatal behaviour to avoid thermal damage. Our results indicate that instantaneous optimisation models have limited capacity to capture leaf gas exchange at high temperatures. Given the increasing occurrence of heatwaves globally, it is important to account for extreme temperatures in predictive stomatal models.