Size-dependent gender modification in a hermaphroditic perennial herb

The size-advantage model predicts that hermaphroditic organisms adjust sex allocation depending on their resource status. We investigated the relation ship between size and sex allocation in the co-sexual perennial herbs Trill ium erectum and Trillium grandiflorum at two sites in southern Ontario, Can ada, by measuring pollen and ovule production and biomass allocation at flo wering and fruiting. In both species, there was a strong relationship betwe en size and gender; larger plants allocated proportionately more biomass to female reproduction and produced fewer pollen grains relative to ovules th an smaller plants. Variation in gender was better explained by size than ag e, although age and size were correlated. While the relationship between si ze and gender was similar between species, T. erectum allocated proportiona tely more to female reproduction than T. grandiflorum, independent of size. In the absence of pollen limitation, there was no evidence of secondary ad justment of gender at fruiting. The results are discussed in the context of models predicting size-dependent gender modification in animal-pollinated plants. Evidence about the pollination and seed dispersal biology of Trilli um spp. suggests that the relative effects of local mate and resource compe tition may be important in driving size-dependent sex allocation in these s pecies.