In European mountain regions land use change, characterised by a concentration of activities towards the more productive and accessible areas and the concomitant cessation or extensification of labour-intensive traditional management in subalpine grasslands, threatens the high plant diversity harboured by these man-made habitats. Two sites in the French Alps were selected with contrasting climatic and land use conditions. This enabled us to test the hypothesis that decreases in plant diversity in response to extensification represented changes in functional composition that could be related explicitely to modifications in nutrient availability and disturbance regimes. Floristic changes in fields with known management history, nutrient availability and current disturbance regime were related to whole-plant morphological and leaf trait measurements. Decreases in nutrient availability along extensification gradients dominated response patterns. They were associated with decreased leaf specific area and nitrogen content and increased dry matter content of leaves and stems. We identified three plant strategies for dominant grasses: exploitative competitors in intensively used manured grasslands; stress tolerators in unfertilised mowed grasslands; and conservative competitors in extensive pastures. Within the matrix they formed, forb diversity decreased when mowing was replaced by grazing and evenness was highest at intermediate levels of fertility.