The CO2 fluxes involved in the mass mortality of large stands of timber on the flanks of Mammoth Mountain in California's Sierra Nevada were measured and spatial and temporal inhomogeneities observed. Our estimate of the integrated flux over the total area of tree-kill is on the order of 400 t day-1. Small scale spatial variability most likely reflects varying degrees of shallow bedrock fracture and soil compaction. Temporal variability suggests a fast response to subsurface conditions such as seismically induced crack propagation, degassing of intruded magma, or changing groundwater/soil moisture conditions. ¿ American Geophysical Union 1996