Will photosynthetic capacity of aspen trees acclimate after long-term exposure to elevated CO2 and O3?

Photosynthetic acclimation under elevated carbon dioxide (CO2) and/or ozone (O3) has been the topic of discussion in many papers recently. We examined whether or not aspen plants grown under elevated CO2 and/or O3 will acclimate after 11 years of exposure at the Aspen Face site in Rhinelander, WI, USA. We studied diurnal patterns of instantaneous photosynthetic measurements as well as A/Ci measurements monthly during the 2004–2008 growing seasons. Our results suggest that the responses of two aspen clones differing in O3 sensitivity showed no evidence of photosynthetic and stomatal acclimation under either elevated CO2, O3 or CO2 + O3. Both clones 42E and 271 did not show photosynthetic nor stomatal acclimation under elevated CO2 and O3 after a decade of exposure. We found that the degree of increase or decrease in the photosynthesis and stomatal conductance varied significantly from day to day and from one season to another.