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The variation of plant water use efficiency (WUE) with water availability has two interacting components: a plastic response, evident when individuals of the same genotype are compared (e.g. wet versus dry years), and an interspecific response, evident when different species living in habitats with different water availability are compared. We analysed the WUE of 25 Patagonian species that belong to four life forms (grasses, shrubs, annual herbs and perennial herbs) in relation to the climatic conditions of 2 years and the mean historic water availability experienced by each species. To estimate water availability, we calculated the effective soil water potential (EWP) of each species, based on available information about soil water dynamics, phenology and root system structure. To estimate WUE, we used isotopic discrimination of leaf C (Delta C-13) and mean annual water vapour difference between leaves and atmosphere (Delta e) measured in situ. For the plastic response, for every species and life form, WUE increased from the dry to the wet year. We hypothesize that photosynthesis was less nutrient limited in the wet than in the dry year, facilitating higher net photosynthesis rates per unit of stomatal conductance in the wet year. For the interspecific response, WUE was lower in species native to drier habitats than in species native to wetter habitats. This response was mostly accounted for by a decrease in Delta e with EWP. Annual herbs, which avoid drought in time (they have the earliest growth cycle), and shrubs, which avoid drought in space (they have the deepest roots), showed the highest EWP and WUE. We conclude that the conventional wisdom which states that the highest WUE occurs within a species during the driest years, and among species in the driest habitats, does not always hold true, and that co-existing life forms drastically differ in water availability and water economy.