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Aridity changes in the eastern Qilian Mountains since AD 1856 reconstructed from tree-rings
Quaternary International 283 (2013) 78e84
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Aridity changes in the eastern Qilian Mountains since AD 1856 reconstructed from tree-rings Yang Deng a, Xiaohua Gou a, *, Linlin Gao a, Zhiqian Zhao a, Zongying Cao a, Meixue Yang b a b
Key Laboratory of Western China’s Environmental Systems (MOE), Research School of Arid Environment and Climate Change, Lanzhou University, Lanzhou 730000, China State Key Laboratory of Cryospheric Sciences, Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences, Lanzhou 730000, China
a r t i c l e i n f o
a b s t r a c t
Article history: Available online 30 April 2012
This paper presents a drought reconstruction for the eastern Qilian Mountains based on a tree-ring width chronology developed from three sites of the Qinghai spruce (Picea crassifolia Kom.). The drought reconstruction, spanning the years 1856e2009, was developed by calibrating tree-ring data with the regional averaged Palmer Drought Severity Index (PDSI). The reconstruction explains 49.9% of the actual PDSI variance during the calibration period from 1951 to 2005. The reconstruction shows droughts occurred in the 1900s, the 1920s and early 1930s, and at the turn of the 21st century. Multitaper method spectral analysis indicates the existence of some significant peaks at 2.6, 4.2 and 56.9 years. Drought variations in the study area are significantly correlated with sea surface temperatures in eastern equatorial Pacific Ocean, northern Indian Ocean, eastern China Sea and the Atlantic Ocean, suggesting a possible connection with the El Niño-Southern Oscillation, the Asian monsoon and the Westerlies. Ó 2012 Elsevier Ltd and INQUA. All rights reserved.
1. Introduction The Minqin Oasis in Gansu Province of northwest China, which is surrounded by the Tengger Desert in the east and the Badain Jaran Desert in the west, is one of the major sources of sandstorms in China. Currently, people in this area are facing the consequences of some of the worst ecological and environmental deterioration in China, which has captured the attention of the Chinese government and the scientific community. The Minqin Oasis is nurtured by Shiyang River which originates in the Qilian Mountains. Therefore, investigation of the climate variations in the upper reaches of Shiyang River is a high research priority for the detection and attribution of climate change. However, the instrumental climatic records in this area are very short, generally begin in the 1950s. In order to better understand the long-term climate history, high-resolution palaeoclimatic records are needed. Precisely dated annual resolution proxy records from tree-rings are of considerable value for developing longer time series for analysis. By using treering data, the long-term climate history has been successfully reconstructed on local, regional and hemispheric scales (Cook et al., 2010; Yang et al., 2010). In recent years, great progress has been made in dendroclimatological studies in China. Numerous tree-ring
* Corresponding author. E-mail address: [email protected] (X. Gou). 1040-6182/$ e see front matter Ó 2012 Elsevier Ltd and INQUA. All rights reserved. doi:10.1016/j.quaint.2012.04.039
based reconstructions of temperature, precipitation, streamflow and drought history have been developed (Liu et al., 2005; Shao et al., 2005; Liang et al., 2007; Gou et al., 2010; Qin et al., 2010; Yang et al., 2011). The Monsoon Asian PDSI variability has also been reconstructed by Cook et al. (2010). Considering that the tree-ring chronologies are still sparse in some regions, additional drought reconstructions are needed. In the eastern Qilian Mountains, several precipitation reconstructions had been conducted (Gou et al., 2001; Chen et al., 2011; Hou et al., 2011) and no drought reconstruction is available. Consequently, it is important to carry out more dendroclimatological studies in this region to obtain climatic variation information in the past. This paper presents a regional drought reconstruction for the past 156 years using three tree-ring width chronologies of spruce from the eastern Qilian Mountains, and 2.5 2.5 gridded Palmer Drought Severity Index (PDSI) data.
2. Materials and methods 2.1. Study area The study area is situated in the eastern part of the Qilian Mountains (Fig. 1), and lies on the present margin of the East Asian summer monsoon (Chen et al., 2006). Regional climate of the study region is arid and continental, with peak warm and wet conditions
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Fig. 1. Map of the study region showing the locations of the tree-ring sampling sites, the Wuwei instrumental station and the four nearby PDSI grid points.
