AbstractWe measured rates of leaf senescence and leaf level gas exchange during autumnal senescence for seedlings of five temperate forest tree species under current and elevated atmospheric CO2concentrations and low‐ and high‐nutrient regimes. Relative indices of whole canopy carbon gain, water loss and water use efficiency through the senescent period were calculated based on a simple integrative model combining gas exchange per unit leaf area and standing canopy area per unit time. Seedlings grown under elevated [CO2] generally had smaller canopies than their current [CO2]‐grown counterparts throughout most of the senescent period. This was a result of smaller pre‐senescent canopies or accelerated rates of leaf drop. Leaf‐level photosynthetic rates were higher under elevated [CO2] for grey birch canopies and for low‐nutrient red maple and high‐nutrient ash canopies, but declined rapidly to values below those of their current [CO2] counterparts by midway through the senescent period. CO2enrichment reduced photosynthetic rates for the remaining species throughout some or all of the senescent period. As a result of smaller canopy sizes and reduced photosynthetic rates, elevated [CO2]‐grown seedlings had lower indices of whole canopy end‐of‐season carbon gain with few exceptions. Leaf level transpiration rates were highly variable during autumnal senescence and neither [CO2] nor nutrient regime had consistent effects on water loss per unit leaf area or integrated whole canopy water loss throughout the senescent period. Indices of whole canopy, end‐of‐season estimates of water use efficiency, however, were consistently lower under CO2enrichment, with few exceptions. These results suggest that whole canopy end‐of‐season gas exchange may be altered significantly in an elevated [CO2] world, resulting in reduced carbon gain and water use efficiency for many temperate forest tree seedlings. Seedling growth and survivorship, and ultimately temperate forest regeneration, could be reduced in CO2‐e