The Alxa Plateau is located to the north of the Hexi Corridor in China's Gansu Province, and west of the Inner Mongolia Plateau,and south of the Gobi Altai Mountain (Fig. 1). Three deserts distribute in this area, including of the Tengger Desert, the Ulan Buh Desert, and the Badain Jaran Desert. Several mountain ranges surround and span the Alxa Plateau. These include the Helan Mountains to the east, the northern mountains of the Hexi Corridor to the south, the outliers of the Altai Mountains to the north, and a series of denuded lower mountain ranges of the Yabulai, Nurgong, Zongnai and Lang mountains in the middle. These mountains not only block the desert's eastward and southward expansion (driven by high-pressure regions from Mongolia), but also form the headwaters for rivers and streams that water the oases on the plateau (Xiao et al., 2024a) (Fig. 1).

The Alxa Plateau is located in the eastern margin of the inland, arid region of Central Asia. Many studies showed that this area is the main interaction zone of the mid-latitude westerly circulation and the Asian monsoon (Xiao et al., 2017; Chen et al., 2019b; Xiao et al., 2024a, b). Its pronounced interannual variability effects on the vegetation cover changes (Ou and Qian, 2006; Tang et al., 2006a, b; Li et al., 2013; Ren et al., 2026).

The Alxa Plateau has a typical continental climate. We have divided the study area into four regions based on geographical distribution characteristics of the three major deserts and the sampling points (Fig. 1): the Yabulai-Nurgong low mountain area (RN), the Ulan Buh Desert area (RW), the Badain Jaran Desert area (RY), and the Tengger Desert area (RT).

Meteorological data for the study regions RN, RY, RW, and RT were obtained from the nearest stations at Nurgong, Youqi, Jilantai, and Zhongwei, respectively. The annual mean temperatures at these stations are 7.4, 8.9, 8.8, and 9.0 °C, respectively (http://data.cma.cn, last access: 26 October 2026). The multi-year average annual precipitation is 103.0, 118.0, 106.9, and 182.9 mm, respectively. Notably, 82 %–88 % of the precipitation concentrated from May to September (Fig. 1).

Z. xanthoxylum is a super-xerophilous and soil-stabilizing species endemic to Central Asia's deserts, and often serves as a dominant or constructive species in the gravelly riparian terraces, gently sloping hills, and pre-hill flood fans in the Alxa Desert (Xiao et al., 2012; Chen et al., 2016). Z. xanthoxylum has well-defined annual growth rings, making it suitable for tree-ring study (Xiao et al., 2019; Ding et al., 2021). Recently, shrub-ring data of this species have been used to conduct climate reconstruction and ecological response studies in the Badain Jaran, Tengger and Ulan Buh deserts, providing a new perspective for studying climate and environmental changes in arid regions (Xiao et al., 2012, 2019; Ding et al., 2021; Peng et al., 2024; Ding et al., 2025).

Regional chronologies exhibit high spatial heterogeneity in interannual variability (Fig. 2). At the decadal scale (11-year moving average), consistent changes are evident in the four regional chronologies around the 1890s, late 1910s to late 1950s, and late 1980s to late 2000s. The period from late 1910s to late 1950s represents a low-growth phase, while the other two periods correspond to consistent high-growth phases.

Correlation analysis between the four regional chronologies and the climate variables reveals spatially heterogeneous correlation patterns. The climate variables include monthly mean temperature, monthly precipitation, and SPEI at different scales (1962–2017). Correlation analysis of SPEI across scales from 1 to 12 months revealed the greatest significance at the 9-month scale (Fig. 3).

Overall, the four regional chronologies exhibit a positive correlation with the SPEI from September of the previous year to September of the current year, always achieving significant levels in the growing season. The four regional chronologies also significantly correlate with the average 9-month scale SPEI from April to August. A predominantly positive correlation was observed between the four regional chronologies and monthly precipitation during the growing season, reaching significant levels, particularly in the RN, RW, and RT, from April to June, with most months achieving significance levels. A significant positive correlation with monthly mean temperature was evident in the RW in February and September of the current year, while the other three chronologies exhibited weak negative correlations with monthly mean temperatures during the growing season.

