Leaf expansion in the fast-growing tree, Populus ϫ euramericana was stimulated by elevated [CO 2 ] in a closed-canopy forest plantation, exposed using a free air CO 2 enrichment technique enabling long-term experimentation in field conditions. The effects of elevated [CO 2 ] over time were characterized and related to the leaf plastochron index (LPI), and showed that leaf expansion was stimulated at very early (LPI, 0-3) and late (LPI, 6-8) stages in development. Early and late effects of elevated [CO 2 ] were largely the result of increased cell expansion and increased cell production, respectively. Spatial effects of elevated [CO 2 ] were also marked and increased final leaf size resulted from an effect on leaf area, but not leaf length, demonstrating changed leaf shape in response to [CO 2 ]. Leaves exhibited a basipetal gradient of leaf development, investigated by defining seven interveinal areas, with growth ceasing first at the leaf tip. Interestingly, and in contrast to other reports, no spatial differences in epidermal cell size were apparent across the lamina, whereas a clear basipetal gradient in cell production rate was found. These data suggest that the rate and timing of cell production was more important in determining leaf shape, given the constant cell size across the leaf lamina. The effect of elevated [CO 2 ] imposed on this developmental gradient suggested that leaf cell production continued longer in elevated [CO 2 ] and that basal increases in cell production rate were also more important than altered cell expansion for increased final leaf size and altered leaf shape in elevated [CO 2 ].Given the importance of forests for global bioproductivity, the consequences of increased atmospheric [CO 2 ] for the global carbon cycle are potentially extremely large (Malhi et al., 1999). Despite this, there are still relatively few large-scale, long-term experiments from which predictions about likely forest responses can be made. Few studies have been completed where trees are allowed to develop to canopy closure and where a "stable" response to [CO 2 ] is likely. Determining the response of leaf area development to elevated [CO 2 ] is important. It is still unknown whether forests of the future will maintain a higher leaf area index (LAI), as implied from smalltree studies (Ceulemans et al., 1997) or whether the long-term (decades) responses will be reduced allocation to foliage and lower LAI, as suggested by some modeling approaches (Medlyn and Dewar, 1996) or involve acclimation to limited nitrogen (Oren et al., 2001).Leaf growth is often stimulated in short-term response to elevated [CO 2 ] (Taylor et al., 1994;Pritchard et al., 1999), and both leaf cell expansion and cell production are sensitive to [CO 2 ] (Taylor et al., 1994). It is likely that these processes respond to additional carbohydrate from photosynthesis and, as such, altered atmospheric [CO 2 ] provides a critical insight into how carbon regulates plant development and growth (Masle, 2000). The importance of leaf develo...