Annual clovers (Trifolium spp.) have different reproductive strategies to achieve persistence in Mediterranean-type climates

Abstract

The aim of this work was to determine whether different species of annual clover (Trifolium spp.), obtained from the same environment, have different reproductive strategies (combinations of reproductive traits) to achieve ecological success. A better understanding of the traits that improve persistence should allow agronomists to narrow the selection criteria for new clover cultivars for ley-farming systems in southern Australia. Seeds of 18 annual clover species were obtained from 3 Australian and 6 Mediterranean sites and were subsequently grown in a common garden in Western Australia. Reproductive traits, including time of flowering, weight per seed, fecundity, pollen to ovule ratio, and pattern of seed softening, were observed. Accessions of different clover species from the same site of collection had different reproductive strategies. Across a range of collection sites, accessions of the same species demonstrated the same broad reproductive strategy; however, some traits, e.g. the timing of flowering, varied within species across collection sites. Principal component analysis suggested that there are 3 broad reproductive strategies demonstrated by these clover species. At one extreme were the relatively large-seeded clovers (T. subterraneum, T. clypeatum, and T. stellatum). The associated cost of these large seeds is reduced fecundity. The large-seeded clovers do not have high long-term hardseededness (the predominant form of seed dormancy in clovers). The relatively small-seeded clovers were all characterised by high fecundity. Many of the small-seeded clovers have high levels of long-term hardseededness, which allow the risk of failure to be spread across seasons (T. spumosum, T. hirtum, T. lappaceum, T. angustifolium, and T. tomentosum). Some of the small-seeded clovers (T. glomeratum, T. nigrescens, T. campestre, T. cernuum, and T. suffocatum) are generalists, producing as many seeds as possible in each season, with very little hardseededness. There are several possible explanations for the apparent success of such different reproductive strategies among clover accessions of different species at the same site. A plant may achieve the same goal by trading one reproductive trait for another. For example, it may either produce many small seeds to spread the risk of failure or produce fewer large seeds with an inherent competitive advantage. Alternatively, temporal and spatial variation may favour clovers with a number of different reproductive strategies. It is likely that a mixture of species with different reproductive strategies will maximise production and persistence of legume-based pastures in ley-farming systems.