We determined vertical oxygen isotope gradients of leaf organic matter for a grassland in Switzerland and a mountain beech forest (Fagus sylvatica) in Northern Italy. A distinctly positive (18)O/(16)O gradient with height above ground was found for the grassland (7.9/1000 m(-1), p < 0.001), whereas the gradient was negative for the forest (-0.077/1000 m(-1), p < 0.001). The results are consistent with microclimatic measurements, although large isotope variations between the species have to be taken into account for the grassland. A conceptual scheme is shown which relates the isotope enrichment to the canopy density, considering the effects of transpiration and canopy structure. We conclude that the analysis of the within canopy variation in delta(18)O of organic matter can be used to provide long-term estimates of leaf water isotope composition, thus improving existing isotope methods to determine the gas-exchange between vegetation and atmosphere.