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

Key issues related to the monitoring by remote sensing of open forest degra dation 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 la nd 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 surfa ce have to be performed. Each set of indicators brings a specific type of i nformation on the land cover. These indicators must therefore be combined t o achieve a comprehensive description of the surface processes. The detecti on of inter-annual changes in landscape spatial structure is more likely to reveal long term and long lasting land-cover changes, while spectral indic ators are more sensitive to fluctuations in primary productivity associated with climatic fluctuations. Different monitoring systems may be optimal fo r different ecosystems. A long: time series of observations is always requi red. The monitoring of the spatio-temporal distribution of biomass burning may also give indications of open forest degradation.