Understanding how drought affects the risk of increased mortality in tropical rain forests

预测气候变化，特别是干旱对热带雨林树木死亡率和相关二氧化碳排放的影响是本项目力求解决的一项紧迫和高度优先的任务。树木死亡率的增加有可能大幅增加大气中二氧化碳的总排放量，但到目前为止，我们的模型还不能可靠地代表干旱期间的死亡率过程，我们建议将新数据和模型结合起来解决这一缺陷。近年来，极端干旱事件的发生率有所增加，气候预测表明，一些热带地区可能在本世纪面临风险。在过去30年里，南美洲热带地区和东南亚的严重干旱与厄尔尼诺事件有关。最近，在过去10年里，亚马逊地区发生了两次百年一遇的干旱事件，加剧了人们对未来气候变化及其影响的担忧。与此同时，与干旱有关的树木死亡率普遍增加的发生率已被认为是全球重要的问题。热带雨林的严重干旱会产生很大的影响。例如，在亚马逊地区，2005年的区域性干旱被认为通过减少树木生长和增加树木死亡率而停止了正在进行的大量净碳汇。在更大的泛热带范围内，对严重干旱对热带雨林影响的观察得出了一个惊人的结果：干旱期间不仅死亡率增加了12倍，而且东南亚和亚马逊地区的影响也大不相同。显然，东南亚雨林的树木比亚马逊雨林的树木更容易受到干旱的影响。此外，一些分类群和树的大小（如种和属，特别是大树）在脆弱性方面存在差异。如果我们要了解干旱对世界雨林的影响，并预测它们未来的组成和功能（例如，它们如何影响大气中的二氧化碳浓度），那么我们需要知道为什么不同地区和物种对干旱的脆弱性不同。为了做出这些预测，我们需要将生态学的理解纳入植被模型，这些模型可以与全球气候模型相结合，形成地球系统模型（ESMs）。使这些植被模型能够正确地代表生态的唯一方法是在天然雨林中进行测量。为了了解干旱的影响，我们必须更进一步，通过实验来控制森林可用的水分，以便了解每个关键过程（例如呼吸作用、光合作用等）的反应。大规模的干旱实验在科学上是强有力的，但在任何生物群系中都非常罕见。我们在这个项目中创造了一个独特的机会，将亚马逊和婆罗洲两个热带雨林干旱实验的结果结合起来。我们团队的实验和建模专业知识的结合特别强大，我们希望利用它在预测干旱对21世纪雨林功能的影响方面取得实质性进展。我们将首先使用我们的模型来测试婆罗洲和亚马逊地区的物理差异（土壤或气候）。其次，我们将重点关注区域内和区域间树木分类群（种或属）之间死亡风险的差异，因为有些分类群比其他分类群更容易受到干旱的影响。我们将重点测量死亡率是否与水或碳供应的减少有关，或者两者兼而有之，并将我们的结果纳入我们的模型。总之，我们将使用热带雨林野外实验和全球植被模型的强大组合来解释婆罗洲和亚马逊地区热带雨林树木对干旱脆弱性的巨大差异。研究结果将在整个热带地区得到应用，我们将利用它来改进对气候变化将如何影响热带雨林在21世纪碳循环中的全球作用的预测。

Predicting the effects of climate change, and especially drought, on rain forest tree mortality and the associated emissions of carbon dioxide (CO2) is an urgent and high-priority task which this project seeks to address. Increases in tree mortality have the potential to substantially increase total CO2 emissions to the atmosphere, but to date our models are not capable of representing the mortality process reliably during drought and we propose to combine new data and modelling to address this deficiency. The incidence of extreme drought events has increased in recent years, and climate predictions suggest that some tropical regions may be at risk this century. Severe drought has been associated with El Nino events in tropical South America and in SE Asia in the last 30 years. More recently, two 1-in-100 yr drought events have occurred in Amazonia in the past 10 years, adding weight to concerns about future shifts in climate and their impacts. At the same time, the incidence of widespread increased tree mortality associated with drought has been recognised as globally important. Severe drought in tropical rain forests can have a large impact. For example, in Amazonia, the regional drought of 2005 is thought to have halted the ongoing large net carbon sink by reducing tree growth and increasing tree mortality. At a larger, pan-tropical scale, observations of the impact of severe drought on tropical rain forests have yielded a startling result: not only do mortality rates increase by up to 12 fold during drought, but the impacts differ substantially between SE Asia and Amazonia. Apparently the rain forest trees of SE Asia are more vulnerable to drought than those of Amazonia. In addition, some taxa and tree sizes (e.g. species and genera, and especially large trees) differ in their vulnerability. If we are to understand the effects of drought on the world's rain forests, and to predict their future composition and functioning (e.g. in how they affect atmospheric CO2 concentration), then we need to know why regions and species differ in their vulnerability to drought. To make these predictions we need to incorporate ecological understanding into vegetation models that can be coupled to global climate models, to form Earth System Models (ESMs). The only way to enable these vegetation models to represent ecology properly is to make measurements in natural rain forests. To understand the impact of drought we must go a step further and experimentally manipulate the moisture available to the forest, in order to understand the responses of each key process (e.g. respiration, photosynthesis etc). Large-scale drought experiments are scientifically powerful, but very rare in any biome. We have created a unique opportunity in this project to combine the results from two tropical rain forest drought experiments, in Amazonia and Borneo. The combination of experimental and modelling expertise in our team is particularly strong and we wish to use it to make a substantial advance in the prediction of the impacts of drought on 21st century rain forest functioning. We will first use our models to test for physical differences (soils or climate) in Borneo and Amazonia. Secondly we will focus on differences in mortality risk among tree taxa (species or genera) within and between regions, as some are more vulnerable than others to drought. We will focus on measuring whether mortality is associated with the loss of supply of water or carbon, or a mixture of both, and incorporate our results into our models.In summary, we will use a powerful combination of tropical rain forest field experiments and global vegetation modelling to explain large observed differences in rain forest tree vulnerability to drought across Borneo and Amazonia. The outcome will have pan-tropical application and we will use it to improve predictions of how climate change will affect the global role of tropical rain forests in the 21st century carbon cycle.