Effects of warming on aboveground tropical plant feedbacks
Tropical forests play a large role in the global carbon cycle. Ongoing climate warming may push them into a climate envelope not currently occupied by closed-canopy forest. It is currently unknown what the consequences will be of such warming for tropical forests and their capacity to take up and store carbon. The uncertainty in model predictions represents, for a large part, our ignorance about the feedbacks between the terrestrial biosphere and the atmosphere in a warming world. The sensitivity of photosynthetic metabolism to temperature is one of the most important uncertainties for land surface models, in particular with respect to the capacity for photosynthetic acclimation of tropical vegetation. We experimentally assessed thermal acclimation of both foliar respiration and photosynthesis of a series of tropical tree species grown in controlled environment chambers at 25°C, 30°C and 35°C. We also grew tropical tree species in sapling mesocosms outdoors under natural tropical conditions in Panama, under ambient and elevated daytime temperatures.
Consistent with previous studies, respiration was down-regulated in warm-acclimated plants, thus increasing their respiratory efficiency compared to non-acclimated plants at the elevated temperature. The optimum temperature of photosynthesis (TOpt) increased with the temperature at which seedlings were grown. However, net photosynthesis rate at TOpt decreased with growth temperature. Underlying this decrease in net photosynthesis rate was a systematic decrease in both the maximum rate of RuBP carboxylation (VCmax) and the rate of RuBP regeneration (Jmax) with increasing temperature.
In sapling mesocosms midday temperatures were on average 30°C, corresponding to the optimum temperature for growth in the growth chamber experiment, while the warmed mesocosms averaged 35°C, a supra-optimal temperature for seedling growth. In line with the growth chamber experiment, VCmax and Jmax tended to be lower for saplings grown at elevated temperatures. However, intra-specific variation in photosynthetic traits was larger than treatment effects on these traits. Despite the decreased VCmax and Jmax, ambient net photosynthesis was similar across mesocosms. Similarly, growth rate did not differ between temperature treatments. These results will be discussed in the context of the fate of tropical carbon fluxes and the potential feedbacks between the terrestrial tropical biosphere and the atmosphere in a warming world.