The success or failure of strains of Trifolium subterraneum L. in a Mediterranean environment

Abstract

Five field experiments, three of which extended over a 9-year period, are described.

Success or failure was assessed on a relative basis in three competitive situations, in each of which associated volunteer annuals were present. These situations were: (1) single-strain swards, (2) single-strain swards in association with an "indicator" strain at low initial density, and (3) mixed swards of several strains.

The results support the hypothesis that the main determinant of success or failure of a strain of T. subterraneum in a mixed pasture is the seed-producing capacity of that strain when grown in ungrazed pure swards. More importantly, the form of the relationship between relative success and seed yield was shown to differ for each of the three competitive situations.

Relative success was also related to "maturity grading" (a measure of earliness of flowering) since seed yield and maturity grading were themselves strongly related. However, partial regression analysis indicated no significant effects of maturity grading independent of seed yield.

The importance of three further possible determinants of success or failure – seed size, petiole length, and hardseededness – is discussed. No evidence for their importance is adduced, except in the case of hardseededness under low rainfall conditions.

Natural selection in multiple-strain swards is also discussed, and the need for further studies on factors associated with success or failure of closely related genotypes is emphasized.