Introduction

All plants need the same resources (water, light and nutrients) for their survival, growth and reproduction. Paradoxically, water is a limiting resource in many tropical rain forests. Variation in community structure, composition and functioning is driven to a large extent by variation in rainfall (Medina 1999; Poorter et al. 2004). Species composition changes continuously and gradually along the rainfall gradient (Hall & Swaine 1981; Bongers et al. 1999; Bongers et al. 2004), and within forests species tend to sort out along slope gradients in water availability (van Rompaey 1993; Webb & Peart 2000; Harms et al. 2001). Within a given forest a range of species coexist that differ in their drought tolerance. These species face to a greater or lesser extent a bottleneck during the dry season, when water is in short supply. Seasonal droughts (with low soil matric potentials) occur annually in tropical moist forest and occasionally in tropical wet forest (Veenendaal et al. 1996a). Even in seemingly aseasonal wet forests dry periods of 15–30 days may occur every other year (Walsh & Newbery 1999).

Within a certain forest type, variation in canopy openness leads to marked gradients in irradiance, temperature and relative humidity (Brown 1993). The light available in gaps can be as high as 30% of full light, whereas in the understorey it can be as low as 1%. Light is therefore the most limiting resource in many humid tropical forests (Whitmore 1996).