Retrieval of biomass in boreal forests from multitemporal ERS-1 and JERS-1SAR images

The response of JERS-1 and ERS-1 synthetic aperture radar (SAR) to the fore st stem volume (biomass) was investigated by employing a digital stem volum e map and weather information. The stem volume map was produced from the Na tional Forest Inventory sample plot data together with a LANDSAT thematic m apper (TM) image. A new indirect inversion method was developed and tested to estimate the forest blockwise stem volume from JERS-1 and/or ERS-1 SAR i mages, The method is based on using a semiempirical backscatter model for i nversion. The model presumes that backscatter from a forest canopy is deter mined by the stem volume, soil moisture, and vegetation moisture. The area of interest is divided into a training and test area, In this study, the tr aining area was 10% of the test site, while the remaining 90% was used for testing the method. The inversion algorithm is carried out in the Following three steps. 1) For the training area, the soil and vegetation moisture parameters are e stimated from the backscattering coefficients and stem volume (must be know n for training areas) with the semiempirical backscatter model. 2) For the area of interest, the stem volume is estimated from the moisture parameters and backscattering coefficients with the semiempirical backscat tering model. 3) If several SAR images are used, the stem volume estimates are combined w ith a multiple linear regression, The regression equation is defined using the stem volume estimates for the training area. The results for the stem volume estimation using L-band and/or C-hand SAR d ata showed promising accuracies: the relative retrieval rms error varied fr om 30 to 5% as the size of the forest area varied from 5 to 30000 ha (the f orest stem volume varied from 0 to 300 m(3)/ha).