Biotic specialization between neighbouring genotypes in Lolium perenne and Trifolium repens from a permanent pasture

SUMMARY (1) Recent studies suggest that the genotype may be an important functional unit of community variation. This raises the principal question addressed in this study: does micro-evolution occur differentially within a single population in response to interactions with different neighbouring genotypes of a second species? (2) Samples of Lolium perenne and Trifolium repens were collected as neighbouring pairs from four different localities in a 40-year-old pasture. Each grass-genet-type was planted, as cloned ramets, in competition with each clover-genet-type in all possible combinations. Cumulative dry-weight production was recorded for the two components separately for 1 year. (3) Each clover-genet-type generally produced more dry weight when grown with the grass-genet-associate with which it was collected than with others. The converse was true for the grass genets. The grass yielded more than the clover in all combinations, but members of natural-neighbouring genet pairs contributed more equitably to the total yield than other pairs. Natural-neighbouring pairs, however, did not differ significantly in total yield from pairs of non-natural neighbours. (4) The results suggest the presence of precisely-defined biotic specialization-in several neighbouring genotype pairs, all belonging to the same two species in different localities of a single community. Because natural neighbours did not have the highest combined yields, biotic specialization cannot, however, be interpreted as indicating niche differentiation. Interpretation instead focuses on the more even distribution of yield in combinations of natural neighbours, reflecting that natural neighbours have more balanced competitive abilities for contested resources than do non-natural neighbours. A mechanism which may generate this relationship between Lolium perenne and Trifolium repens is proposed, based on the relatively high mobility of T. repens clones through stolon extension, and the opportunity for concomitant competition and beneficence (through nitrogen enrichment from symbiotic Rhizobium) between these two species. (5) This study suggests that natural selection, in contexts of competition, may result in more balanced competitive abilities for contested resources instead of niche differentiation and that this may be an important evolutionary mechanism of coexistence in plant communities.

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