Figure 1. Annual weeds interfere with pumpkin harvest and reduce fruit yield. The abundant seed rain ensures an abundant weed population in future years. Photo: E. Gallandt.

Figure 2. The effects of plant density on biomass in single species populations (‘monoculture’). At very low densities, there is no competition and total biomass increases linearly with density (a). As density increases, competition becomes important and the increase in biomass is less than proportional to density (curved part of graph a). At high densities further increases in density do not lead to any increase in biomass (‘Law of Constant Final Yield’). The effects of density on mean individual plant biomass are best illustrated with a log–log plot (b). The proportional increase in total biomass with density at low densities on graph (a) corresponds to the flat part of graph (b). The flat part of graph (a) corresponds to the linear decrease (slope=–1) on graph (b).

Figure 3. The right-rectangular hyperbolic model relating crop yield loss to weed density (Cousens, 1985). Crop yield loss (Y_L=iN/1+iN/A), where N=weed density, i and A are fit parameters with i representing the slope as N approaches zero and A the slope as N approaches infinity.

Figure 4. Results from an experiment designed to determine the ‘critical period’ for weed control. Data from two complementary experiments are plotted on the same axes. Results from one experiment show cabbage yield when the crop is maintained weed-free for 0, 1, 2, 4, 6 and 14 weeks. Results from the second experiment show cabbage yield when the crop is unweeded for 0, 2, 4, 6 and 14 weeks. Weeds present for only a short period of time have no effect on yield, and the end of the critical period marks the point at which the crop has a sufficient size advantage to be unaffected by subsequently emerging weeds. Data from Table 1, p. 854, Miller AB and Hopen HJ (1991) Critical weed-control period in seeded cabbage (Brassica oleracea var. capitata). Weed Technology 5, 852–857.

Figure 5. Effect of sequence of seed sowing, and resulting sequence of seedling emergence, over 10 consecutive days on space, estimated as proportional to biomass, occupied by individual plants. Panels show the randomly selected positions in which seeds were sown, and the area occupied on day 4, 8, 12, 16 and 20 days, respectively. Earlier-emerging individuals preempt space from those emerging later, evident by the small amount of space occupied by seedlings sown on days 7 through 10. This figure was published by Harper JL (1977) The influence of density on yield and mortality. In: Population Biology of Plants, chap. 6. Copyright Elsevier.