The role of specialization in nutrient-use efficiency as a mechanism driving species diversity in a tropical rain forest

The search for explanations of how large numbers of tree species in species-rich tropical forests can exist has been a predominant focus of ecological research. Within biogeographic regions, lower tree species richness occurs in forests both on very nutrient-poor soils and on nutrient-rich soils. The role of soil heterogeneity, however, in the maintenance of local species richness in tropical forests has yet to be elucidated. In this study, the role of specialization in nutrient-use efficiency was examined as a mechanism by which plant species have specialized to soils of varying fertility in an ever-moist rain forest in northwest Borneo. The hypothesis that differences in nutrient-use would determine species ability to grow in the more nutrient poor soil type was tested. Seedlings of six tree species predominantly found on one or the other of the two dominant soil types in the study site were transplanted in the forest and in controlled nursery experiments. The nutrient pools and fluxes of the two soils were studied and the distributions of soil types and canopy openings were mapped in 52 ha of forest. Four of the six species showed better growth in their native soil than in the soil where they were uncommon. One species native to the more nutrient poor soil had significantly higher nitrogen-use and phosphorus-use efficiencies than a congeneric species native to the more nutrient rich soil. Another congeneric species pair showed no significant differences in nutrient-use efficiency. These species showed characteristics that suggested soil moisture had a more influential role in determining their demographic patterns within the landscape than nutrients. Overall, species growth responses and the dynamics of the nutrient cycles suggested that phosphorus was limiting plant growth on the more nutrient poor soil, but not on the more nutrient rich soil. Results from this study imply that small-scale differences in soil nutrients and texture have a strong influence on species demographic patterns and diversity of tropical forests. The effect of small-scale soil nutrient heterogeneity and its effect on the dynamics of biotic and abiotic resources, therefore, need to be considered as part of management practices in tropical ecosystems.