عنوان مقاله [English]
Changes in climate conditions and the frequency and intensity of extreme climate events like droughts and heatwaves have pronounced impacts on forest health and diversities around the world. Among the climate extremes and hazards, drought has the most influential impacts in forests and can adversely alter their density and spatial extent around the globe, particularly in mid-latitudes of both hemispheres. By investigating the impacts of droughts and diseases on forest trees, Loustau et al., (2006) stated that although most pathogens can resist water deficiency in drought periods in general droughts have no negative impact on forest diseases. By simulating historical long-lasting droughts in moderate climate forests, Borken et al., (2006) reported that a reduction in severe summer droughts significantly increases the CO2 conservation in forest soil. Hogg et al., (2008) investigated the impacts of regional droughts on the productivity, biomass and dieback of the Aspen forest of western Canada and concluded that the forest dieback and decline have a good correlation with annual relative humidity index which is related to short term droughts.In recent decades, oak forest dieback becomes one of the most important environmental challenges in western Iran. Climate change, pests, physiographic and anthropogenic factors were introduced as the main causes of Zagros forests dieback in the preliminary studies. The present study investigates the oak forest dieback in Lorestan province, Iran, concerning drought occurrences and their characteristics.
The data used in this study include 1) field surveys using GPS, 2) MODIS satellite data for the period 2000-2017, and 3) monthly precipitation records of meteorological stations for the period 1980 - 2017. Using GPS, the coordinates of the areas encountered canopy level dieback were recorded by the authors through several field surveys. The monthly precipitation records of 16 meteorological stations well distributed over the study area were acquired from the meteorological organization of Iran. The 30 years of 1980 – 2017 was chosen as it has complete data records in all considered stations and meets the minimum record length required for SPI computation in the arid and semi-arid climates. For all considered stations, the SPI time series were computed for 3-, 6-, 9-, 12-, and 24- month time scales from which drought characteristics (i.e., drought frequency, duration, severity, and magnitude) were subsequently computed. The MODIS satellite images with a 16-day interval, totally accounting for 411 images during the eighteen years of 2000-2017 time period were also retrieved from its website and processed by ENVI software for computing NDVI index as the representative of forest health and freshness. The NDVI time series were then divided into three categories based on the density of the forest coverage, namely forest with low, medium, and maximum densities. The mean annual time series of NDVI were then correlated with the SPI time series of each time scale ended at each calendar month. The maps of the long-term mean of drought characteristics were also drawn to spatially analyze drought characteristics over the study area and assess their linkage with oak trees canopy level dieback hot spots over the region.
The field study and the literature review showed that the dieback has occurred in Lorestan forests with different intensity and spatial extent, being not related to a specific land elevation, geographic location, and mountain slope and direction. The canopy level dieback of the oak trees was also found as a more common feature than the forest decline in the study area. Investigating the freshness of the forests represented by the Normalized Difference Vegetation Index (NDVI) time series derived from MODIS data showed that the first widespread forest dieback occurred in 2004 and then more intensified in 2008. The results show that the mean annual NDVI as a signal of forests trees health and freshness have the highest correlation with the SPI time series ended in all spring and summer months, particularly at 9- and 12- month time scales that showed the highest correlations with the mean annual NDVI time series. This relationship is relatively similar in most of the studied stations, more specifically in those located in or near the forest dieback hotspots that showed the highest correlation coefficients being significant at 5% significance level. The computed SPI time series also reveals that droughts are more frequent, long-lasting, and severe in the southwest, central, and northeast areas of Lorestan province where more hotspots of oak trees dieback were observed. Investigating the relationship between drought events and the spatial and temporal variations of NDVI time series indicates that a significant reduction in precipitation amount, as represented by SPI, is the main cause of oak trees canopy level dieback in the Lorestan forest, explaining 20% to 70% of the variation of the dieback phenomenon depending on the location over the province. The more widespread, severe and long-lasting droughts in the province were observed in 2004, 2005, and 2008 that coincide well with the years identified with the more pronounced oak trees canopy level dieback in the Lorestan province, as reported by literature and the local habitants.
Based on the results achieved in this study, it can be concluded that the oak trees canopy level dieback occurred in many parts of the Lorestan oak forests rather than forest decline. It was also found that drought occurrences explain 20% to 70% of the variation of this phenomenon depending on geographical locations s and SPI time scales considered. Investigating the relationship between drought events and spatial and temporal variations of the NDVI showed that a significant reduction in precipitation amount, as represented by SPI, is one of the main causes of oak trees canopy level dieback in the Lorestan forest.