Seed mass and the competition/colonization trade-off: a sowing experiment

1 A seed-addition experiment using seven co-occurring annual plant species with a range of seed masses was carried out in a limestone grassland in Sou th Wales. 2 If seedlings compete for establishment sites, then large seed size may co nfer enhanced competitive ability. However, the simple reciprocal relations hip found between seed mass and per capita seed output showed that species producing larger seeds suffer reduced fecundity. Seed size may therefore ac t as a surrogate in a competition/colonization trade-off. 3 Equal numbers of seeds of all species were sown in a mixture over a range of densities. As sowing density increases, all species should reach a high er proportion of the available microsites. If large-seeded species are the best competitors they are expected to win all the sites they reach, and hen ce to occupy an increasing proportion of sites as sowing density increases. 4 The three species with the largest seeds made up 49% of individuals at lo w-density sown plots but 83% of individuals in high-density sown plots. In addition, seed mass and plant density were not correlated in unsown plots, but were strongly correlated in high-density sown plots. However, all small -seeded species maintained a presence in sown plots. 5 Although species were sown at random with respect to one another, individ uals were up to five times more likely than expected to have a conspecific as a nearest neighbour. This could be caused by interspecific competition a nd/or by environmental heterogeneity that favours different species in diff erent patches. 6 The results suggest that seedlings do compete for establishment sites and that large-seeded species generally win when in direct competition. In uns own areas small-seeded species win many sites by forfeit (because large-see ded species are strongly recruitment limited) but there may be a restricted subset of potential sites for which they are the best competitors and whic h they can win outright.