Biotic Specialization Between Neighbouring Genotypes in Lolium Perenne and Trifolium Repens from a Permanent Pasture

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Abstract
A study was conducted to determine whether micro-evolution occurs differentially within a single population in response to interactions with different neighboring genotypes of a second species. Samples of L. perenne and T. repens were collected as neighboring pairs from 4 different localities in a 40-yr 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 2 components separately for 1 yr. Each clover-genet-type generally produced more dry weight when grown with the grass-genet-associate with which it was collected than with others. Th converse was true for the grass genets. The grass yielded more than the clover in all combinations, but members of naturel-neighboring genet pairs contributed more equitably to the total yield than other pairs. Natural-neighboring pairs did not differ significantly in total yield from pairs of non-natural neighbors. The results suggest the presence of precisely-defined biotic specialization in several neighboring genotype pairs, all belonging to the same 2 spp. in different localities of a single community. Because natural neighbors did not have the highest combined yields, biotic specialization cannot be interpreted as indicating niche differentiation. Interpretation instead focuses on the more even distribution of yield in combinations of natural neighbors, reflecting that natural neighbors have more balanced competitive abilities for contested resources than do non-natural neighbors. A mechanism which may generate this relationship between L. perenne and T. repens is proposed, based on the relatively high mobility of T. repens clones through stolon extension, and the opporunity for concomitant competition and beneficence (through N enrichment from symbiotic Rhizobium) between these 2 spp. Natural selection, in contexts of competition, may results in more balanced competitive abilities for contested resources instead of niche differentiation and this may be an important evolutionary mechanism of coexistence in plant communities.