Nothofagus pumilio (Poepp. et Endl.) Krasser is a broadleaved deciduous tree that dominates high-elevation forests in the southern Andes. We evaluated the degree to which differences in stomatal density and physiological traits (net assimilation, conductance and water use efficiency) were related to environmental and genetic influences with elevation by comparing plants growing under field and common garden conditions. Low-elevation leaves under field conditions had fewer stomata, although this pattern was not maintained in the common garden. Assimilation rates were >40% higher for high-elevation plants in the field, and 18% higher in the common garden, than those for low-elevation plants. In addition, under field conditions high-elevation plants tended to have higher stomatal conductance and lower instantaneous water use efficiency than did low-elevation plants; however, these differences were not significant in the common garden. Thus, assimilation seems to be under genetic control whereas ecophysiological traits related to the use of water appear to be more responsive to environmental cues. Our results suggest that plants growing along elevational gradients may show complex ecophysiological patterns. These patterns may be acquired by genetically driven responses to conditions that are fixed throughout the life span of individuals, such as soil nutrients. Also plastic adjustments may favour opportunistic use of available water during the dry season, particularly under Mediterranean-type climate regimes with summer drought.