Controls on Annual Forest Carbon Storage: Lessons from the Past and Predictions for the Future

F are a critical component of the global carbon (C) cycle, storing more than 1 × 1015 metric tons of carbon in biomass, detritus, and soils (Dixon et al. 1994). Forest C storage is an important ecosystem service, locking upC thatmight otherwise exist in the atmosphere as carbon dioxide (CO2), a potent greenhouse gas. In the northern hemisphere, forests are estimated to sequester up to 7 × 108 metric tons of C annually (Goodale et al. 2002), or nearly 10% of current global fossil fuel C emissions (IPCC 2007). However, there is a great deal of variation in the capacity of individual forest ecosystems to take up and store C.Annual rates of forest C storage vary across latitudes because of broad gradients in community composition, growing season length, precipitation, temperature, and solar radiation (figure 1a).On average, boreal forests are close to C neutral, whereas those atmidlatitudes store approximately fourmetric tons of C per hectare (ha) per year, but with substantial variation in annual C storage recorded at all latitudes. The difference between maximum and average annual C storage, often exceeding 100%, suggests that forests generally storeC at rates below their potential (figure 1). The broad flexibility in forest C storage rates offers opportunities as well as challenges for those considering forests as an important component of strategies to mitigate rising atmospheric CO2. Multiyear empirical observations of forest C fluxes at the ecosystem scale help provide a quantitative understanding of the biotic and abiotic constraints on C uptake and loss. For example, the broadly distributed terrestrial C cycling research network of theAmericas,AmeriFlux,has supported long-term observations of forest C storage to determine how climate, land use, management, disturbance, and forest-stand structure influence rates of C sequestration (http://public.ornl.gov/ ameriflux).AmeriFlux is a partner in the worldwide network of C cycling research sites called FLUXNET,which coordinates global syntheses of observations from an extensive array of ecosystems (Baldocchi et al. 2001).Understanding how variation in annual C storage is controlled at the ecosystem scale