The Loess Plateau is a fragile ecological zone that is sensitive to climate change. The response, adaptation, and feedback of tree growth in forest ecosystems to global warming and CO2 enrichment are urgent scientific issues. Intrinsic water use efficiency (iWUE) is an important indicator for understanding forest ecosystem adaptability to climate change and CO2 enrichment. In this study, tree-ring width, tree-ring stable carbon isotope ratio (δ13C), and iWUE of P. tabulaeformis Carr. were established. Climate response analysis showed that temperature was the main limiting factor affecting radial tree growth and that relative humidity significantly affected the stable carbon isotope fractionation of tree rings. During 1645–2011, the iWUE increased by 27.1%. The responses of iWUE to climate factors and atmospheric CO2 concentrations (Ca) showed that the long-term variation in iWUE was affected by Ca, which could explain 69% of iWUE variation, and temperature was the main factor causing iWUE interannual variation. The ecosystem of P. tabulaeformis showed a positive response to rising Ca, as its carbon sequestration capacity increased. In response to global warming and CO2 enrichment, rising Ca promoted increases in iWUE but ultimately failed to offset the negative impact of warming on tree growth in the study area.

To explore the history of the changes in monsoon precipitation and their driving mechanisms in the context of global warming, climatology studies using tree-ring stable oxygen isotopes (δ18O) were carried out in Shanxi Province, China. Based on a tree-ring δ18O series from Pinus tabulaeformis Carr. on Heng Mountain, a 230-year June‒July precipitation sequence from 1784 to 2013 AD was reconstructed that explained 45% of the total variance (44% after adjusting the degrees of freedom). The reconstructed sequence captured the characteristics of the variations in precipitation. Periods of drought occurred mainly in 1820‒1840 AD, 1855‒1865 AD, 1895‒1910 AD, 1925‒1930 AD, and 1970‒1995 AD, and wet periods occurred mainly in 1880‒1895 AD, 1910‒1925 AD, and 1935‒1960 AD. The dry and wet years in the precipitation reconstruction corresponded well to the years in which disaster events were documented in historical records. A spatial correlation analysis with Climatic Research Unit (CRU)-gridded precipitation data indicated that the reconstructed precipitation provided good regional representation and reflected large-scale June‒July precipitation changes in northern China. In addition, the reconstructed precipitation sequence was also significantly correlated with the dry and wet index (DWI) and other tree-ring dry/wet reconstructions from the surrounding areas. The correlation between the reconstructed precipitation and the Asian monsoon index showed that the precipitation can indicate the intensity of the Asian summer monsoon. Moreover, a significant negative correlation was found between the El Niño–Southern Oscillation (ENSO) and the reconstructed precipitation. At the decadal scale, the negative phase of the Pacific Decadal Oscillation (PDO) and the positive phase of the Atlantic Multidecadal Oscillation (AMO) may co-promote summer precipitation in the study area.

Paleoclimatic data has often been applied to demonstrate the influence of anthropogenic activities on runoff to desert rivers, and such features as reservoirs are clearly known to influence available water in downstream riparian settings. The impact of human activities on the hydrology of a downstream desert oasis is, however, an open question. High‐resolution hydroclimate paleo‐reconstructions for desert oases are lacking, partly because paleoclimate proxies in extremely dry deserts are insensitive to extremely low precipitation. Here, we first attempt to reconstruct regional precipitation (r = −0.743, p < 0.001, n = 49) for the interval 1886–2009 from the stable oxygen isotope ratio (δ18O) of five Populus euphratica trees growing in the Ejina Oasis, on the lower reaches of the Heihe River in extremely arid Northwest China. The δ18O series for P. euphratica in the oasis (∼900 m a.s.l) abruptly increases relative to an δ18O series of Qinghai spruce (Picea crassifolia) growing in upper limit of forest (∼3,000 m a.s.l) in Longshou Mountain (middle reaches of the Heihe River) after about 2000, when the Ecological Water Diversion Project on the Heihe River was implemented. This finding implies that the Ejina Oasis suffered from serious drought during the last decade, or at least that the Oasis is stressed more than expected by current climate conditions. Other evidence also indicates that human activities contributed to a decrease in air moisture in the Ejina Oasis after 2000. To mitigate water stress on the oasis, we recommend some practical measures to ensure the rational development of the desert oasis.