Elevated carbon dioxide does not affect average canopy stomatal conductance of Pinus taeda L

While photosynthetic responses of C₃ plants to elevated CO₂ are fairly well documented, whole-plant water use under such conditions has been less intensively studied. Woody species, in particular, have exhibited highly variable stomatal responses to high CO₂ as determined by leaf-level measurements. In this study, sap flux of Pinus taeda L. saplings was periodically monitored during the 4th year of an open-top chamber CO₂ fumigation experiment. Water use per unit sapwood area did not differ between treatments. Furthermore, the ratio of leaf area to sapwood area did not change under high CO₂, so that average canopy stomatal conductance (on a unit leaf area basis) remained unaffected by the CO₂ treatment. Thus, the only effect of high CO₂ was to increase whole-plant water use by increasing sapling leaf area and associated conducting sapwood area. Such an effect may not directly translate to forest-level responses as the feedback effects of higher leaf area at the canopy scale cannot be incorporated in a chamber study. These feedbacks include the potential effect of higher leaf area index on rainfall and light interception, both of which may reduce average stomatal conductance in intact forest canopies.