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Many studies have documented that the delta C-13 values of plants increase with altitude both on a global scale and locally in humid climates, while in semi-arid areas the opposite trend has been found. The study reported herein was conducted in a mountainous area of China characterized by a temperate semi-humid climate. The delta C-13 values of C-3 species do not exhibit a consistent variation along an altitudinal gradient and the observations suggest that the pattern of increasing delta C-13 with altitude cannot be generalized. In the study area, in addition to environmental factors such as changing air pressure and light, the interaction between temperature and plant water balance determines the delta C-13-altitude variations in C-3 plants. The delta C-13 of the leaves of C-4 plants is found to increase with altitude with a mean gradient of 0.9 parts per thousand/km. The altitudinal trend of C-4 plants is attributed to the combined influences of water availability and other factors rather than temperature. Copyright (C) 2010 John Wiley & Sons, Ltd.