Death by dehydration: How savanna trees die in drought

植物如何应对干旱正在成为一个严重的不确定性，它限制了我们预测生态系统将如何应对气候变化的能力。最近的研究对既定的理论提出了质疑，该理论认为水力故障、碳饥饿和生物相互作用的组合可以预测干旱死亡率(麦克道尔模型)。我们的目标是检验另一种假说：在干旱期间，脱水而不是水分运输失败会导致生理表现下降，从而导致死亡。为了实现这一目标，我们建议监测个别稀树草原树并控制它们的供水。我们将在稀树大草原生态系统中进行实验，因为稀树大草原树的特征(高水平的再发芽和高的非结构性碳储量)似乎违反了麦克道尔模型的前提。我们将监测54棵稀树草原树，它们在物种(6个物种)、大小和竞争邻居方面存在差异。对于每一棵树，监测变量将包括树液流量、树干水分含量、叶水势、叶面积和树干直径。还将监测每棵树附近的草和土壤蒸散量。我们将使用一个新的状态空间模型来解释个体树木在三个生长季节的动态。在过程层面上，我们将通过使用状态空间建模来推断难以观察的过程，例如从树干液流、茎干水分和土壤水分的测量观测中推断从根表面吸收水分的过程，从而加深理解。因为状态空间方法依赖于观测数据序列背后的过程的变化，所以我们通过使用BACI(控制影响之前)实验设计在个别树木上施加干旱来创造额外的变化。这项拟议的研究的主要目标是检验我们的工作假设(即干旱期间导致死亡的是脱水，而不是水运输失败)。我们将通过检验在我们的时间序列中观察到的茎生长、茎死亡率和全株死亡率来检验这一假说，是用植物的模型储水水库还是植物的模型水分运输赤字来最好地解释这一假设。这项研究将评估麦克道尔模型是否需要通过正式考虑脱水死亡假说来扩展。

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.