Climate change and the Amazon: assessing the impact of climate on tree growth using tree ring widths and isotopes

The Amazon rainforest may be very vulnerable to climate change. If the earth warms as predicted, much of the Amazon rainforest could disappear by the end of the century, as tropical trees should be unable to cope with higher temperatures and frequent droughts in the future. Such a 'die-back' of the Amazon rainforest could be disastrous for global warming, as the vast stores of carbon contained in the Amazon would be emitted back into the atmosphere as greenhouse gases. This will speed-up global warming. Such a scenario is predicted by computer models that simulate responses of vegetation to climate. However, it is still very uncertain how sensitive rainforest trees are to changes in climate like increasing atmospheric CO2 concentrations, higher temperatures, and droughts. One way of getting insights into how trees will respond to future climate is by looking at how trees have responded to similar conditions in the past. An effective approach of doing this is by using tree rings, as they provide an historical archive of growth rates over their entire life (i.e., 200 years or more), which can be related to the climate conditions at the time of ring-formation. Therefore, I propose to use tree rings of tropical trees to assess how sensitive trees have been to changes in temperature, drought, solar radiation, and increases in atmospheric CO2-concentrations over the last century. I will measure ring widths and stable isotope concentrations in tree rings to analyse the responses of tree growth to climatic and atmospheric changes. Ring width measurements will give insight in growth responses, and carbon isotopes will help understand the underlying processes behind any growth responses by giving detailed information on photosynthesis and plant water use. For example, by studying long-term trends in growth rates and plant water use, I will be able to test whether increases in CO2 concentrations over the past century have indeed stimulated tropical tree growth, as has often been proposed. By linking year to year variation in growth rates to variation in droughts, temperature and solar radiation, I will determine the main climatic factors controlling tropical tree growth. My study will improve our understanding of sensitivity of tropical trees to predicted changes in climate and increasing atmospheric CO2 and allow a better assessment of the likelihood of the predicted disappearance of the Amazon rainforest over the next century.