Seedling survival in a northern temperate forest understory is increased by elevated atmospheric carbon dioxide and atmospheric nitrogen deposition

We tested the main and interactive effects of elevated carbon dioxide concentration (CO₂]), nitrogen (N), and light availability on leaf photosynthesis, and plant growth and survival in understory seedlings grown in an N‐limited northern hardwood forest. For two growing seasons, we exposed six species of tree seedlings (Betula papyrifera, Populus tremuloides, Acer saccharum, Fagus grandifolia, Pinus strobus, and Prunus serotina) to a factorial combination of atmospheric CO₂ (ambient, and elevated CO₂ at 658 μmol CO₂ mol⁻¹) and N deposition (ambient and ambient +30 kg N ha⁻¹ yr⁻¹) in open‐top chambers placed in an understory light gradient. Elevated CO₂ exposure significantly increased apparent quantum efficiency of electron transport by 41% (P<0.0001), light‐limited photosynthesis by 47% (P<0.0001), and light‐saturated photosynthesis by 60% (P<0.003) compared with seedlings grown in ambient CO₂]. Experimental N deposition significantly increased light‐limited photosynthesis as light availability increased (P<0.037). Species differed in the magnitude of light‐saturated photosynthetic response to elevated N and light treatments (P<0.016). Elevated CO₂ exposure and high N availability did not affect seedling growth; however, growth increased slightly with light availability (R²=0.26, P<0.0001). Experimental N deposition significantly increased average survival of all species by 48% (P<0.012). However, seedling survival was greatest (85%) under conditions of both high CO₂] and N deposition (P<0.009). Path analysis determined that the greatest predictor for seedling survival in the understory was total biomass (R²=0.39, P<0.001), and that carboxylation capacity (V_cmax) was a better predictor for seedling growth and survival than maximum photosynthetic rate (A_max). Our results suggest that increasing CO₂] and N deposition from fossil fuel combustion could alter understory tree species recruitment dynamics through changes in seedling survival, and this has the potential to alter future forest species composition.