A 1232-Year Tree-Ring Record of Climate Variability in the Qilian Mountains, Northwestern China

IAWA Journal, Vol. 30 (4), 2009: 407–420

A 1232-year tree-ring record of climate variability in the Qilian Mountains, Northwestern China

Yong Zhang1, Xiaohua Gou1, Fahu Chen1*, Qinhua Tian2, Meilin Yang1, Jianfeng Peng3 and Keyan Fang1

SUMMARY A millennium-long tree-ring width chronology for the middle Qilian Mountains in northwestern China has been developed back to A.D. 775. Correlation analysis indicates that the tree-ring width reflects growing- season moisture variability. Our chronology reveals three distinct periods based on the prevailing moisture anomalies: A.D. 775–1101 (wetness persistence), 1101–1831 (dryness persistence) and 1831–2006 (wetness persistence). A 31-year running mean through the tree-ring index series clearly shows seven obvious dry spells and eight wet spells. Compared with the proxies associated with the East Asian monsoon and the westerlies in the past millennium, our moisture-sensitive tree-ring chronology revealed that the East Asian summer monsoon had a strong influence on tree growth before A.D. 1300. From about A.D. 1450–1750, the westerlies strongly affected the Qilian Mountains. After A.D. 1750, a combined influence of both East Asian monsoon and westerlies was apparent. In the past century, the effect of westerlies has become stronger. Our results suggest that tree rings can preserve the information on the advance and retreats of the westerlies and the East Asian summer monsoon. Addition- ally, this research is helpful for understanding the driving mechanism of the Asian monsoon and the westerlies in northwestern China over the past thousand years. Key words: Qilian Mountains, tree rings, PDSI, Asian monsoon, westerlies, Juniperus (Sabina) przewalskii.

INTRODUCTION

Northwestern China (NW China) is located in the interior of Eurasia and is dominated by an arid climate. This area is particularly sensitive to global change, as it is the tran- sitional region for the East Asian summer monsoon and Northern Hemisphere westerly winds (Lehmkuhl & Haselein 2000; Morrill et al. 2003; Yang & Williams 2003), which makes it an opportune target for studying variations of both climate systems.

1) Center for Arid Environment and Paleoclimate Research (CAEP), Key Laboratory of West China’s Environmental System, Ministry of Education, Lanzhou University, Lanzhou 730000, P.R. China. 2) The State Key Laboratory of Loess and Quaternary Geology, the Institute of Earth Environment, Chinese Academy of Sciences, Xi’an 710075, P.R. China. 3) College of Environment and Planning, Henan University, Kaifeng 475004, P.R. China. *) Corresponding author [E-mail: [email protected]].

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Unfortunately, few reliable instrumental records extend back a century in China (Bradley 1999), and the situation is even worse in NW China where weather records extend back only 60 years at most. Thus, to obtain long, high-resolution paleoclimatic archives, such as historical documents, tree rings, ice cores, and stalagmites, it is im- perative to provide a long-term context for assessing modern-day climate variability in China. Considerable improvements in understanding paleoclimatic changes and the causing mechanisms, using various continental proxies on orbital-scale or even longer time scales, have been achieved in many parts of the world including NW China (Sun et al. 2004; Xiao et al. 2004; Chen et al. 2006; Vandenberghe et al. 2006; Yang et al. 2006; Hartmann & Wünnemann 2007; Zhao et al. 2007). However, this research is insufficient to examine the climate variation of the past 2000 years because of the low resolution of those proxies. Since high-resolution paleoclimatic records are rare in NW China, more high-resolution proxies are needed to study climate changes in the westerly and Asian monsoon region, especially the interplay of these two circulations over the past millennium. Tree rings play an important role in global change studies because of their annual dating, potential for millennium-long records, and wide geographical extent. Numerous examples of tree-ring chronologies being used for reconstructing climatic variability over the last 1,000 or more years worldwide are described by Briffa et al. (1995), Mann et al. (1999), Esper et al. (2002), Mann & Jones (2003), and Cook et al. (2004). In NW China, especially in the Qilian Mountains and adjacent areas, fortunately many millennium-long tree-ring width chronologies have been developed to investigate climate change for the past thousand years (Kang et al. 2003; Zhang et al. 2003; Sheppard et al. 2004; Liu et al. 2005; Shao et al. 2005; Huang & Zhang 2007; Shao et al. 2009). Other multi-centennial tree-ring chronologies were constructed to see the variation of temperature, precipitation or moisture in the historic period (Liu et al. 2003; Li et al. 2007; Tian et al. 2007; Xiao et al. 2007; Bhattacharyya & Shah 2009; Fang et al. 2009; Guo et al. 2009; Shao et al. 2009; Tian et al. 2009). However, few tree-ring-based studies have investigated the possible forcing mechanism of climate change in NW China. Here we present a millennium-long tree-ring chronology developed at three sites of Qilian juniper (Juniperus przewalskii – better known in China under its syno- nym Sabina przewalskii) in the middle Qilian Mountains, where long-living conifer trees are abundant. We discuss the moisture variation for the past thousand years. We especially aim at extracting the climate signal contributed by the Asian monsoon and the westerly system, respectively, by comparing the tree-ring chronology with climatic proxies, and try to present the regional forcing mechanism of the westerly and Asian monsoon in NW China.

