Investigating the impact of drought and dust on oak trees decline in the West of Iran

Persian oak ( Quercus brantii ) is a dominant species in the ecosystems of western Iran. Over the past decade, its decline has become a cause of concern, and still, the effect of some factors such as drought and dust on this decline is unknown. So this study monitored the impact of those factors on declining patches of Q. brantii in the Zagros region in western Iran via the improved Temperature Vegetation Drought Index (iTVDI) and aerosol optical depth (AOD) indices and examined their relationship with Normalized Difference Vegetation Index (NDVI). The indices were obtained from the moderate-resolution imaging spectroradiometer (MODIS) products for declining patches in northern Khuzestan and Ilam provinces from 2001 to 2021. In addition, the spatiotemporal variations in NDVI, iTVDI, SPI, and AOD were also used, and were evaluate their correlations with NDVI. According to the trend results, the highest increased slope value from 20001 to 2021 was to iTVDI and AOD, while NDVI and SPI had a decreased trend value in all declined patches except for the Malekshahi3. In the most decline iTVDI, AOD had an upward slope while the slope of the drought index decreased in most of them. The slope of the NDVI index was increasing in the most decline patches. Pearson correlation analysis also showed NDVI has a negative relationship with AOD. Results indicate that AOD, SPI, and iTVDI influence the forest dynamics and, as a result, contribute to forest degradation. The use of indices derived from the MODIS time series to monitor forest areas might help to improve long-term decision-making. As a result, it may be concluded that oak decline occurs in the Zagros area as dust events and drought become more prevalent, and soil water and vegetation products are scarce. So drought and dust occurrences should be considered for managing these forests.