in July and August. According to the nearest meteorological station, Wuwei (37.92 N, 102.66 E, 1532 m a.s.l.) for the period 1951e2008, the annual average temperature and annual total precipitation are 8.0 C and 167 mm, respectively (Fig. 2). The main tree species in this region are the Qinghai spruce (Picea crassifolia Kom.) and Qilian juniper (Sabina przewalskii Kom.). 2.2. Tree-ring data Qinghai spruce tree-ring samples were collected from living trees in the eastern Qilian Mountains in July, 2010. Cores were taken from the living trees by an increment borer at breast height (1.3 m), and one or two cores were obtained per individual tree. In total, 43 cores were collected from 27 trees at BSB (Baishu, 37.70 N, 101.91 E, 2748e2770 m a.s.l.), 29 cores from 19 trees at LTB (Longtan, 37.80 N, 102.05 E, 2680e2720 m a.s.l.), and 44 cores from 23 trees at DAG (Daan, 37.79 N, 102.7 E, 3072e3085 m a.s.l.). Core samples were mounted, air-dried, and then polished with progressively finer sandpapers. Exact calendar years of each
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individual ring were assigned by visual cross-dating (Fritts, 1976). The visually cross-dated tree-rings were then measured with a precision of 0.001 mm. The quality of visual cross-dating was further checked by the computer program COFECHA (Holmes, 1983). As the three sampling sites were very close (maximum distance between the three sites is about 15 km) and all ring-width series cross-dated well, all increment cores collected at the three sites were used to build the composite chronology. Measured ring-width series were standardized into a tree-ring chronology using the program ARSTAN (Cook, 1985). Approximately 4/5 of the sampled series were detrended with negative exponential curves or straight lines. The remainder was detrended with a cubic smoothing spline with a 0.5 frequency response at 67 percent of the series length or the Friedman supersmoother with an a-parameter between 7 and 9. The chronology was calculated using a bi-weight robust mean (Cook, 1985). Sample depth within the chronology typically decreases back in time and may result in timedependent variance changes in the earlier part of the chronology. The variance in chronologies was stabilized in the chronology compilation process using the Briffa RBAR-weighted method, which uses average correlations between series in combination with the sample size each year to make adjustments in the variance for changes in sample size (Osborn et al., 1997). The Expressed Population Signal (EPS), computed over 30 years lagged by 15 years, were used to estimate the signal strength of the final record (Wigley et al., 1984). 2.3. Climate data Monthly temperature and precipitation records were obtained from the nearest meteorological station in Wuwei and are available from the year AD 1951. A gridded (2.5 2.5 ) version of the Palmer Drought Severity Index (PDSI; Dai et al., 2004) was also used to investigate the relationship between tree growth and regional moisture availability on the eastern part of the Qilian Mountains. Monthly PDSI values from the four closest grid points to the sampling site (centered over 36.25 N and 101.25 E, 36.25 N and 103.75 E, 38.75 N and 101.25 E, 38.75 N and 103.75 E) were averaged to represent the regional drought variations. Because most of the earliest instrumental records around these grids do not begin until the 1950s, the PDSI data before 1950 were truncated in
Fig. 2. Plots of (a) the composite STD chronology developed from the merged ring-width data from the three sampling sites, (b) sample depth through time, and (c) running EPS statistics. The EPS were calculated using a 30-year window that lags 15 years.
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3. Results and discussion 3.1. Chronology Cross-dated measurements extend to AD 1808. However, the composite chronology was truncated at AD 1856 (Fig. 3), where the sample depth falls below five trees. The EPS statistic is considered acceptable at levels above 0.85 (Cook and Kairiukstis, 1990) and the regional chronology attains EPS values above 0.85 throughout the entire period. 3.2. Correlations with climate
Fig. 3. Mean monthly rainfall during the period 1951e2008 (black bars) and temperature during the period 1951e2008 (solid line) for Wuwei.
order to only use the most reliable data (1950e2005). Simple correlation analyses between ring-width index and monthly climate data spanning a ‘dendroclimatic year’ (Fritts, 1976) that begins in May of the previous growing season and ends in August of the current growing season were employed. In order to identify large scale forcing of drought in the study area, spatial correlation fields for the reconstructed and actual PDSI of the previous September to current August were generated, with concurrent gridded Sea surface temperature (SST) from the HadISST1 dataset (Rayner et al., 2003). The KNMI Climate Explorer (http://www.knmi.nl/) (van Oldenborgh and Burgers, 2005) was used for this purpose of generating spatial correlation maps.
The correlation for the composite STD chronology with the climate records from the Wuwei station and the four-grid-point averaged monthly PDSI is shown in Fig. 4a. Tree-rings correlated negatively with temperature, and significant correlations (at the 0.05 level) were found for most of the months analyzed. For precipitation, no significant correlations (at the 0.05 level) were found. As the nearest meteorological station is about 70 km away, and the altitude is only 1532 m a.s.l., about 1300 m lower than the sampling sites, the precipitation data in Wuwei station may not be a good indicator for the sampling sites. Correlations improved with PDSI data, a direct metric of moisture conditions (Dai et al., 2004). As shown in Fig. 4b, tree growth was positively correlated with PDSI for all months from the previous May to the current August. More specifically, 14 consecutive months from prior-year July to current August were significant (at 0.01 level), indicating that moisture availability during these seasons is primarily limiting radial growth of spruce (Picea crassifolia) in the study area. Highest correlation of 0.71 is found with previous September to current August PDSI data.
