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

Abstract

Summary 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 South Wales. 2 If seedlings compete for establishment sites, then large seed size may confer enhanced competitive ability. However, the simple reciprocal relationship found between seed mass and per capita seed output showed that species producing larger seeds suffer reduced fecundity. Seed size may therefore act 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 higher proportion of the available microsites. If large‐seeded species are the best competitors they are expected to win all the sites they reach, and hence 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 low‐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, individuals were up to five times more likely than expected to have a conspecific as a nearest neighbour. This could be caused by interspecific competition and/or by environmental heterogeneity that favours different species in different patches. 6 The results suggest that seedlings do compete for establishment sites and that large‐seeded species generally win when in direct competition. In unsown areas small‐seeded species win many sites by forfeit (because large‐seeded species are strongly recruitment limited) but there may be a restricted subset of potential sites for which they are the best competitors and which they can win outright.