Environmental and stomatal control of photosynthetic enhancement in the canopy of a sweetgum (Liquidambar styraciflua L.) plantation during 3 years of CO ₂ enrichment

Light-saturated photosynthetic and stomatal responses to elevated CO ₂ were measured in upper and mid-canopy foliage of a sweetgum (Liquidambar styraciflua L) plantation exposed to free-air CO ₂ enrichment (FACE) for 3 years, to characterize environmental interactions with the sustained CO ₂ 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 CO ₂ assimilation (A) averaged 46% higher in the +200 μmol mol ^-1 CO ₂ treatment, in mid- and upper canopy foliage. Stomatal conductance (g _s) averaged 14% (mid-canopy) and 24% (upper canopy) lower under CO ₂ enrichment. Variations in the relative responses of A and g _s were linked, such that greater relative stimulation of A was observed on dates when relative reductions in g _s were slight. Dry soils and high VPD reduced g _s and A in both treatments, and tended to diminish treatment differences. The absolute effects of CO ₂ on A and g _s were minimized whenever g _s was low (<0-15 mol m ^-2 s ^-1), but relative effects, as the ratio of elevated to ambient rates, varied greatly under those conditions. Both stomatal and non-stomatal limitations of A were involved during late season droughts. Leaf temperature had a limited influence on A and g _s, and there was no detectable relationship between prevailing temperature and CO ₂ effects on A or g _s. The responsiveness of A and g _s to elevated CO ₂, both absolute and relative, was maintained through time and within the canopy of this forest stand, subject to seasonal constraints and variability associated with limiting air and soil moisture.