Experimental warming across a tropical forest canopy height gradient reveals minimal photosynthetic and respiratory...

Tropical forest canopies cycle vast amounts of carbon, yet we still have a limited understanding of how these critical ecosystems will respond to climate warming. We implemented in situ leaf-level + 3°C experimental warming from the understory to the upper canopy of two Puerto Rican tropical tree species, Guarea guidonia and Ocotea sintenisii. After approximately 1 month of continuous warming, we assessed adjustments in photosynthesis, chlorophyll fluorescence, stomatal conductance, leaf traits and foliar respiration. Warming did not alter net photosynthetic temperature response for either species; however, the optimum temperature of Ocotea understory leaf photosynthetic electron transport shifted upward. There was no Ocotea respiratory treatment effect, while Guarea respiratory temperature sensitivity (Q₁₀ ) was down-regulated in heated leaves. The optimum temperatures for photosynthesis (T_opt ) decreased 3-5°C from understory to the highest canopy position, perhaps due to upper canopy stomatal conductance limitations. Guarea upper canopy T_opt was similar to the mean daytime temperatures, while Ocotea canopy leaves often operated above T_opt . With minimal acclimation to warmer temperatures in the upper canopy, further warming could put these forests at risk of reduced CO₂ uptake, which could weaken the overall carbon sink strength of this tropical forest.