Linking Intra-Specific Trait Variation And Plant Function: Seed Size Mediates Performance Tradeoffs Within Species

Substantial intra-specific trait variation exists within plant communities, and in theory this variation could influence community dynamics. Although recent research has focused on intra-specific variation in traits themselves, it is the influence of this variation on plant performance that makes intra-specific trait variation relevant to ecological dynamics within or among species. Understanding the links between trait and performance variation, and the role of traits in mediating relationships among multiple components of performance, is critical for assessing the importance of intra-specific trait variation for community dynamics. Seed size is thought to affect aspects of plant performance including fecundity, seedling growth, dispersal and tolerance of natural enemies. For two tropical tree species, we assessed how seed size was related to performance variation within each species and determined whether intra-specific trait variation mediates intra-specific performance tradeoffs. We used field seed rain collection to characterize size-dependent outcomes of dispersal, sowed seeds of known size in soil collected near or far from conspecifics to characterize susceptibility to soil pathogens, and monitored growth of seedlings from seeds of known size. We found that intra-specific seed size variation caused intra-specific performance variation. The degree of trait-based performance variation was consistently smaller than the degree of trait variation, and seed size influenced different components of performance for each species. One species exhibited a tradeoff in which small seeds had a fecundity advantage (more seedlings per unit reproductive mass) but produced smaller seedlings, whereas the other species exhibited a tradeoff in which small seeds dispersed to areas of low conspecific density but were less tolerant of density-responsive natural enemies. Our results indicate that a single trait can influence multiple components of performance and can mediate different tradeoffs in co-occurring species. Complex and heterogeneous effects of a single trait in multidimensional niche space may favour inter-specific niche differentiation and coexistence.