CARRYOVER EFFECTS ON INTERCLONAL COMPETITION IN THE GRASS HOLCUS-LANATUS - A RESPONSE-SURFACE ANALYSIS

The effects of past (carryover) and present (direct) clipping environment on the competitive interaction between two clones of the grass Holcus Ianatus were investigated in a glasshouse experiment using response surface analysis. Two carryover treatments, previous clipping or no clipping, were applied to plants for eight weeks. Tillers from these plants were planted in mixtures of the two clones over a range of frequencies and tiller densities, between 44-40000 tillers m(-2). During the competition experiment two direct treatments, clipping or no clipping, were applied. Carryover and direct treatments were applied factorially in two replicate blocks. After ten weeks of growth plant mortality and yield of biomass and tiller number by each clone were measured in each treatment. Plant mortality was very low. The biomass and tiller yield data sets for each clone in each clipping treatment combination were analysed using a non-linear competition model. Every data set gave an r(2) < 0.99. Statistical comparison of response surfaces showed significant clonal differences within each combination of carryover and direct treatments in the model parameters for both biomass and tiller number. The short-term outcome of competition was determined by calculating the growth rates of clones, in terms of biomass accumulation and tiller production, using the model parameter estimates. In most clipping treatments the planting densities of the competitors affected the outcome of competition. Both carryover treatment and direct treatment significantly affected the model parameter estimates of both clones and changed the equivalence coefficients of the competitive interaction. The responses to the carryover treatment were affected by the direct treatment and vice versa. Therefore, the short-term outcome of competition and the effects of the competitor densities on the outcome were different in each of the four treatment combinations. These results may explain the high genotypic diversity previously observed in the study population of H. lanatus. Spatial and temporal environmental heterogeneity, e.g. in grazing levels, may cause intra-population variation in the outcome of interclonal competition and thus promote genotypic coexistence. Carryover effects from past environmental conditions will magnify this process, effectively increasing the environmental heterogeneity experienced by the population.