Recent studies show that many old forests continue to be carbon sinks long past their presumed peak. What are the mechanisms behind recently observed high levels of carbon storage in old forests? How much carbon will forests of the future store? How will nitrogen cycling interact with and constrain carbon storage? U.S. DOE and NSF-supported FLUXNET investigators at the University of Michigan Biological Station (UMBS) are addressing these questions by combining experimental and observational studies that use meteorological and ecological approaches.