Facts and figures of Silver fir dieback in France are given (extent, key factors, dynamics). An increasing decline of fir stand health status in the Pyrenees and southern Alps is expected from ecoclimatic modelling inspired by climate change scenarios. The dieback causes a range of symptoms on declining silver fir trees, and subsequently drastic changes in habitat conditions, especially for forest insects. The study sampling design aimed at monitoring biodiversity response to silver fir dieback will be replicated in 2 regions in the Pyrenees mountains, and stratified on factors such as the dieback intensity at the plot and the landscape levels, and the the management mode of declining plots (salvage logging or not). In each region, a balanced sampling design of about 30 plots will be set up. In each plot, environmental and entomological data will be collected using standardized protocols. The dieback intensity will be measured at the local stand scale, using the ARCHI method based on tree architecture, and at a larger scale, using remote sensing data.