All organisms face tradeoffs in they way they allocate resources. Trees, for example, can allocate resources in ways that promote rapid growth (e.g., more leaves with higher nitrogen content) or in ways that protect them from herbivores (e.g., thorns, defensive chemicals). This project addresses those tradeoffs and asks if they contribute to specialization of tree species on different soil types in an extremely diverse Bornean rain forest. Environmental gradients in the availability of resources may contribute disproportionately to tree diversity because tree species often specialize on particular habitats. A reciprocal transplant experiment, combined with a defoliation treatment and observations of resource allocation in the field, will be used to test for differences in allocation to growth, storage, and defense among trees on different soil types. Plant available mineral nutrients and water will be quantified to document the variation in belowground resources. In addition to advancing knowledge of the determinants of fundamental life history trade-offs, this research will improve understanding of tree species' responses to belowground resources and of mechanisms whereby soil-specific tree species distributions arise in forests. This project is an international collaboration with several worthy broader impacts. Several local Malaysians and Iban (an indigenous people) will be trained in data collection and management. This research will provide valuable information on forest regeneration and the environmental requirements of tree species, which can be applied directly in forest conservation and reforestation efforts in Borneo. Public lectures and development of secondary school education materials will communicate the broader implications of this work and emphasizing the vital functions of tropical forests and the ecological services they provide to human society.
Intellectual Merit: All organisms face trade-offs in how resources are allocated during a lifetime. For example, a juvenile tree growing in the understory of a closed-canopy forest accumulates carbohydrates assimilated via photosynthesis. Those carbohydrates could be used to make new leaves, promoting further growth, or they could be stored for future use during periods of resource-deprivation. The evolutionary responses to these unavoidable trade-offs have produced a variety of species’ ecological strategies that influence patterns of biodiversity. We used a field experiment with seedlings of 13 tee species to investigate the role of trade-offs in allocation of carbohydrates to traits enabling fast growth versus storage in determining the soil habitat specialization and patterns of diversity in a hyper-diverse rain forest in Borneo. We found strong evidence that this trade-off exists, since both fast growth and storage carried survival costs and benefits, depending on how stressful the environment was. In addition, the soil habitat specialization of tree species was strongly affected by the capacity of a species to plastically respond to environmental variation in its allocation to growth versus storage to function. For example, species characteristic of fertile, moist soil grew faster due to greater allocation to total leaf area, but their greater plasticity also helped avoid risks of insufficient storage in order to survive stress. The trade-off that we observed between allocation to fast growth versus storage provides a potential mechanism to promote biodiversity: at times or locations where foliar herbivory or other forms of carbon-deficit stress are high, species or individuals with greater allocation to storage would have increased survival, but under less stressful conditions, species or individuals with greater allocation to growth would have increased survival. In addition to the main project described above, several other projects were conducted under this award. Two undergraduate research thesis projects resulted from this award. A journal article submitted based on one of the theses found that Bornean tree species that display soil habitat specialization do not differentially take up different chemical forms of nitrogen in soil (ammonium versus nitrate). Instead, these tree species appear to have flexible nitrogen acquisition strategies that would enable them to take up the nitrogen forms that are most readily available in the soil. Another study in review for publication found that trees growing on different soil types in this Bornean rain forest have different tactics for deploying fine root surface area, which is critical for the uptake of mineral nutrients and water from soil. The tree community on the poorer soil achieved fine root absorptive area through faster growth and greater investment in root mass, whereas trees on the richer soil achieve a similar standing absorptive area through slower growth of less-dense fine root tissues. One master's thesis was supported through this award. This thesis examines plasticity in the allocation to roots versus stems and leaves in response to different soil and light environments. Finally, this award made possible a study that uses next-generation DNA sequencing methods to describe the variation in the composition of ectomycorrhizal symbionts (fungi that live on the surface of roots and facilitate nutrient and water aquisition) that colonize tree species that are characteristic of the different soil habitats . Broader Impacts: Research conducted under this award has advanced scientific discovery while promoting teaching, training, and learning by (1) supporting the research training and career development of one female postdoctoral fellow, (2) supporting research training and education of eight undergraduates, including five women and four underrepresented minorities in STEM fields, and (3) supported research training and education of one master's degree student. The research findings have been and will continue to be disseminated broadly through publications, presentations at professional meetings, and academic seminars by the PI and project personnel. In addition, the PI has increased public scientific literacy and engagement with the environmental biology by giving public lectures through several venues, conducing public outreach in collaboration with museums and educational organizations, and participating in secondary school programs to increase participation of under-represented groups in STEM fields. Research conducted under this award has also improved the intellectual well-being of individuals in society by increasing our undersanding of tropical rain forests, a biome that has captivated the imagination of humanity for hundreds of years.
