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The rising atmospheric CO 2  concentration ( C  a ) has increased tree growth and intrinsic water-use efficiency (iWUE). However, the magnitude of this effect on long-term iWUE and whether this increase could stimulate the growth of riparian forests in extremely arid regions remain poorly understood. We investigated the relationship between growth [ring width; basal area increment (BAI)] and iWUE in a riparian  Populus euphratica  Oliv. forest to test whether growth was enhanced by increasing CO 2  and whether this compensated for environmental stresses in the lower reaches of the inland Heihe River, northwestern China. We accomplished this using dendrochronological methods and carbon (δ 13 C) and oxygen (δ 18 O) isotopic analysis. We found an increase in BAI before 1958, followed by a decrease from 1958 to 1977 and an increase to a peak around 2000. Tree-ring carbon discrimination (Δ) and δ 18 O indicated significant negative overall trends from 1920 to 2012. However, the relationship shifted in strength and direction around 1977 from significantly negative to a weak connection. The seasonal minimum temperature in April to July showed strong influence on Δ, and δ 18 O was controlled by relative humidity (negatively correlated) and temperature (positively correlated) in June and July. The patterns of internal to atmospheric CO 2  ( C  i  /C  a ) suggest a specific adaptation of tree physiology to increasing CO 2 . Intrinsic water-use efficiency increased significantly (by 36.4%) during the study period. The increased iWUE explained 19.8 and 39.1% of the observed yearly and high-frequency (first-order difference) variations in BAI, respectively, after 1977. Our results suggest significant CO 2  stimulation of riparian tree growth, which compensated for the negative influences of reductions in river streamflow and a drying climate during the study period.

tree physiology

Tree growth and intrinsic water-use efficiency of inland riparian forests in northwestern China: evaluation via δ 13 C and δ 18 O analysis of tree rings