Change detection of thinned Norway spruce stands using optical SPOT-4 satellite data

The possibility to detect thinning of Norway spruce (Picea abies) stands using multitemporal SPOT-4 satellite data was investigated. The test site is located in southern Sweden (58 degrees 30'N, 13 degrees 40'E). A field inventory was carried out for 20 stands subjected to mechanical tree thinning and 17 unthinned stands. The reduction in forest stem volume was on average 20%. SPOT satellite data from the years 2000 and 2002 (before and after the thinning) were radiometrically normalized and a difference image was computed. Pixel values from the difference image showed that the reflectances were significantly lower in the near-infrared band for the thinned stands compared with the unthinned stands. In addition, an increase in the mid-infrared band was observed for the thinned stands. However, no significant change could be detected in the green or red bands. Discriminant analysis using a combination of bands in the difference image and the image acquired prior to thinning showed that approximately 83%-95% (using between 2 and 6 bands) of the stands could be spectrally separated. For practical applications, detection of thinnings could be useful for improving the efficiency of timber harvest accounting.