Including Tree Diversity In Predictions Of Tropical Forest Drought Responses

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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.