Including Tree Diversity In Predictions Of Tropical Forest Drought Responses

热带森林是世界上最重要的生态系统之一；它们是生物多样性的热点，储存了大量的碳，缓解了气候变化，并影响了全球气候模式。预测这些热带森林将如何应对气候变化是制定全球政策决策的优先事项。据预测，土壤水分的长期减少和严重干旱事件将是下个世纪热带森林面临的主要威胁。如果热带森林未来遭受干旱，命运将取决于哪些类型的树木死亡，哪些树木能够从幼苗存活到成年。要理解这一点，需要了解树木具有哪些特性可以改变它们在干旱期间的死亡风险或生存能力，以及这些特性在不同类型、年龄或高度的树木之间是如何变化的。目前，用于预测热带森林对未来气候变化响应的植被模型大多只考虑存在一种或两种固定类型的热带树木。它们只解释了不同树冠高度和位置的树木对干旱的反应差异非常有限。因此，这些模型目前不太可能捕捉到真实世界树木特性多样性对干旱反应的变化，而这些变化最终将决定热带森林对干旱胁迫的恢复能力。在植被模型中描述热带森林干旱响应对树木特性多样性的依赖性是复杂的。它需要关于不同功能类型和发育阶段的树木如何应对长期干旱胁迫的新数据。我的目的是收集必要的数据，以了解热带树木的干旱生存能力如何与树木特性及其在干旱条件下树木发育过程中的变化有关。我将使用这些数据来开发植被模型，并对它们表示热带森林干旱反应的方式进行阶段性改变。我有一个令人兴奋的机会来使用世界上持续时间最长的热带森林干旱实验，在过去的15年里，这个地方50%的降雨量被排除在外。在这里，我将测试不同发育阶段和不同特性的热带树木是否对长期干旱胁迫做出不同的反应，以及这如何影响死亡风险。我将对树木在干旱中生存的能力、不同大小级别的重要树木分类群进行详细的生理测量；这些分类群将包括被发现对干旱胁迫高度敏感和抵抗的物种。为了配合这项研究，来自焦点分类群的幼苗将在苗木干旱实验中生长，该实验将用于测试苗木在先前暴露于干旱后是否增强了抗旱性。最后，将在实验室条件下种植已经历15年实验干旱的母树幼苗和未暴露于干旱环境中的树苗，并在各种土壤干旱条件下进行种植。我的研究将创建唯一的数据集，能够测试功能特性、发育阶段和干旱暴露如何控制干旱导致死亡的风险，以及哪些类型的树最有可能从幼苗存活到成年，以在未来干旱条件下维持热带森林。这将为我的模型开发工作提供一个独特的机会，使用从单一树木模型到动态全球植被模型的模型层次结构。我的最终目标是根据树冠的位置和所拥有的特性，展示干旱反应中的重要差异，以便能够更准确地预测热带森林将如何应对未来的气候变化。

Tropical forests are one of the world's most important ecosystems; they are a biodiversity hot spot, store vast quantities of carbon, mitigate climate change, and influence global weather patterns. Predicting how these tropical forests will respond to climate change is a priority for making global policy decisions. Prolonged reductions in soil moisture and acute drought events are predicted to be a key threat to tropical forest in the coming century. The fate of tropical forests if exposed to drought in the future will depend on which types of trees die and which can survive from seedling to adult. Understanding this requires knowledge of which properties trees possess that alter their risk of dying or their survivorship during drought, as well as how these properties change between trees of different types, ages or heights. Currently most vegetation models used to predict the responses of tropical forest responses to future climate change only consider there to be one or two fixed types of tropical tree. They only account for very limited differences in how trees of different canopy heights and positions respond to drought. Therefore these models are currently unlikely to capture the variations in drought responses from real-world diversity in tree properties and which will ultimately determine the resilience of tropical forests to drought stress. Representing the dependency of tropical forest drought responses on the diversity of tree properties in vegetation models is complex. It requires new data on how trees of different functional types and developmental stages respond to prolonged exposure to drought stress. My aim is to collect the data necessary to understand how drought survivorship of tropical trees is related to tree properties and their variation throughout a trees development in drought conditions. I will use this data to develop vegetation models and provide a step change in how they represent tropical forest drought responses. I have the exciting opportunity to use the longest running tropical forest drought experiment in the world, a site where 50% of incoming rainfall has been excluded for the last 15 years. Here I will test if tropical trees at different developmental stages and with different properties respond differently to long-term drought stress and how this influences mortality risk. I will make detailed physiological measurements of properties associated with a trees ability to survive in drought, on important tree taxa of different size classes; these taxa will include species found to be highly sensitive and resistant to drought stress. To accompany this study, seedlings from the focal taxa will be grown in a seedling drought experiment which will be used to test if drought resistance in seedlings increases following prior exposure to drought. Finally seedlings from mother trees which have been exposed to 15 years of experimental drought and seedlings from those which have not been exposed to drought will be grown in lab conditions and subjected to various soil drought conditions. This will test if prior exposure to drought in mother trees induces production of seedling which are more drought-resistant.My research will create the only data-set which is able to test how functional properties, developmental stage and drought exposure control the risk of drought-induced mortality and which types of tree are most likely to survive from seedling to adult to sustain tropical forests under future drought conditions. This will provide a unique opportunity for my model development work, using a hierarchy of models from the scale of a single tree model to a dynamic global vegetation model. My ultimate goal is to represent the important differences in drought responses, based on a trees canopy position and the properties it possesses, to enable more accurate predictions of how tropical forests will respond to future climate change.