Death by dehydration: How savanna trees die in drought

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How plants respond to drought is emerging as a critical uncertainty that limits our ability to forecast how ecosystems will respond to climate change. Recent studies question the established theory which posits that a combination of hydraulic failure, carbon starvation and biotic interactions can predict drought mortality (the McDowell model). We aim to test an alternative hypothesis: during drought dehydration rather than water transport failure induces reductions in physiological performance which can lead to mortality. To achieve this, we propose to monitor individual savanna trees and manipulate their water supply. We will perform our experiment in a savanna ecosystem because the traits of savanna trees (high levels of resprouting and high non-structural carbon reserves) appear to violate the premises of the McDowell model. We will monitor 54 savanna trees that differ in species (6 species), size and competitive neighbourhood. For each individual tree, monitoring variables will include sap flow, stem water content, leaf water potential, leaf area and stem diameter. Grass and soil evapotranspiration will also be monitored in each trees' neighbourhood. We will use a novel state-space model to interpret the dynamics of individual trees over three growing seasons. On a process level we will deepen understanding by using state space modelling to infer difficult to observe processes, such as the uptake of water from the root surface, from measured observations of sap flow, stem water content and soil water content. Because the state space approach relies on variation in processes underlying the observational data series, we create additional variation by experimentally imposing drought on individual trees using a BACI (before after control impact) experimental design. The proposed research's primary objective is to test our working hypothesis (i.e. dehydration rather than water transport failure leads to mortality during droughts). We will test this hypothesis by examining whether stem growth, stem mortality and whole plant mortality observed in our time series are best explained by the plant's modelled water storage reservoir or by the plant's modelled water transport deficit. The study will evaluate whether the McDowell model needs to be expanded by formally considering the death by dehydration hypothesis.