Despite its ecological importance, broad-scale use of foliar nitrogen as an indicator of ecosystem response to atmospheric N deposition has heretofore been obscured by its poorly understood intrinsic variability through time, space, and across species. We used a regional survey of foliar N conducted within a single growing season to observe that eight of nine major canopy tree species had increased foliar N in response to a gradient of N deposition in the Adirondack Park, New York. These results (1) add important foliar N evidence to support N saturation theory, (2) strongly reinforce the conclusion that N deposition is affecting the N status of forest ecosystems in the northeastern U.S., and (3) extend N saturation theory by identifying that temperate forest canopy species differ in their foliar N response to N deposition. Interestingly, species-specific differences were strongly related to two functional traits that arise from within-leaf allocations of N resources--leaf mass per area (LMA) and shade tolerance. Thus, combining species-specific knowledge of these functional traits with existing foliar N-centered remote sensing and ecosystem modeling approaches may provide a much-needed avenue to make broad-scale assessments of how persistently elevated rates of N deposition will continue to affect temperate forest ecosystems.