A 3585-YEAR RING-WIDTH DATING CHRONOLOGY OF QILIAN JUNIPER FROM THE NORTHEASTERN QINGHAI-TIBETAN...

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IAWA Journal, Vol. 30 (4), 2009: 379–394 A 3585-YEAR RING-WIDTH DATING CHRONOLOGY OF QILIAN JUNIPER FROM THE NORTHEASTERN QINGHAI-TIBETAN PLATEAU Xuemei Shao 1 * , Shuzhi Wang 2 , Haifeng Zhu 1 , Yan Xu 1 , Eryuan Liang 3 , Zhi-Yong Yin 4 , Xinguo Xu 5 and Yongming Xiao 5 SUMMARY This article documents the development of a precisely dated and well- replicated long regional tree-ring width dating chronology for Qilian juniper (Juniperus przewalskii Kom.) from the northeastern Qinghai- Tibetan Plateau. It involves specimens from 22 archeological sites, 24 living tree sites, and 5 standing snags sites in the eastern and northeastern Qaidam Basin, northwestern China. The specimens were cross-dated suc- cessfully among different groups of samples and among different sites. Based on a total of 1438 series from 713 trees, the chronology covers 3585 years and is the longest chronology by far in China. Comparisons with chronologies of the same tree species about 200 km apart suggest that this chronology can serve for dating purposes in a region larger than the study area. This study demonstrates the great potential of Qilian juniper for dendrochronological research. Key words: Northeastern Qinghai-Tibetan Plateau; Qilian juniper; den- drochronology; cross-dating; 3585-year chronology; archeological wood. INTRODUCTION One of the aims of dendrochronology is to construct long-term chronologies covering hundreds to thousands of years. These chronologies have major applications to climatic interpretations, radiocarbon analysis, and dating of past events (Lara & Villalba 1993; Scuderi 1993; Hughes & Graumlich 1996; Stahle et al. 1998, 2007; Grudd et al. 2002; Helama et al. 2002; Naurzbaev et al. 2002; Friedrich et al. 2004; Bhattacharyya & Shah 2009; Fang et al. 2009; Guo et al. 2009; Liang et al. 2009; Park & Lee 2009; Sho et al. 2009; Tian et al. 2009; Zhang et al. 2009). Long chronologies have been established in many regions of the world (Ferguson 1968; Hantemirov & Shiyatov 2002; Eronen et al. 2002; Grudd et al. 2002). However, the development of long tree-ring chronologies in China has been limited by the lack of long-lived species and widespread land-use changes over the past few centuries due to rapid population growth. 1) Institute of Geographical Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing 100101, P.R. China. 2) Institute of Archaeology, Chinese Academy of Social Sciences, Beijing 100710, P.R. China. 3) Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100085, P.R. China. 4) Department of Marine Science and Environmental Studies, University of San Diego, 5998 Alcala Park, San Diego, CA 92110, U.S.A. 5) Qinghai Provincial Institute of Cultural Relics and Archaeology, Xining 810007, P.R. China. *) Corresponding author [E-mail: [email protected]].

Transcript of A 3585-YEAR RING-WIDTH DATING CHRONOLOGY OF QILIAN JUNIPER FROM THE NORTHEASTERN QINGHAI-TIBETAN...

IAWA Journal, Vol. 30 (4), 2009: 379–394

A 3585-YEAR RING-WIDTH DATING CHRONOLOGY OF QILIAN JUNIPER FROM THE NORTHEASTERN QINGHAI-TIBETAN

PLATEAU

Xuemei Shao1 *, Shuzhi Wang2, Haifeng Zhu1, Yan Xu1, Eryuan Liang3, Zhi-Yong Yin4, Xinguo Xu5 and Yongming Xiao5

SUMMARY

This article documents the development of a precisely dated and well-replicated long regional tree-ring width dating chronology for Qilian juniper (Juniperus przewalskii Kom.) from the northeastern Qinghai-Tibetan Plateau. It involves specimens from 22 archeological sites, 24 living tree sites, and 5 standing snags sites in the eastern and northeastern Qaidam Basin, northwestern China. The specimens were cross-dated suc-cessfully among different groups of samples and among different sites. Based on a total of 1438 series from 713 trees, the chronology covers 3585 years and is the longest chronology by far in China. Comparisons with chronologies of the same tree species about 200 km apart suggest that this chronology can serve for dating purposes in a region larger than the study area. This study demonstrates the great potential of Qilian juniper for dendrochronological research.Key words: Northeastern Qinghai-Tibetan Plateau; Qilian juniper; den- drochronology; cross-dating; 3585-year chronology; archeological wood.