Fig. 4. Correlations between the composite STD chronology, monthly temperature and monthly total rainfall (a), mean monthly PDSI (b). Dotted lines and dashed lines indicate the 0.05 and 0.01 significance levels, respectively. p: previous year; c: current year.
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Fig. 5. Plots showing the comparison between the actual and reconstructed PDSI during the common period of 1951e2005.
As the study location is located in an arid region, annual precipitation is very low and can hardly meet the trees’ demand, trees growing in these areas are mostly moisture-stressed. The PDSI, which considers both precipitation and temperature, is a better indicator of the moisture than simply precipitation. 3.3. PDSI reconstruction From the above analyses, tree growth in the sampling sites was mainly influenced by moisture, so we selected the PDSI from the previous September to current August as the predictand. A simple linear regression model was used to reconstruct the PDSI of the eastern Qilian Mountains:
PDSI98 ¼ 5:18*STD 6:08 Where PDSI98 is the average PDSI from previous September to current August and STD represents the standard tree-ring chronology. The reconstruction captured 49.9% of the actual PDSI variance during the period 1951e2005, the highest so far documented for the eastern Qilian Mountains. The comparison between reconstructed and actual data showed a very close similarity (Fig. 5). In order to further evaluate the statistical fidelity of the reconstruction
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model, a split period calibration-verification scheme was applied. The PDSI data were split in two sub-periods (1951e1978 and 1979e2005). Both sub-period calibration models captured very high a variance in the actual data (Table 1). Rigorous calibrationeverification statistics (Cook and Kairiukstis, 1990) were used to test the statistical significance and reliability of calibration models. These included the reduction of error (RE), coefficient of efficiency (CE), sign test (ST), and Pearson’s correlation coefficient (Fritts, 1976; Cook and Kairiukstis, 1990). As shown in Table 1, all tests are passed, even for the two most rigorous tests of model validation, RE and CE. These tests demonstrate the validity of the regression model. On the basis of the model, the drought history in the study area has been reconstructed for the period AD 1856d2009 (Fig. 6a). The mean of the reconstructed PDSI from 1856 to 2009 is 1.03, which is significantly below the defined value of zero mean for normal moisture conditions. A significantly negative mean PDSI was also noted in other studies in the central Tien Shan area (Li et al., 2006), the Ortindag Sand Land (Liang et al., 2007) and the Xinglong Mountains (Fang et al., 2009), indicating the need to improve the regional PDSI model. In order not to overestimate the drought conditions, PDSI values of mean þ1s (standard deviation, s ¼ 1.03) or mean 1s were defined as wet or drought conditions. The following discussions are based on the rescaled drought indices. The drought reconstruction shows a strong inter-annual variability throughout the entire period, and the most noteworthy feature in the reconstruction is the low frequency variations. The reconstructed PDSI is above the mean in the first two decades, below the mean until the 1930s, then above the mean again until the end of the 20th century and with an extreme drought echo in the early of the 21st century. Droughts were found in the 1900s, the 1920s and early 1930s, and at the turn of the 21st century. The drought in the 1920s and early 1930s shown in the reconstruction appears to have persisted longer than any drought in the observational record. This drought has been widely reported in the semiarid and arid areas of north China (Liang et al., 2006; Liu et al., 2006, 2009, 2010; Tian et al., 2007; Fang et al., 2009; Yang et al., 2011; Zhang et al., 2011), and led to major economic and life losses. The dry epoch at the turn of the 21st century was shown in both the actual and reconstructed data (Fig. 5), and has also been recorded in the Xinglong Mountains (Fang et al., 2009), Guiqing Mountains (Fang et al., 2010) and the west part of the Qilian Mountains (Yang
Fig. 6. The reconstructed annual PDSI in east Qilian Mountains (a), and the reconstruction of regional summer PDSI (JuneeJulyeAugust) of eastern Qilian Mountains after merging the same four-grid points derived from the Monsoon Asia Drought Atlas (Cook et al., 2010) (b). The three horizontal lines in (a) are the mean of the reconstruction and mean s. The thick lines indicate the five-plot running averaged data.
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Table 1 Calibration and verification period statistics for the reconstructed average PDSI values of the four nearby grid points.
r r2 RE CE Sign test
Calibration (1951e1978)
Verification (1979e2005)
Calibration (1979e2005)
Verification (1951e1978)
Full calibration (1951e2005)
0.58 (df ¼ 26) 0.34 / / 21/7*
0.73 (df ¼ 25) 0.53 0.62 0.43 20/7*
0.73 (df ¼ 25) 0.53 / / 22/5**
0.58 (df ¼ 26) 0.34 0.57 0.28 21/7*
0.71 (df ¼ 53) 0.50 / / /
Note that the df means degree of freedom, the RE indicates the reduction error and CE designates the coefficient of efficiency. One and two asterisks indicate the 0.05 and 0.01 significance levels, respectively.
