Different degrees for different trees: variation in the acclimation of photosynthetic capacity to climate change among seedlings of three boreal trees

Background/Question/Methods

Boreal forests provide crucial ecological and economic services and are important ecosystems for modelling global carbon dynamics. We expect anthropogenic CO2 emissions to warm boreal regions by up to 8ºC by 2100, yet we have relatively little data on the physiological responses of boreal trees to these climatic changes. In particular, we do not know the extent to which photosynthetic capacity in boreal trees will acclimate to changes in atmospheric CO2 and warming. To resolve this, we grew three species of North American boreal tree (paper birch, tamarack, and black spruce) from seed at either ambient (AC, 400 ppm) or elevated CO2 (EC, 750 ppm), and at either ambient temperatures (T0) or with +4ºC (T4) or +8ºC (T8) warming. After four months, we measured the maximum rate of Rubisco carboxylation (Vcmax) at multiple leaf temperatures.

Results/Conclusions

Each species showed unique responses. In paper birch, the temperature response of Vcmax was unaffected by warming, and only weakly acclimated (i.e. decreased when measured at a common leaf temperature) to elevated CO2. In tamarack, Vcmax acclimated to warming under current CO2, but was enhanced by moderate warming (T4) under high CO2. Vcmax also acclimated to warming under current CO2 in black spruce, but decreased in T0 plants and increased in T8 plants when grown under high CO2. These results suggest that different boreal tree species have different photosynthetic responses to climate change. This has implications for our models of CO2 exchange between boreal trees and the atmosphere, and for the composition and productivity of Canada's boreal forests in the future.