Leaf angle as a strategy for light competition: Optimal and evolutionarily stable light-extinction coefficient within a leaf canopy

To analyze the effect of leaf angle on light competition among individual plants, a simple model for photosynthesis of an individual plant in a dense stand was developed. The model assumes that each plant has a particular leaf angle that determines the light-extinction coefficient (K). Light climate of a target plant is determined by the K values both of its own and of its neighbors, while light absorption of a target is determined by its own K. Evolutionarily stable K (ESK) values (the K values at which photosynthesis of a target plant having any other K values is smaller than that of its neighbors) were calculated. The ESK value was found to depend both on total leaf area of the stand and on the degree in which neighboring plants affect light climate of each other. The ESK value is always higher than the K value that maximizes canopy photosynthesis when light interception by neighbors occurs. Ecological roles of vertical (low K) and horizontal (high K) leaves are discussed. Horizontal leaves may be advantageous in light competition because the ability of light interception is higher in such leaves. Vertical leaves may be favored in clonal plants whose ramets are closely spaced (phalanx type) because photosynthetic capacity of the whole clone is greater if K is low.