Elevated atmospheric CO2 (eCO2) often increases photosynthetic CO2 assimilation (A) in field studies of temperate tree species, although there is evidence that the increases may decline through time due to biochemical acclimation and ontogenic and environmental forces. Indeed, at the free air CO2 enrichment (FACE) study in Oak Ridge, TN, photosynthesis was significantly increased in the canopy of 12-year-old sweetgum trees following two years of ~40% enhancement of CO2. Prior to the 2009 conclusion of the sweetgum FACE research study, photosynthesis was re-assessed in late July 2008 and 2009 to determine if the early enhancement of CO2 was sustained after an additional 9-10 years of treatments. Maximum rates of photosynthesis were assessed at prevailing CO2 and temperature for mid- and upper canopy leaves on detached, re-hydrated branches using a portable gas exchange system. Light-saturated photosynthetic CO2 response curves (A-Ci curves) were determined and results contrasted with earlier measurements using a consistent set of leaf photosynthesis model equations. Relationships between maximum electron transport rate, Jmax, maximum Rubisco activity, Vcmax, foliar nitrogen (N) and chlorophyll content, and CO2 treatments were assessed.
In 1999, maximum photosynthesis under eCO2 treatment was 14.5 ± 1.8 μmol m-2 s-1, 40% higher than under aCO2 treatment, but the stimulation of Amax declined through time such that there was no significant differences by 2009 (Amax=6.84 or 6.09 ± 0.52 μmol m-2 s-1 for eCO2 or aCO2, respectively). In 1999, there was no treatment effect on area-based foliar N or chlorophyll content; however, by 2008, N and chlorophyll were significantly less under eCO2 (15 and 25%, respectively). Foliar N content declined 35-40% from 1999-2008, with values as low as 0.091 mg cm-2. Concurrently, chlorophyll content declined by 11% (aCO2) or 38% (eCO2), possibly associated with nutrient-dependent leaf senescence. Leaf area-based N use efficiency (A:N) was greater in eCO2 in 1999 resulting in greater A despite similar N content, but the enhanced efficiency in eCO2 trees was lost as foliar N declined to sub-optimal levels. There was no treatment difference in the ratio of Jmax:Vcmax through time. There was no treatment difference in the declining linear relationships between Jmax or Vcmax with declining N; however, as N content approached 0.1 mg cm-2 the slope of the regressions were not significantly different from zero. This research suggests that the initial enhancement of photosynthesis to elevated CO2 will not be sustained through time if nitrogen becomes limited during stand development.