Satellite-based radar mapping of British forest age and Net Ecosystem Exchange using ERS tandem coherence

A method of mapping forest age structure using satellite remote sensing data in combination with ground data is used to form an age structure map of mainland Britain’s forests. Age structure is then used to demonstrate a method of calculating Net Ecosystem Exchange (NEE) for a region of forest. Synthetic Aperture Radar (SAR) coherence data provides an indication of forest biomass, which is related to forest age using forest management data at five independent locations. Coherence data is sensitive to time varying environmental effects and hence requires extensive calibration of the function relating coherence to forest age. The calibration approach appropriate in Britain makes use of the extensive ground GIS data available. Age structure information for nearly 3 million hectares of forest is generated, of which 70% is privately owned and age information is otherwise unavailable. The resulting map has a spatial resolution that can differentiate individual forest stands and provides detailed regional age and biomass estimates. Comparison of stand age estimates with ground data is has potential to provide growth rate and felling information. The age structure map is used in combination with an exemplary function relating forest age to NEE to estimate atmospheric carbon exchange for England, Scotland and Wales. This method predicts threefold higher (10.87 M t a−1) forest carbon uptake than national inventory figures. The remote sensing data also indicates age estimates that conflict with the ground data in Wales, which is explainable by the introduction of partial felling practices in this region during the time period of the coherence acquisitions.