Four treatments (control, N, NP, NPK) of an ongoing long-term fertilizer study in an interior Alaska aspen (Populus tremuloides Michx.) stand were chosen to characterize the response of selected attributes of the leaves to supplemental nutrients. Vertical differentiation of the canopy with respect to mean and total leaf area and mass, total leaf number, foliar nutrient concentrations, and indices of leaf size (length, width), shape (length/width), and internal anatomy (specific leaf weight, SLW) were considered for each of the treatments. In addition, seasonal changes in levels of chlorophylls, foliar nutrients, and nonstructural carbohydrates were determined. Canopy and leaf morphology and foliar nutrient levels responded markedly to increased soil nitrogen. Augmenting soil P or P and K did not greatly alter the nitrogen response. Canopy leaf area and leaf biomass were approximately doubled by nitrogen fertilization due to increases in leaf numbers rather than increases in mean leaf area or mass. A pronounced vertical differentiation in mean leaf area and SLW occurred within fertilized treatments resulting in a gradation from small leaves of low SLW at the bottom of the canopy to large leaves with greater SLW at the top of the canopy. Specific leaf weight and foliar nutrients (N, P, K) increased with canopy height in the fertilized plots but this trend was less evident or lacking in the control plots. These responses probably reflect a much greater intracanopy gradient in irradiance in the fertilized plots than existed in the more open control plots where vertical differentiation was virtually absent. The general effect of fertilizer on a tree basis (particularly nitrogen) was to increase levels of chlorophyll and nutrients per unit leaf area or leaf mass (CO2 fixation potential) and to augment total leaf surface (light capturing potential) by increasing the number of leaves. Within the canopy, the photosynthetic potential on an individual leaf basis as indicated by nutrient concentrations and SLW was positively correlated with height thus predicting that large leaves originating from leader shoots at the top of the canopy are the most productive. Forest Sci. 23:92-102.