These findings indicate that radial growth variation of Z. xanthoxylum on the Alxa Desert Plateau is primarily constrained by climatic aridity, resulting from low precipitation during the growing season.

To investigate the impact of two major circulation systems on the radial growth of desert shrubs on the Alxa Plateau, correlation analyses were conducted at the interannual scale, using the entire regional chronology and grouped circulation indices. The results indicate that no significant correlations were observed among the four regional chronologies or their groupings at the interannual scale (Fig. 4). Analysis of the two major circulation indices revealed relatively consistent correlations between the medium index group and the four regional chronology indices: negative with WCI, and positive with EASMI. The low WCI index group showed positive correlations with RN and RT, and negative correlations with RW and RY. The high WCI index group exhibited positive correlations with the four regional chronologies. The low EASMI index group exhibited negative correlations with the four regional chronologies. For the high EASMI index group, negative correlations were observed with all four regional chronologies, except RT.

Overall, shrub diameter growth in the medium index group showed a negative correlation with WCI, and a positive correlation with EASMI. For the high and low index groups, however, shrub growth showed inconsistent correlations with WCI and EASMI.

Correlation analysis between the two circulation indices and the four regional chronologies at the interdecadal scale (Fig. 5) indicates a negative correlation between the four chronologies and the EASMI (p < 0.01), especially in the RW and RN. For the WCI, the RW and RN showed a significant positive correlation. Conversely, the RT and RY showed insignificant negative correlations.

Further partial correlation analysis of the two circulation indices and four regional chronologies at the decadal scale revealed that all four chronologies exhibited significantly negative correlations with EASMI when controlling for the WCI variable. Controlling for the EASMI variable produced significant positive correlations between the RN and RW in the northern Alxa Plateau and the WCI, while the RY and RT in the southern Alxa Plateau showed negative correlations, with only the RY reaching statistical significance (see Table 1).

As shown in Fig. 5, the WCI exhibits four discernible phases on a decadal scale: a gradual strengthening phase (1956–1963), followed by a weakening phase (1964–1979), a renewed strengthening period (1980–1994), and a subsequent weakening phase (1995–2011). The highest index values occurred primarily during two intervals: 1962–1967 and 1989–2007. In contrast, the lowest values were observed between 1971–1985 and 1956–1958.

In addition, we applied SEM to analyze the relationships among the two circulation indices, SPEI9, and four regional chronologies at both interannual and decadal (11-year moving average) scales. The results show correlation patterns and significance levels that are generally consistent with the aforementioned results (Fig. S1).

The EASMI exhibited a strengthening phase from 1955 to 1963, followed by a weakening phase from 1964 to 1993, another strengthening phase from 1994 to 1982, and then a weakening phase from 1983 to 2011. During the entire study period, the strongest EASMI occurred from 1955 to 1978, while the weakest values were observed from 1990 to 1998 and from 2011 to 2012.

Four patterns can be identified based on the intensity characteristics of the two circulation systems over the decadal scale: both circulations are strong (e.g., 1960–1966); both circulations are weak (e.g., 1980–1988 and 2005–2015); the WCI is strong and the EASMI is weak (e.g., 1990–1998); and the WCI is weak and the EASMI is strong (e.g., 1955–1959) (Fig. 5).

When both circulation indices were strong, the chronology indices of the RN, RW, and RY were relatively low, while the RT indices were relatively high. Under the pattern of a strong WCI and weak EASMI, the chronology indices of the RN and RW were relatively high, while those of the RY and RT were relatively low. When both circulation indices were weak, the values across the four chronologies were inconsistent. For example, during 1980–1988, the indices of the RN, RW, and RT showed an upward trend, whereas the RY exhibited a downward trend. Conversely, during 2005–2015, the indices in the RN, RW, and RY declined, while an upward trend was observed in the RT. When the WCI was weak and the EASMI was strong, the chronology indices in the RN, RW, and RT were relatively low, while the RY indices were close to the mean level.

