The Fate of Seed Banks: Factors influencing Seed Survival for Light-demanding Species in Moist Tropical Forests

Seed mass in species-rich moist tropical forests often varies over six or more orders of magnitude (Foster, 1982; Foster and Janson, 1985; Hammond and Brown, 1995). Most of these species, including some with the minutest seeds (Metcalfe, 1996; Metcalfe and Grubb, 1997), can be classified as ‘shade-tolerant’ with the ability to establish beneath a closed canopy. Seeds of the shade-tolerators vary widely in morphology and physiology, but many are recalcitrant, and most germinate within a few months of dispersal (Hall and Swaine, 1980; Ng, 1980; Hopkins and Graham 1983; Garwood 1983). This chapter concerns the approximately 10–20% of tree species in moist tropical forests that can be classified as ‘light-demanding’ or ‘gap-dependent’ (Dalling et al., 1998a; Molino and Sabatier, 2001). These species require higher levels of irradiance than are found in closed forest understories for successful seedling establishment. Variation in seed mass, morphology and physiology among these species can be as great or greater than that of shade tolerators (Hammond and Brown, 1995). Establishment of light-demanding species occurs when seeds are dispersed directly to canopy openings, or perhaps more frequently, when disturbance cues the germination of seeds present in the soil seed bank (e.g. Putz and Appanah, 1987; Lawton and Putz, 1988; Dalling and Hubbell, 2002). Successful recruitment from a persistent seed bank depends on surviving an array of predators on the soil surface, remaining buried or being returned to shallow soil depths that permit seedling emergence, and avoiding predation or infection by soil invertebrates and pathogenic microbes. The additional interactions that seeds encounter during their stay in the soil set apart studies of seed fate for lightdemanding species from those of rapidly germinating shade-tolerant species in tropical forests. Greater duration of the seed stage of the regeneration cycle might also imply that sources of seed mortality play a more important role as a demographic filter influencing population growth and niche partitioning for these species. Despite this, however, seed fate in the soil remains poorly explored, with most studies restricted to relatively simple experimental assays of seed survival (e.g. Hopkins and Graham,