ORIGINAL PAPER Climate warming and increasing atmospheric CO 2 have contributed to increased intrinsic water-use efficiency on the northeastern Tibetan Plateau since 1850 Guobao Xu • Xiaohong Liu • Dahe Qin • Tuo Chen • Wenling An • Wenzhi Wang • Guoju Wu • Xiaomin Zeng • Jiawen Ren Received: 20 March 2012 / Revised: 27 January 2013 / Accepted: 31 January 2013 / Published online: 20 February 2013 Ó Springer-Verlag Berlin Heidelberg 2013 Abstract We investigated the physiological responses of Tibetan juniper (Sabina tibetica) to changes in the atmo- spheric CO 2 concentration (C a ) and climate on the north- eastern Tibetan Plateau based on annual tree-ring d 13 C values since 1850. Intrinsic water-use efficiency (iWUE) increased, and the internal to ambient CO 2 ratio (C i /C a ) showed no significant trend from 1895 to 1974 in the study region, indicating an active response to changing C a . The long-term trends in iWUE in the naturally occurring trees were mainly caused by the anthropogenic increase in C a . However, from 1975 to 2002, iWUE increased rapidly at the study site (by 12.4 % compared with the overall mean from 1850 to 2002), which is greater than the expected increase due only to an active response to C a . Our analysis showed that decreased water availability caused by greater evaporation due to decreased precipitation and a warming growth environment from 1975 to 2002 may have reduced stomatal conductance, leading to a higher iWUE. The warming climate and increased C a accounted for 83.6 % of the variance in iWUE of Tibetan juniper on the northeast- ern Tibetan Plateau from 1975 to 2002. Keywords Sabina tibetica  Intrinsic water-use efficiency  Climate warming  Tree-ring d 13 C  Ecophysiological parameters Introduction Trees show a physiological response to their environment through e.g. increased intrinsic water-use efficiency (iWUE) (Andreu-Hayles et al. 2011) or accelerated growth (Silva et al. 2010). Based on the carbon isotope discrimination (D) by biological processes, which affects the d 13 C in tree rings, long-term changes in iWUE can be evaluated (Saurer et al. 2004). Some studies have suggested that iWUE in trees changes in response to variations in water availability (e.g. drought), the ratio of the intercellular (C i ) to atmospheric CO 2 concentrations (C i /C a ) or increasing atmospheric CO 2 con- centration (C a ) (Duquesnay et al. 1998; Feng 1999; Leavitt et al. 2003, 2010). Therefore, iWUE, inferred from changes in tree-ring d 13 C, can give insight into how naturally growing trees respond, or have responded to increased C a and climate change (Farquhar et al. 1982; Duquesnay et al. 1998; Feng 1998, 1999; Tang et al. 1999; Waterhouse et al. 2004). The Tibetan Plateau is sensitive to climate change and is an ecologically fragile zone (Sun and Zheng 1998). Tem- peratures on the Tibetan Plateau increased throughout the 20th century, especially since the 1970s (Wang et al. 1998; Wang and Gaffen 2001). This trend has caused large changes in the water and heat balances of trees, which in turn has affected their growth and iWUE. However, little research on the Tibetan Plateau has described the long-term Communicated by A. Braeuning. Special topic: Dendroecology in Asia. Electronic supplementary material The online version of this article (doi:10.1007/s00468-013-0855-3) contains supplementary material, which is available to authorized users. G. Xu  X. Liu (&)  D. Qin  T. Chen  W. An  W. Wang  G. Wu  X. Zeng  J. Ren State Key Laboratory of Cryospheric Sciences, Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences, Donggang West Road No. 320, Lanzhou 730000, China e-mail: liuxh@lzb.ac.cn G. Xu  W. An  W. Wang  G. Wu  X. Zeng University of Chinese Academy of Sciences, Beijing 100049, China 123 Trees (2013) 27:465–475 DOI 10.1007/s00468-013-0855-3