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Keywords:

  • biochemical capacity;
  • deciduous trees;
  • FACE;
  • forest;
  • Liquidambar styraciflua;
  • nitrogen;
  • photosynthetic adjustment;
  • photosynthetic capacity

Summary

  • • 
    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 CO2 Enrichment (FACE) facility in eastern Tennessee, USA.
  • • 
    Net photosynthetic rates (Agrowth) 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 Amax, Vcmax or Jmax) of L. styraciflua leaves, despite increased area-based leaf sugar (10%) and starch content (27%), and reduced mass-based leaf nitrogen concentration (NM; 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.