The canopy structure of a stand of vegetation is determined by the growth patterns of the individual plants within the stand and the competitive interactions among them. We analyzed the carbon gain of individuals in two dense monospecific stands of Xanthium canadense and evaluated the consequences for intra-specific competition and whole-stand canopy structure. The stands differed in productivity, and this was associated with differences in nitrogen availability. Canopy structure, aboveground mass, and nitrogen contents per unit leaf area (Narea) were determined for individuals, and leaf photosynthesis was measured as a function of Narea. These data were used to calculate the daily carbon gain of individuals. Within stands, photosynthesis per unit aboveground mass (Pmass) of individual plants increased with plant height, despite the lower leaf area ratios of taller plants. The differences in Pmass between the tallest most dominant and shortest most subordinate plants were greater in the high-nitrogen than in the low-nitrogen stand. This indicated that competition was asymmetric and that this asymmetry increased with nitrogen availability. In the high-nitrogen stand, taller plants had a higher Pmass than shorter ones, because they captured more light per unit mass and because they had higher photosynthesis per unit of absorbed light. Conversely, in the low-nitrogen stand, the differences in Pmass between plants of different heights resulted only from differences in their light capture per unit mass. Sensitivity analyses revealed that an increase in Narea, keeping leaf area of plants constant, increased whole-plant carbon gain for the taller more dominant plants but reduced carbon gain in the shorter more subordinate ones, which implies that the Narea values of shorter plants were greater than the optimal values for maximum photosynthesis. On the other hand, the carbon gain of all individual plants, keeping their total canopy N constant, was positively related to an increase in their individual leaf area. At the same time, however, increasing the leaf area for all plants simultaneously reduced the carbon gain of the whole stand. This result shows that the optimal leaf area index (LAI), which maximizes photosynthesis of a stand, is not evolutionarily stable because at this LAI, any individual can increase its carbon gain by increasing its leaf area.