THE EFFECT OF CLIPPING ON INTERCLONAL COMPETITION IN THE GRASS HOLCUS-LANATUS - A RESPONSE-SURFACE ANALYSIS

1 The effect of clipping on the outcome of interclonal competition in the clonaI grass Holcus lanatus was investigated using a response surface analysis in a greenhouse pot experiment. All binary combinations of three clones were planted in mixtures at a range of frequencies and tiller densities from 18.5 to 40 000 tillers m(-2). Two treatments, unclipped or clipped, were applied in two blocks. After 10 weeks of growth the biomass, tiller production and plant mortality of each clone in each treatment were measured. 2 Plant mortality was low but was increased by increased density and clipping. The plant biomass and tiller number data sets were fitted to a competition model in the form of a non-linear difference equation. In all data sets the value of r(2) was over 0.99 and the biomass data sets showed non significant lack of fit to the model. However, all the tiller number data sets showed significant lack of fit to the model. This was probably due to plasticity in tiller sizes and in tiller production rates, which also brought about differences in the parameter estimates for the two variables, biomass and tiller number. 3 Parallel curve analyses showed that there were significant clonal differences in the parameter estimates for intrinsic growth rates, density responses and equivalence coefficients within each clipping treatment in terms of biomass gain and tiller production. 4 The parameter estimates of each clone were significantly affected by the clipping treatments. Clipping decreased the intrinsic growth rates and changed the density responses. In two of the three clone combinations the clipping treatment had significant quantitative effects on the equivalence coefficient although there were no qualitative effects. In the third clone combination the rank order of the equivalence coefficients was reversed by clipping. 5 The response surface analyses allowed the complete description of competitive interactions under all densities and frequencies. The outcome of interclonal competition was affected by the clipping treatment and the relative densities of the competitors. These results may explain the high genotypic diversity of the study population. Environmental heterogeneity, e.g. in grazing levels, may vary the outcome of intergenotypic competition over space and time and thus allow clones to coexist.