Abstract: By using C-isotope(leaf δ¹³C value) as an index of long-term water use efficiency(WUE), we evaluated WUE of three typical trees of Pinus sylvestris var. mongolica(P.sylvestris), P.tabulaeformis(P.tabulaeformis) and Q.mongolica Fischer ex Ledebour(Q.mongolica) in Liaoning Province and their responses to environmental factors involving annual average precipitation, annual average air temperature and altitude. The δ¹³C values of these three trees were -30.37‰- -25.10‰, -30.32‰- -24.07‰ and -29.85‰- -23.51‰, respectively, and the δ¹³C values in three trees leaf were decreased with the longitudes. By statistical analysis, δ¹³C of three trees were evident higher in western part of Liaoning province than the eastern part, indicating that trees in western Liaoning was higher. Further analysis found that the δ¹³C values in three trees leaf were greatly influenced by the average annual precipitation and altitude, and decreased significantly with annual average precipitation, and increased with altitude, but the δ¹³C values in three trees leaf were less influenced by the temperature. The WUE were much robust influenced by the altitude in P.sylvestris(P<0.01). Besides, much more significant effective in WUE-precipitation was found in P.sylvestris and P.tabulaeformis compared with Q.mongolica. Therefore, it is difficult to identify WUE inter-species differences based on leaf δ¹³C, which were greatly response to environmental factors as rainfall and altitude. Our findings will lay the foundation for future scientific studies of WUE based on the δ¹³C.

Key words: δ¹³C, water use efficiency, precipitation, air temperature, altitude