Vegetation recovery assessment at the Jou-Jou Mountain landslide area caused by the 921 Earthquake in Central Taiwan

The catastrophic earthquake, 7.3 on the Richter scale, occurred on September 21, 1999. The 921 Earthquake was a terrifying disaster that caused numerous casualties in Central Taiwan. It is the severest earthquake in a recent hundred years. The land morphology and topography changed greatly with such features as land upheaval, river blockades and lakes dammed by debris in many locations. Severe disaster districts were found in the Jou-Jou Mountain landslide area in Nan-Tou County. A method was developed to assess the landslide characteristics and monitor the vegetation recovery at Jou-Jou Mountain. SPOT satellite imageries were used to analyze the Normalized Difference Vegetation Index (NDVI) derived from SPOT imagery before and after the earthquake for comparison. The vegetation recovery index (C) was proposed to calculate and evaluate the vegetation recovery rate. The results showed that the vegetation restoration rate has achieved 47.1% in natural plant succession one year after the earthquake. The poor and very poor locations were distributed mainly in mountain ridge, scoured slope base and acidic sulfate soil areas. Mountain ridges and acidic sulfate soil had lower recovery rates affected by the impacts of soil moisture and SO42−. Scoured slope bases next to the concave banks might need foundation engineering first to establish a stable habitat for plants.