MATERIAL AND METHODS

The Qilian Mountains (93.52°–103.00° E, 36.50°–39.50° N) are located along a bound- ary area of the Tibetan Plateau, the Inner Mongolia-Xinjiang Plateau, and the Loess Plateau (Fig. 1). This mountain system is about 850 km long and 200–300 km wide with peaks over 4,000 m (Tuanjie Peak, 5,826 m), which creates a strong rain-shadow

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50° 100° 98° 100° 102° 104° 40°

50° f e 38° b c

d PDSI grid point Meteorological station Tree-ring site N 0 50 100 km 0° 36° 50° 100° Figure 1. (1) Sites of different climatic proxies: a, ice core in Greenland (Mayewski & Maasch 2006); b, our tree-ring chronology in the Qilian Mountains; c, stalagmite in the Wanxiang Cave (Zhang et al. 2008); d, sediments of Lake Huguang Maar (Yancheva et al. 2007); e, tree-ring data from Jiuquan (Tian et al. 2007); f, tree-ring data from Tien Mountain (Li et al. 2006); g, tree-ring data from Helan Mountain (Li et al. 2007); the rectangle indicates northwestern China and dashed lines indicate the modern extent of the East Asian and Southwest Asian monsoon (Morrill et al. 2003). (2) Map of tree-ring sampling sites and meteorological stations; also shown are two PDSI grid points, developed by Dai et al. (2004). effect for monsoon moisture coming from the southeast (Gou et al. 2005). The mean precipitation is 150–410 mm distributed unevenly in both space and time, i.e., precipitation decreases from east to west and is concentrated in summer. The mean temperature is 0–5 °C between 2,000–3,000 m elevation. The main tree species in this region are Qilian juniper (Juniperus przewalskii Kom.), Qinghai spruce (Picea crassifolia Kom.), aspen (Populus davidiana Dode), and birch (Betula platyphylla Suk.). Qilian juniper grows in open stands on dry, exposed slopes at elevations ranging from 2,700 to 3,400 m. Forest soils are typically montane, grey brownish, and are subjected to serious ero- sion related to sparse vegetation cover (Liu et al. 2005). Tree-ring samples were collected from living Juniperus przewalskii trees generally growing on steep sunny slopes in southeast Sunan County, Gansu Province (Fig. 1). Our three sampling sites are close to each other (Table 1), and their elevation ranged

Table 1. Tree-ring sampling sites, the nearest meteorological station, and the PDSI grid point, developed by Dai et al. (2004).

Data type Site Longitude Latitude Elevation Sample size Time span code E N m (cores/trees) A.D.

