Climate warming and increasing atmospheric CO2 have contributed to increased intrinsic water-use efficiency on the northeastern Tibetan Plateau since 1850

We investigated the physiological responses of Tibetan juniper (Sabina tibetica) to changes in the atmospheric CO2 concentration (C a) and climate on the northeastern Tibetan Plateau based on annual tree-ring δ13C values since 1850. Intrinsic water-use efficiency (iWUE) increased, and the internal to ambient CO2 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 northeastern Tibetan Plateau from 1975 to 2002.