INTRODUCTION

One of the aims of dendrochronology is to construct long-term chronologies covering hundreds to thousands of years. These chronologies have major applications to climatic interpretations, radiocarbon analysis, and dating of past events (Lara & Villalba 1993; Scuderi 1993; Hughes & Graumlich 1996; Stahle et al. 1998, 2007; Grudd et al. 2002; Helama et al. 2002; Naurzbaev et al. 2002; Friedrich et al. 2004; Bhattacharyya & Shah 2009; Fang et al. 2009; Guo et al. 2009; Liang et al. 2009; Park & Lee 2009; Sho et al. 2009; Tian et al. 2009; Zhang et al. 2009). Long chronologies have been established in many regions of the world (Ferguson 1968; Hantemirov & Shiyatov 2002; Eronen et al. 2002; Grudd et al. 2002). However, the development of long tree-ring chronologies in China has been limited by the lack of long-lived species and widespread land-use changes over the past few centuries due to rapid population growth.

1) Institute of Geographical Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing 100101, P.R. China.

2) Institute of Archaeology, Chinese Academy of Social Sciences, Beijing 100710, P.R. China.3) Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100085, P. R. China.4) Department of Marine Science and Environmental Studies, University of San Diego, 5998 Alcala

Park, San Diego, CA 92110, U.S.A.5) Qinghai Provincial Institute of Cultural Relics and Archaeology, Xining 810007, P.R. China.*) Corresponding author [E-mail: [email protected]].

IAWA Journal, Vol. 30 (4), 2009380

Recently, Qilian junipers (Juniperus przewalskii Kom., better known in China under its synonym Sabina przewalskii) growing in the northeastern part of the Qinghai-Tibetan Plateau have been reported to be several centuries to more than a millennium old (Liu et al. 2005; Liu et al. 2006; Gou et al. 2006; Zhang & Qiu 2007; Zhang et al. 2009). This species provides a unique opportunity to develop long tree-ring chronologies in China. The longest published chronology in China was developed using live and archeological wood of Qilian junipers from sites located near Dulan (Fig. 1) with a length of 2326 years (Zhang et al. 2003), hereafter named as the Dulan Chronology. Later it was extended to 515 BC by Sheppard et al. (2004). However, this chronology suffers from low sample depths before 100 BC and from AD 700 to 900. The availability of long-living trees and numerous logs excavated from the tombs of the Tang Dynasty (approximately AD 618–907) make the eastern Qaidam Basin an ideal region for developing long tree-ring chronologies. During recent investigations in the northeastern and eastern Qaidam Basin, we had the opportunity to develop a Qilian juniper chronology that is longer than the published chronology in China. This work serves two purposes: first, to provide a dating chronology specifically for the archeological wood in the study area, and secondly to examine the spatial extent over which good cross-dating can be expected for archeological dating chronologies and for living juniper chronologies.

Figure 1. Locations of the 51 tree-ring sample sites in the study area (A). Site numbers are identi- fied in Table 1. The two sites in the Qilian Mountains used for comparison are shown in the inset map of China (B).

