The effect of below-ground resources on above-ground allometric relationships in tropical tree species

Background/Question/Methods To quantify sources and sinks in the global carbon cycle, the magnitude of natural carbon sequestration must be estimated, especially within highly productive tropical rainforests. As woody biomass is difficult to quantify directly, measurements of above-ground allometry in tree species are often used to evaluate the potential for above-ground biomass within a forest stand, but these allometric equations often average over large taxonomic groups and forest types. The extent to which this biases estimates of carbon storage is poorly understood, especially with respect to the influence of soil resources on above-ground growth. We examined this effect on a 52-ha forest dynamics plot in Lambir Hills National Park in Sarawak, Borneo, which sits upon an edaphic resource gradient ranging from more fertile, moist clay soil to less fertile, well-drained sandy loam. Since tree species' distributions are closely tied to this soil gradient and total basal area on sandy loam is significantly greater than on clay soil, possible differences are suggested in the allometries of species specializing on the two soils. To examine the influence of soil resources on above-ground allometry, we quantified variation in height and crown allometries of six tree species, three from the slower-growing, shade-tolerant Knema genus and three from the faster-growing, light-demanding Macaranga genus. Study species included two soil generalists, one from each genus, to provide intraspecific comparisons across soil types and two sets of paired congeners, each consisting of a sandy loam and clay specialist, for phylogenetically-controlled interspecific contrasts. For each species, power functions were fit to the relationships between stem diameter and height, crown area, and crown depth. Results/Conclusions Pairwise comparisons of parameter estimates of the allometric relationships between congeneric specialists yielded no significant interspecific differences in any allometric relationship across soil types in either genus. Similarly, soil generalists exhibited no intraspecific differences in the parameters defining the allometries of trees growing on sandy loam versus clay soils. Our results indicate that variation in allometries within these soils is as great as that between soils. Broad soil-type effects may be obscured by plastic responses of trees to even small changes in resource availability within a soil type. Although we evaluated relatively few of the large number of species in this forest, our results suggest that for some forests, lack of consideration of soil resources in allometric equations for above-ground biomass may not produce large biases in the estimation of carbon sequestration within tropical forests.