Samples of two moss species, Sphagnum and Polytrichum, were collected at 19 points selected along vertical transects in the Karkonosze Nits. (739-1393 m) and the Izerskie Mts. (500-1100 m), SW Poland. Continuous automatic monitoring of temperature and humidity at the sampling points was carried out (I h intervals) through the entire growth season (2004). A strong correlation between delta C-13 value of plant tissue cellulose and mean air temperature was noted for both areas and sampling periods (R-2 between 0.92 and 0.64). It was calculated that a 1 degrees C increase in air temperature during the growing season results in a -1.6 parts per thousand. (Sphagnum) and a -1.5 parts per thousand. (Polytrichum) decrease in delta C-13. In contrast, it seems that humidity shows no clear influence on the carbon isotope composition of mosses. (c) 2007 Elsevier Ltd. All rights reserved.