Water defi cit index (WDI) を用いた丹沢山地におけるブナ群落の衰退状況の評価

概要

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We have estimated natural beech forest decline at Tanzawa mountains spreading over Kanagawa, Yamanashi and Shizuoka prefectures, in which the decline has become a serious problem since 1980s, using multi-temporal 8day-composite data obtained from MODIS aboard the Terra satellite, AMeDAS data in 2007 and GDEM (Global Digital Elevation Model) obtained from ASTER aboard Terra. The NDVI (Normalized Difference Vegetation Index) and WDI (Water Deficit Index) maps, which indicate vegetation density and the difference of transpiration rates between similar vegetation density areas, were derived from these data and compared with an existing mortality-map of beech forests at the study area in order to verify their accuracy. To produce WDI-map, we calculated maps of air temperature using AMeDAS data and GDEM. The interpolation method using the environmental lapse rate calibrated air temperature maps with the good accuracy of RMSE = 0.49K. The WDI-map could detect the mortality more accurately than NDVI-map in summer although both maps were effective in spring. Considering the characteristic of WDI, the index detects forest decline inducing the reduction of transpiration rates caused by air pollution and water deficit. Therefore WDI could be expected as an index for monitoring vegetation decline.