Experimental Manipulations of Albedo and Mortality of Upper Canopy Leaves in a Tropical Forest Diverge From Earth System Model Results

How tropical forest leaves respond to climate change has important implications for the global carbon cycle and biodiversity. Climate change could impact the energy balance properties of tropical forest canopies through (a) long-term trait changes and (b) abrupt disruptions/damage to leaf/photosynthetic machinery. We assessed the radiative and evaporative impacts of two recently proposed impacts of climate change on tropical forest canopies: (a) long-term leaf darkening and (b) leaf death through high temperature extremes. We darkened leaves to absorb 138 Wm⁻² more energy in the upper canopy of a seasonally dry tropical moist forest in Panama. 20% of this extra energy went toward heating leaves by ∼4°C, 3% went toward warming the air, and 77% went toward evaporative cooling. This leaf warming led to the appearance of necrosis across 9 ± 5% of the leaf area on certain species. In contrast, brightening leaves decreased energy absorbed by an average of 58 Wm⁻², which mainly reduced evaporation (88%) with only 12% reducing leaf temperatures (and no change in sensible heat flux). This asymmetrical result suggests leaves may be close to hydraulic limitations to support transpirational cooling toward the end of the dry season. Similar albedo increases in a model (CLM 4.0) did not diverge between brightening and darkening leaves and generally showed sensible heat flux to dominate although there were strong geographic trends. Heat death in leaves generally heated nearby leaves (by an average of ∼1.35°C) and air temperature (by 0.5°C) but less than hypothesized because leaf albedo increased. Overall, our canopy top experiments question important potential climate feedbacks but need further study.