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-C_i curves) were determined and results contrasted with earlier measurements using a consistent set of leaf photosynthesis model equations. Relationships between maximum electron transport rate, J_max, maximum Rubisco activity, V_cmax, foliar nitrogen (N) and chlorophyll content, and CO₂ treatments were assessed.

Results/Conclusions

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 A_max declined through time such that there was no significant differences by 2009 (A_max=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 J_max:V_cmax through time. There was no treatment difference in the declining linear relationships between J_max or V_cmax 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.