Population Demography of Pastinaca sativa (Apiaceae): Effects of Seed Mass on Emergence, Survival, and Recruitment

The effect of seed mass on emergence, survival, and recruitment to flowering in Pastinaca sativa, a species that produces progressively smaller seeds on the primary, secondary, and tertiary umbels, was analyzed in two field experiments begun in 1982 and 1983. In both experiments, overall emergence was positively related to seed mass, but the effects of seed mass on emergence in fall cohorts was relatively unimportant. Initial seed mass was positively related to overwintering survival in one experiment but not the other, possibly due to differences in winter conditions. Survival to flowering was positively related to seed mass in the 1982 experiment but negatively related to seed mass in the 1983 experiment. This difference is due to greater survival of seedlings from small seeds during short-term droughts, a previously unrecognized advantage of small seeds. Because of differences in survival in the two experiments, recruitment (number of flowering plants/number of seeds sown) was positively related to seed mass in the 1982 experiment but not in the 1983 experiment. The effects of timing of emergence on survival generally disappeared within 12 months. Our results suggest that the relationship between initial seed mass and recruitment in Pastinaca represents a distribution of outcomes, variable from year to year. However, because short-term droughts in Iowa are common, we hypothesize that in most years, seed mass has little impact on recruitment. Furthermore, small scale heterogeneity may often overwhelm any potential effects of seed mass and timing of emergence on recruitment.