Tropical tree species vary in their capacity to utilize resources, and their distribution is strongly influenced by spatial and temporal heterogeneity of resource availability (Ricklefs 1977; Whitmore 1978; Bazzaz 1983; Denslow 1987; Fowler 1988; Kohyama 1993; Bazzaz & Wayne 1994). Spatial heterogeneity of soil resources influences broad-scale patterns of tree species distribution (Vitousek 1984; Vitousek & Denslow 1986; Denslow et al. 1990; Clark et al. 1998). Within forests, heterogeneity of light availability across the gap–understorey continuum can determine local distributions (Swaine & Whitmore 1988; Bazzaz 1991). Fine-scale variation in soil conditions, for example within tree-fall gaps, can also influence species distribution patterns (Orians 1982; Putz 1983; Vitousek & Denslow 1986; Núñez-Farfan & Dirzo 1988). Furthermore, gradients in light availability down through the forest canopy influence vertical distributions (Chazdon et al. 1988; Canham et al. 1994; Thomas 1995; Aiba & Kohyama 1996). Few studies, however, have considered an array of both horizontal and vertical components of resource availability and how they might interact to influence the distribution and coexistence of sympatric tropical trees (although see Clark & Clark 1992).
Many studies of horizontal heterogeneity of resource availability have focused on the importance of light levels, and the consequent variation in tree life histories in relation to the gap–understorey continuum (Denslow 1987). Pioneer and non-pioneer tree species have been shown to differ in distribution and performance at various light levels (Augspurger 1984; Bongers & Popma 1990; Clark et al. 1993), but few studies have examined ecologically ‘similar’ groups of species such as non-pioneers (but see Clark & Clark 1992; Denslow et al. 1998), and rarely have resources other than light been considered. Kohyama (1993) suggested that incorporating both horizontal and vertical structure into models of forest dynamics increases the conditions under which stable species coexistence occurs, even when the species are all non-pioneers. We investigated the distributions of 11 pioneer species of Macaranga (Euphorbiaceae) in Borneo, in relation to a range of horizontal and vertical components of resource heterogeneity. We argue that an understanding of life-history variation and its role in the maintenance of high diversity in tropical rain forests, must involve detailed analyses of species’ responses to the full spectrum of resource heterogeneity throughout their life cycles (see also Clark & Clark 1992).
We investigated whether horizontal tree distribution was, as expected, closely correlated with canopy openings and high light environments (Brokaw 1985; Alvarez-Buylla & Martinez-Ramos 1992). We also tested whether distributions were biased towards particular physical microsites for germination and establishment, for example tip-up mounds or nutrient-rich sites (as seen by Orians 1982; Brandani et al. 1988; Uhl et al. 1988; Denslow et al. 1990; Molofsky & Augspurger 1992; Itoh et al. 1995), or to slightly broader scale variation in soil texture.
Light availability declines two orders of magnitude from above the canopy to the understorey of a tropical forest (Yoda 1974; Chazdon 1986) and access to this vertical heterogeneity is largely determined by plant size and allocation patterns to above-ground organs (Kohyama 1993; Thomas 1995; Aiba & Kohyama 1996; Zipperlen & Press 1996). Maximum tree size differs among pioneer species (Sarukhán et al. 1985; Swaine & Whitmore 1988) and although this may be indicative of different longevities and hence temporal differences in the occupation of successional sites, the relationship to resource acquisition has not been considered. We quantified maximum tree size and sapling height–diameter allometry to investigate vertical distributions of Macaranga species.