Sensitivity of stomatal and canopy conductance to elevated CO2 concentration - interacting variables and perspectives of scale

The hydrological response of forests to rising CO2 is a critical biotic feedback in the study of global climate change. Few studies, however, have investigated this highly dynamic response at relevant temporal and spatial scales. A combination of leaf and whole-tree measurements and stand-level extrapolations were used to assess how stomatal conductance, canopy transpiration and conductance, and evapotranspiration might be affected by future, higher CO2 concentrations. Midday measurements of stomatal conductance for leaves sampled in a 12-yr-old sweetgum (Liquidambar styraciflua) stand exposed to free-air CO2 enrichment were up to 44% lower at elevated than at ambient CO2 concentrations, whereas canopy conductance, averaged over the growing season, was only 14% lower in stands exposed to CO2 enrichment. The magnitude of this response was dependent on vapor pressure deficit and soil water potential. Annual estimates of evapotranspiration showed relatively small reductions due to atmospheric CO2 enrichment. These data illustrate that the hydrological response of a closed-canopy plantation to elevated CO2 depends on the temporal and spatial scale of observation. They emphasize the importance of interacting variables and confirm that integration of measurements over space and time reduce what, at the leaf level, might otherwise appear to be a large and significant response.