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Increasing concentrations of atmospheric carbon dioxide (CO 2 ) and tropospheric ozone (O 3 ) have the potential to affect tree physiology and structure, and hence forest feedbacks on climate. Here, we investigated how elevated concentrations of CO 2  (+45%) and O 3  (+35%), alone and in combination, affected conductance for mass transfer at the leaf and canopy levels in pure aspen ( Populus tremuloides  Michx.) and in mixed aspen and birch ( Betula papyrifera  Marsh.) forests in the free-air CO 2 –O 3  enrichment experiment near Rhinelander, Wisconsin (Aspen FACE). The study was conducted during two growing seasons, when steady-state leaf area index ( L ) had been reached after &gt; 6 years of exposure to CO 2 - and O 3 -enrichment treatments. Canopy conductance ( g  c ) was estimated from stand sap flux, while leaf-level conductance of sun leaves in the upper canopy was derived by three different and independent methods: sap flux and  L  in combination with vertical canopy modelling, leaf  13 C discrimination methodology in combination with photosynthesis modelling and leaf-level gas exchange. Regardless of the method used, the mean values of leaf-level conductance were higher in trees growing under elevated CO 2  and/or O 3  than in trees growing in control plots, causing a CO 2  × O 3  interaction that was statistically significant ( P  ≤ 0.10) for sap flux- and (for birch)  13 C-derived leaf conductance. Canopy conductance was significantly increased by elevated CO 2  but not significantly affected by elevated O 3 . Investigation of a short-term gap in CO 2  enrichment demonstrated a +10% effect of transient exposure of elevated CO 2 -grown trees to ambient CO 2  on  g  c . All treatment effects were similar in pure aspen and mixed aspen-birch communities. These results demonstrate that short-term primary stomatal closure responses to elevated CO 2  and O 3  were completely offset by long-term cumulative effects of these trace gases on tree and stand structure in determining canopy- and leaf-level conductance in pure aspen and mixed aspen-birch forests. Our results, together with the findings from other long-term FACE experiments with trees, suggest that model assumptions of large reductions in stomatal conductance under rising atmospheric CO 2  are very uncertain for forests.

Leaf and canopy conductance in aspen and aspen-birch forests under free-air enrichment of carbon dioxide and ozone