Collaborative Research: Testing the Role that Biotic Interactions Play in the Latitudinal Diversity Gradient: A Chemical Community Ecology Approach to Understanding Tree Diversity

Washington University, Saint Louis MO

Investigators

Abstract

One of the most striking features of our planet is the dramatic increase in biodiversity from the poles to the equator. This global diversity gradient is especially noticeable among trees. Forest plots measuring 0.25 km2 contain 10 tree species in subarctic Canada, between 10 and 70 tree species in the continental United States, and >1,200 tree species in Amazonia. Since Darwin and Wallace, biologists have hypothesized that environmental stress predominates in temperate and seasonal climates, but the stresses experienced by plants in warm, wet, tropical rainforests are dominated by their interactions with other organisms, including insect herbivores and microbial pathogens. Plants are thought to evolve chemical defenses against their enemies, and their enemies to evolve counters to these defenses, more rapidly in the tropics, resulting in greater variation in defensive chemistry among plant species in the tropics, and ultimately greater species diversity. These ideas have gone largely untested at large scales because of a lack of tools to study the astonishing diversity of plant chemistry. This study will overcome this historical obstacle to chemical ecology by taking advantage of recent innovations in metabolomics—the study of the chemical profiles of organisms. The scientists will examine species differences in chemical profiles, or metabolomes, for >2,000 tree and shrub species in 20 long-term forest plots ranging from subarctic Canada to the Amazon Rainforest, their effects on the growth and survival of individual trees, their contribution to the maintenance of diversity. The project will support two Ph.D. students from underrepresented groups at the University of Texas at Austin and Washington University in St. Louis, undergraduate research opportunities, and capacity building in the larger research community. Other broader impacts include offering workshops at international meetings and the creation of a publicly available metabolome database for the research community. These data will support future research at US Government-supported sites within the National Science Foundation’s National Ecological Observatory Network (NEON) and the Smithsonian Institution’s Forest Global Earth Observatory. The researchers will couple recent innovations in ecological metabolomics with existing data on the diversity and dynamics of forest-tree communities that span large-scale gradients in climate, latitude, and tree-species diversity from boreal forest in Canada (11 species in 21 hectares [ha]) to tropical rainforest in Amazonia (>1,200 species in 25 ha). This project will leverage existing data on the growth, survival, and mapped distributions of more than 3,600 tree species in 20 large-scale forest-dynamics plots coordinated by the Smithsonian Forest Global Earth Observatory (ForestGEO) and San Diego Zoo Global, as well as existing metabolomics data from 13 forest plots. Building on these data, the researchers will collect and analyze leaves of 1,367 species in 7 new forest plots that represent unique climates and/or provide key seedling performance datasets. The researchers will combine metabolomics data, neighborhood demographic models, and theoretical simulations to determine i) how leaf-secondary chemistry shapes local species interactions within tree communities, ii) how the effects of leaf-secondary chemistry on local species interactions vary across latitudinal and climatic gradients, and iii) the importance of chemically mediated niche differences in maintaining species diversity across latitudinal and climatic gradients. Ultimately, the combination of long-term forest plots and novel techniques in metabolomics will provide unprecedented answers, in scope, scale, and substance, to questions that are fundamental to understanding Earth’s biodiversity. In addition to mentoring undergrad and graduate students, the researchers will lead a forest-metabolomics workshop at international meetings to train students and other scientists in the collection, analysis, and application of metabolomic data to community-scale ecology. The workshop will provide capacity building for early-career researchers from US and developing countries in Africa, Asia, and Latin America. Finally, the project will generate a public database of metabolites for over 200 North American and over 2,000 Neotropical tree species, most of which are understudied tropical rainforest trees. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.