Leaf functional traits and functional diversity of multistoried agroforests in West Java, Indonesia

We examined leaf trait variations among mixed-planting woody perennial crops in traditional but partially commercialized agroforest in West Java, Indonesia. We hypothesized that the structure of multistoried agroforests consisting of many species was sustained by differential use of the light resource by species with different leaf traits, and that functional diversity would decline as species diversity decreased due to commercialization of traditional agroforests. We focused on morphological and chemical traits of the fully exposed sun leaves and basal-interior leaves of 22 common perennial crops. The species varied as widely in their sun leaf traits as in previous studies of tropical lowland rain forests and in worldwide datasets. From one-third to half of the species exhibited significant differences in specific leaf area and in the nitrogen concentration per unit leaf mass between sun leaves and basal-interior leaves, likely in response to differences in irradiance and leaf age. Species within a product-type group (e.g., timber bamboos) generally had similar leaf traits. The diversity of functional leaf traits was significantly positively linearly correlated with species diversity, with no asymptote for functional diversity even at high levels of species diversity; thus, species diversity in the agroforests appears to maintain high ecosystem functioning through differential use of the available light resource. This also means that changing from the traditional multipurpose production system based on perennial crops into a specialized single-purpose system may damage ecosystem functions such as carbon sequestration. Understanding the leaf trait spectrum provides insights into other cash crops or cultivars that can fill gaps within the trait space in the plant assemblage, thereby improving profits from the agroforest while maintaining ecosystem functioning.