Light acquisition and use by individuals competing in a dense stand of an annual herb, Xanthium canadense

The importance of light acquisition and utilization by individuals in intra specific competition was evaluated by determining growth and photosynthesis of individual plants in a dense monospecific stand of an annual. Xanthium canadense. Photosynthesis of individual plants in the stand was calculated using a canopy photosynthesis model in which leaf photosynthesis was assume d to be function of leaf nitrogen content and light availability. The estim ated photosynthetic rates of individuals were strongly correlated with the measured growth rates. Photosynthetic rates per unit aboveground mass (RPR, relative photosynthetic rate) increased with increasing aboveground mass, suggesting asymmetric (one-sided) competition in the stand. However, larger individuals had similar RPRs, suggesting symmetric (two-sided) competition . These results were consistent with the observation that size inequality o ver the whole stand increased with growth, but it remained stable among the larger individuals. The RPR of an individual was calculated as the product of absorbed photon flux per unit aboveground mass (Phi(mass)) and light us e efficiency (LUE, photosynthesis per unit absorbed photon flux). Phi(mass) indicates the efficiency of light acquisition, and was higher in larger in dividuals in the stand, while LUE was highest in individuals with intermedi ate aboveground mass. LUE depends on leaf nitrogen content. At an early sta ge, leaf nitrogen contents of smaller individuals were similar to those tha t maximize LUE. Light availability to smaller individuals decreased as they grew, while their nitrogen contents did not change markedly, which decreas ed their LUE. We con eluded that asymmetric competition among individuals i n the stand resulted mainly from lower efficiencies in both light acquisiti on and light use by smaller individuals.