Light-saturated photosynthetic and stomatal responses to elevated CO 2 were measured in upper and mid-canopy foliage of a sweetgum ( Liquidambar styraciflua L) plantation exposed to free-air CO 2 enrichment (FACE) for 3 years, to characterize environmental interactions with the sustained CO 2 effects in an intact deciduous forest stand. Responses were evaluated in relation to one another, and to seasonal patterns and natural environmental stresses, including high temperatures, vapour pressure deficits ( VPD ), and drought. Photosynthetic CO 2 assimilation ( A ) averaged 46% higher in the +200 µ µ µ µ mol mol − 1 ), but relative effects, as the ratio of elevated to ambient rates, varied greatly under those conditions. Both stomatal and non-stomatal limitations of A were involved during late season droughts. Leaf temperature had a limited influence on A and g s , and there was no detectable relationship between prevailing temperature and CO 2 effects on A or g s . The responsiveness of A and g s to elevated CO 2 , both absolute and relative, was maintained through time and within the canopy of this forest stand, subject to seasonal constraints and variability associated with limiting air and soil moisture.

Leaf area index (LAI) and its seasonal dynamics are key determinants of terrestrial productivity and, therefore, of the response of ecosystems to a rising atmospheric CO(2) concentration. Despite the central importance of LAI, there is very little evidence from which to assess how forest LAI will respond to increasing [CO(2)]. We assessed LAI and related leaf indices of a closed-canopy deciduous forest for 4 years in 25-m-diameter plots that were exposed to ambient or elevated CO(2) (542 ppm) in a free-air CO(2) enrichment (FACE) experiment. LAI of this Liquidambar styraciflua (sweetgum) stand was about 6 and was relatively constant year-to-year, including the 2 years prior to the onset of CO(2) treatment. LAI throughout the 1999-2002 growing seasons was assessed through a combination of data on photosynthetically active radiation (PAR) transmittance, mass of litter collected in traps, and leaf mass per unit area (LMA). There was no effect of [CO(2)] on any expression of leaf area, including peak LAI, average LAI, or leaf area duration. Canopy mass and LMA, however, were significantly increased by CO(2) enrichment. The hypothesized connection between light compensation point (LCP) and LAI was rejected because LCP was reduced by [CO(2)] enrichment only in leaves under full sun, but not in shaded leaves. Data on PAR interception also permitted calculation of absorbed PAR (APAR) and light use efficiency (LUE), which are key parameters connecting satellite assessments of terrestrial productivity with ecosystem models of future productivity. There was no effect of [CO(2)] on APAR, and the observed increase in net primary productivity in elevated [CO(2)] was ascribed to an increase in LUE, which ranged from 1.4 to 2.4 g MJ(-1). The current evidence seems convincing that LAI of non-expanding forest stands will not be different in a future CO(2)-enriched atmosphere and that increases in LUE and productivity in elevated [CO(2)] are driven primarily by functional responses rather than by structural changes. Ecosystem or regional models that incorporate feedbacks on resource use through LAI should not assume that LAI will increase with CO(2) enrichment of the atmosphere.

Summary• The photosynthetic response of trees to rising CO 2 concentrations ([CO 2 ]) can be affected by plant source -sink relations, in addition to seasonal changes in environmental conditions. Characterization of biochemical and morphological feedbacks is important for understanding ecosystem responses to elevated atmospheric [CO 2 ].• The seasonal responses of leaf gas exchange and related biochemical parameters were measured during 3 yrs of exposure on established plantation sweetgum ( Liquidambar styraciflua ) trees at a Free-Air CO 2 Enrichment (FACE) facility in eastern Tennessee, USA.• Net photosynthetic rates ( A growth ) of upper-canopy leaves were 44% higher in trees grown in elevated [CO 2 ] compared with ambient [CO 2 ] over the 3-yr period. There were no significant CO 2 treatment effects on photosynthetic or biochemical capacity (i.e. no change in A max , V cmax or J max ) of L. styraciflua leaves, despite increased area-based leaf sugar (10%) and starch content (27%), and reduced mass-based leaf nitrogen concentration ( N M ; 10%).• These results suggest that established L. styraciflua trees in closed-canopy forests might exhibit a long-term positive response to elevated [CO 2 ] without reductions in photosynthetic capacity.