Since the original Darwin's definition of ''functional traits'' as predictors (proxies) of organism performance, a growing scientific community, mainly over the last three decades, used the ''traitbased'' approach to address fundamental ecological and multi-scale questions. The strong link between plant functional traits (PFTs), vegetation processes and ecosystem services, makes this approach particularly promising in the study of vegetation responses to environmental changes (i.e. land use change, climate change, management pressures, etc.). Despite the high amount of papers available, there are many questions still open. In this PhD thesis I describe several applications of the trait-based approach in forest and grassland mountain ecosystems, to explore: (i) the patterns and functional clonal groups and (ii) of community-level PFTs of the herb layer along a coppice forest succession; (iii) the intraspecific variability of PFTs in contrasting grasslands habitats; (iiii) the effects of simulated extreme climatic events on grassland ecosystems.