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Previous studies have suggested foliar delta C-13 generally increases with altitude. However, some observations reported no changes or even decreased trends in foliar delta C-13. We noted that all the studies in which delta C-13 increased with elevation were conducted in the human regions, whereas those investigations in which delta C-13 did not vary or decreased were conducted in areas with water stress. Thus, we proposed that the pattern of increasing delta C-13 with elevation is not a general one, and that delta C-13 may remain unchanged or decrease in plants grown in arid environments. To test the hypothesis, we sampled plants along altitude gradients on the shady and sunny slopes of Mount Tianshan characterized by arid and semiarid climates. The measurements of foliar delta C-13 showed no altitudinal trends for the plants grown on either of the slopes. Therefore, this study supported our hypothesis. In addition, the present study addressed the effect of atmospheric pressure on plant delta C-13 by accounting for the effects of temperature and precipitation on delta C-13. This study found that the residual foliar delta C-13 increased with increasing altitude, suggesting that atmospheric pressure played a negative role in foliar delta C-13.