Competition and Stand Structure in Some Even-Aged Plant Monocultures

Abstract

Clements defined competition in the following terms: 'When the immediate supply of a single factor necessary (for growth) falls below the combined demands of the individual plants competition begins' (Donald 1963). It is a difficult process to study directly and proof of the causal link between the growth of some individuals, environmental depletion and the reduction in relative growth rate of others has not been obtained. Work on competition in monocultures has concentrated on studies of population structures or on changes in individual plant characteristics observed under conditions where competition is thought to take place; it is the assumed outcome of competition which is studied. The technique most frequently used is to establish a series of communities with different planting densities and to observe them over a time interval and from such studies three symptoms have been described which Harper (1967) considered as indicative of a community under 'density stress'. Firstly, from a frequency distribution of seed or seedling weight which is normal a skewed distribution of individual plant weights develops, there being a large number of plants slightly smaller than the mean and a small number of plants much greater than the mean, referred to as a 'hierarchy of exploitation' by Harper (1967). This type of distribution has a positive value of the third moment, g1 (Sokal & Rohlf 1969), and is commonly referred to in the plant competition literature as 'log normal', an expression used by Koyama & Kira (1956) although they did not attempt to fit this particular distribution to their skewed frequency distributions. The closer the initial spacing of plants the sooner skewness appeared from which Koyama & Kira inferred that the competitive effect developed sooner. Skewness is commonly found in stem size distributions of even-aged forest monocultures, e.g. Bliss & Reinker (1964), Jack (1971), Ford & Newbould (1970). A second phenomenon taken as indicative of density stress is density dependent mortality. The proportion of plants which die in a set interval of community development increases the closer that initial plantings are made above a threshold value, e.g. Davidson & Donald (1958), Yoda et al. (1963). Thirdly, under closer spacings there are alterations in the morphology of the 'mean' plant, a phenomenon referred to as a 'plastic response' by Harper (1967). The proportions of the plants change; mean height increases, there is an alteration in mean leaf weight/area and a reduction in weight of seed produced by each unit of weight of vegetative tissue, e.g. Donald (1963), Hiroi & Monsi (1966). The phrase 'hierarchy of exploitation' suggests that when competition is taking place