Maximization of Daily Canopy Photosynthesis: Effects of Herbivory on Optimal Nitrogen Distribution

Recent models have shown that higher leaf nitrogen concentration per unit area maximizes nitrogen use efficiency with increasing light intensity. As a result, total canopy photosynthesis is maximized when nitrogen concentrations are higher towards the top of a canopy. Expanding upon this previous work, a model of daily canopy photosynthesis was constructed based on distributions of light, leaf nitrogen, and folivory. The model indicates that the optimal distribution of nitrogen depends significantly upon both the severity and location of folivory. Relative to nitrogen distributions that maximized daily canopy photosynthesis without herbivory, the optimal nitrogen distribution shifted towards either more uniform or skewed distributions when herbivores fed on high nitrogen foliage at the top of the canopy or on low nitrogen foliage towards the bottom of the canopy, respectively. These results suggest that, because foliar losses are balanced by increased irradiance of remaining leaves, plants' nitrogen allocation patterns should depend on how severe defoliation is and whether damage is concentrated towards the top or bottom of a canopy. Moreover, the critical importance of nitrogen distribution to photosynthesis implies that plants should not necessarily minimize loss of leaf area to folivores, but should protect the ratio of total nitrogen to leaf area and the distribution of nitrogen within the canopy. As a corollary to the nutrient stress hypothesis of plant defense theory, the model suggests that plants may need to translocate nutrients to maintain an optimal distribution in the canopy following herbivory. The model reinforces the point that leaf area loss alone is a poor indicator of loss of photosynthetic capacity when nitrogen is non-uniformly distributed among leaves. To accurately assess damage to a plant, one must consider not only what resources have been removed, but what resources remain.