Persistent stimulation of photosynthesis by elevated CO2 in a sweetgum (Liquidambar styraciflua) forest stand

The photosynthetic response of trees to rising CO2 concentrations ([CO2]) 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 [CO2]. 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, Enrichment (FACE) facility in eastern Tennessee, USA. Net photosynthetic rates (A(growth)) of upper-canopy leaves were 44% higher in trees grown in elevated [CO2] compared with ambient [CO2] over the 3-yr period. There were no significant CO2 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 [CO2] without reductions in photosynthetic capacity.