COMPETITIVE EFFECTS AND COMPETITIVENESS IN ANNUAL PLANT STANDS .1. MEASUREMENT METHODS AND PROBLEMS RELATED TO PLANT-DENSITY

The author's basic research on competitive effects in annual plant sta nds is reviewed, with particular emphasis on measurement methods. This paper deals with density influences on biomass production, especially regarding crop-weed and mixed crop stands. Density is defined here as the number of individuals, or ''plant units':, per unit area in an ea rly stage of stand development. Different production-density relations hips are characterized. Experimental results indicate problems in deve loping simple mathematical expressions suited as standard models for p roduction-density relationships covering wide density ranges, in one-s pecies stands as well as in mixed stands. The production per plant, or plant unit, is called unit production (UP). The ratio of the UP of on e plant category to that of another category in a mixed stand is calle d the unit production ratio (UPR). The relative competitiveness of two categories is assessed on the basis of the direction and magnitude of the change of UPR when plant densities in a mixed stand are changed, thereby changing the competitive pressure. The plant category that gai ns in response to an increase of this pressure relative to another cat egory, as assessed using the UPR, is considered the superior competito r. If the UPR remains unchanged when densities are altered, the plant categories compared are regarded as being equally competitive under th e given environmental conditions, irrespective of the UPR value. The u nit reduction ratio (URR), analogous to UPR, is used for comparing two plant categories with regard to their ability to reduce the biomass p roduction in another plant through competition. Reference models are d erived from biomass-density equations for one-species stands. In these stands, plants occurring at density X are assumed to be mixtures of t wo or more categories of plants, A, B, etc., at densities X-A, X-B, et c., where X-A + X-B + ... = X. The influence of these densities on bio masses produced, as well as on various biomass-related indices, can th us be determined on the basis of well-defined biomass-density relation ships, and free of experimental errors. Reference models illustrate de nsity influences on biomass production that are confounded with influe nces related to relative competitiveness. These influences are pitfall s in attempts to assess relative competitiveness based on various rela tions and indices. Ways to reduce the risks of misjudgements are illus trated.