Fig. 7. Spatial Pearson correlation plots for the actual and reconstructed PDSI with previous September to current August averaged HadISST1 SST during the period 1950e2005.
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et al., 2011). Comparison of this PDSI reconstruction with the summer PDSI reconstruction derived from Cook et al. (2010) shows that there are significant discrepancies between the two reconstructions (Fig. 6). The discrepancies may be caused by the different season reconstructed and/or some remote tree-ring chronologies used in Cook’s reconstruction as they used the “search radius” method with a 1000 km circle. According to Shi et al. (2007), a strong signal of climate change from a warm-dry to a warm-wet pattern has already been observed in northwest China, based on an analysis of the hydrological and meteorological data base, basic circulation patterns and modeling studies for 40 years. This trend has been also found in some treering based drought or streamflow reconstructions (Li et al., 2006; Liu et al., 2010; Qin et al., 2010). However, this trend was more prevalent in the Xijiang, the central-west part at the north flank of the Qilian Mountains and the southeast of Qaidam Basin (Shi et al., 2007). No such trend was found in this drought reconstruction, the rainfall reconstruction of Jiuquan region (Yang et al., 2011) and precipitation or drought reconstructions in the eastern regions (Fang et al., 2009; Hou et al., 2011).
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annual variability and low frequency variations, and droughts were found in the 1900s, the 1920s and early 1930s, and at the turn of the 21st century. Multi-taper method spectral analysis indicates the existence of some significant peaks at 2.6, 4.2 and 56.9 years. Meanwhile, drought variations in the study area are significantly correlated with SSTs in the eastern equatorial Pacific Ocean, northern Indian Ocean, eastern China Sea and the Atlantic Ocean, suggesting possible a connection with the ENSO, the Asian monsoon and the Westerlies.
Acknowledgements The authors thank Emily Derbyshire for her kind help in editing the English writing. The authors also thank reviewers for useful comments and suggestions to improve the manuscript. This research was supported by the National Basic Research Program of China (973 Program) (No. 2009CB421306), the National Science Foundation of China (No. 40971119, No. 41171039 and No. 40890051) and the One Hundred Talents Program of CAS (No. 29O827B11).
3.4. Links with the remote oceans Significant peaks were found at 2.6 (p < 0.01), 4.2 (p < 0.01), and 56.9 years (p < 0.01). The 56.9 year cycle may be related to the Atlantic Multi-decadal Oscillation (Schlesinger and Ramankutty, 1994). The 2e4 year cycle falls within the range of ENSO variability, and has been noted in many moisture-related tree-ring reconstructions in surrounding areas (Tian et al., 2007; Fang et al., 2009, 2010; Liu et al., 2009). The negative correlation of the PDSI with eastern equatorial Pacific Ocean SSTs also suggests a possible connection to ENSO (Fig. 7). The spatial correlations between PDSI and SST during the instrumental period were roughly consistent with correlation patterns identified for the northeastern Tibetan Plateau (Li et al., 2008) and the Guiqing Mountains (Fang et al., 2010). The PDSI also significantly correlated with SSTs of the northern Indian Ocean, East China Sea and Atlantic Ocean. The significant correlations with northern Indian Ocean SSTs suggest a possible connection with the Indian monsoon. The most significant correlations were found with the East China Sea SSTs, suggesting the impacts of East Asian summer monsoon (EASM) on droughts in the study region (Li and Zeng, 2002). Although the study area is located to the west of 105 E, it is also affected by the EASM. In a far western site, Xu et al. (2011) also found the EASM signal. Additionally, a drying trend which has been observed since the late 1950s was found in the drought reconstruction, which is consistent with the decline of the EASM (Liu et al., 2003; Jiang and Wang, 2005). The drying trend has also been noted in other treering studies in the north margin of the EASM region (Liu et al., 2003; Li et al., 2007; Liang et al., 2007). The significant correlations with the Atlantic Ocean SSTs suggest a connection with the NAO, an indicator of the strength of the Westerlies. Located in the north margin of the EASM region, the climate of the eastern Qilian Mountains may also have been affected by the Westerlies at the same time as the EASM was in decline. In the middle and western Qilian Mountains, Zhang et al. (2009) and Liu et al. (2009) found that the Westerlies might have an effect on those areas. 4. Conclusion Based on the three tree-ring width chronologies from Qinghai spruce (Picea crassifolia Kom.) in the eastern Qilian Mountains, the mean PDSI during the period from the previous September to the current August of four nearby grid points were reconstructed back to AD 1856. The drought reconstruction shows a strong inter-
