Effects of temperature and canopy location on the photosynthetic capacity of early and late successional tree species

Background/Question/Methods

Forests are an important regulator of Earth’s climate because they remove up to a quarter of human-emitted CO₂ annually. However, it is uncertain what this amount of CO₂ uptake will be in the future as forests are altered due to human and natural shifts in forest species composition. Models can project the future of forest CO₂ uptake, but their calculations depend on how accurately models parameterize photosynthetic processes. In particular, models require values for the maximum rate of carboxylation (V_cmax) and rate of electron transport (J) and often use the same values for species within the same plant functional group. To test for species differences and to determine whether the future potential of forest CO₂ uptake in the Great Lakes region will change as forests undergo succession, we conducted net assimilation versus stomatal CO₂ concentration (A/C_i) curves on adult sun and shade leaves of early successional Populus grandidentata (bigtooth aspen) and late successional Quercus rubra (red oak) and Acer rubrum (red maple) in northern lower Michigan. To test how these photosynthetic parameters will change in a warming climate, we conducted these curves across a range of temperatures and derived V_cmaxand J values specific to species, canopy-location, and temperature.         

Results/Conclusions

Preliminary analysis indicates that V_cmax and J increase with temperature (p < 0.05) across a spectrum from 23 to 32°C. We saw no species differences in the response of V_cmax to temperature, but the response of J to temperature did differ between P. grandidentata and A. rubrum sun leaves (p < 0.05) and between Q. rubra and A. rubrum shade leaves (p < 0.01). In addition, there was a difference in the response of V_cmax to temperature between sun and shade leaves of Q. rubra. The response of J to temperature was also different between sun and shade leaves of both later-successional species. These results suggest that models should consider incorporating effects of species, canopy location, and temperature on photosynthetic parameters used in calculating CO₂ uptake in forests. In addition, they suggest that forest succession will cause changes in ecological variation that may impact total canopy photosynthesis.