Title: Carbon and water exchange in Amazonian rainforests
Description: The Amazon rainforest contains approximately 25% of the earth's terrestrial carbon stores and is responsible for cycling vast amounts of carbon and water between the atmosphere and biosphere. This thesis addresses several important issues surrounding the controls on carbon and water exchange and ecosystem stability in Amazonian rainforests through the use of eddy-covariance data, ground-based measurements, long-term climate records, and numerical models. Based on four years of eddy-covariance data for CO 2 and H 2 O fluxes in an evergreen, old-growth tropical rainforest we found that this forest was a small net source of carbon to the atmosphere with an average loss of 0.9 ± 0.22 Mg C ha -1 yr -1 . This estimate was independently confirmed through biometric methods which also showed the forest to be losing carbon at a rate of 1.5 ± 0.57 Mg C ha -1 yr -1 due to excess respiratory losses. The annual carbon balance was very sensitive to weather anomalies, particularly the timing of the dry-to-wet season transition, reflecting modulation of light inputs and respiration processes.

We found that this forest maintained high rates of photosynthesis throughout the year due to adequate water supplies, high year-round temperatures, and high light levels. Canopy carbon uptake rates were largely controlled by phenology and light with no indication of seasonal water limitation during the 5-month dry season. Photosynthetic efficiency declined late in the wet season, before leaf senescence, and increased after new leaf elongation midway through the dry season. However, ecosystem respiration was inhibited by moisture limitations on heterotrophic respiration during the dry season. Historical records and charcoal found in soils show that fires have occurred in many evergreen tropical forests. Future climate scenarios suggest that temperatures in the Amazon may increase while precipitation decreases, likely decreasing water availability and increasing drought and flammability. We found that over 600,000 km 2 , more than 11% of the Brazilian Amazon, could shift to transitional forests or savanna, if aridity increases as predicted by climate change models. Our analysis showed that increased aridity may lead to bisection of Amazonian equatorial forests.