Tree ring Hyg01 99.70° 38.68° 3100–3200 75/40 540–2006 Hyg02 99.68° 38.70° 2900–3000 50/28 1042–2006 Hyg03 99.67° 38.71° 3012 13/5 776–2006 Meteorological data Sunan 99.62° 38.83° 2312 – 1957–2006 PDSI Point A 98.75° 38.75° – – 1953–2005

Downloaded from Brill.com09/25/2021 06:50:02PM via free access 410 IAWA Journal, Vol. 30 (4), 2009 from 2,900 to 3,200 m, which is close to the lower juniper tree-line limit in the Qilian Mountains, where tree growth is generally sensitive to moisture availability (Fritts 1976). All sampled trees appeared healthy and isolated, representing optimal conditions for maximizing climate signals contained in the growth rings. Standard 5-mm increment cores were taken from the trees and returned to the lab for processing. After mounting and sanding the transverse surface to a fine polish, the core samples were cross-dated, and ring widths were measured to a precision of 0.001 mm using standard dendrochronological techniques. The quality of visual crossdating was further checked statistically with the COFECHA program (Holmes 1983; Grissino-Mayer 2001). Since the three sampling sites were very close, all ring-width series cross-dated well. The mean of correlation coefficients between each series and the master series was 0.639. Hence, all three tree-ring index series could be merged to develop one robust mean chronology. After some fragmented cores of poor quality were discarded, 78 cores (49 trees) were standardized together to develop a single ring-width chronology capturing the regional climate signal. The ARSTAN program (Cook 1985) was used to remove biological growth trends while preserving variation that was likely to be related to climate. Most of the series were conservatively detrended either by fitting a negative exponential function (33 series) or a linear regression function of any slope (18 series). A cubic spline with a 50% frequency-response cut-off equal to 67% of the series length was used in a few cases when abrupt growth changes resulted in a poor match of the con- servative detrending function. To reduce the potential influence of the changing sample size, the variance of the chronology was also stabilized using the method described by

STD chronology 0 80 800 1000 1200 1400 1600 1800 2000 60 40 20 0 Number of cores 800 1000 1200 1400 1600 1800 2000 Year Figure 2. Standard chronology and its sample depth from A.D. 775–2006.

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Osborn et al. (1997). The running robust average correlation (RBAR) and the expressed population signal (EPS) (Wigley et al. 1984) were used together to evaluate the quality of our chronology. The reliable portion of the chronology extends from A.D. 775 to 2006 with an EPS ≥ 0.85 (Fig. 2). Monthly temperature and precipitation records from 1957–2006 were obtained from the nearest meteorological station (Sunan; Fig. 1). These data show an annual mean temperature of 3.9 °C, with a monthly mean temperature over 13.5 °C in May- September (Fig. 3). January (-9.9 °C) and July (16.3 °C) are the coldest and warmest months, respectively. The mean annual precipitation is 255.8 mm, 86% of which falls from May-September (Fig. 3). July is the month with the most precipitation (61.5 mm). Thus, the seasonality of temperature and precipitation at this location reflects a typical monsoonal climate (Wang 2006).

70 18 Precipitation 16 Temperature 60 14 12 50 10 8 40 6 4 30 2 0 Temperature (°C) Temperature Precipitation (mm) 20 -2 -4 10 -6 -8 0 -10 -12 1 2 3 4 5 6 7 8 9 10 11 12 Month Figure 3. Average monthly sum of precipitation (bars) and average mean temperature (line) at the Sunan meteorological station (1957–2006).

We used the monthly Palmer Drought Severity Index (PDSI; Palmer 1965) dataset in order to investigate the relationship between tree growth and regional moisture availability on the middle part of the Qilian Mountains. The PDSI function is a measure of meteorological drought and is suitable for describing soil moisture and stream flow changes in the warm season (Dai et al. 2004). We used the updated monthly PDSI dataset developed by Dai et al. (2004; version 3, updated in November 2006), which was designed for global coverage based on a 2.5° × 2.5° grid system with a maximum time span from 1870–2005. The grid point used is located at 38.75° N, 98.75° E, which is the nearest one to our sampling sites (Fig. 1(2)). Because the earliest instrumental records in this grid did not begin until 1953 in our study area, we truncated the PDSI data before this time in order to use only the most reliable data (1953–2005).

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RESULTS AND DISCUSSION

Climate–tree growth relationships were assessed for the common period of tree rings and climate data (Blasing et al. 1984). The common period of tree rings, temperature and precipitation is 1957–2006, and their monthly correlations were calculated from the prior September to the current September. Likewise, the common period between tree rings and the PDSI is 1953–2005, and their monthly correlations were calculated for the warm season (i.e. April–October).

