Density effects on the size structure of annual plant populations: An indication of neighbourhood competition

Size variability among plants has been observed to increase with higher stand density, leading to the speculation that resource distribution among competing plants is primarily asymmetric rather than symmetric. The relationships between size variability, stand density, and type of resource distribution among competing plants were investigated using a spatially explicit, individual-plant model of annual plant population dynamics. When plants varied in neighbourhood competition, size variability increased with higher stand densities whether shared resources were symmetrically or asymmetrically distributed among competing plants. Size variability did not increase with higher stand densities when neighbourhood competition was constant for all plants. These simulations indicate that increased size variability among competing plants does not distinguish between symmetric and asymmetric resource distribution, but rather is direct evidence for neighbourhood competition.