Forest Greening Weakens from Spring to Summer Under Shifting Climatic Controls Across the Zagros Forests

Forest greening may vary considerably within the growing season, particularly in seasonally water-limited ecosystems. This study examined how long-term greening varies from spring to summer across the Zagros forests of western Iran and whether its environmental associations change seasonally. MODIS NDVI time series (2003–2024) were analyzed for March–August and separately for March–April, May–June, and July–August. Trends in land surface temperature, precipitation, and soil moisture were assessed, and generalized additive models were used to examine their associations with seasonal NDVI trends and topography. More than 93% of the forest area showed increasing NDVI during March–August, but greening weakened markedly from spring to summer. Mean NDVI trends declined from 0.0293 decade−1 in March–April to 0.0121 in May–June and 0.0101 in July–August. Model explanatory power also declined from 45.4% to 34.0% and 28.2%, respectively. Temperature-related effects were more prominent early and mid-season, while topographic influences became relatively more important toward summer. Although increasing heat and water limitation later in the season is consistent with other seasonally dry ecosystems, our results show that widespread Zagros greening masks substantial within-season differences in both trend magnitude and environmental associations, which are not apparent from growing-season trends alone.