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

Abstract

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 treatmentswas 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.