Consequences of CO 2 and light interactions for leaf phenology, growth, and senescence in Quercus rubra

Summary We investigated how light and CO 2 levels interact to influence growth, phenology, and the physiological processes involved in leaf senescence in red oak (Quercus rubra) seedlings. We grew plants in high and low light and in elevated and ambient CO 2 . At the end of three years of growth, shade plants showed greater biomass enhancement under elevated CO 2 than sun plants. We attribute this difference to an increase in leaf area ratio (LAR) in shade plants relative to sun plants, as well as to an ontogenetic effect: as plants increased in size, the LAR declined concomitant with a decline in biomass enhancement under elevated CO 2 Elevated CO 2 prolonged the carbon gain capacity of shade‐grown plants during autumnal senescence, thus increasing their functional leaf lifespan. The prolongation of carbon assimilation, however, did not account for the increased growth enhancement in shade plants under elevated CO 2 . Elevated CO 2 did not significantly alter leaf phenology. Nitrogen concentrations in both green and senesced leaves were lower under elevated CO 2 and declined more rapidly in sun leaves than in shade leaves. Similar to nitrogen concentration, the initial slope of A / C i curves indicated that Rubisco activity declined more rapidly in sun plants than in shade plants, particularly under elevated CO 2 . Absolute levels of chlorophyll were affected by the interaction of CO 2 and light, and chlorophyll content declined to a minimal level in sun plants sooner than in shade plants. These declines in N concentration, in the initial slope of A / C i curves, and in chlorophyll content were consistent with declining photosynthesis, such that elevated CO 2 accelerated senescence in sun plants and prolonged leaf function in shade plants. These results have implications for the carbon economy of seedlings and the regeneration of red oak under global change conditions.