Collaborative Research: Are Amazon forest trees source or sink limited? Mapping hydraulic traits to carbon allocation strategies to decipher forest function during drought

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Understanding how large expanses of tropical forests will respond to climate variation, including increasing frequency of droughts, is important since these forests play a big role in the planet's carbon cycle. This award provides funds to study how the Amazon forest will respond to droughts such as experienced in the recent El Nino. Researchers will examine how individual species capture and use carbon and water, and use that data to understand how biodiversity of species makes the whole forest more resilient to droughts. The results will be integrated to model and predict how differences in forest species composition result in differences in whole-forest responses to drought. This award will evaluate how composition of biological communities influences their overall function and the effects of future changes on entire tropical forests. The project will advance international collaboration and the training of U.S. undergraduates to do research in Brazil. Increased public awareness of these issues will occur through outreach programs at the Biosphere 2, and a Science Festival which facilitates targeted presentations about the Amazon to 100+ middle-high school students/year. The science of plant drought response has largely assumed that environment regulates photosynthetic source of carbon ('C-source regulation'), which is then allocated to growth and other functions according to fixed allometric rules. There is an emerging view, however, that within-plant C-sink processes (growth, storage, and respiration) are directly regulated by the environment and by within-plant feedbacks (the 'C-sink regulation' hypothesis), such that changes in allocation and sink processes are a key mechanism of drought response. Realistically propagating such mechanisms to the ecosystem scale requires accounting for species-specific strategies that vary across the plant economic spectrum. To realistically test this hypothesis, the following challenges will be me through integrating measurements of tree C-source (photosynthesis) and C-sink (growth, storage, respiration) processes across a range of hydraulic traits to identify regulating mechanisms; Upscaling the mechanisms of drought response to the ecosystem level by focusing on distributions of both resources and plant hydraulic traits mapped to the plant economic spectrum; and Observing the ecosystem-scale photosynthetic C source (Gross Ecosystem Productivity, GEP) and water loss by transpiration. This evaluation of the controls on tree C-source and C-sink processes - including the Kok effect, Non-Structural C storage, and plant water relations - will transform our understanding of Amazon forest C dynamics and assessment of the forest's future under climate change.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.