The above results indicate that the radial growth of Z. xanthoxylum in different regions of the Alxa Plateau varies spatially in response to the interplay between the two circulation systems. When the EASMI is weak and the WCI is strong, radial growth is enhanced in the northern parts of the plateau (RN and RW), but suppressed in the southern parts (RY and RT). Conversely, when the EASMI is strong and the WCI is weak, radial growth is generally inhibited across all four regions. When both circulation systems are strong, growth is favorable only in the RT region, and unfavorable in the other three regions. When both circulation systems are weak, radial growth is suppressed in the northern regions (RN and RW), but promoted in the southern regions (RY and RT).

Overall, the study area is predominantly influenced by the EASM. However, the Westerlies exert a significant impact on the central-southern, central-northern, and north-eastern parts of the Alxa Plateau, except in the south-eastern region (RT), where its influence is weaker. Consequently, we propose that the interaction zone between the EASM and the Westerlies is primarily located in the central and northern parts of the Alxa Plateau (see Fig. 6). Furthermore, the northern boundary of the EASM extends approximately 400 km northwestwards to the Yabulai-Nurgong-Lang Mountains line, covering the entire Tengger and Ulan Buh deserts in the process. Meanwhile, the Westerlies can advance eastwards, influencing areas up to the Helan Mountains.

Similar to the findings of other dendroclimatic studies in arid and semi-arid regions (Xiao et al., 2012, 2019, 2024a; Ding et al., 2025), the radial growth of Z. xanthoxylum in the four regions of the Alxa Plateau is primarily limited by low precipitation and high temperatures. There is a significant positive correlation between radial growth and the growing-season drought index. Consequently, the four regional chronologies primarily reflect regional moisture variations (Fig. 4).

On an interannual scale, the four regions exhibit coherent dry/wet patterns, as well as considerable spatiotemporal heterogeneity (Fig. 2). Arid conditions dominate for a larger proportion of the time across the four regions. This is particularly evident in northern Alxa Plateau, where the longest continuous drought occurred from the 1910s to the 1980s, with the most severe drought happening during the 1850s–1860s. A relatively consistent humid phase occurred mainly from the late 1980s to the late 2000s across the four regions, though the timing, duration, and intensity of this phase varied between regions. These characteristics reflect the inherently arid nature of the inland desert climate.

On a broader (interdecadal) scale, an extreme drought period in the 1920s–1930s was shared by much of northern China (Liang et al., 2006; Fang et al., 2009, 2010). On a decadal scale, a comparison of the four desert chronologies from the Alxa Plateau and three chronologies from the surrounding mountains (Xiao et al., 2024a) indicates that all of them experienced a sustained dry phase from the 1920s to the early 1930s (Fig. 7). This drought event persisted for approximately half a century.

A humid phase during the 1990s–2000s was revealed by both the two northern chronologies of Alxa Plateau (RN and RW) and the chronologies from nearby Helan Mountains. This humid period lasted longer and was relatively more pronounced in the desert regions than in the mountains, especially in the northwestern Alxa Plateau (RN). The RN region also recorded a synchronous dry period with the Helan Mountains around the 1860s. In RN region, this drought persisted until the late 1880s, whereas in the Helan Mountains, it transitioned to a humid phase in the mid-1860s, which continued until the mid-1880s (Fig. 7). During the 1900s and 1980s, the two southern chronologies from the Alxa Plateau (RY and RT) indicated periods of increased humidity, which coincided with the northern mountains of the Hexi Corridor (DS). Once again, the desert regions exhibited higher moisture levels.

From the 1930s to the late 1950s, the Alxa Plateau experienced a dry phase, while the surrounding mountains were in a humid phase, reflecting an opposite climatic tendency between the two types of terrain. This opposite climatic condition persisted for nearly half a century, or even longer, in this area. The dry periods captured in the desert chronologies are consistent with earlier single-site climate reconstructions (Xiao et al., 2012, 2019; Ding et al., 2025). These humid periods were also documented in the monsoon-influenced mountainous areas east of the Alxa Plateau, as evidenced by local historical records (Yuan, 1994) and tree-ring chronologies from Mt. Xinglong (Fang et al., 2009; Chen et al., 2013) and Mt. Guiqing (Fang et al., 2010).

