Detection of Changes in Forest Landcover Type after Fires in Portugal

We present a methodology to detect terrain mobilization in the years following a fire using multitemporal Landsat TM imagery. Furthermore, we show the spectral evolution of the land cover since fire occurrence, and analyze the feasibility of detecting the type of land cover a number of years after fire occurrence. The methodology developed here is a spectral change identification technique based on vegetation index differencing. Spectral change identification has the advantage that it is based on the detection of physical changes between image dates, which avoids the propagation of accuracy errors. The vegetation index used was the Atmosphere Resistant Vegetation Index (ARVI). Although ARVI differencing was obtained through a simple subtraction between two different dates, the critical element was to decide the thresholds between change and no-change. The spectral evolution of the land cover was done through a multitemporal analysis from 1990 until 1998, and can be seen as a first step to help identifying the post-fire vegetation species. To assess the efficiency of this methodology in detecting the different land cover changes after a fire, a change detection error matrix was used. The reference data used for the validation was a land use map from 1990 in order to detect the land cover type before the fire, ortho-rectified aerial photographs from 1995 in order to detect the land cover type after the fire, and also burned area maps for each year. This study demonstrated the feasibility of using earth observation data to assess land cover changes in burnt forest areas.