Water availability is not generally considered the most limiting resource at high elevations, especially at timberline area due to the higher soil water content and more abundant precipitation, which may result in no difference in leaf δ13C among plants. Variations in leaf δ13C in conifer, evergreen and deciduous shrubs and forbs were studied at the timberline in Sergyemla mountain, on the south-eastern edge of the Tibetan Plateau. We aimed at investigating the water use strategy at both plant species and life-form (tree, shrub or forb) levels to determine if life form could be used to group plants with similar physiological characteristics, e.g. the balance between CO2 and water fluxes under extremely environmental conditions. The results suggested that the δ13C values of 31 species belonging to 18 families and 28 genera ranged from -30.24‰ to -25.39‰, with a mean of -27.68‰, showing that all species perform photosynthesis through the C3 pathway. At the species level, no significant differences in δ13C values were found in evergreen shrubs (Rhododendron wardii, R. pingianum) or deciduous shrubs (Sorbus rehderiana, Salix oritrepha and Ribes glaciale) (P>0.05). However, life form-specific difference in δ13C values was highly significant (P < 0.001), with the sequence of evergreen trees (-27.27‰) > evergreen shrubs (-27.56‰) > deciduous shrubs (-27.93‰) and forbs (-2791‰). The results indicated that although soil water availability is high at the timberline, significant differences in isotope fractionation occur in individuals, suggesting different water use efficiency (WUE). There is no difference in δ13C in plants with the same life forms but a significant difference between different life forms, suggesting that the variation of WUE is dependent on life form. The δ13C values of different life forms could be an integrated indicator of changes in water use strategy among functional groups at the timberline.