title: Tree growth and intrinsic water-use efficiency of inland riparian forests in northwestern China: evaluation via δ13C and δ18O analysis of tree rings.
abstract: The rising atmospheric CO2 concentration (Ca) 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 CO2 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 CO2 (Ci/Ca) suggest a specific adaptation of tree physiology to increasing CO2. 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 CO2 stimulation of riparian tree growth, which compensated for the negative influences of reductions in river streamflow and a drying climate during the study period.
authors.affiliation: State Key Laboratory of Cryospheric Sciences, Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences, No. 320 Donggang West Road, Lanzhou 730000, China liuxh@lzb.ac.cn.
authors.forename: Xiaohong
authors.initials: X
authors.lastname: Liu
authors.affiliation: State Key Laboratory of Cryospheric Sciences, Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences, No. 320 Donggang West Road, Lanzhou 730000, China University of the Chinese Academy of Sciences, Beijing 100049, China.
authors.forename: Wenzhi
authors.initials: W
authors.lastname: Wang
authors.affiliation: State Key Laboratory of Cryospheric Sciences, Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences, No. 320 Donggang West Road, Lanzhou 730000, China University of the Chinese Academy of Sciences, Beijing 100049, China.
authors.forename: Guobao
authors.initials: G
authors.lastname: Xu
authors.affiliation: State Key Laboratory of Cryospheric Sciences, Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences, No. 320 Donggang West Road, Lanzhou 730000, China University of the Chinese Academy of Sciences, Beijing 100049, China.
authors.forename: Xiaomin
authors.initials: X
authors.lastname: Zeng
authors.affiliation: State Key Laboratory of Cryospheric Sciences, Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences, No. 320 Donggang West Road, Lanzhou 730000, China University of the Chinese Academy of Sciences, Beijing 100049, China.
authors.forename: Guoju
authors.initials: G
authors.lastname: Wu
authors.affiliation: State Key Laboratory of Cryospheric Sciences, Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences, No. 320 Donggang West Road, Lanzhou 730000, China University of the Chinese Academy of Sciences, Beijing 100049, China.
authors.forename: Xuanwen
authors.initials: X
authors.lastname: Zhang
authors.affiliation: State Key Laboratory of Cryospheric Sciences, Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences, No. 320 Donggang West Road, Lanzhou 730000, China.
authors.forename: Dahe
authors.initials: D
authors.lastname: Qin
mesh_terms: D002245:Carbon Dioxide / Q000378:metabolism*; D002247:Carbon Isotopes / Q000032:analysis; D002681:China; D065928:Forests; D010103:Oxygen Isotopes / Q000032:analysis; D032107:Populus / Q000254:growth & development* / Q000378:metabolism*; D013997:Time Factors; D014867:Water / Q000378:metabolism*
chemical_list: D002247:Carbon Isotopes; D010103:Oxygen Isotopes; D014867:Water; D002245:Carbon Dioxide
keywords: CO2 fertilization; stable isotopes