Elevated [CO2] and forest vegetation: more a water issue than a carbon issue?

Studies of responses of forest vegetation to steadily increasing atmospheric concentrations of CO2 have focussed strongly on the potential of trees to absorb extra carbon; the effects of elevated [CO2] on plant-soil water relations via decreased stomatal conductance and increased ambient temperature have received less attention, but may be significant in the long term at the ecosystem level. CO2 augmentation experiments with young trees demonstrate small increases in aboveground carbon content, but these increases tend to diminish as trees get older. By contrast, several experiments suggest continued decreases in transpiration and increased soil water content under these conditions. In tropical forests, the major cause of increases in aboveground biomass observed in the recent past is not necessarily elevated [CO2]. Undoubtedly, the potential of monitoring trees in forest dynamics plots to deduce CO2-specific alterations in forest structure and standing biomass will unfold in the decades to come. The comprehensive understanding of responses of forest vegetation to elevated [CO2] in the Anthropocene will depend upon the inclusion of detailed measurements of soil water pools and water fluxes through the soil-plant-atmosphere continuum in future tree CO2 augmentation experiments and forest dynamics plot studies.