An In Situ Leaf and Branch Warming Experiment in the Amazon

Leaves and branches of mature trees, lianas, and gap species were warmed in an Amazonian forest for 4 mo to observe the effect of warming on photosynthesis, stomatal conductance, and transpiration. Electric resistance heaters increased air temperatures near the leaves by approximately 2 degrees C. Sunlit leaf temperatures increased by 2-3 degrees C on average, but during some periods leaf temperatures increased by >5 degrees C. Maximum photosynthesis (A(max)) decreased significantly in the warmed leaves vs. the control leaves over the 13-wk study period with an average decrease in A(max) of 1.4 mu mol/m(2)s (19% decrease from a mean A(max) of 7.2 mu mol/m(2)s) when measured at 30 degrees C and there were no signs of acclimation to higher temperatures within existing leaves. The decline in A(max) was likely due to irreversible temperature damage caused by very high leaf temperatures and not due to C-i limitation of carboxylation. Warming had a larger negative impact on A(max) in canopy level tree species than other tested functional groups such as lianas or gap species. Transpiration did not significantly increase in the warmed leaves compared with the control group. This study indicates that increased temperatures due to global warming could potentially decrease future tropical forest carbon uptake by a significant amount.