Shoot competition and root competition.
The effects of shoot and root competition in 23 studies including additive and replacement techniques, radial and linear layouts, several of methods of partitioning and a range of grass, crop and weed species are reviewed. Effects of competition were measured by the competitive balance (relative competitive ability) of the components of the mixture using the competitive balance index, Cb, the intensity of competition (as determined by plant size) using the competitive intensity index, C1, the efficiency of resource use by the mixture using the relative yield total, relative yield of the mixture and overyield. Root competition usually affected the balance between components more than shoot competition and was usually more intense than shoot competition. In crop-weed experiments, shoot competition was often more intense than root competition; crops usually had a greater competitive ability than weeds. Generally the relative importance of root competition increased with time. Increased yield of a mixture can indicate niche separation, which is of potential agricultural significance as a means of obtaining higher yields than with monocultures. Increased yield of mixtures tended to occur in experiments that allowed different species to root at different depths, and root interaction was usually the more important cause. Although positive interaction between shoot and root competition has often been considered a basic feature of competition, such interactions occurred rarely; in the case of intensity of competition the interaction was usually negative. There was little evidence to justify the common assumption that adding environmental resources reduced competitive effects; competitive imbalance was often greater at higher resource levels.