Germination Ecology of Neotropical Pioneers: Interacting Effects of Environmental Conditions and Seed Size

Abstract

Abstract. 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 m 2 ) from larger gaps. Germination response was significantly related to seed size. Small-seeded species (seed mass Ͻ2 mg) showed significantly greater germination in response to irradiance of 22.3 mol·m Ϫ2 ·s Ϫ1 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 Ͼ2 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 smallseeded 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.

Short dry spells in the wet season increase mortality of tropical pioneer seedlings

“…Although seed size effects on drought sensitivity were not detected in this study, a comparable field experiment that tracked both the emergence and the initial survival of pioneer seedlings at the same study site found lower survivorship of small-seeded species (Dalling and Hubbell 2002). We suggest that drought might contribute to such size-dependent seedling mortality in pioneer seedlings, since the maximum soil depth from which successful seedling emergence can take place is strongly correlated with seed mass (Bond et al 1999;Pearson et al 2002). Consequently, the very smallest-seeded pioneers such as M. argentea (0.063 mg) are only able to emerge through a few millimeters of soil (Pearson et al 2002), which exhibit the most severe drought conditions.…”

“…We suggest that drought might contribute to such size-dependent seedling mortality in pioneer seedlings, since the maximum soil depth from which successful seedling emergence can take place is strongly correlated with seed mass (Bond et al 1999;Pearson et al 2002). Consequently, the very smallest-seeded pioneers such as M. argentea (0.063 mg) are only able to emerge through a few millimeters of soil (Pearson et al 2002), which exhibit the most severe drought conditions. Such seedlings are therefore likely to be more vulnerable to drought and to remain susceptible for a longer period post-emergence relative to larger seeded species.…”

“…Low water potentials can be explained by high soil surface temperatures (up to >40°C; Fig. 1; Pearson et al 2002) that develop in sun-exposed gaps. High temperatures lead to steep vapor pressure deficit gradients between the air and the soil, causing high evaporative water loss from the soil surface.…”

“…Sensitivity to drought stress during emergence and establishment may provide an additional mechanism contributing to species coexistence among pioneers. Since small seeded species are only capable of successful emergence from the surface few millimeters of soil (Pearson et al 2002) we predict that they will be particularly susceptible to surface drying events.…”

Post-dispersal removal of seeds of pioneer species from five Panamanian forests

“…Small-seeded species (< 1 mg seed mass) are frequently removed by litter ants at the soil surface , are susceptible to fungal pathogens (Dalling et al 1998), and are only capable of successfully emerging from the surface few millimetres of soil (Pearson et al 2002). Seed relocation by litter-dwelling ants in the forest understorey may increase chances of seedling recruitment for pioneer species since microsites suitable for germination and establishment are often patchy and scarce.…”