Trend analysis of greening and browning in Hyrcanian forests and their responses to climate change

Recognizing the impact of climate change on the temporal and spatial variations in forests is crucial for sustaining them in the face of climate change. Here, we aimed to: (1) analyses the greening and browning trends in HFs based on time-series VIs, focusing on foliage trends observable through remote sensing; (2) explore the temporal and spatial trends of climatic factors; and (3) identify the relationship between the greening and browning of the forests and climate change. In this regard, we generated an 18-year (2003-2020) time series with an 8-day temporal resolution, encompassing MODIS vegetation indices (EVI and NDVI) and four climatic and hydrological factors: day and night temperature (LSTd, LSTn), precipitation (PRE), and actual evapotranspiration (ET). Subsequently, we used spatial statistical methods for analysis. EVI and NDVI trend analyses over the study period revealed greening in 77.02% and 92.32% of the study area, respectively. The statistical test confirmed significance (p < 0.05) for this greening in around 41.59% (EVI trend) and 75.11% (NDVI trend). Regarding the climatic and hydrological factors, PRE exhibited a declining trend, whereas LSTd, LSTn, and ET showed an increasing trend. Conclusively, the results reveal a positive correlation, ranging between 0.7 and 0.9, between temperature (LSTd and LSTn) and vegetation indices, indicating a close association between the greening process in HFs and rising temperatures (LSTd and LSTn). These results contribute to the understanding of the ecological resilience of HFs, aiding in the development of strategies to enhance ecosystems' resilience in the face of climate change.