Dietary constraints for large herbivores tend to be most strongly linked to quality of the forage available. In highly seasonal environments, such as mountain areas, both plant quality and available biomass may act as constraints. However, studies addressing the nutritional basis of diet selection of wild large herbivores under harsh conditions in sufficiently large spatial and temporal frameworks are scarce. We studied the functional importance of relative variability in plant quality and biomass for diet selection by a migratory population of Alpine red deer (Cervus elaphus) at the landscape scale and across an annual cycle. Botanical diet composition at plant group level did not show a particular ‘Alpine pattern’ but was similar to known patterns from lowland areas. Sources of variability were season, habitat (either open land or forest) and sex. Red deer foraged selectively in all seasons, and preferences for plant groups were negatively linked to plant abundances. Use and selection of plant groups were associated with high nutritional value (high crude protein and organic matter, low fibre), but partly also with high levels of active tannins. In the cold season, deer made strong nocturnal use of fertilized valley floor meadows offering high-quality grass, but still showed some selection for tannin- and fibre-rich coniferous browse, indicating a need for supplementing grass intake. Altogether, the nutritional value of the diet exceeded that of the forage available in the forested habitat, which was at or below the lower threshold for fulfilling metabolic needs of red deer. High-quality grass on farmed meadows may thus be a critical source of food in mountainous areas during winter.