Introduction. Modeling the global response of photosynthesis to climate change is limited by our understanding of the major component processes of net photosynthesis (Anet) and their temperature responses within and among species. While the mechanisms underpinning the temperature responses of RuBP carboxylation and regeneration and stomatal conductance (gs) are relatively well understood at temperate latitudes, more work is needed to characterize these patterns within lower latitude species. Objectives. We addressed two research questions: (1) Do subtropical trees acclimate their optimum temperature of Anet (ToptA) and Anet at Topt (Aopt) to elevated growth temperatures? And (2) What limits Aopt in subtropical trees? Methods. We measured the acclimation of Anet within six commonly planted tree species across steep (~5°C) urban thermal gradients in Miami, Florida (USA), taking advantage of an underutilized “natural experiment” for testing species responses to potential future conditions. We performed nested temperature-CO2 response curves of Anet for ≥ 20 individuals each of each species, measuring Anet, maximum RuBP carboxylation, maximum RuBP regeneration, and gs, and modeling the Topt and maximum process rate of each parameter. Results. Contrary to most other studies of intraspecific acclimation of photosynthesis, we did not find acclimation of ToptA, Aopt, Anet, or any of the underlying photosynthetic parameters to growth temperature in any of our six tree species across the thermal gradient. There was significant involvement of all three underlying parameters in limiting photosynthetic performance across all species, but model selection for the best single-predictor model of Anet revealed that almost all trees across our study species were best predicted by gs. Implications/Conclusions. The lack of observed acclimation of ToptA was surprising given the relatively large range of growth temperatures captured. Also surprising was the fact that Anet did not decline in any of the study species as a result of the lack of ToptA acclimation. While it is possible that our expected results were obscured by microsite variations in growth temperature that we did not capture, our results suggest that Anet in some tree species may be resilient to long-term changes in temperature (i.e., over the lifetime of a tree) even if ToptA is static. We also found that gs usually limited Anet in all six species, evidence that controls on Anet in the subtropics may be more similar to those in the tropics than those in more temperate zones and highlighting the need for continued investigation of the environmental controls to gs.


photosynthesis, thermal optima, Vcmax, Jmax, stomatal conductance, thermal gradient, acclimation

Alyssa Kullberg

Presentation within symposium:

S-19 Tropical forest response to temperature: a pantropical synthesis of elevation gradients, and leaf thermoregulation studies

Photosynthetic thermal optima controlled by stomatal conductance in six subtropical species and do not acclimate across an urban thermal gradient.