Seed Mass and Seedling Performance Within Eight Species of Psychotria (Rubiaceae)

Large seeds have been hypothesized to confer survival advantages in the shaded forest, and small seeds to confer growth advantages in open habitats. We explored these hypotheses using experimental studies in both field and controlled conditions. Our experiments examined intraspecific effects of seed mass on seedling demography in eight sympatric woody species of Psychotria. We compared the effects of seed mass on seedling size (biomass), maximum relative growth rate (RGR), the proportion of emerged seedlings that survived until one year of age, and the proportion of sown seeds reaching the stage of one-year-old seedlings in gaps compared with shaded rain forest habitats, as well as in contrasting greenhouse light conditions. For each species, seeds were classified into four seed mass categories and introduced to three pairs of shaded forest gap sites. In the greenhouse, emerged seedlings from different seed mass categories were grown in contrasting light environments. A positive effect of seed mass on seedling survival was observed in six of seven species in the shaded forest but only in two species in gaps. Final seedling biomass was positively related to seed mass in five species in the shaded forest, but only in two species in gaps. In some species, RGR decreased with seed mass. Consequently, the biomass achieved by small-seeded seedlings with relatively fast growth rates was similar to that of seedlings derived from large seeds. The probability of recruitment increased with seed mass in four of seven species in the shaded forest, but only in two species in gaps. Negative effects of seed mass on recruitment occurred only in one species, and only in gaps. The positive effects of seed mass on seedling biomass and survival observed under shaded greenhouse conditions were weaker than those observed in the shaded forest. Overall, our results indicate that: (1) seed mass influences recruitment in natural forest habitats, (2) larger seeds confer higher probability of recruitment in the shaded forest by producing seedlings with larger size and higher probability of survival, and (3) in natural habitats ecological factors in addition to light (e.g., seed predators) operated selectively on seedlings emerging from seeds differing in mass. Corresponding Editor: W. J. Platt