The Effects of Plant Age on the Ability to Predict Mixture Performance from Monoculture Growth

(1) Austin (1982) has proposed a method for predicting relative mixture performance from relative monoculture performance. In this paper I evaluate the method of Austin using data from experiments on Typha spp. competing along gradients in water depth. (2) The data used to evaluate Austin's method are from two studies; one of competition between Typha latifolia and Typha domingensis seedlings along a miniature water-depth gradient; and the second of competition between adult plants of both species in a large, experimental pond. The seedling experiment compared species over a fourteen-level gradient that ranged from 18 cm above the water to 21 cm deep. The adult experiment included eleven levels ranging from 5 cm above water to 115 cm deep, and extended over a 2 5 year period. Both experiments included monocultures as well as 50:50 mixtures. (3) The ability to predict relative mixture performance from relative monoculture performance using Austin's method varied, depending upon the age of the plants. The correlation between mixture and monoculture results was greatest for one-year-old adults and declined strongly with increasing age. Relative monoculture performance at 12 months was significantly correlated with mixture performance at 12, 13, 14, 15 and 24 months, but not beyond. Except for the oldest plants (28 months old), mixture performance at any point in time correlated significantly with the monoculture performance at some earlier time. For plants older than 24 months, their mixture performance correlated with the monoculture performance 11-12 months before. (4) The results show that the ability to predict mixture performance from monoculture performance in Typha depends upon the age of the plants. Monoculture growth early in the life of the plant predicts subsequent mixture performance for a substantial period (in this study, up to one year later). As plants grow older, their mixture performance becomes more difficult to predict from monoculture growth.