Reproductive patterns in annual legume species on an aridity gradient

Abstract

Reproductive patterns are analysed in annual legumes of west Asia, and their relationships to increasing aridity determined by multivariate analysis. Dormancy, seed size, dispersal and fecundity are shown to be partially substitutable in terms of their effect on survival and population growth. The range of patterns show greatest diversity under mesic conditions in coastal, mediterranean areas with high winter rainfall, low incidence of frost and long growing season. Increasing aridity leads to increasing reproductive homogeneity, in which a subset of patterns - those composed of high levels of seed dormancy, high seed to pod ratios, restricted dispersal capabilities and early flowering - become predominant. These findings corroborate earlier theoretical and empirical evidence concerning desert annuals. The majority of widespread species are shown to possess ''arid-type'' patterns. This exerts considerable influence on both the type and degree of ecotypic differentiation within species. The only feature that responds consistently to climatic change is flowering time, particularly among species characterized by high seed dormancy. In the few widespread species that do not display ''arid-type'' characteristics, ecotypic differentiation appears more frequently in a larger suite of traits. The unequal environmental demands made on species showing different levels of dormancy (e.g. variation in length of growing season from year to year and place to place) leads to strong asymmetries governing the relationships between reproduction and ecological amplitude. In this respect ''arid-type'' strategists with high seed dormancy appear to have greater chances of expansion than others. This has implications when choosing pasture legumes to improve mediterranean grasslands.