Changes of ndvi associated with typhoon disturbance in subtropical forest ecosystem in central taiwan

Typhoon disturbances have major impact on the structure, function and dynamics of forest ecosystems. To evaluate impact of typhoon disturbances on forest ecosystems in central Taiwan, we used 8 SPOT images of Lien-hua-chi Experimental Forest together with geographical information system to calculate changes in the normalized difference vegetation index (NDVI). The temporal variations of changes in NDVI in relation to typhoon disturbances were closely related to the intensities of typhoons. After the attack of strong Typhoon Herb in the summer of 1996 NDVI decreased 5.8% causing the NDVI to be lower than that in winter. After medium Typhoon Toraji in the summer of 2001, NDVI decreased 3.0% and one month after light Typhoon Aere in the summer of 2004 NDVI increased 2.8%. For the comparable period in 2003, when there was no typhoon disturbance, NDVI rose 4.7 %. The results suggest that there is a threshold typhoon intensity above which typhoons could lead to detectable decrease in NDVI one month after the typhoon attack and that summer NDVI is expected to be higher without typhoon disturbance. NDVI changes caused by typhoon disturbances were closely related to landscape-level geomorphology and topography at finer scale. The topographic sheltering is key to the severity of wind disturbances. NDVI showed higher decrease at higher elevations and windward slopes (north and southwest slopes) than in lower elevation and leeward slope. In contrast, the disturbance of the heavy rainfall brought by typhoons increase from ridge to valley. Typhoon induced NDVI decrease differed between conifer plantation and natural hardwood forests but the patterns were not consistent among typhoons as such patterns observed from on typhoon should not be over-generalized. Our study also suggest that to better understand effects of typhoon disturbance on forest dynamics, the role of both wind velocities and torrential rain must both be examined.