The relative importance of winter harshness and early summer foraging conditions are of prime interest when assessing the effect of global warming on Arctic and mountainous ecosystems. We explored how climate and vegetation onset (satellite-derived normalized difference vegetation index data) determined individual performance in three reindeer populations (data on 27814 calves sampled over 11 years). Snow conditions, spring temperatures and topography were the main determinants of the onset of the vegetation. An earlier onset positively affected the body mass of calves born the following autumn, while there was no significant direct negative impact of the previous winter. This study underlines the major impact of winter and spring climatic conditions, determining the spring and summer food availability, and the subsequent growth of calves among alpine herbivores.