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Aridity changes in the eastern Qilian Mountains since AD 1856 reconstructed from tree-rings Yang Deng a, Xiaohua Gou a, *, Linlin Gao a, Zhiqian Zhao a, Zongying Cao a, Meixue Yang b a b
Key Laboratory of Western China’s Environmental Systems (MOE), Research School of Arid Environment and Climate Change, Lanzhou University, Lanzhou 730000, China State Key Laboratory of Cryospheric Sciences, Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences, Lanzhou 730000, China
a r t i c l e i n f o
a b s t r a c t
Article history: Available online 30 April 2012
This paper presents a drought reconstruction for the eastern Qilian Mountains based on a tree-ring width chronology developed from three sites of the Qinghai spruce (Picea crassifolia Kom.). The drought reconstruction, spanning the years 1856e2009, was developed by calibrating tree-ring data with the regional averaged Palmer Drought Severity Index (PDSI). The reconstruction explains 49.9% of the actual PDSI variance during the calibration period from 1951 to 2005. The reconstruction shows droughts occurred in the 1900s, the 1920s and early 1930s, and at the turn of the 21st century. Multitaper method spectral analysis indicates the existence of some significant peaks at 2.6, 4.2 and 56.9 years. Drought variations in the study area are significantly correlated with sea surface temperatures in eastern equatorial Pacific Ocean, northern Indian Ocean, eastern China Sea and the Atlantic Ocean, suggesting a possible connection with the El Niño-Southern Oscillation, the Asian monsoon and the Westerlies. Ó 2012 Elsevier Ltd and INQUA. All rights reserved.
1. Introduction The Minqin Oasis in Gansu Province of northwest China, which is surrounded by the Tengger Desert in the east and the Badain Jaran Desert in the west, is one of the major sources of sandstorms in China. Currently, people in this area are facing the consequences of some of the worst ecological and environmental deterioration in China, which has captured the attention of the Chinese government and the scientific community. The Minqin Oasis is nurtured by Shiyang River which originates in the Qilian Mountains. Therefore, investigation of the climate variations in the upper reaches of Shiyang River is a high research priority for the detection and attribution of climate change. However, the instrumental climatic records in this area are very short, generally begin in the 1950s. In order to better understand the long-term climate history, high-resolution palaeoclimatic records are needed. Precisely dated annual resolution proxy records from tree-rings are of considerable value for developing longer time series for analysis. By using treering data, the long-term climate history has been successfully reconstructed on local, regional and hemispheric scales (Cook et al., 2010; Yang et al., 2010). In recent years, great progress has been made in dendroclimatological studies in China. Numerous tree-ring
* Corresponding author. E-mail address: [email protected] (X. Gou). 1040-6182/$ e see front matter Ó 2012 Elsevier Ltd and INQUA. All rights reserved. doi:10.1016/j.quaint.2012.04.039
based reconstructions of temperature, precipitation, streamflow and drought history have been developed (Liu et al., 2005; Shao et al., 2005; Liang et al., 2007; Gou et al., 2010; Qin et al., 2010; Yang et al., 2011). The Monsoon Asian PDSI variability has also been reconstructed by Cook et al. (2010). Considering that the tree-ring chronologies are still sparse in some regions, additional drought reconstructions are needed. In the eastern Qilian Mountains, several precipitation reconstructions had been conducted (Gou et al., 2001; Chen et al., 2011; Hou et al., 2011) and no drought reconstruction is available. Consequently, it is important to carry out more dendroclimatological studies in this region to obtain climatic variation information in the past. This paper presents a regional drought reconstruction for the past 156 years using three tree-ring width chronologies of spruce from the eastern Qilian Mountains, and 2.5 2.5 gridded Palmer Drought Severity Index (PDSI) data.
2. Materials and methods 2.1. Study area The study area is situated in the eastern part of the Qilian Mountains (Fig. 1), and lies on the present margin of the East Asian summer monsoon (Chen et al., 2006). Regional climate of the study region is arid and continental, with peak warm and wet conditions
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Fig. 1. Map of the study region showing the locations of the tree-ring sampling sites, the Wuwei instrumental station and the four nearby PDSI grid points.
