OOS 47-4 - Experimental warming across a tropical forest canopy height gradient reveals lack of physiological acclimation

Background/Question/Methods
Tropical forest canopies cycle vast amounts of carbon, yet we have limited understanding of how these critical ecosystems will respond to climate warming. Recent evidence shows that lowland tropical forest canopies may already be exceeding thermal maxima for photosynthesis. Height can strongly influence both the microclimate and physiology of forest canopy foliage, yet vertical trends in canopy micrometeorology and physiological acclimation are rarely examined in any forest. In a Puerto Rican tropical wet forest, we implemented in situ leaf-level + 3°C experimental warming from the understory to the upper canopy. After approximately 1 month of continuous warming, we assessed adjustments in photosynthesis, stomatal conductance, and foliar respiration. To investigate vertical variability of foliar thermoregulation, we also assessed microclimate, leaf temperature, and the difference between leaf and air temperature (ΔT) on control leaves from forest floor to canopy top.

Results/Conclusions
Experimental warming did not affect temperature responses of net photosynthesis or respiration for upper or mid-canopy foliage, revealing a lack of physiological acclimation. 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. Leaf temperature did not significantly differ across the shaded foliage from 0-16 m in height until the uppermost layer (20 m), which was up to 4°C hotter than the rest of the foliage. Temperatures of shaded leaves were no different from air temperatures, while sun-lit leaf temperatures were frequently greater than air temperature. In addition, the uppermost canopy was the only stratum to exceed mean T_opt for this site (30.2 ± 1.1°C). With minimal physiological acclimation to warmer temperatures in the 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. However, the shaded leaves may be able increase carbon uptake with further warming because they appear to be able to maintain temperatures below photosynthetic optima, possibly with the help of radiation shielding provided by the uppermost canopy layer.