(a) Precipitation 0.5 Temperature

0.4

0.3

0.2

0.1

0.0

-0.1

-0.2

-0.3 Preceding year Current year -0.4 Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Month -0.5

(b) 0.5

0.4

0.3

0.2 Correlation coefficient Correlation coefficient

0.1

0.0 Apr May Jun Jul Aug Sep Oct Month Figure 4. Correlation of tree rings with (a) precipitation (black bars) and temperature (grey bars) from prior July to current September from 1957–2006, and with (b) the monthly PDSI data for the current growth season from 1953–2005; dashed lines indicate the 95 % confidence level.

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As shown in Figure 4a, significant correlations between tree-ring width and temperature occur only in prior and current September with coefficients of 0.278 and 0.365, respectively. Precipitation correlated positively with tree growth in most months of the current year, especially in May (0.30) and June (0.41) (at the 0.05 level), and negatively from prior October-December. From Figure 4b, we can see that tree growth positively correlated with the PDSI in the warm season (April-October), and statistically significant correlations (at the 0.05 level) are found from June to October, with the highest value (0.42) in July. Generally, past experience indicates that a seasonally averaged climate index is more representative than the climate of just one month, so we used the seasonally average index for further analysis. Thus, the correlations between these seasonal temperatures, precipitation, PDSI and tree-ring width were calculated. As shown in Table 2, both the preceding winter season for precipitation and PDSI correlated positively with tree growth; however, none of these correlations are statistically significant. Additionally, temperature in the preceding winter correlated significantly positively with tree growth. From Table 2 it is evident that the correlation of tree-ring width with precipitation and soil moisture in the growing season is strongly positive. Table 2. Correlation of tree-ring indices with multi-month seasons of temperature, precipitation and PDSI.

Sunan Point A Months ––––––––––––––––––––––––––––––– Temperature Precipitation PDSI P9-C3 0.30* 0.14 0.19 P12-C2 0.30* 0.17 0.19 C3-C5 -0.07 0.27 0.21 C3-C9 0.10 0.43* 0.33* C6-C8 -0.10 0.42* 0.40* C6-C9 0.05 0.34* 0.39* C5-C7 -0.16 0.52* 0.36* C6-C7 -0.15 0.45* 0.40* P9-C8 0.15 0.44* 0.29* P9-C3: September of previous year to March of current year, etc.; asterisks indicate correlations ≥ 95% confidence.

According to these results, we inferred that tree growth at our sampling sites is mainly influenced by moisture in the growing season. Shao et al. (2005) indicated that the cambium of Qilian juniper likely becomes active in May, and the earlywood should be largely formed by the end of June or early July. During this period of cambial activity, soil moisture should be necessary for tree growth, but the low precipitation and high temperature in the warm season (Fig. 3) will lead to physiological drought stress in the trees. Under these conditions, the available moisture supply cannot meet the needs of tree growth, thus resulting in a narrow ring. Therefore, the significant positive correlation of tree-ring width with moisture indicated that our tree-ring series represent the moisture variability in our study area during the growing season, especially from May to July, when precipitation is greatly effected by Asian monsoon and/or westerlies.

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(a)

(b) Accumulated Tree-ring anomalies index

Year Figure 5. (a) Standardized ring-width indices series (thin line) and 31-year running mean (thick grey line); (b) accumulated anomalies of tree-ring indices; vertical dotted lines indicate inflec- tions of long-term change of drought and wetness.

Smoothing the tree-ring index series by a 31-year running mean clearly shows multi- year and decadal variations (Fig. 5a, dark line). Based on the mean value and standard deviation (σ) of the smoothed series, the level of dry and wet conditions can be divided into three categories. The mean of this smoothed series was 0.99, and 1σ = 0.12. Here, we defined a “wet spell” as the mean plus 1σ and a “dry spell” as the mean minus 1σ as follows: wet spell >1.11; normal, 0.87 to 1.11; dry spell < 0.87 (Fig. 5a, dashed lines). Consequently, 7 dry spells with a duration of more than 2 years, A.D. 830–872, 1116–1166, 1269–1296, 1465–1503, 1706–1726, 1774–1801, 1921–1960, as well as 8 wet spells with a duration of more than 2 years were identified, A.D. 781–788, 884– 904, 965–1005, 1079–1097, 1236–1343, 1840–1866, 1889–1911, 1975–2006. Most of the dry and wet spells occurred in the early and late periods of our chronology, i.e., before about A.D. 1350 and after about A.D.1800. Furthermore, the amplitude of the dry and wet fluctuations in the two periods is larger than in the intermediate period. In the climate time-series analysis, an accumulative anomaly chart is often used to determine the trend of a period, i.e., whether a negative anomaly or a positive anomaly is dominant (Whetton & Rutherfurd 1994; Wei 1999; Lavin et al. 2006). We used this technique to show the state of drought (or wetness), from which an increase of the accumulated anomalies of the ring-width index indicates the persistence of wetness and a decrease reflects the persistence of drought. As shown in Figure 5b, the accumulated anomalies of the tree-ring index increased before A.D. 1101 and then decreased until A.D. 1831; after 1831, it increased again. Thus our ring-width chronology can be

