The effects of belowground resources on aboveground allometric growth in Bornean tree species

Tree height and crown allometries reflect adaptations for resource acquisition and structural stability, as well as plastic responses to a heterogeneous environment. While both light and soil resources limit growth and influence competitive responses in tropical forests, the effects of belowground resources on allometries are less understood, especially within the understory. To characterize outcomes of tree competition along an edaphic resource gradient, we quantified variation in height and crown allometries of six Bornean tree species from contrasting regeneration niches (light-demanding vs. shade-tolerant) on two soil habitats (clay-fine loam and sandy loam) within a 52-ha forest dynamics plot. Using empirically-fit allometric parameters and diameter growth rates from plot census data, we modeled tree height and crown area growth over the projected life span of each species. Based on resource competition theory, we hypothesized that tree species specializing on and populations of generalist species growing on the relatively moister, more fertile clay-fine loam soil habitat would have faster height and crown growth rates, compared to those on the sandy loam habitat, regardless of regeneration niche. Among soil specialists and within generalists of both genera, trees growing on clay-fine loam had taller stems and larger crowns at a given age and faster height and crown area growth rates at most sizes than trees on sandy loam. Differences in height and crown growth were driven by the faster diameter growth rates of trees on clay-fine loam, not by differences in height- and crown-diameter allometries, as trees on sandy loam were significantly taller at a given diameter, and differences in crown allometry were not consistent across soil habitats. Characterizing the height and crown growth responses of trees along resource gradients provides insight into the mechanisms that maintain diversity in tropical forests. Our results point to the importance of adaptive and plastic responses to both above and belowground resource availability in determining the allometric growth of trees and suggest that this diversity of responses may contribute tree species coexistence through competition-based trade-off mechanisms and variation in growth among individuals. Additionally, as the importance estimating natural carbon sequestration increases with the escalating effects of anthropogenic climate change, differences in tree growth and architecture across soil habitats also have implications for the approximation of forest carbon storage on heterogeneous tropical soils.