in July and August. According to the nearest meteorological station, Wuwei (37.92 N, 102.66 E, 1532 m a.s.l.) for the period 1951e2008, the annual average temperature and annual total precipitation are 8.0 C and 167 mm, respectively (Fig. 2). The main tree species in this region are the Qinghai spruce (Picea crassifolia Kom.) and Qilian juniper (Sabina przewalskii Kom.). 2.2. Tree-ring data Qinghai spruce tree-ring samples were collected from living trees in the eastern Qilian Mountains in July, 2010. Cores were taken from the living trees by an increment borer at breast height (1.3 m), and one or two cores were obtained per individual tree. In total, 43 cores were collected from 27 trees at BSB (Baishu, 37.70 N, 101.91 E, 2748e2770 m a.s.l.), 29 cores from 19 trees at LTB (Longtan, 37.80 N, 102.05 E, 2680e2720 m a.s.l.), and 44 cores from 23 trees at DAG (Daan, 37.79 N, 102.7 E, 3072e3085 m a.s.l.). Core samples were mounted, air-dried, and then polished with progressively finer sandpapers. Exact calendar years of each
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individual ring were assigned by visual cross-dating (Fritts, 1976). The visually cross-dated tree-rings were then measured with a precision of 0.001 mm. The quality of visual cross-dating was further checked by the computer program COFECHA (Holmes, 1983). As the three sampling sites were very close (maximum distance between the three sites is about 15 km) and all ring-width series cross-dated well, all increment cores collected at the three sites were used to build the composite chronology. Measured ring-width series were standardized into a tree-ring chronology using the program ARSTAN (Cook, 1985). Approximately 4/5 of the sampled series were detrended with negative exponential curves or straight lines. The remainder was detrended with a cubic smoothing spline with a 0.5 frequency response at 67 percent of the series length or the Friedman supersmoother with an a-parameter between 7 and 9. The chronology was calculated using a bi-weight robust mean (Cook, 1985). Sample depth within the chronology typically decreases back in time and may result in timedependent variance changes in the earlier part of the chronology. The variance in chronologies was stabilized in the chronology compilation process using the Briffa RBAR-weighted method, which uses average correlations between series in combination with the sample size each year to make adjustments in the variance for changes in sample size (Osborn et al., 1997). The Expressed Population Signal (EPS), computed over 30 years lagged by 15 years, were used to estimate the signal strength of the final record (Wigley et al., 1984). 2.3. Climate data Monthly temperature and precipitation records were obtained from the nearest meteorological station in Wuwei and are available from the year AD 1951. A gridded (2.5 2.5 ) version of the Palmer Drought Severity Index (PDSI; Dai et al., 2004) was also used to investigate the relationship between tree growth and regional moisture availability on the eastern part of the Qilian Mountains. Monthly PDSI values from the four closest grid points to the sampling site (centered over 36.25 N and 101.25 E, 36.25 N and 103.75 E, 38.75 N and 101.25 E, 38.75 N and 103.75 E) were averaged to represent the regional drought variations. Because most of the earliest instrumental records around these grids do not begin until the 1950s, the PDSI data before 1950 were truncated in
Fig. 2. Plots of (a) the composite STD chronology developed from the merged ring-width data from the three sampling sites, (b) sample depth through time, and (c) running EPS statistics. The EPS were calculated using a 30-year window that lags 15 years.
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3. Results and discussion 3.1. Chronology Cross-dated measurements extend to AD 1808. However, the composite chronology was truncated at AD 1856 (Fig. 3), where the sample depth falls below five trees. The EPS statistic is considered acceptable at levels above 0.85 (Cook and Kairiukstis, 1990) and the regional chronology attains EPS values above 0.85 throughout the entire period. 3.2. Correlations with climate
Fig. 3. Mean monthly rainfall during the period 1951e2008 (black bars) and temperature during the period 1951e2008 (solid line) for Wuwei.
order to only use the most reliable data (1950e2005). Simple correlation analyses between ring-width index and monthly climate data spanning a ‘dendroclimatic year’ (Fritts, 1976) that begins in May of the previous growing season and ends in August of the current growing season were employed. In order to identify large scale forcing of drought in the study area, spatial correlation fields for the reconstructed and actual PDSI of the previous September to current August were generated, with concurrent gridded Sea surface temperature (SST) from the HadISST1 dataset (Rayner et al., 2003). The KNMI Climate Explorer (http://www.knmi.nl/) (van Oldenborgh and Burgers, 2005) was used for this purpose of generating spatial correlation maps.
The correlation for the composite STD chronology with the climate records from the Wuwei station and the four-grid-point averaged monthly PDSI is shown in Fig. 4a. Tree-rings correlated negatively with temperature, and significant correlations (at the 0.05 level) were found for most of the months analyzed. For precipitation, no significant correlations (at the 0.05 level) were found. As the nearest meteorological station is about 70 km away, and the altitude is only 1532 m a.s.l., about 1300 m lower than the sampling sites, the precipitation data in Wuwei station may not be a good indicator for the sampling sites. Correlations improved with PDSI data, a direct metric of moisture conditions (Dai et al., 2004). As shown in Fig. 4b, tree growth was positively correlated with PDSI for all months from the previous May to the current August. More specifically, 14 consecutive months from prior-year July to current August were significant (at 0.01 level), indicating that moisture availability during these seasons is primarily limiting radial growth of spruce (Picea crassifolia) in the study area. Highest correlation of 0.71 is found with previous September to current August PDSI data.
