Photosynthesis of tropical forest trees represents an enormous flux of carbon from the atmosphere to the terrestrial biosphere. Tropical forests have been hypothesized to be close to exceeding their thermal threshold, but data on the temperature responses of tropical tree physiology are limited; as such, there is an urgent need to accurately assess the rates of photosynthesis of tropical trees in relation to temperature. We recently reported distinct differences in the temperature response characteristics of net photosynthesis between seedlings and canopy trees of one tropical tree species. As canopy trees contribute much more to the total amount of carbon taken up by tropical forests than more commonly studied seedlings, there is a particular need for photosynthetic temperature response data in the upper canopy. We took in situ measurements of net photosynthesis in relation to leaf temperature for upper-canopy leaves of 30 tropical tree and liana species at two sites in Panama, making use of construction cranes for repeated canopy access. For each species we calculated the optimum temperature for photosynthesis (TOpt), the rate of photosynthesis at TOpt (AOpt), the high-temperature CO2 compensation point, and the temperature range over which the trees can maintain a photosynthesis rate of >80% of AOpt.
Results/Conclusions
Consistent with previous, much smaller studies, we found across species that the optimum temperature of net photosynthesis was close to mean ambient daytime air temperatures, suggesting that much of the time, tropical forest trees operate at supra-optimal temperatures for photosynthesis. However, there was no steep drop-off of photosynthesis above TOpt, enabling the maintenance of a positive net carbon balance up to leaf temperatures ≥40°C. The high-temperature carbon compensation point was quite variable, with net photosynthesis being reduced to zero near 40°C in some species, and near 50°C in others. Consequently, the temperature range over which leaves were able to maintain 80% or more of AOpt varied considerably across species. Systematic differences in temperature response traits between early-and late-successional species, and between trees and lianas, suggest that late-successional species are more sensitive to high temperatures than early-successional species and lianas. Global warming could thus potentially lead to changes in forest community structura and carbon dynamics by providing a competitive advantage for fast-growing, early-successional species and lianas that have low wood density, short lifespans, and limited carbon storage potential. Future work will need to determine whether early- and late-successional species similarly differ in their capacity to physiologically acclimate to elevated temperatures.