Increases in atmosphericCO(2) have little influence on transpiration of a temperate forest canopy

Models of forest energy, water and carbon cycles assume decreased stomatal conductance with elevated atmospheric CO2 concentration ([CO2]) based on leaf-scale measurements, a response not directly translatable to canopies. Where canopy-atmosphere are well-coupled, [CO2]-induced structural changes, such as increasing leaf-area index (L-D), may cause, or compensate for, reduced mean canopy stomatal conductance (G(S)), keeping transpiration (E-C) and, hence, runoff unaltered. We investigated G(S) responses to increasing [CO2] of conifer and broadleaved trees in a temperate forest subjected to 17-yr free-air CO2 enrichment (FACE; +200molmol(-1)). During the final phase of the experiment, we employed step changes of [CO2] in four elevated-[CO2] plots, separating direct response to changing [CO2] in the leaf-internal air-space from indirect effects of slow changes via leaf hydraulic adjustments and canopy development. Short-term manipulations caused no direct response up to 1.8xambient [CO2], suggesting that the observed long-term 21% reduction of G(S) was an indirect effect of decreased leaf hydraulic conductance and increased leaf shading. Thus, E-C was unaffected by [CO2] because 19% higher canopy L-D nullified the effect of leaf hydraulic acclimation on G(S). We advocate long-term experiments of duration sufficient for slow responses to manifest, and modifying models predicting forest water, energy and carbon cycles accordingly.