With global warming and increasing human activities, exploring the impact of the rising atmospheric carbon dioxide concentration and climate change on forest ecosystems is crucial. In this study, we focus on Euphrates poplar (Populus euphratica Oliv.) in the upper reaches of the Tarim River in the Alaer region of Xinjiang. We use dendrochronological methods, tree-ring width, and stable carbon isotope series to explain basal area increment (BAI) and intrinsic water use efficiency (iWUE) changes. We further explore the influence of past climate change and human activities on the radial growth and iWUE of P. euphratica through stable oxygen isotope analysis combined with historical literature records. The results showed that relative humidity had an essential effect on Δ¹³C and δ¹⁸O fractionation in P. euphratica tree rings, whereas the vapor pressure deficit (VPD) was considered the main factor influencing the inter-annual variability of the iWUE and BAI. Since 1850, long-term variations in iWUE have exhibited an upward trajectory correlated with rising atmospheric CO₂ levels. Approximately 13% of this iWUE increase can be attributed to changes in carbon-concentration-induced water use efficiency (^cciWUE). Although Δ¹³C and δ¹⁸O were generally uncorrelated between 1850 and 2018, around 1918, their relationship changed from being weakly correlated to being significantly negatively correlated, which may record changes related to the upstream Tarim River diversion. During the period from 1850 to 2018, both the BAI and iWUE showed an increasing trend for P. euphratica growth; however, the relationship between them was not stable: during 1850–1958, both variables were mainly influenced by climatic factors, while during 1959–2018, the most important influence was due to human activities, specifically agricultural development and irrigation diversions. An abrupt surge in the BAI was observed from 1959 to 1982, reaching its peak around 1982. Surprisingly, post-1983, the escalating iWUE did not correspond with a continuation of this upward trajectory in the BAI, highlighting a divergence from the previous trend where the enhanced iWUE no longer facilitated the growth of P. euphratica. Despite P. euphratica having adapted to the continuously rising C_a, improving its iWUE and growth capacity, this adaptive ability is unstable and may easily be affected by human activities. Overall, the increase in C_a has increased the iWUE of P. euphratica and promoted its growth at a low frequency, while human activities have promoted its development at a high frequency.

Reconstruction of relative humidity changes in the upper Tarim River using carbon isotopic tree-ring chronology bridges the gap in historical observations on the Tarim River Basin in Arid Central Asia. Populus euphratica Olivier (P. euphratica), growing in the Tarim River Basin of Xinjiang, is an excellent record of past climate change. Based on precise dating, we analysed alpha-cellulose stable carbon isotopes in four cores of P. euphratica taken from the Alaer region of the upper Tarim River Basin. The four stable carbon isotope series records were corrected by the “pin method” and then combined into a carbon isotopic discrimination (Δ¹³C) series by the “numerical mix method”. The discrimination (Δ¹³C) series were clearly correlated with the mean relative humidity (RH_AS) in April–September of the growing season (n = 60, r = −0.78, p < 0.001), and according to the climate response analysis, we designed a simple regression equation to reconstruct the mean relative humidity (RH_AS) in April–September from 1824 to 2018 on the Alaer region. The reconstructed sequence showed mainly dry periods in the last 195 years, 1857–1866 and 1899–1907, while primarily wet periods from 1985 to 2016. Due to increased global warming and human activities, the climate shifted from “warm–dry” to “warm–wet” in the mid-to-late 1980s, when there were signs of a shift from “warm–wet” to “warm–dry” in the 2010s, with an increasing trend towards aridity. The RH_AS series of Alaer compares well to other hydroclimate series’ surrounding the research area, and the spatial correlation analysis indicates that the reconstructed series has good regional representativeness. On an interdecadal scale, the revamped RH_AS series is positively correlated with the Atlantic Multidecadal Oscillation (AMO) and negatively correlated with the North Atlantic Oscillation (NAO), reflecting the influence of westerly circulation on regional wet and dry variability. At the same time, the RH_AS may also be influenced by The Pacific Decadal Oscillation (PDO).

Since the 1950s, divergence problems have reduced the temporal stability of tree rings in response to climate, shaken the foundations of dendroclimatology, and affected the reliability of reconstructed models based on tree rings and the accuracy of historical climate series. Therefore, it is of great importance to investigate divergence problems, which will help us to better understand the growth strategies of trees in response to climate warming and provide a scientific basis for accurate climate reconstruction and simulation of forest dynamics. This paper aims to elucidate the mechanism of divergent growth of Picea schrenkiana at high altitudes in the western Tianshan from three aspects: variations in atmospheric circulation, changes in climatic factors, and the coping strategy of trees with climate change. High spring temperatures accelerate the melting of snow cover. Large amounts of snowmelt initially replenish soil water, leading to the rapid growth of trees, but later, the continuous consumption of snowmelt reduces the available water capacity of soil, resulting in water stress on trees. The pattern of trees’ response to changes in climate ranges from a pure temperature limitation to a collaborative temperature–moisture limitation. Since the 1990s, the weakening of the westerly circulation and the North Atlantic Oscillation has reduced their impacts on the hydroclimate in the Tianshan Mountains of Central Asia. The combination of heat-induced water deficit and a long-term weakening trend in atmospheric circulation has slowed tree growth over the past decade.