AmeriFlux: Its Impact on our understanding of the ‘breathing of the biosphere’, after 25 years

For over two decades, the AmeriFlux network has quantified the breathing of the biosphere, as defined by measuring the flux densities of carbon dioxide and water vapor exchange between ecosystems and the atmosphere. The network includes natural and managed ecosystems that span broad climatic and ecological gradients. In this review, we provide an overview of key discoveries by this network and highlight important open questions. Such discoveries are possible through the network's shared and open access database that is used to interpret these fluxes; it includes a comprehensive set of carbon dioxide and water vapor flux measurements and a complementary set of biophysical variables, including meteorological and soil conditions, along with structural and functional properties of the ecosystems. Topics covered in this paper include interannual variability, trends and magnitude of carbon dioxide fluxes, and the response of these fluxes to biophysical drivers and extreme events. We examine results both within individual sites and across natural gradients. A subset of long-term flux measurements produced by the network provide quantitative insights into how whole ecosystems are responding to climate variability and global change, including warming, rising CO2 and changes in rainfall distribution. By reviewing and synthesizing these insights, we underscore the value of long-term, bottom-up research coalitions. In closing, we hope this review can guide the future research directions necessary to deepen understanding of climate impacts on ecosystems in the decades ahead.