Regenerating Temperate Forests under Elevated CO2 and Nitrogen Deposition: Comparing Biochemical and Stomatal Limitation of Photosynthesis

• Photosynthesis of temperate trees growing in a competitive environment was investigated here in a factorial design of community composition (deciduous, coniferous and mixed species), carbon dioxide and nitrogen treatments. This study included seedlings of three deciduous (Betula alleghaniensis, Quercus rubra, Acer rubrum) and three coniferous (Pinus strobus, Picea rubens, Tsuga canadensis) species. • Nitrogen partitioning changed significantly in response to the treatments. Higher area-based nitrogen concentrations ( N a) in the conifer needles, however, did not induce higher growth rates. • Analysis of biochemical limitations of photosynthesis revealed that deciduous trees invested more nitrogen into carboxylation ( V cmax), electron transport ( J max) and P i regeneration capacity, but at much lower absolute concentrations for N a than conifers; consequently conifers maintained much higher rates for all three parameters. Deciduous species showed a strong stomatal limitation, whereas conifers maintained higher stomatal conductance at increasing mesophyll internal carbon dioxide concentration, indicating a much stronger assimilatory response to elevated carbon dioxide. • Differences between the biochemical and stomatal response to elevated carbon dioxide and nitrogen indicate that within mixed stands, individual plant responses do not fully characterize community response.