Due to global warming and climate changes forest ecosystems in Romania experienced environmental degradation. Climate models scenaria evidenced that some of the most severe weather events could become more frequent in Romania over the next 50 to 100 years. In the case of Carpathian mountain forests, winter storms, heavy rains, summer draughts and heat waves are considered main keys of climate risks, particularly in prealpine and alpine areas. Forest phenology is a multi-scale phenomenon, arising from processes in leaves and trees, which controls carbon and water cycles, observable at the ecosystem scale (e.g. eddy flux measurements), very important to characterize the relation between phenophase transition events at different spatial scales. Through time series of the Normalized Difference Vegetation Index (NDVI), provided by MODIS Terra/Aqua data is possible to examine trends and seasonality in forest vegetation Leaf Area Index (LAI) and in primary productivity, as well as to identify the impacts of human disturbances (clearcutting, etc.). In order to extract forest phenological changes, this paper applied fusion technique to multitemporal satellite imagery data (Landsat TM/ETM/OLI, Sentinel 2 and MODIS Terra/Aqua satellite data) for a mountain forest ecosystem placed in Prahova Valley, South Carpathians in Romania over 2000-2018 period. Effects of climate extremes on forests can have both short-term and long-term implications for standing biomass, tree health and species composition. The preservation and enhancement of mountain forest vegetation cover in natural, semi-natural forestry ecosystems is an essential factor in sustaining environmental health and averting natural hazards.