[Tree-ring δC and water use efficiency of Platycladus orientalis in mountains of Beijing].

Water use efficiency (WUE) is different among species and regions. Few literatures have been reviewed related to long-term WUE of Platycladus orientalis in mountainous areas of Beijing, China. Tree-ring δC of P. orientalis was used to determine the long-term variation of annual intrinsic water use efficiency (WUE) and its response to environmental change. Combining with quantification of tree-ring width, the relationship between net carbon sequestration and WUE of P. orientalis was eventually explored. The results showed that mean annual temperature increased with the increase of time from 1918 to 2013, whereas annual precipitation fiercely fluctuated. Tree-ring δC decreased and WUE increased over time. WUE was positively related and more sensitive to air temperature increasing than temperature decreasing. Correlation between WUE and fluctuated annualprecipitation was ambiguous, which indicated the precipitation was not the main factor affecting WUE. The de-trend tree-ring width of P. orientalis increased initially and then decreased, especially in recent 20 years. According to the correlation between WUE and environmental factors, temperature resulted in stomatal conductance (g) decreasing, which caused a reduction in evapotranspiration and an increase in respiratory loss, leading to the increase of WUE and a down trend in net carbon sequestration and tree growth.