Canopy development, leaf demography and growth dynamics of wheat and three weed species growing in pure and mixed stands.
Wheat cv. Concorde, Chenopodium album, Agropyron repens [Elymus repens] and Spergula arvensis were grown in pure stands and in mixtures of weeds with wheat at normal sowing rates in greenhouse conditions. Leaf demography and growth dynamics were studied by continuous monitoring, using non-destructive methods. The vertical distribution of leaves was monitored at weekly intervals. Leaf size was measured in pure stands of each species at regular intervals. In the pure wheat stands, plant survival followed an almost perfect Pearl (1928) type 1 curve, typical of organisms which die mainly from physiological senescence. Demographic patterns of leaves changed abruptly after ear emergence when leaf production virtually ceased and leaf deaths increased sharply for all cohorts. Quite small differences in individual plant size, early in the experiments, resulted in significant differences in final grain yield. The individual leaf area of wheat, C. album and S. arvensis declined after the onset of flowering. Increased density caused a reduction in individual leaf area in all weed species. The distribution of wheat leaves in the canopy was affected by the presence and density of weeds, more than the total number of wheat leaves. Only C. album at high density caused a reduction in wheat grain yield. Wheat reduced the number of weed leaves and changed their distribution within the canopy, with proportionately fewer weed leaves in the lower part of mixed stands than pure stands. Weeds of mixed stands were constantly shaded by wheat leaves and produced less biomass than weeds of pure stands.