Persistent stimulation of photosynthesis by elevated CO 2 in a sweetgum ( Liquidambar styraciflua ) forest stand

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.