GERMINATION ECOLOGY OF NEOTROPICAL PIONEERS: INTERACTING EFFECTS OF ENVIRONMENTAL CONDITIONS AND SEED SIZE

Germination provides many potentially unrecognized sources of variation in the regeneration niche. In this study we relate germination requirements and seed size for 16 species of pioneer trees to microclimatic conditions present in gaps in semi-deciduous rain forest in Panama. We found that, whereas increased duration of direct irradiance can be an effective indicator of the presence of a canopy gap across all scales of canopy openness, diel fluctuations in soil temperature effectively discriminate both understory sites and small gaps (25 m2) from larger gaps. Germination response was significantly related to seed size. Small-seeded species (seed mass <2 mg) showed significantly greater germination in response to irradiance of 22.3 μmol·m−2·s−1 than in complete darkness. Their germination was unaffected by an increasing magnitude of diel temperature fluctuation up to a species-specific threshold, above which it declined. Large-seeded species (seed mass >2 mg) germinated equally in light and darkness (with one exception) and either showed a positive germination response to an increasing magnitude of temperature fluctuation (four species) or no significant response (four species). The maximum seed burial depth from which seedlings could emerge successfully was strongly positively associated with seed mass. We conclude that photoblastic germination of tropical pioneer trees results in small-seeded species germinating in gaps only when seeds are located in microsites that are suitable for seedling emergence. A positive germination response to increasing temperature fluctuation can stimulate germination of larger-seeded species in larger gaps and when they are buried beneath an opaque soil or litter layer. Therefore, seed size differences constrain the physiological mechanisms of canopy gap detection in tropical pioneer trees and might contribute to observed differences in the distribution of adult plants in relation to canopy gap size.