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

Summary

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  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].
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  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 CO₂ Enrichment (FACE) facility in eastern Tennessee, USA.
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  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%).
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  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.