Fig. 4. Correlations between the composite STD chronology, monthly temperature and monthly total rainfall (a), mean monthly PDSI (b). Dotted lines and dashed lines indicate the 0.05 and 0.01 significance levels, respectively. p: previous year; c: current year.
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Fig. 5. Plots showing the comparison between the actual and reconstructed PDSI during the common period of 1951e2005.
As the study location is located in an arid region, annual precipitation is very low and can hardly meet the trees’ demand, trees growing in these areas are mostly moisture-stressed. The PDSI, which considers both precipitation and temperature, is a better indicator of the moisture than simply precipitation. 3.3. PDSI reconstruction From the above analyses, tree growth in the sampling sites was mainly influenced by moisture, so we selected the PDSI from the previous September to current August as the predictand. A simple linear regression model was used to reconstruct the PDSI of the eastern Qilian Mountains:
PDSI98 ¼ 5:18*STD 6:08 Where PDSI98 is the average PDSI from previous September to current August and STD represents the standard tree-ring chronology. The reconstruction captured 49.9% of the actual PDSI variance during the period 1951e2005, the highest so far documented for the eastern Qilian Mountains. The comparison between reconstructed and actual data showed a very close similarity (Fig. 5). In order to further evaluate the statistical fidelity of the reconstruction
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model, a split period calibration-verification scheme was applied. The PDSI data were split in two sub-periods (1951e1978 and 1979e2005). Both sub-period calibration models captured very high a variance in the actual data (Table 1). Rigorous calibrationeverification statistics (Cook and Kairiukstis, 1990) were used to test the statistical significance and reliability of calibration models. These included the reduction of error (RE), coefficient of efficiency (CE), sign test (ST), and Pearson’s correlation coefficient (Fritts, 1976; Cook and Kairiukstis, 1990). As shown in Table 1, all tests are passed, even for the two most rigorous tests of model validation, RE and CE. These tests demonstrate the validity of the regression model. On the basis of the model, the drought history in the study area has been reconstructed for the period AD 1856d2009 (Fig. 6a). The mean of the reconstructed PDSI from 1856 to 2009 is 1.03, which is significantly below the defined value of zero mean for normal moisture conditions. A significantly negative mean PDSI was also noted in other studies in the central Tien Shan area (Li et al., 2006), the Ortindag Sand Land (Liang et al., 2007) and the Xinglong Mountains (Fang et al., 2009), indicating the need to improve the regional PDSI model. In order not to overestimate the drought conditions, PDSI values of mean þ1s (standard deviation, s ¼ 1.03) or mean 1s were defined as wet or drought conditions. The following discussions are based on the rescaled drought indices. The drought reconstruction shows a strong inter-annual variability throughout the entire period, and the most noteworthy feature in the reconstruction is the low frequency variations. The reconstructed PDSI is above the mean in the first two decades, below the mean until the 1930s, then above the mean again until the end of the 20th century and with an extreme drought echo in the early of the 21st century. Droughts were found in the 1900s, the 1920s and early 1930s, and at the turn of the 21st century. The drought in the 1920s and early 1930s shown in the reconstruction appears to have persisted longer than any drought in the observational record. This drought has been widely reported in the semiarid and arid areas of north China (Liang et al., 2006; Liu et al., 2006, 2009, 2010; Tian et al., 2007; Fang et al., 2009; Yang et al., 2011; Zhang et al., 2011), and led to major economic and life losses. The dry epoch at the turn of the 21st century was shown in both the actual and reconstructed data (Fig. 5), and has also been recorded in the Xinglong Mountains (Fang et al., 2009), Guiqing Mountains (Fang et al., 2010) and the west part of the Qilian Mountains (Yang
Fig. 6. The reconstructed annual PDSI in east Qilian Mountains (a), and the reconstruction of regional summer PDSI (JuneeJulyeAugust) of eastern Qilian Mountains after merging the same four-grid points derived from the Monsoon Asia Drought Atlas (Cook et al., 2010) (b). The three horizontal lines in (a) are the mean of the reconstruction and mean s. The thick lines indicate the five-plot running averaged data.
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Table 1 Calibration and verification period statistics for the reconstructed average PDSI values of the four nearby grid points.
r r2 RE CE Sign test
Calibration (1951e1978)
Verification (1979e2005)
Calibration (1979e2005)
Verification (1951e1978)
Full calibration (1951e2005)
0.58 (df ¼ 26) 0.34 / / 21/7*
0.73 (df ¼ 25) 0.53 0.62 0.43 20/7*
0.73 (df ¼ 25) 0.53 / / 22/5**
0.58 (df ¼ 26) 0.34 0.57 0.28 21/7*
0.71 (df ¼ 53) 0.50 / / /
Note that the df means degree of freedom, the RE indicates the reduction error and CE designates the coefficient of efficiency. One and two asterisks indicate the 0.05 and 0.01 significance levels, respectively.
