Recent studies have provided evidence of a large flux of root-respired CO 2 in the transpiration stream of trees. In our study, we investigated the potential impact of this internal CO 2 transport on aboveground carbon assimilation and CO 2 efflux. To trace the transport of root-respired CO 2, we infused a (13)C label at the stem base of field-grown Populus deltoides Bartr. ex. Marsh trees. The (13)C label was transported to the top of the stem and throughout the crown via the transpiration stream. Up to 17% of the (13)C label was assimilated by chlorophyll-containing tissues. Our results provide evidence of a mechanism for recycling respired CO 2 within trees. Such a mechanism may have important implications for how plants cope with predicted increases in intensity and frequency of droughts. Here, we speculate on the potential significance of this recycling mechanism within the context of plant responses to climate change and plants currently inhabiting arid environments.