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

Abstract

Abstract 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 µ mol 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 (&lt;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.