Variation in reproductive characteristics of Poa pratensis across a successional chronosequenc

Many aspects of reproductive behavior of plants are plastic and can vary greatly among individuals of the same species living in different habitats. The number of seeds produced by a plant is generally more plastic than the mean individual seed mass (Harper et al., 1970), although the size of individual seeds has been observed to be influenced by environmental factors such as light, temperature, soil nutrients, and level of competition (reviewed in McGinley et al., 1987; McGinley and Charnov, 1988). Because both biotic and abiotic conditions change during secondary succession, the reproductive behavior of a species may differ among successional stages. Here I report on the effect of successional stage on the number of seeds, the total mass of seeds, and the mean individual seed mass produced by individuals of the introduced grass Poa pratensis at Cedar Creek Natural History Area in Minnesota. Soil nutrient availability and plant biomass increase over time, whereas light availability decreases over time during secondary succession at this site (Inouye et al., 1987). Because Poa pratensis colonizes fields rapidly, reaches its greatest proportional abundance in intermediate-aged fields, and remains abundant in the oldest fields at this site, it is an important species in all stages of secondary succession at Cedar Creek (Tilman, 1988).