Reproductive yield of individuals competing for light in a dense stand of an annual, Xanthium canadense

In a dense stand, individuals compete with each other for resources, especially for light. Light availability decreases with increasing depth in the canopy, thus light competition becoming stronger with time in the vegetative phase. In the reproductive phase, on the other hand, leaves start senescing, and the light environment, particularly of smaller individuals, will be improved. To study the effect of change in light climate on reproduction of individuals, we established an experimental stand of an annual, Xanthium canadense, and assessed temporal changes in whole plant photosynthesis through the reproductive phase with particular reference to light availability of individuals. At flowering, 83% of individuals were still alive, but only 27% survived to set seeds. Most of the individuals that died in the reproductive phase were smaller than those that produced seeds. Individuals that died at the early stage of the reproductive phase had a lower leaf to stem mass ratio, suggesting that the fate of individuals was determined partly by the pattern of biomass allocation in this period. At the early stage of the reproductive phase, leaf area index (LAI) of the stand was high and larger individuals had higher whole plant photosynthesis than smaller individuals. Although light availability at later stages was improved with reduction in LAI, whole plant photosynthesis was very low in all individuals due to a lower light use efficiency, which was caused by a decrease in photosynthetic N use efficiency. We conclude that light competition was still strong at the early stage of the reproductive phase and that later improvement of light availability did not ameliorate the photosynthesis of smaller individuals.