||Understanding responses of ecosystem carbon cycles to climatic and atmospheric change is the focus of the ORNL Terrestrial Ecosystem Science Scientific Focus Area (TES SFA). Overarching science questions include: (1) How will interactions among the physical climate, biogeochemical cycles, ecological processes, fossil fuel emissions and land use evolve and influence one another over decades and centuries? (2) How do terrestrial ecosystem processes, interactions and feedbacks control the magnitude and rate of change of greenhouse gases? and (3) How will the magnitude and rate of atmospheric and climatic change alter the structure and function of terrestrial ecosystems and their capacity to provide goods and services to society? The proposed science includes large-scale manipulations, carbon cycle observations, process-level studies, and an integrating suite of modeling efforts. ORNL's climate change manipulations are organized around a single climate change experiment focusing on the combined response of multiple levels of warming at ambient or elevated CO2 in a black spruce - Sphagnum ecosystem in northern Minnesota. The experiment allows the evaluation of mechanisms controlling vulnerability of organisms and ecosystem processes to climate change variables. The TES SFA addresses fundamental processes controlling terrestrial vegetation function and change to improve mechanistic representation of ecosystem processes within terrestrial carbon (C) cycles and Earth system models. Integration of biophysical, biochemical, physiological, and ecological processes in ecosystem models is optimally constrained by historical and contemporary observations. The TES SFA plan is structured to eliminate artificial distinctions between experimental or observational studies and model building, parameter estimation, evaluation, and projection.