Anthropogenically derived carbon dioxide (CO2) and reactive nitrogen (N) compounds are altering the composition of Earth’s atmosphere and impacting ecosystem function. I investigated the response of northern hardwood tree species to atmospheric CO2 and N deposition in order to determine how these global change factors might impact future forest functioning. I exposed six species of tree seedlings (Betula papyrifera Marshall, Populus tremuloides Michx, Acer saccharum Marsh, Fagus grandifolia Ehrh., Pinus strobus Linnaeus and Prunus serotina Ehrh.) to a factorial combination of atmospheric CO2 (ambient, and elevated CO2 at 658 umol CO2/mol) and N deposition (ambient and ambient + 30kg N/ha/yr) in open-top chambers palced in an understory light gradient. My research determined that elevated [CO2] exposure increases understory photosynthesis by approximately 41-60%; but increases were similar for shade tolerant and shade intolerant species. I also found that understory seedlings growing in N-limited soils and with elevated atmospheric [CO2] downregulated photosynthesis 10% over two years. Greater N deposition increased seedling photosynthesis and alleviated the downregulation evident under conditions of elevated atmospheric [CO2]. Atmospheric N deposition also increased seedling survival by 45%. My results indicate that N deposition and atmospheric CO2 will interact to influence understory seedling survival because plants exposed to elevated CO2 and high N deposition had the greatest survival rates. My results also indicate that [CO2], N deposition and understory light availability will interact and strongly impact symbiotic arbuscular mycorrhizal fungi associations. My results showed that experimental N deposition substantially decreased total AM colonization, AM hyphal abundance, and arbuscules. I conclude that increases in atmospheric [CO2] and reactive N deposition will enhance seedling photosynthesis and survival in N-limited temperate forests because seedlings will use these atmospheric compounds as additional plant resources. However, N deposition will decrease arbuscular mycorrhizal (AM) fungal colonization and this may alter future biogeochemical cycling and forest functioning. Alteration of forest functioning due to change in plant resource availability can ultimately cause large-scale changes in forest composition, which we need to be able to anticipate if we are to manage forests effectively for a sustainable future.