Abstract
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 and morphological acclimation, and environmental constraints. Indeed, at the free air CO2 enrichment (FACE) study in Oak Ridge, Tennessee, A was increased in 12-year-old sweetgum trees following two years of ~40% enhancement of CO2. A was re-assessed a decade later to determine if initial enhancement of CO2 was sustained through time. Measurements were conducted at prevailing CO2 and temperature on detached, re-hydrated branches using a portable gas exchange system. Photosynthetic CO2 response curves (A-Ci curves) were contrasted with earlier measurements using consistent leaf photosynthesis model equations. Relationships between maximum electron transport rate (Jmax), maximum Rubisco activity (Vcmax) and foliar nitrogen (N) and chlorophyll content were assessed. In 1999, light-saturated photosynthesis (Asat) for eCO2 treatments was 15.4 ± 0.8 μmol m-2 s-1, 22% higher than aCO2 treatments (P<0.01). By 2009, Asat declined to <50% of 1999 values, and there was no longer a significant effect of eCO2 (Asat = 6.9 or 5.7 ± 0.7 μmol m-2 s-1 for eCO2 or aCO2, respectively). In 1999, there was no treatment effect on area-based foliar N; however, by 2008, N content in eCO2 foliage was 17% less than in aCO2 foliage. Photosynthetic N use efficiency (Asat:N) was greater in eCO2 in 1999 resulting in greater Asat 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 declining linear relationships between Jmax or Vcmax with declining N, or in the ratio of Jmax:Vcmax through time. Results suggest that initial enhancement of photosynthesis to elevated CO2 will not be sustained through time if nitrogen becomes limited.
