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

While photosynthetic responses of C-3 plants to elevated CO2 are fairly wel l documented, whole-plant water use under such conditions has been less int ensively studied. Woody species, in particular, have exhibited highly varia ble stomatal responses to high CO2 as determined by leaf-level measurements . In this study, sap flux of Pinus taeda L. saplings was periodically monit ored during the 4th year of an open-top chamber CO2 fumigation experiment. Water use per unit sapwood area did not differ between treatments. Furtherm ore, 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) rem ained unaffected by the CO2 treatment. Thus, the only effect of high CO2 wa s to increase whole-plant water use by increasing sapling leaf area and ass ociated conducting sapwood area. Such an effect may not directly translate to forest-level responses as the feedback effects of higher leaf area at th e canopy scale cannot be incorporated in a chamber study. These feedbacks i nclude the potential effect of higher leaf area index on rainfall and light interception, both of which may reduce average stomatal conductance in int act forest canopies.