| We investigate the sensitivity of tropical forests to high temperatures using several methods. In the second chapter, we used leaf gas exchange, sap flow, and eddy covariance measurements to investigate whether high temperature currently limits CO2 uptake at the LBA-ECO (Large-scale Biosphere-Atmosphere) km-83 tropical forest site in Brazil. Leaf-level temperature-photosynthesis curves, and comparisons of whole-canopy net ecosystem CO2 exchange (NEE) with air temperature, showed that CO2 uptake declined sharply during warm periods.;In the third chapter, we then warmed leaves and branches of mature trees, vines, and gap species by ∼2°C and found that maximum photosynthetic capacity and stomatal conductance of the warmed leaves in tree species declined significantly relative to the control group over the 13 week study period. We saw no obvious signs of acclimation to higher temperatures, but all species tested showed high rates of photorespiration.;The fourth chapter shows that tropical forest photosynthesis is controlled by internal rhythms as well as such environmental conditions as light, temperature, and water.;In the fifth chapter, we observed that NEE increased during smoky periods. We calculate that the increase in CO2 uptake during smoky periods is mainly (∼62%) due to decreased canopy temperature and partially (∼38%) due to increased diffuse light in the sub-canopy.;In the sixth chapter, we observed that canopy EVI (Enhanced Vegetation Index) and canopy CO2 uptake increase during the dry season when temperatures are warmest. This seems contrary to our previous results, but find that instead, this is due to changing forest phenology. New leaves flush in the dry season, causing the increase in greenness, and these new leaves have higher maximum photosynthetic capacity, causing the increase in canopy CO2 uptake.;In the seventh chapter, we simulate climate by modifying leaf crop albedo to understand the effect increased albedo will have on regional air temperatures.;Overall, we find that tropical forest photosynthesis is currently sensitive to high temperatures. As temperatures rise due to global warming, tropical forest photosynthesis may decrease due to increased temperatures but may increase due to reduced photorespiration at high CO2 concentrations. |
