Over the past century, forests in the eastern United States have been growing and accumulating more large trees, providing valuable biomass storage and other ecosystem services. This trend in biomass accumulation is widely expected to continue with longer growing seasons. However, long-term studies at the Smithsonian Conservation Biology Institute (SCBI) in Virginia suggest the opposite, biomass in this temperate deciduous forest is declining. With 15 years of data, project researchers have found that tree growth is now slowing and more trees are dying, and these changes are not explained by forest aging or competition. Researchers hypothesize that the changes are caused by a combination of reduced nutrient availability, changes in weather such as more intense heat waves, and non-native insects and diseases. Over the next ten years, they will test their hypotheses in detail at the SCBI site and compare with other forests in the region to understand whether findings from this site apply more broadly. The research will be conducted with up to 19 interns and students to build the future workforce in forestry, forest research, and conservation. Researchers will share knowledge gained with forest researchers, managers, and policymakers around the world through the GEO-TREES network. The project will also contribute to databases used to understand the functioning and management of Earth’s forests and help to raise public awareness about forest research and conservation. Reproducible data and results from this project have potential to reduce uncertainty in Earth system models and improve accuracy and precision of rapidly growing global carbon markets. At the SCBI’s 25.6-ha forest dynamics plot, part of the Forest Global Earth Observatory (ForestGEO) network, 15 years of detailed observations have revealed (1) declining tree growth rates and a striking 25% decline in woody productivity; (2) increasing mortality across multiple tree species and a 94% increase in woody