THE EFFECTS OF DENSITY AND SPATIAL, DISTRIBUTION ON SELECTION FOR EMERGENCE TIME IN PRUNELLA-VULGARIS (LAMIACEAE)

We investigated the effects of both overall density and variation in l ocal density on the relationship between emergence time and final biom ass in Prunella vulgaris. The relationship between emergence time and final biomass was used to quantify the pattern of selection on emergen ce time. Seeds were planted in flats in three different spatial distri butions (hexagonal, random, high variance) at each of three overall de nsities (308, 769, and 3,077 seeds/m(2)). Individual seedlings were ma rked upon emergence, and their final biomass was determined after 90 d ays of growth. With increasing overall density, mean plant biomass dec reased, but the coefficient of variation in biomass and the magnitude of directional selection for early emergence increased. Increasing var iation in the spatial distribution of the plants had no effect on mean plant biomass but did significantly increase the coefficient of varia tion in biomass at both low and medium densities. Both the magnitude o f directional selection and the curvature in the relationship between emergence time and final biomass tended to increase with increased var iation in the spatial distribution. Our results suggest that both over all plant density and the spatial distribution of individuals can affe ct the pattern of selection on plant traits.