Tropical forests have evolved under very narrow temperature regimes, and therefore may be more susceptible to global climate change than areas of higher latitude. Upper canopy leaves may already be near or at their thermal maxima, and increasing temperatures could cause permanent damage. Height can impact both the microclimate and physiology of a tropical canopy. We assessed vertical gradients of air temperature, vapor pressure deficit, leaf temperature, and photosynthetic photon flux density in a Puerto Rican tropical forest. We measured within-canopy microclimate using dataloggers distributed along a canopy access tower. Based on previous work, we expected a linear increase in in leaf temperature with height and very little air temperature change with height.
Results/Conclusions
Unexpectedly, leaf temperature was consistent with air temperature at most heights, except for in the upper canopy. Leaf temperature only greatly exceeded air temperature above 17m. The upper canopy was the only location to drastically exceed the known optimal photosynthetic temperature of the measured species by more than a degree. As warming continues, temperatures might continue to exceed the thermal optima of photosynthesis in tropical canopies and potentially cause the tropics to become a weaker carbon sink. This is critical to the global carbon balance as the tropics cycle more carbon than any other biome and the upper canopy cycles a disproportionate amount relative to the remainder of the canopy.