Abstract
Elevated atmospheric CO2 (eCO(2)) often increases photosynthetic CO2 assimilation (A) in field studies of temperate tree species. However, there is evidence that A 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 2 years of similar to 40 % enhancement of CO2. A was re-assessed a decade later to determine if the initial enhancement of photosynthesis by eCO(2) 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 versus the CO2 concentration in the intercellular air space (Ci); or A-Ci curves) were contrasted with earlier measurements using leaf photosynthesis model equations. Relationships between light-saturated photosynthesis (Asat), maximum electron transport rate (Jmax), maximum Rubisco activity (Vcmax), chlorophyll content and foliar nitrogen (N) were assessed. In 1999, Asat for eCO(2) treatmentswas 15.4+ 0.8 mmol m 22 s 21, 22 % higher than aCO(2) treatments (P, 0.01). By 2009, Asat declined to,50 % of 1999 values, and there was no longer a significant effect of eCO(2) (Asat similar to 6.9 or 5.7+ 0.7 mmol m 22 s 21 for eCO(2) or aCO(2), respectively). In 1999, there was no treatment effect on area-based foliar N; however, by 2008, N content in eCO(2) foliage was 17 % less than that in aCO(2) foliage. Photosynthetic N-use efficiency (Asat : N) was greater in eCO(2) in 1999 resulting in greater Asat despite similar N content, but the enhanced efficiency in eCO(2) 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 the initial enhancement of photosynthesis to elevated CO2 will not be sustained through time if N becomes limited.