Downloaded from Brill.com09/25/2021 06:50:02PM via free access Zhang et al. — Tree-ring record of climate in northwestern China 415 mainly divided into three intervals according to the characteristics of the accumulated anomalies, A.D. 775–1101 (wetness persistence), 1101–1831 (dryness persistence) and 1831–2006 (wetness persistence), although there are yet many different, short-term, high-frequency changes within these three stages (Fig. 5a). These variations of dryness and wetness are closely related to two climate systems, i.e. the westerlies and the East Asian monsoon. The westerlies mainly contribute to dry conditions and the monsoon contributes to wet conditions. However, the interaction of these two systems in the summer (June-August) determines wetness during the growing season in NW China. Thus, the advances and retreats of the westerlies and the East Asian monsoon intimately influence summer precipitation in NW China. The relative strength of the westerlies and the East Asian Monsoon, as the dominant climate systems influencing the moisture variation in NW China, experienced different stages in the past nearly 1200 years. Two continental sediment records from the Asian monsoon area as well as tree-ring data from the Helan Mountain were employed as representative of variation of Asian monsoon climate for the past thousand years (Li et al. 2007; Yancheva et al. 2007; Zhang et al. 2008). Similarly, ice core data from Greenland and tree-ring data from the Tien Mountains were used to represent variations of the westerlies in the past millennium (Li et al. 2006; Mayewski & Maasch 2006). Westerlies experienced two stages in the past thousand years according to the study of ice cores from central Greenland (Mayewski & Maasch 2006): before about A.D. 1450 it was weak, while after about A.D. 1500 it became strong (Fig. 6a). The East Asian summer monsoon underwent three stages, based on the δ18O record of the Wanxiang Cave stalagmite (Zhang et al. 2008) (Fig. 6c): before about A.D. 1300 (strong), from about A.D. 1300 to 1750 (weak), and after about A.D. 1750 (strong again). Similarly, Ti content series (Yancheva et al. 2007) from sediment in Lake Huguang Maar also indicated the variation of the East Summer Monsoon in a decadal-centennial time scale from about A.D. 780–1430 (Fig. 6d). Both proxies from the Asian Monsoon area before about A.D. 1300 showed three obvious periods with a weakening of East Asian Summer monsoon (Fig. 6, grey bars): about A.D. 800–920, 1010–1090 and 1100–1180, which are clearly coincidental with dry periods reflected by our tree-ring series during about A.D. 780–1300. Generally speaking, a weak summer monsoon leads to low precipitation. However, variation in the East Asian summer monsoon is out of accord with our tree-ring series after about A.D. 1300. Considering the characteristics of variation of westerlies and the East Asian summer monsoon in the past millennium, we infer that moisture variation in NW China is strongly related to the interaction of westerlies and East Asian summer monsoon. Before about A.D. 1300, the East Asian summer monsoon was strong while westerlies were weak; thus, our study area, i.e. the middle part of the Qilian Mountains, was greatly affected by the East Asian summer monsoon. In this period, water vapor related with the East Asian summer monsoon would enter the middle part of the Qilian Mountains, the intensity of the monsoon would notably affect tree growth in this area and the fluctuations of dry and wet change became larger. During the period of about A.D. 1300–1450, the westerlies and the East Asian summer monsoon were significantly weak; their signals could not be clearly preserved by our tree-ring series (Fig. 6, light

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et al.

et al. 2008)

et al.

