Effects of stem fraction on the optimization of biomass allocation and maximum photosynthetic capacity

Abstract

1. A model was developed to examine effects of the stem biomass fraction on the optimal responses of plants to soil nitrogen availability. 2. Our model predicts that the optimal leaf : root ratio and optimal photosynthetic capacity (Pmax) increase with soil N availability. For a given N availability, the optimal leaf : root ratio decreases and the optimal Pmax increases with increasing stem fraction. As a result, the increase in optimal leaf : root ratio is smaller, and that in optimal Pmax is greater, in response to increasing N availability when stem fraction is large. 3. To test these predictions we grew two herbs with different stem fractions: Polygonum cuspidatum Sieb, et Zucc. and Chenopodium album L. Showing excellent agreement with the simulation results, the leaf N concentration and leaf : root ratio of the two herbs increased with increasing N availability, and leaf N concentration was larger for C. album with higher stem fraction than P. cuspidatum. 4. The general tendency for plants with larger stem fractions also to have greater leaf N concentrations and Pmax was demonstrated for a wide range of temperate herbs. This suggests that stem fraction may be a source of variation in Pmax among plants in the same functional group.