Biogeochemical asymmetry and the diversification of tropical rain forests in Borneo: The endmember hypothesis

Abstract Mixed dipterocarp lowland tropical rain forests on Oxisols and Ultisols and heath forests on Spodosols are considered distinct vegetation types in Borneo. Here we present the endmember hypothesis of ecosystem diversification, which describes a continuum of forest structure and composition between these two forest types linked to divergent pathways of pedogenesis and nutrient limitation. One endmember involves the formation of Oxisols or Ultisols via desilication and accumulation of secondary iron and aluminium oxides at moderately acidic pH. The other endmember involves the formation of Spodosols (podzols) through the accumulation of humic material and the eluviation of iron and aluminium at strongly acidic pH. Phosphorus (P) availability decreases towards Ultisols and Oxisols due to phosphate fixation on iron and aluminium oxides, but nitrogen (N) availability does not decrease. In contrast, N availability decreases relative to P availability towards Spodosols because strong acidity suppresses N mineralization. Forests in Borneo occur on soils at different pedogenic stages derived from various parent materials. The endmember hypothesis predicts that these ecosystems align along a continuum between the two pedogenic endmembers. The availability of P and N is therefore asymmetric along the pedogenic axis and forests differ in P and N availability and limitation. The ecosystem continuum between the two endmembers in Borneo can be explained by differential tree adaptations to P versus N deficiency based on ecophysiology principles of nutrient acquisition and the concentrations, locations, and mobility of nutrients in plants. These differences drive turnover of tree species along the pedogenic axis between the two endmembers, leading to a continuum of diverse ecosystems reflecting the asymmetric availability of soil P and N. Diversification in tropical rainforests is a consequence of biogeochemical processes leading towards two divergent pedogenic endmembers under a humid tropical climate. Our hypothesis based on observations of the tropical forests of Borneo highlights the importance of biogeochemical feedbacks for ecological adaptation and community composition in a hyper‐diverse biome. The mechanisms underpinning pedogenic divergence towards the endmembers and the pan‐tropical applicability of this hypothesis merit further study to support our understanding of patterns and drivers of tropical biodiversity. Read the free Plain Language Summary for this article on the Journal blog.