Soil seed bank from lowland rain forest in Singapore: canopy-gap and litter-gap demanders

Abstract

The vast array of species found in the humid lowland tropical forests of the world are frequently divided into two general groups, light-demanders and shade-tolerators (Denslow 1980, Swaine & Whitmore 1988). In this dichotomy the characteristics of the gap-demanders are often given to include small seed size, dormancy mechanisms, potentially rapid growth, and requirement of direct sunlight for germination and establishment. Conversely, shade-tolerators are considered to have large, short-lived seeds, the potential to germinate in canopy shade and an ability to grow slowly in deep shade (Swaine & Whitmore 1988). Failing to conform to either general pattern, small-seeded shade-tolerant species have been documented from both the Central American and South-east Asian tropics (Ellison at al., Metcalfe & Grubb 1995). Some of these small-seeded species have been shown to be able to germinate in filtered light approximating to canopy shade (Metcalfe 1996), and seedlings of other small-seeded species have been recorded growing under a closed canopy (Grubb 1996, Kiew 1988, Raich & Gong 1990). Several taxa shown by Metcalfe (1996) and Metcalfe & Grubb (1997) to be small-seeded and shade-tolerant were found in considerable numbers in the soil seed bank in Sabah by Kennedy (1991), and one (Urophyllum glabrum) by Putz & Appanah (1987) in Peninsular Malaysia. Species with small seeds (less than about 1-10 mg) do not have the resources to emerge from underneath a covering of leaf litter, and require litter-free sites for successful establishment (Guzmán-Grajales & Walker 1991, Molofsky & Augspurger 1992, Putz 1983).