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 local density on the relationship between emergence time and final biomass in Prunella vulgaris. The relationship between emergence time and final biomass was used to quantify the pattern of selection on emergence time. Seeds were planted in flats in three different spatial distributions (hexagonal, random, high variance) at each of three overall densities (308, 769, and 3,077 seeds/m2). Individual seedlings were marked upon emergence, and their final biomass was determined after 90 days of growth. With increasing overall density, mean plant biomass decreased, but the coefficient of variation in biomass and the magnitude of directional selection for early emergence increased. Increasing variation in the spatial distribution of the plants had no effect on mean plant biomass but did significantly increase the coefficient of variation in biomass at both low and medium densities. Both the magnitude of directional selection and the curvature in the relationship between emergence time and final biomass tended to increase with increased variation in the spatial distribution. Our results suggest that both overall plant density and the spatial distribution of individuals can affect the pattern of selection on plant traits.