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In recent decades, oak forest dieback is one of the most important environmental challenges in western Iran. This phenomenon has occurred with varying intensity and weakness in the western provinces of Iran. In preliminary studies, Climatic conditions, pests, physiographies and anthropogenic factors were introduced as the causes of Zagros forests dieback. Therefore, the purpose of the present study is to investigate oak forest dieback in relation to drought in Lorestan. The data used include field surveys using GPS, Modis satellite imagery, data of meteorological stations for Lorestan province a 30 - year period (1980 - 2016). The results of field studies and reports showed that dieback occurred in different parts of the forests of the Lorestan province with different severity and weakness and this phenomenon was not related to specific direction, elevation or slope. Also, it was found that in Lorestan province, the dieback of trees is more common than their decline. The results of the study of freshness values of the forests in the study area showed that the first palpable vegetation decline occurred in 2005 and then more strongly in 2008. The results of the processing of precipitation data and SPI calculation in the study area showed that droughts haven’t a same and droughts occurred with varying magnitude, severity, intensity and duration in different areas of Lorestan province. These conditions are visible at different time scales across the province. According to the results of the Standardized Precipitation Index, the occurrence of drought is more frequent in southwest, central and northeast areas of Lorestan province and severe droughts have occurred in these areas. Investigation of the relationship between drought events and its spatial and temporal variations with changes in freshness of the forests in the study area showed that in general, changes in precipitation amount and its reduction is one of the main reasons for decreasing the freshness of Lorestan forest trees. Increased periods of rainfall reduction and drought duration, especially at the 12- and 9-month time scales, showed a high correlation between the effects of drought on decreasing the freshness of forests in Lorestan province.Keywords: Precipitation, SPI, Severity and Duration, NDVI, Forest Dieback..
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