Comparative ecology of 11 sympatric species of Macaranga in Borneo: tree distribution in relation to horizontal and vertical resource heterogeneity

Abstract

1 Horizontal and vertical heterogeneity of resource availability, coupled with the specialized use of resources by tree species, results in complex patterns of tree species distributions in tropical rain forests. We studied the horizontal and vertical distributions of 4014 individuals in 11 species of early successional Macaranga (Euphorbiaceae) in tropical rain forest in Sarawak, Malaysia. 2 The horizontal distribution of individual trees was assessed with respect to crown light levels, establishment microsites, and broader scale variation in soil textural properties. Vertical distribution was assessed using an allometric approach to estimate maximum tree height (Hmax) and the slope of the sapling height–diameter relationship. 3 Average light levels intercepted and the proportion of individuals in each of five crown illumination classes varied significantly among the 11 species. Species ranged from extremely high‐light demanding, to quite shade tolerant. Average light levels intercepted by trees generally increased through ontogeny, but the ranking of species did not change significantly. 4 Fewer individuals of the more shade‐tolerant species established on disturbed microsites, irrespective of light levels. Among the more high‐light demanding species, the proportion of trees on different types of disturbed sites varied. 5 Trees of seven species were significantly more common on clay‐rich soils, two preferred sand‐rich soils, and two were not strongly affected by soil texture. 6 Hmax ranged from 5.5 to 31.3 m and was negatively correlated with shade tolerance among species, although among the more high‐light demanding species there was a wide range of tree sizes. Among species, Hmax was negatively correlated with both the slope and y‐intercept of the sapling height–diameter relationship, indicating that small‐statured species (also more shade tolerant) had more slender saplings than larger statured species. 7 Heterogeneity of resource availability leads to differences in horizontal and vertical tree distribution, which are important for the coexistence of 11 Macaranga species.