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

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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.