## Introduction

The dynamics and abundance of forest tree species are determined in part by the relationships between tree growth, crown dimensions and light. Trees of wet tropical forests show much variation in growth rate, with many slow-growing individuals, particularly in the smallest size classes (Condit, Hubbell & Foster 1993; Clark & Clark 1999). Among saplings this variation in growth has been associated with variation in leaf area and light level (Sterck *et al*. 1999, 2003; Poorter 2001). Such factors have been incorporated into spatially explicit forest simulation models, which calculate growth rates as empirical functions of neighboring tree positions, or as functions of light as estimated from neighbor position and size (Lorimer 1983; Shugart 1984; Pacala *et al*. 1996). However, the individual tree growth rates predicted by such models are rather weakly correlated with observed growth rates on mapped plots, particularly in tropical forests (Vanclay 1995; Gourlet-Fleury & Houllier 2000; Uriarte *et al*. 2004). The models perform better for structurally simpler temperate forests, particularly when measures of the effects of neighbors on crown dimensions are included (Monserud 1975; Cole & Lorimer 1994). Likely reasons for the inability of spatial models to reproduce observed growth rates in diverse tropical forests include unexplained variation in light interception due to irregularities in tree height, crown size and vine loads.

We hypothesize that much of the observed variation in growth, both within and among species, can be related to two central factors: the amount of light intercepted by a tree; and the density of its wood. More specifically, we expect an inverse relationship between growth rate and wood density, given that the thickness of the shell of stem wood associated with a given biomass increment is inversely proportional to its density. To test this hypothesis, we estimated crown dimensions and exposure to light among 727 pole-sized trees (8–20 cm d.b.h.) of 21 species of contrasting wood density in long-term forest dynamics plots at Pasoh, Peninsular Malaysia and Lambir Hills National Park, Sarawak, Malaysia (north-west Borneo). This size range was chosen to include a more equitable distribution of crown light exposure than would be encountered among trees of smaller or larger size classes. From these measurements we developed a composite predictor of growth, which is indicative of whole-tree light interception. As all trees ≥1 cm in diameter have been tagged and mapped on the plots, we were also able to compute neighborhood-related competition indices and compare their predictive capacity with the derived predictor.