In four mature northern hardwood forests in the northern lower and western upper Michigan, simulated atmospheric nitrogen (N) deposition (30 kg N ha-1 yr-1 as NaNO3 for 14 years) has increased aboveground tree growth, but has not affected canopy leaf biomass or leaf area index. In order to understand the mechanism behind the increased growth, we hypothesized that the NO3- additions increased foliar N concentrations and leaf-level photosynthesis in the mature sugar maple (Acer saccharum) trees that dominate these forests. The NO3- additions significantly increased foliar N. However, there was no significant difference between the ambient and +NO3- treatments in two growing seasons (2006-2007) of instantaneous photosynthesis measurements from either excised branches or canopy towers. In measurements on excised branches, photosynthetic nitrogen use efficiency (μmol CO2 s-1 g-1 N) was significantly decreased by NO3- additions. Furthermore, we found no consistent NO3- effect across all sites in either current foliage or leaf litter collected annually throughout the study (1993-2007) and analyzed for 13C and 18O, isotopes that can be used together to integrate changes in photosynthesis over time. These data together suggest that NO3- additions have not stimulated photosynthesis. Rather than increases in C assimilation, the observed increases in aboveground growth at our study sites are more likely due to shifts in C allocation. Because N deposition and tropospheric ozone (O3) are linked through anthropogenic NOx emissions, we also tried to examine whether or not there were interactive effects between N deposition and O3. To do this, we tried to compare photosynthesis in the two N deposition treatments during periods of normal and high tropospheric O3 during the 2007 growing season. Unfortunately, this research was unsuccessful because high ozone episodes were rare during the 2007 growing season.