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97°E 98°E

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36°Nliving treesdead treesarchaeological wood

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381Shao et al. — Tree-ring chronology of Qilian juniper

STUDY AREA

The study area is located in the eastern part of Haixi Prefecture of Qinghai province, northwestern China (Fig. 1). It is situated in the Qaidam Basin that lies on the north-eastern Qinghai-Tibetan Plateau, approximately one hundred km west of the Qinghai Lake, the largest inland lake in China. Branches of the Qilian Mountains are found in the north of the Qaidam Basin, and in the east and south are the Kulun Mountains. Ac-cording to meteorological records from the stations in the study area, the climate is arid and semi-arid and continental in nature. The annual mean temperatures range from -1 to 5°C, with mean January temperatures from -10 to -14°C and mean July temperatures from 10 to 17°C. The annual total precipitation is approximately 50–340 mm, declin-ing from east to west and from high to low elevation. Up to 84–89% of this total falls during May to September. Soils in the study area range from poorly developed loess of 20–50 cm thickness on gentle slopes to thin soils on steep slopes. Some sand dunes are also seen in places northeast of the study area. The study area is characterized by a desert steppe (Zheng 1996) with natural vegeta-tion consisting of various herbs and shrubs of desert species. Trees are almost exclusively conifers, composed of Qilian juniper and Qinghai spruce (Picea crassifolia). They are limited to the mountains between 3450 and 4230 m above sea level (a.s.l.). Due to the lack of meteorological measurements, we can only assume that the presence of the trees in this elevation zone indicates higher precipitation due to the orographic effect. Qilian juniper is an endemic species of northwestern China. As the name implies, the center of distribution is the Qilian Mountains, located to the east of our study area. It is a species with a wide ecological amplitude. Zongwulong Mountains in the study area has been thought as the westernmost limit of Qilian juniper’s natural range (Wang 1993). However, we observed Qilian junipers growing on the westernmost Buerhan- buda Mountains located in the south of Golmud city (94.29 °E) in 2006. The species has been found as far east as the west margin of the Loess Plateau and as far south as the Songfan area in Sichuan Province. This species is hardy and drought tolerant, mak-ing it the dominant species on dry sunny mountain slopes with thin rocky soils in arid and semi-arid regions. Junipers in the study area are widely spaced with tree heights ranging from 3 to 6 m and diameters at breast height from 30 to 50 cm. Occasionally, trees with diameters larger than 50 cm were seen in some stands. On steep slopes, severe soil erosion caused exposure of trees’ roots, while other trees simply grow in large rock fissures. We also observed a few trees with abrasion scars possibly caused by falling rocks and dead trees by lightning. Only at relatively wet sites in the east of the study area do they co-exist with Qinghai spruce, forming dense forests. Genetically similar juniper species include Sabina centrasiatica, S. vulgaris var. jarkendensis, and S. pseudosabina var. turkestanica, which are mostly found in northwestern China. Due to the harsh environment, Qilian juniper growth is extremely slow, forming wood with very narrow annual rings. Around local settlements, junipers have been used for the construction of houses and other structures. Known for its resistance to decay, juniper wood was considered as the top choice for coffin and tomb construction when burial ceremonies were popular. Local people reported that junipers were heavily logged along highways, railways and roads, for construction and heating/cooking purposes.

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2 32

4.5

1.51

0.

40

0.64

ML

= m

ean

leng

th; P

MR

= p

erce

ntag

e of

mis

sing

ring

s; M

S =

mea

n se

nsiti

vity

; MC

= m

ean

corr

elat

ion.

