A new evaluation of the influence of climate change on Zagros oak forest dieback in Iran

Abstract

Zagros forest, the second most important renewable source of cellulose for Iran, is one of the best known oak forest reserves worldwide. These vital ecosystems hold great environmental and ecological importance in protecting water resources in the climatic dry conditions of the country. In recent years, however, oak dieback has decimated these forests. The purpose of our work is to evaluate and analyze the relationship of climate variables and the dieback of the Zagros oak forest in Lorestan Province, Iran. Dieback is defined by dieback indices, such as DVI, GEMI, IPVI, MSAVI2, NDVI, EVI, OSAVI, Sarvi2, SR, and VHI, which were obtained for 110 locations in the study area in the period between 2001 and 2017. On the other hand, daily climate variables influencing oak tree dieback were extracted in the same time period. As a first step, the most important climate variables affecting oak tree dieback were detected using the factor analysis method. Next, the so-called G* spatial correlation index was used to compute the covariance of dieback indices with climate variables as well as the spatial patterns of the changes of these covariances. The results of factor analysis showed that three factors, namely, humidity, temperature, and precipitation, were the most significant variables affecting dieback indices. Spatial autocorrelation patterns indicated that humidity played a major role regarding the widespread dieback in the study area. The mismatch of areas of low humidity and dieback indices, however, prompts that other factors, such as fungal diseases, affected the dieback alongside climatic variables. A similar investigation relating temperature with dieback indices indicated that changes in the match of spatial autocorrelation patterns of temperature and other dieback indices were less than 30% in terms of matching area except for NDVI and EVI. The effect of temperature on dieback indices was more limited to the southeast part of the study area. However, the covariance change pattern relating precipitation with dieback indices was related to northwest areas. Among all studied dieback indices, the NDVI was most correlated with all three factors of humidity, temperature, and precipitation, and temperature along humidity was found to play the most important role concerning the dieback of the Zagros oak forest.