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 mol^-1 CO₂ treatment, in mid- and upper canopy foliage. Stomatal conductance (g_s) averaged 14% (mid-canopy) and 24% (upper canopy) lower under CO₂ 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 CO₂ 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 CO₂ effects on A or g_s. The responsiveness of A and g_s to elevated CO₂, 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.