|The primary target of CO2 is photosynthesis and in most plants this process is stimulated in leaves exposed to elevated CO2. But early observations noted that after some time in elevated CO2, the rate of photosynthesis declined, a phenomenon called acclimation, and it was thought by some that if this occurred under long-term exposure to elevated CO2, there might be no stimulation of growth by elevated CO2. In a series of papers (Li, et al., 1999; Hymus et al., 2001 and 2002; Ainsworth et al., 2002; Dore et al, 2003, Hymus et al, In Press; Li et al In Review) we have shown that acclimation occurs in both species but that it does not obviate a strong response of photosynthesis in the scrub oak ecosystem.
We have shown that elevated CO2 caused a stimulation of net ecosystem CO2 exchange in the scrub oak ecosystem of 60-100% per unit ground area and 35-40% stimulation per unit leaf area (Li et al. 1999; Ainsworth, et al., 2002; Hymus et al, In Press). This response has been sustained throughout the 6-year study. In scrub oak, the combination of high temperature and increased growth of leaves (Hymus et al. 2002b) is most likely responsible for the relatively high stimulation of CO2 assimilation. Our use of the open top chamber as a cuvette gives results that are very similar to those obtained in the same ecosystem using eddy flux (Dore et al. 2003). Use of the open top chamber to determine net ecosystem gas exchange has also tested favorably against the output of an ecosystem model in our study of a Chesapeake Bay wetland (Rasse et al. 2003). In recent, unpublished work, Jiahong Li has shown that, when measurements were made in the field on attached leaves, acclimation of both of the dominant species, Q. myrtifolia and Q. geminata, always occurs and that the proximate cause is feedback inhibition by accumulation of non-structural carbohydrates rather than failure of the supply of nitrogen from soil to keep pace with demand of growth. Although Ainsworth et al. (2003) reported that acclimation did not occur in Q. myrtifolia, this conclusion is based on the fact that measurements were made on detached leaves in the lab. Rates of photosynthesis were higher and the differences between CO2 treatments for the critical properties, Vcmax and Jmax, larger on attached leaves in the field than on detached leaves measured simultaneously in the lab.