Figure 6. Comparison of our tree-ring series and climate proxies related with strength of westerlies and the East Asian Summer Monsoon. a, non-sea salt (nss) Ca+2 content in ice core of Greenland Ice Sheet Project Two (GISP2) (Mayewski & Maasch 2006); b, our tree-ring chronology for the middle of the Qilian Mountains; c, δ18O record from Wanxiang Cave stalagmites (Zhang et al. 2008); d, titanium content of sediment in Lake Huguang Maar (Yancheva et al. 2007); e, PDSI reconstruction in Jiuquan (Tian et al. 2007); f, PDSI reconstruction for the Tien Mountain(Li et al. 2006); g, PDSI reconstruction for Helan Mountain (Li et al. 2007). Dark line indicates the 5-year running mean of each series; dotted lines indicate the mean of each series. Dark grey bars indicate three distinct dry periods before about A.D. 1300 and light grey bar area indicates a period when the East Asian Monsoon and westerlies are both weak. grey bar). After that, the westerlies became strong while the East Asian summer monsoon was weak until about A.D. 1750. The middle part of the Qilian Mountains was affected greatly by westerlies and the climate in this area represented the comparatively dry and stable moisture variability of this period. After about A.D. 1750, westerlies were generally strong with a slight decreasing trend; East Asian summer monsoon became strong again, so the signal of our tree-ring series was integrated by those associated with westerlies and the Asian summer monsoon. It is noteworthy that the proxies from the Asian monsoon zone show a decreasing summer monsoon in the most recent 100 years (Fig. 6c, g) (Li et al. 2007; Zhang et al. 2008), yet our series indicate a moisture increase during this period, consistent with the trend of the tree-ring series from the western part of the Qilian Mountains (Fig. 6e) (Tian et al. 2007) as well as of the Tien Mountain (Fig. 6f) (Li et al. 2006), where climate is mainly controlled by westerlies (Aizen et al. 1996). Therefore, the influence of westerlies seemed to become stronger on the Qilian Mountains. Published research on Holocene climate change indicates that the advance and retreat of westerlies and the Asian monsoon greatly affected the regional climate in

Downloaded from Brill.com09/25/2021 06:50:02PM via free access Zhang et al. — Tree-ring record of climate in northwestern China 417 arid central Asia (An et al. 2000; Zhao et al. 2007; Chen et al. 2008). Therefore, the comparison of our tree-ring series with those records can provide a spatial evolution pattern of westerlies and the East Asian summer monsoon in the Qilian Mountains during the past millennium.

CONCLUSIONS

A millennium-long tree-ring-width chronology has been developed for the Qilian Mountains, NW China. Correlation analysis results indicate that moisture in the growing season significantly influences tree growth. Compared with other proxies of westerlies and the East Asian summer monsoon, our reconstructed series preserve the information of advance and retreat of westerlies and the East Asian summer monsoon. NW China, especially the Qilian Mountains, was influenced greatly by the East Asia summer monsoon before about A.D. 1300 and was controlled by westerlies during the period from A.D. 1450–1750. Since about A.D. 1750, a combined influence of westerlies and East Asian summer monsoon is preserved by our tree-ring series. However, the effect of westerlies seems to have become stronger in the past century. Our preliminary results reveal the shift of westerlies and the East Asian summer monsoon and the interactions between them in NW China in different periods of the past millennium. This work will improve the understanding of the complex circulation mechanisms involving westerly and Asian monsoon systems. More work on methods and theories is required to refine models of climate forcing in this area. Because of the lack of high-resolution westerly-related proxies for the past 1000 years and the anthropogenic warming in the most recent 100 years, more careful analyses are required to verify our result. Additionally, further studies on interaction between subtropical (monsoon), mid-latitude (westerly) atmospheric circulation systems and local topography in determining regional climate should also be conducted.

ACKNOWLEDGEMENTS

We were very grateful to Prof. Steven W. Leavitt’s and Ms. Nonie Bell’s help to improve our manuscript and to the reviewers who gave valuable comments on our manuscript. This research was sup- ported by the Chinese NSFC Innovation Team Project (40721061), National Science Foundation of China (40671191 and 90502008), One Hundred Talent Program of the Chinese Academy of Sciences (29O827B11), Chinese 111 Project (B06026), and Program for New Century Excellent Talents in University (NCET-05-0888).

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