Tropical forest upper canopies cycle a disproportion amout of carbon compared to middle and lower canopy leaves. There is a growing body of evidence suggesting that tropical forest upper canopies are approaching temperature thresholds, where climate warming could reduce upper canopy carbon uptake. Even so, we know little about how a forest canopy might acclimate across the gradient from the shaded understory to the sunlit upper canopy. We investigated this tropical forest canopy gradient acclimation potential on two tropical canopy trees, Guarea guidonia and Ocotea sentenisii, in a Puerto Rican forest located at the Tropical Responses to Altered Climate Experiment (TRACE) site. We measured photosynthetic and respiratory responses to temperature after one month of experimental, leaf-level warming. Overall, we found limited evidence for physiological acclimation to experimental warming. Only Guarea showed respiratory acclimation through lowered temperature sensitivity (Q10). Despite the high stress conditions of the upper canopy, net photosynthesis did not acclimate for either species. However, Ocotea understory leaves acclimated toward higher rates of electron transport with experimental warming. Neither species photosynthetic temperature sensitivity, or thermal niche, acclimated or varied throughout the canopy height gradient. Surprisingly, the optimum temperature for photosynthesis (Topt) decreased as canopy height increased, due to the greater stomatal sensitivity to high temperatures in the middle and upper canopies. Declined Topt in the mid and upper canopies contributed to these leaves operating beyond their Topt greater than 50% of the time during daylight hours in the upper canopy and greater than 20% of the time in the mid canopy. Leaf area did not vary across the canopy height gradient and stomatal conductance was low at high temperatures for the upper canopy leaves, suggesting that these leaves did not have high rates of transpiration and experienced little to no thermoregulation in the upper canopy. The lack of Topt acclimation and frequent exposures to temperatures above Topt put these upper canopy leaves at a high risk of reduced carbon uptake under climate warming.
Forest canopy, photosynthesis, respiration, thermoregulation, height gradient, temperature response