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 ([CO ₂]), 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 CO ₂ (ambient, and elevated CO ₂ at 658μmolCO ₂ mol ^-1) and N deposition (ambient and ambient +30kgNha ^-1 yr ^-1) in open-top chambers placed in an understory light gradient. Elevated CO ₂ 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 [CO ₂]. 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 CO ₂ exposure and high N availability did not affect seedling growth; however, growth increased slightly with light availability (R ² = 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 [CO ₂] and N deposition (P <0.009). Path analysis determined that the greatest predictor for seedling survival in the understory was total biomass (R ² = 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 [CO ₂] 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.