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 µmolCO2 mol-1) and N deposition (ambient and ambient 130 kgNha-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 (Vcmax) was a better predictor for seedling growth and survival than maximum photosynthetic rate (Amax). 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.