Environmental and stomatal control of photosynthetic enhancement in the canopy of a sweetgum (Liquidambar styraciflua L.) plantation during 3 years of CO₂ enrichment

Light-saturated photosynthetic and stomatal responses to elevated CO₂ were measured in upper and mid-canopy foliage of a sweetgum (Liquidambar styraciflua L) plantation exposed to free-air CO₂ enrichment (FACE) for 3 years, to characterize environmental interactions with the sustained CO₂ 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 CO₂ assimilation (A) averaged 46% higher in the +200 µmol mol1 CO₂ treatment, in mid- and upper canopy foliage. Stomatal conductance (gs) averaged 14% (mid-canopy) and 24% (upper canopy) lower under CO₂ enrichment. Variations in the relative responses of A and gs were linked, such that greater relative stimulation of A was observed on dates when relative reductions in gs were slight. Dry soils and high VPD reduced gs and A in both treatments, and tended to diminish treatment differences. The absolute effects of CO₂ on A and gs were minimized whenever gs was low (