Impacts of Drought Severity and Frequency on Natural Vegetation Across Iran

Drought recurrence is increasing in arid and semi-arid regions, and its effects are becoming more complicated due to climate change. Despite the increasing frequency of drought events, the sensitivity of natural vegetation to different levels of drought frequency and severity is not fully understood. Here, we aim to characterize the regional spatio-temporal patterns of drought frequency and severity and the response of vegetation across Iran at a high spatial resolution (5 km × 5 km). We examined the responses of three natural vegetation types (forest, grassland, and shrubland) to drought conditions across Iran using the Normalized Difference Vegetation Index (NDVI) and the Standardized Precipitation Evapotranspiration Index (SPEI) at different time scales and temporal lags from 2001 to 2022. Our results showed that drought severity increased in 15%, decreased in 1%, and remained stable in 84% of the study area. The severity and frequency of drought showed spatial patterns across Iran (i.e., increased from northwest to southeast and central Iran). The correlation between the monthly NDVI anomaly and SPEI varied across vegetation types, SPEI accumulation period (SPEI-1-3-6-9-12), and temporal lags, revealing different sensitivities of vegetation to drought in Iran. All natural vegetation types showed the strongest responses two months after drought events. Forests, mostly located in northern Iran, showed lower sensitivity to drought onset and responded slower to drought severity than other vegetation classes (i.e., grasslands and shrublands). These findings highlight the importance of analyzing the sensitivity of natural vegetation at different levels of drought severity and frequency for land use planning and mitigation efforts.