Functional significance of photoblastic germination in neotropical pioneer trees: a seed’s eye view

1. We investigated the relationship between seed mass and the suitability of microsites for germination for five small-seeded (< 2·4 mg fresh mass) photoblastic neotropical pioneer trees. We determined the germination response of these species to the ratio of red to far-red irradiance (r:fr) and compared it to r:fr measured under varying conditions of canopy openness, litter and soil thickness. 2. At a constant temperature the germination percentage of each of the species increased sharply with increasing r:fr above a species-specific threshold r:fr. The smallest-seeded species, Miconia argentea , had the lowest r:fr threshold for germination (0·12) while the larger-seeded Cecropia species and Solanum hayesii possessed higher values (0·21–0·27). The largest seeded species, Solanum hayesii , also showed a positive germination response to a 10 ° C temperature fluctuation, which was independent of the response to r:fr. 3. The mean r:fr at ground level declined with decreasing gap size but not sufficiently to suppress the germination of these five species. However, a covering of one litter leaf in direct sunlight reduced the r:fr to between 0·18 and 0·83 depending on the species and leaf wetness. The top-soil at our study site was aggregated and irradiance was transmitted or reflected between soil aggregates with little change in r:fr. Light did not penetrate the aggregates. 4. We suggest that photoblastic germination in neotropical pioneers has evolved to inhibit germination in response to conditions most likely to alter within the life of an individual seed (i.e. superficial burial by leaf litter or incorporation into the surface soil), rather than to fine scale variation in canopy openness. Among photoblastic species, the pattern of response to r:fr suggests that smaller-seeded species would germinate in a broader range of microsites than larger-seeded species. This lower degree of discrimination may be associated with their higher risk of mortality and therefore more limited persistence in the soil seed bank.