PREDICTING GROWTH IN STANDS OF MIXED-SPECIES FROM THAT IN INDIVIDUAL-SPECIES

The growth of spring cabbage (Brassica oleracea var. capitata L.) and carrot (Daucus carota L.) in mixed species stands is compared with growth curves predicted by two forms of a dynamic competition model, which uses a conductance relationship (Aikman and Scaife, 1993) to allow for the constraints on growth from a set of environmental variables. While a plant is isolated, light interception is assumed to occur within a zone whose area is a function of plant weight. Lateral foliage expansion is constricted when the available space is filled. One form of the model assumes that all plants are of similar height (Aikman and Benjamin, 1994). The second form assumes the crown zones are in separate vertical layers, allowing greater lateral expansion in each layer but imposing shading on the underlayer. Parameter values of the model were estimated from the growth within even-aged monocrops. The first form of the model gave the best prediction of growth in the intercrops, often producing a close agreement between observed and predicted weights. Only at the highest density used, 0.05 m spacing, did the different height form of model give a better prediction of growth. Many mixed species stands may be approximated by one or other of the forms of the model, and the relevant form can easily be calibrated from the growth observed in monocrops. Hence, simple models may be sufficiently accurate to predict growth in mixed species systems such as intercrops, or crops and weeds. (C) 1995 Annals of Botany Company