The Growth, Distribution and Neighbour Relationships of Trifolium Repens in a Permanent Pasture Iv. Fine-scale Biotic Differentiation

(1) Ramets of Trifolium repens were sampled from sites in a field of permanent pasture dominated by a mosaic of Lolium perenne, Holcus lanatus, Cynosurus cristatus and Agrostis tenuis. After multiplication of the ramets in a glasshouse, they were transplanted back into the original swards in all combinations of clover 'type' and site of origin. The pre-existing vegetation on the sites was either destroyed by herbicide or left as a control. (2) Clover 'types' were also transplanted into experimental sown swards of the four grass species. (3) Differences were found in aggressiveness of the grass swards and species, and between the clover 'type', and these were most clearly expressed in the growing swards. Most striking was a 'principal diagonal effect', in which each clover 'type' made most growth when transplanted back into its sward of origin-or into a pure stand of the grass that had dominated that sward. (4) This is interpreted as a micro-evolutionary esponse of clover to the diversifying selection imposed by a variety of grass neighbours, and is discussed in relation to the ecological significance of the outbreeding habit and 'Sisyphean fitness'.