Changes in the normalized difference vegetation index (NDVI) at the Fushan Experimental Forest in relation to typhoon Bilis of 2000

Abstract

Frequent typhoon disturbances have major impacts on the dynamics of forest ecosystems in Taiwan. By analyzing multi-temporal satellite images, it is possible to detect large-scale forest canopy changes caused by typhoon disturbances. We calculated normalized difference vegetation index (NDVI) from 3 SPOT images (10 Aug. 2000, 5 Sept. 2000 and 1 July 2001) to detect vegetation changes in the Fushan Experimental Forest before and after typhoon Bills struck Taiwan on 21-23 Aug. 2000. Results indicated that the standard deviation of the NDVI was very small suggesting that the vegetative cover was rather homogenous at the Fushan Experimental Forest. Following typhoon Billis, the NDVI decreased 4.8%, but it subsequently increased 4.8% and therefore had returned to pre-typhoon levels by July 2001. The NDVI tended to be higher at higher elevations and on east-facing slopes than at lower elevations and on slopes with other aspects. There was a positive correlation between the rates of NDVI decrease caused by typhoon Bilis and the post-typhoon recovery. For each of the 3 images, the variation of the NDVI within the same category of elevation, slope, or aspect was typically higher than that among batch variations. Thus, a better characterization of small-scale variations is critical to our understanding of the spatial variations of the vegetation distribution in the Fushan Experimental Forest.