A Reinterpretation of Yield Relationships in Replacement Series Experiments

Abstract

Replacement series are commonly used to study interactions between component species in a mixture. The conventional analysis of data from replacement series experiments compares actual yields with expected yields that would occur if competition between species were equal. Comparisons with expected yields are of limited value, since they do not assess the contributions of intraspecific and interspecific interference to the determination of mixture yields. An analogy is drawn with competitive enzyme inhibition to show that expected yields represent 1 state in a continuum of possible results which develop as the intensity of interference is increased in a mixture. An alternative approach is proposed involving different comparisons. At any given planting density of a species, intraspecific interference is related to the difference between the actual monoculture yield and the yield projected from the initial slope of the monoculture yield: density curve. The difference between the monoculture and mixture yields of a species is the combined result of interspecific interference and any altered intraspecific interference which may occur in the mixture. Two indices, the relative monoculture response and relative mixture response, are proposed as measures of plant interference in monocultures and mixtures. Analysis of data from a representative experiment with green foxtail (Setaria viridis (L.) Beauv.) and barnyard grass (Echinochloa crus-galli (L.) Beauv.) indicates that the latter species caused greater interference in both monocultures and mixtures. Total yields projected from the monoculture yield; density relationships greatly exceeded mixture yields. Considerable scope may thus exist for developing mixtures which give greater yields than monocultures.