Some Aspects of Competition Between Pasture Species: The Effect of Environment and Defoliation

The majority of experimental investigations designed to analyse plant competition have concerned growth in pure and mixed model stands sown at varying density, fertility levels or spatial arrangements. These experiments have emphasized the environmental factors for which plants may compete but they have rarely stressed the influence of other parameters such as management, temperature, light intensity and day-length which in practice affect growth. Competitive interrelations between pasture plants are influenced by seasonal weather conditions (some species being physiologically adapted for maximum growth at different periods of the growing season) and the frequency and intensity of defoliation. Hence assessment of the importance of competition as a factor affecting the botanical composition of a mixed sward is complicated by seasonality of growth and by the system of pasture management. It is also difficult under sward conditions to measure the effect of neighbouring plants upon the growth of an individual. An approach to this problem was made by Clements, Weaver & Hanson (1929) who used the 'denuded quadrat' to measure competition in pasture. In these quadrats the herbage surrounding an individual plant within the sward was removed to ground level in a circle 15-2 cm in diameter, and its growth was compared with a similar individual in competition with its surrounding herbage. In this manner competition for light was eliminated, though not competition for water and nutrients. However, this method has been criticized by Harper (1964) who has shown that the behaviour of individuals of a species in isolation may be irrelevant to understanding their behaviour in the community. In fact experimental designs for the detection of competition in natural situations are very few; even long term observations of pasture coupled with time-consuming botanical separations to assess the proportional contribution of a species tell us little of competitive interrelationships. Proportional yields can only be regarded as an indication of competitive success at a particular moment of time.