Seasonal changes in competitive ability of contrasting populations of Dactylis glomerata.
Populations of D. glomerata from Norway and Portugal were grown in pure stands and in 50:50 mixtures under natural conditions in Aberystwyth. Stands were sown at monthly intervals between Sept. 1969 and Sept. 1970. Monthly harvests from each sowing were taken either from stands which had been defoliated regularly or from stands with uninterrupted growth from sowing. In both managements the environmental conditions during stand establishment had a major influence on the subsequent relative competitive abilities of the 2 populations. This was most pronounced for the Sept. sowings in which the Portuguese population was more aggressive initially and this aggressiveness was maintained throughout the following summer even though environmental conditions at that time did not favour this population. Similarly, the Norwegian population was more aggressive in stands sown in May and June. Again, this aggressiveness was often maintained throughout the series of harvests even though this population was dormant during the winter. In other monthly sowings the initial competitive relationships were modified by the effect of the subsequent environmental conditions on the seasonal growth patterns in the 2 populations. These reversals of aggressiveness were more marked in stands that were defoliated regularly than in those with uninterrupted growth.