Monitoring forest degradation in tropical regions by remote sensing: some methodological issues

Key issues related to the monitoring by remote sensing of open forest degradation in a tropical context are discussed. Degradation of forest-cover is often a complex process, with some degree of ecological reversibility and a strong interaction with climatic fluctuations. Only a representation of land cover as a continuous field of several biophysical variables can lead to an accurate detection of forest degradation. For this purpose, repetitive measurements of spectral, spatial and temporal indicators of the land surface have to be performed. Each set of indicators brings a specific type of information on the land cover. These indicators must therefore be combined to achieve a comprehensive description of the surface processes. The detection of inter-annual changes in landscape spatial structure is more likely to reveal long term and long lasting land-cover changes, while spectral indicators are more sensitive to fluctuations in primary productivity associated with climatic fluctuations. Different monitoring systems may be optimal for different ecosystems. A long: time series of observations is always required. The monitoring of the spatio-temporal distribution of biomass burning may also give indications of open forest degradation.