The westerly jet axis is situated near 40° N, and its zonal variation generates two pairs of cross-coupled positive and negative anomaly centers, one over the Iranian Plateau and the Mongolian Plateau, and the other over the Eastern Europe and Eastern China. The Alxa Plateau lies at the transitional margin between the Mongolian Plateau and Eastern China anomaly centers. Northward shifts of the westerly jet are generally associated with a slight increase in precipitation over the region, whereas southward shifts correspond to reduced precipitation (Sun et al., 2003; Guo et al., 2025).

When the westerlies intensify, their core shifts southwards, leading to increased precipitation in the central and northern parts of the Alxa Plateau. Conversely, a weakening of the westerlies results in reduced precipitation. At the same time, the cold, dry westerly airflow lowers regional temperatures, thereby reducing vegetation evapotranspiration and alleviating drought stress. Consequently, the radial growth of Z. xanthoxylum in the northern Alxa Plateau (RN and RW) shows a significant positive correlation with the WCI, whereas the two chronologies in the southern Alxa Plateau (RY and RT) show a negative correlation (with RY reaching statistical significance) (Table 1). This finding is further supported by the significant positive correlation observed on a decadal scale between the WCI and the radial growth of P. crassifolia in the Helan Mountains, east to the Alxa Plateau (Xiao et al., 2024a).

Research indicates that some of the moisture carried by the westerlies moves southwards after traversing northern Xinjiang, reaching northern Qinghai, the Hexi Corridor, northern Ningxia, and northern Shaanxi (Li et al., 2012). Western China lies within the zone of the prevailing westerlies, with the majority of moisture transported by these winds (Chen et al., 2008, 2009, 2010; Liu et al., 2017). Analyses indicate that the WCI is a reliable indicator of moisture transport and the moisture budget in northwestern China, primarily in summer. This is based on reanalysis data from the American National Environmental Protection Committee/National Center for Atmospheric Research (Feng et al., 2004; Wang et al., 2005; Li et al., 2008).

When the EASM intensifies, its warm, moist airflows are obstructed by high mountain ranges such as the Helan Mountains and the eastern Qilian Mountains at the monsoon boundary. This orographic lifting leads to precipitation on the windward side, while the descending air on the leeward side becomes warm and dry. This further exacerbates the high temperatures and aridity of the Alxa Plateau. Consequently, the radial growth of Z. xanthoxylum in the four regions of the plateau shows a significant negative correlation with the EASMI (Table 1). Dendroclimatological studies of P. crassifolia in the Helan and Changling Mountains, the southeastern part outside the Alxa, corroborate this mechanistic interpretation by reporting a positive correlation between radial growth and the EASMI on a decadal scale. This confirms the interception of warm, humid monsoon airflows by the surrounding mountains (Xiao et al., 2024a). This phenomenon is referred to as the foehn effect within a rain shadow area (where foehn refers to a type of dry, warm, and down-slope wind). Wind tunnel experiments and simulation indicate that the tropical semi-arid climate and the formation of desertified savanna in western Hainan Island, China, are primarily caused by the foehn effect associated with multiple mountain ranges in the rain shadow region (Li et al., 2006). Meteorological observations on both sides of the Southern Alps in New Zealand have also confirmed the occurrence of lee-side foehn effects resulting from orographic blocking (Mcgowan and Andrew, 1994). The Alxa Plateau is surrounded by high mountain ranges, including the Qilian Mountains, the North Mountains of the Hexi Corridor, the extensions of the Altai Mountains, and the Helan Mountains. These ranges have an average elevation of between 3000 and 4000 m. These ranges block both major atmospheric circulation systems, creating significant foehn effects, which subsequently influence climate change and vegetation growth in the desert regions of the Alxa Plateau.

The southeastern Tengger Desert lies within a gap between the southern Helan Mountains and the eastern Qilian Mountains. This creates a topographic funnel effect that intensifies the interaction between the weakened edges of the two circulation systems. On a decadal scale, when the two circulation systems show contrasting intensities (one strong and one weak), Z. xanthoxylum radial growth in this region tends to be lower (Fig. 5). Conversely, when both circulations are either strong or weak, radial growth tends to be higher. This pattern suggests that a more balanced state between the two circulation systems may reduce or counteract regional aridity, thereby favoring shrub growth. Decadal-scale correlation and partial correlation analyses further indicate that this area is primarily influenced by the EASM (Table 1).

