Carbon dioxide stimulation of photosynthesis in Liquidambar styraciflua is not sustained during a 12-year field experiment

Elevated atmospheric CO(2) (eCO(2)) often increases photosynthetic CO(2) 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 CO(2) enrichment (FACE) study in Oak Ridge, Tennessee, A was increased in 12-year-old sweetgum trees following 2 years of ∼40 % enhancement of CO(2). 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 CO(2) and temperature on detached, re-hydrated branches using a portable gas exchange system. Photosynthetic CO(2) response curves (A versus the CO(2) concentration in the intercellular air space (C(i)); or A–C(i) curves) were contrasted with earlier measurements using leaf photosynthesis model equations. Relationships between light-saturated photosynthesis (A(sat)), maximum electron transport rate (J(max)), maximum Rubisco activity (V(cmax)), chlorophyll content and foliar nitrogen (N) were assessed. In 1999, A(sat) for eCO(2) treatments was 15.4 ± 0.8 μmol m(−2) s(−1), 22 % higher than aCO(2) treatments (P < 0.01). By 2009, A(sat) declined to <50 % of 1999 values, and there was no longer a significant effect of eCO(2) (A(sat) = 6.9 or 5.7 ± 0.7 μmol m(−2) s(−1) 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 (A(sat) : N) was greater in eCO(2) in 1999 resulting in greater A(sat) 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 J(max) or V(cmax) with declining N, or in the ratio of J(max) : V(cmax) through time. Results suggest that the initial enhancement of photosynthesis to elevated CO(2) will not be sustained through time if N becomes limited.