LTREB: Disentangling mechanisms driving biomass decline in a temperate deciduous forest

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 mortality; and (3) a recent shift from biomass accrual to loss. Ten more years of measurements – including tree censuses, dendrometer bands, and tree cores – will be critical to disentangling underlying mechanisms of biomass decline. Researchers will test hypotheses that (1) global change drivers – specifically nutrient dilution and increasing meteorological extremes – are reducing tree growth; (2) non-indigenous insects and diseases, nutrient dilution, and increasing meteorological extremes are driving sustained increases in tree mortality; and (3) the net result is reduced biomass accumulation. Hypothesis testing will focus on SCBI’s ForestGEO plot and draw on external data to contextualize across the mid-Atlantic region and beyond. They will use explainable AI approaches to take advantage of large machine-ready datasets to uncover complex, nonlinear relationships between environmental drivers and biological responses. Data will be contributed to the databases of ForestGEO and GEO-TREES, a new global reference system designed to provide much-needed calibration and validation of satellite-based biomass estimates to scientists, policymakers and other end users. 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. NSF Award ID: 2533379 | Program: 01002627DB NSF RESEARCH & RELATED ACTIVIT | Principal Investigator: Kristina Teixeira | Institution: Smithsonian Institution, WASHINGTON, DC | Award Amount: $600,000 View on NSF Award Search: https://www.nsf.gov/awardsearch/show-award/?AWD_ID=2533379 View on Research.gov: https://www.research.gov/awardapi-service/v1/awards/2533379.html