A study of Holocene lake-level evolution in the ancient Zhuye Lake, on the central Alxa Plateau, showed that lake-level change was subject to the combined effects of EASM and the arid climate of Central Asia (Li, 2009). The radial growth of Z. xanthoxylum in the RY and RT is generally similar. However, due to its more westerly location, the RY is influenced by both the EASM and the Westerlies, as indicated by a significant partial correlation with the WCI (Table 1). Dendroclimatological studies from nearby mountains further illustrate this: on a decadal scale, the radial growth of P. crassifolia in the Changling Mountains (on the southern edge of the Tengger Desert) shows a positive correlation with the EASMI and a non-significant negative correlation with the WCI. In contrast, spruce growth in the Dongda Mountains (northern mountains of the Hexi Corridor, and southern edge of the Badain Jaran Desert) correlates positively with the EASMI, yet exhibits a significant negative correlation with the WCI (Xiao et al., 2024a).

Paleoclimate records from Qinghai Lake on the northern Tibetan Plateau indicate that the cold, dry Westerlies and the warm, humid EASM exhibited an anti-phase relationship on glacial-interglacial to millennial scales during the late Quaternary. Another study of the nearby Toson Lake area also showed that the out-of-phase moisture transport and interaction between the Westerlies and EASM drive asynchronous lake evolution between Central and East Asia over centennial-millennial timescales (Li et al., 2022). Their interplay likely represented a dominant climatic mode in eastern Central Asia for much of this period (An et al., 2012). This anti-phase pattern is also evident in the instrumental indices of the two circulation systems over the past five decades (Fig. 5). This interaction mechanism has also been confirmed by studies on the arid regions of Central Asia (Luo et al., 2026). These indices are constructed based on the seasonal to interannual positional shifts, boundary variations, and intensity changes of the atmospheric circulations (Zhang et al., 2003; Li and Zeng, 2005; Li et al., 2008; Chen and Chen, 2017; Chen et al., 2018). Consequently, the tree-ring chronologies from the four regions of the Alxa Plateau predominantly show inverse correlations with the two circulation indices on both interannual and decadal scales (Fig. 5; Table 1).

Research using proxy indicator cycles has shown that the Alxa Plateau and its surroundings area are influenced by large-scale ocean-atmosphere oscillations on interannual and interdecadal scales, such as the North Atlantic Oscillation, Pacific Decadal Oscillation, El Niño-Southern Oscillation, and solar activity (Gou et al., 2015a, b; Liu et al., 2016; Wang et al., 2017). The intensification of the EASM in humid monsoon regions and arid inland monsoon margins may be associated with rising temperatures in the Northern Hemisphere (An et al., 2012). However, all of the aforementioned large-scale climate and ocean-atmosphere changes affect the EASM and westerly circulation through different pathways (Li, 2009). These influences, in turn, have various effects on the northwestern margin of the EASM and the interaction zone between the two major atmospheric circulations, often with a lagged response (Ou and Qian, 2006). Consequently, over the past five decades, the phase relationship and interaction between the two circulations have exhibited additional strong-weak combinatorial patterns in response to global-scale climatic changes (Fig. 5). This complexity has contributed to the heterogeneity of the spatiotemporal response of Z. xanthoxylum radial growth to climate across the four desert regions of the Alxa Plateau.

In conclusion, this dendroclimatological study, based on tree-ring width chronologies from 26 Z. xanthoxylum shrub sites across four desert regions of the Alxa Plateau (RN, RW, RY, and RT), demonstrates that the radial growth of this desert shrub is primarily limited by drought conditions resulting from low precipitation during the growing-season. The dry/wet variations reflected in the four chronologies exhibit notable spatiotemporal heterogeneity, which is largely governed by the interaction between the EASM and Westerlies. While the entire study area is predominantly influenced by the EASM, and the northern and southwestern parts are also affected by the westerly jet. Consequently, the interaction between the westerly jet and the EASM occurs mainly over the central and northern Alxa Plateau, thereby driving regional vegetation dynamics and desertification processes.