While photosynthetic carbon fixation is the most often discussed target for elevated CO2, the other direct target of CO2 is the stomatal apparatus. It is well known that stomata close in response to exposure to elevated CO2 and that growth of some species in elevated CO2 results in few stomata per unit leaf area. Reduced stomatal aperture in scrub oak has been confirmed for Scrub Oak by Lodge et al.(2001). Our studies are equivocal about the possibility that the number of stomata on Quercus species are reduced in elevated CO2 (unpublished). The effects of stomatal closure on plant and ecosystem water balance are complex as shown by early results of evapo-transpiration which indicate reduced water loss was accompanied by increased soil water within the first 5 cm of soil (Hungate et al. 2002). Subsequently, Li et al.(2003) have shown that transpiration, measured by stem flow, is reduced by 37-49% between March and October 2000 in elevated CO2 . This effect was reversible, as expected if the effect occurred primarily as a result of partial closure of stomatal aperture. While Li et al. (2003) favor the interpretation that this is due mainly to increased shade, the possibility remains that acclimation of stomata to elevated CO2 is an important response with wide ranging consequences including increased local warming. Increased leaf area index throughout the 6 year growing season has caused the effect of elevated CO2 on ecosystem ET to disappear and in some case to be reversed. In fact, Hungate et al. clearly showed that the effect of elevated CO2 on transpiration is tightly coupled to CO2 effects on leaf area. When LAI was below about 2.0, elevated CO2 reduced ET and increased soil water. But when elevated CO2 also stimulated LAI (see Growth above), the effect of CO2 was to stimulate water loss. Earlier interpretation (Hungate et al. 2002) suggested that there might be a "mulch" effect of leaf litter on ET. This remains one possibility to explain the long-term effect of elevated CO2 on soil water balance. Our recent analyses show that acclimation of stomatal response to elevated CO2 makes transpiration in the scrub oak canopy less sensitive to VPD.
Water plays a complex role in regulating ecosystem carbon balance. Hymus et al. (2003) have shown that daily maximum NEE is positively dependent on soil water content suggesting an additional layer of complexity in the interaction between the effect of elevated CO2 on carbon cycling and environmental factors.