Fig. 7. Spatial Pearson correlation plots for the actual and reconstructed PDSI with previous September to current August averaged HadISST1 SST during the period 1950e2005.
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et al., 2011). Comparison of this PDSI reconstruction with the summer PDSI reconstruction derived from Cook et al. (2010) shows that there are significant discrepancies between the two reconstructions (Fig. 6). The discrepancies may be caused by the different season reconstructed and/or some remote tree-ring chronologies used in Cook’s reconstruction as they used the “search radius” method with a 1000 km circle. According to Shi et al. (2007), a strong signal of climate change from a warm-dry to a warm-wet pattern has already been observed in northwest China, based on an analysis of the hydrological and meteorological data base, basic circulation patterns and modeling studies for 40 years. This trend has been also found in some treering based drought or streamflow reconstructions (Li et al., 2006; Liu et al., 2010; Qin et al., 2010). However, this trend was more prevalent in the Xijiang, the central-west part at the north flank of the Qilian Mountains and the southeast of Qaidam Basin (Shi et al., 2007). No such trend was found in this drought reconstruction, the rainfall reconstruction of Jiuquan region (Yang et al., 2011) and precipitation or drought reconstructions in the eastern regions (Fang et al., 2009; Hou et al., 2011).
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annual variability and low frequency variations, and droughts were found in the 1900s, the 1920s and early 1930s, and at the turn of the 21st century. Multi-taper method spectral analysis indicates the existence of some significant peaks at 2.6, 4.2 and 56.9 years. Meanwhile, drought variations in the study area are significantly correlated with SSTs in the eastern equatorial Pacific Ocean, northern Indian Ocean, eastern China Sea and the Atlantic Ocean, suggesting possible a connection with the ENSO, the Asian monsoon and the Westerlies.
Acknowledgements The authors thank Emily Derbyshire for her kind help in editing the English writing. The authors also thank reviewers for useful comments and suggestions to improve the manuscript. This research was supported by the National Basic Research Program of China (973 Program) (No. 2009CB421306), the National Science Foundation of China (No. 40971119, No. 41171039 and No. 40890051) and the One Hundred Talents Program of CAS (No. 29O827B11).
3.4. Links with the remote oceans Significant peaks were found at 2.6 (p < 0.01), 4.2 (p < 0.01), and 56.9 years (p < 0.01). The 56.9 year cycle may be related to the Atlantic Multi-decadal Oscillation (Schlesinger and Ramankutty, 1994). The 2e4 year cycle falls within the range of ENSO variability, and has been noted in many moisture-related tree-ring reconstructions in surrounding areas (Tian et al., 2007; Fang et al., 2009, 2010; Liu et al., 2009). The negative correlation of the PDSI with eastern equatorial Pacific Ocean SSTs also suggests a possible connection to ENSO (Fig. 7). The spatial correlations between PDSI and SST during the instrumental period were roughly consistent with correlation patterns identified for the northeastern Tibetan Plateau (Li et al., 2008) and the Guiqing Mountains (Fang et al., 2010). The PDSI also significantly correlated with SSTs of the northern Indian Ocean, East China Sea and Atlantic Ocean. The significant correlations with northern Indian Ocean SSTs suggest a possible connection with the Indian monsoon. The most significant correlations were found with the East China Sea SSTs, suggesting the impacts of East Asian summer monsoon (EASM) on droughts in the study region (Li and Zeng, 2002). Although the study area is located to the west of 105 E, it is also affected by the EASM. In a far western site, Xu et al. (2011) also found the EASM signal. Additionally, a drying trend which has been observed since the late 1950s was found in the drought reconstruction, which is consistent with the decline of the EASM (Liu et al., 2003; Jiang and Wang, 2005). The drying trend has also been noted in other treering studies in the north margin of the EASM region (Liu et al., 2003; Li et al., 2007; Liang et al., 2007). The significant correlations with the Atlantic Ocean SSTs suggest a connection with the NAO, an indicator of the strength of the Westerlies. Located in the north margin of the EASM region, the climate of the eastern Qilian Mountains may also have been affected by the Westerlies at the same time as the EASM was in decline. In the middle and western Qilian Mountains, Zhang et al. (2009) and Liu et al. (2009) found that the Westerlies might have an effect on those areas. 4. Conclusion Based on the three tree-ring width chronologies from Qinghai spruce (Picea crassifolia Kom.) in the eastern Qilian Mountains, the mean PDSI during the period from the previous September to the current August of four nearby grid points were reconstructed back to AD 1856. The drought reconstruction shows a strong inter-
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