Collaborative Research: The other side of tropical forest drought: Do shallow water table regions of Amazonia act as large-scale hydrologic refugia from drought?

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The vegetation that covers the Earth has many important functions. One of the biggest impacts of vegetation is trapping and storing carbon dioxide from the atmosphere. The vast Amazon rainforest makes a significant contribution to this carbon storage function. However, the Amazon forest is being threatened by recent changes to climate, which is making droughts stronger and more frequent. A big question is whether the Amazon will be resilient to these droughts, or whether they will slow forest growth and kill trees, which would release carbon. A significant challenge is the lack of information about a large portion of the Amazon forest—about 30%—with shallow water tables, where trees live in an environment of excess water. In these water-logged forests, tree growth is slow because soil conditions are poor for tree roots. Here, drought could actually be beneficial to trees by reducing this water-logging. On the other hand, these forests have roots that grow close to the soil surface and require wet conditions. In this case, strong droughts that last a long time dry the upper soil and could reduce growth more, killing more trees than in forests with well-drained soils. This project will answer questions about the role of belowground water sources as an important control on response of tropical forest to drought. Data about forest growth and carbon dioxide exchange will be made available to the broader research community. This project will also help train students in how to measure forests, and provide information about tropical forests to the public.This project will measure forest canopy response to drought at sites in the Brazilian Amazon. The approach is to unify field ecology with remote observation of ecosystems from canopy towers and satellites. Canopy towers constructed in this project—representing the first of their kind in water-logged soils of the interior Amazon—provide detailed information about tree growth. The capacity of the forest canopy for photosynthesis will be measured as well as leaf demography and phenology. This new knowledge will be combined with networks of existing canopy towers in deep water table depth sites, satellite images spanning the Amazon, data on tree growth and death from a network of forest surveys, and detailed measurements of soil water and other environmental factors. This project will build an accurate new understanding of the factors that impact forest carbon cycling, while contrasting water-logged with well-drained soil responses. A data-based model of ecosystem structure and function will offer a quantitative accounting of the contribution of these water-logged forests in the Amazon to climate change responses. These results have the potential to transform our current perspective on drought effects in tropical forests.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.