In the coming decades the tropics will experience ongoing warming, and this temperature change influences the rate at which photosynthesis operates. The unprecedented temperature regimes and the large contribution of tropical forest to the global carbon cycle makes a better understanding of how tropical trees respond to climate change critically important. Photosynthesis measurements are generally standardized by using only sun leaves, but given that there are five shade leaves for every sun leaf in tropic forests, data on sun leaves alone are not enough to model the actual dynamics of carbon uptake of tropical forests. The aim of this project is to compare the temperature sensitivity of biochemical and stomatal parameters between sun and shade leaves. We determined stomatal conductance and biochemical parameters on sun and shade leaves of three tropical tree species. Biochemical parameters (the maximum rates of Rubisco carboxylation and regeneration) were derived from measurements of curves of photosynthesis in relation to CO2 concentration across temperature ranges of at least ten degrees. The data were fitted with a mechanistic model to determine the optimal temperature for all parameters.
Results/Conclusions
The temperature optima of the biochemical parameters and stomatal conductance did not differ significantly between sun and shade leaves and for most photosynthetic parameters the shade and sun leaves had similar temperature sensitivity. Stomatal conductance for sun leaves decreased with an increase in temperature for all species. This pattern changed for shade leaves, which had very low stomatal conductance and less pronounced decrease when temperature increased. Interestingly, while photosynthesis of sun leaves was increasingly limited by stomatal conductance with increasing temperature, shade leaves tended to be more constrained by the biochemical parameters, which were much lower in shade leaves. Despite considerable differences in leave temperatures, our results suggest that the temperature sensitivity of photosynthetic parameters is similar in sun and shade leaves, and that accurate representation of tropical forests in ecosystem models does not require separate parameterization of sun and shade leaves. Nonetheless, the apparent difference in the parameters that limit photosynthesis at high temperature requires further study.