IAWA Journal, Vol. 30 (4), 2009384

MATERIAL AND SAMPLING

Qilian juniper trees that constitute the chronology presented here can be divided into three distinct groups: subfossil trees from archeological tombs, dead trees or snags on the hills, and living trees. Archeological wood was collected from the tombs located mainly on the alluvial fans at the foothills of mountains. In 1999, four tombs located in Dulan County were excavated by archeologists from Peking University, Beijing, China. These tombs are thought to be built in the time of the Tubo Kingdom based on the materials preserved inside (School of Archaeology and Museology 2005). There-after, a few other tombs, also located in Dulan County, were excavated by archeologists from the Qinghai Institute of Archaeology, and the chambers made of Qilian juniper logs in the tombs were preserved. In 2002, numerous juniper logs from two tombs in Xiatatu (XTT) of the town of Guolimu, 25 km east of Delingha, were collected by archeologists. These tombs are again dated to the time of the Tubo Kingdom based on the excavated materials (Cheng 2001; Xu 2001, 2002, 2005). In recent years, many tombs have been robbed and scattered juniper logs and blocks are sometimes seen on the ground. We collected wood discs and cores from such robbed tombs at 16 sites and from excavated tombs at six sites during the period of 2003 through 2007. A total of 22 archeological sites are included in this study (Table 1). Since 1998, tree-ring cores have been extracted from living junipers and standing snags at sites on the Zongwulong, Shalike, Qinghainan, Ela, and Buerhanbuda Moun-tains (Fig. 1). A total of 24 Qilian juniper sites distributed up to 200 km across have been sampled (Table 1). Except for two eastern sites (WL1 and WL2), where mixed forests of Qinghai spruce and Qilian juniper are present, Qilian juniper is the only dominant species at the other 22 sites. Since the percentage of missing rings is high for Qilian juniper growing in this arid region (Shao et al. 2003), we sampled many trees of dif-ferent ages at some sites. We also selected stands from a variety of microenvironments, including hill slope hollows that can collect and preserve more runoff than ridgelines and top slopes, and the upper and lower limits of tree growth. Our sampling strategy facilitates the cross-dating of ring widths and reduces the chance that all collected specimens miss a ring for the same year at a site. To develop a chronology as long as possible for living trees, we have been going back to some sites several times to search for old trees in hard-to-reach locations. We also collected cores from standing snags at five sites, including three at the upper limit of tree growth (sites 23–25). Standing snags were collected to strengthen the sample depth for the earlier part of the period covered by the living trees, so that robust ring width estimations can be produced for the overlapping period between the archeological wood and living trees.

CROSS-DATING

The increment cores were air-dried and then glued to wooden mounts with the trans-verse surface facing up. The cores were sanded with increasingly fine grades of sand paper, to at least 600-grit so that all cellular details of the annual rings can be seen clearly under microscopes. We utilized the skeleton-plot technique (Stokes & Smiley

385Shao et al. — Tree-ring chronology of Qilian juniper

1968; Fritts 1976) to cross-date the rings and the widths were measured to the nearest 0.01 mm using a Uni-versity Model 4 system and a Lintab ring-width measurement system. To confirm that the rings in Qilian juniper in the study area are formed annually, we inspected the cellular characteristics of the rings microscopically and found that the boundary between the latewood of one ring and earlywood of the next was almost invariably sharp, with no evidence of gradation in cell size or wall thickness in most cases (Fig. 2A). Further unambiguous evidence of an-nual rings came from the comparison of the samples taken in 2001 with those taken in 2004 from several sites. Three more rings corresponding to the number of years lapsed were observed in the 2004 samples than in 2001 samples by comparing the narrow-wide ring patterns. It could be concluded that the Qilian juniper in the study area formed only one growth ring in each year under most environmental conditions. For Qilian juniper growing in the arid region, difficulties of cross-dating arise from missing rings associated with the extremely slow growth rates of this species. Therefore, cores from complacent and young trees were dated first since they contained fewer missing rings. The year-to-year variations in ring widths from these trees composed a rough ring pattern to serve as the basis for dating old trees. If there were inter-vals in a core from an old tree in which

Figure 2. Samples of annual rings from Qilian juniper. A: normal narrow rings; B: rings that are too narrow to date; C: a false ring indicated by the arrow (magnified at × 40).

many missing rings occurred or extremely low average growth rates had produced sequences of rings of only one or two cells in width (Fig. 2B), we would separate the core into two or more sections by discarding the problematic segments. A few false rings were identified by their un-sharp boundary between the latewood of one ring and earlywood of the next (Fig. 2C). To locate the missing rings rationally, a specimen with missing rings was verified by cross-dating its ring pattern with other cores in which

A

B

C

IAWA Journal, Vol. 30 (4), 2009386

Figure 3. Patterns of ring-width variations. A: during the period of 850 BC to 600 BC for standing snags and archeological wood (separated by a dotted line), and B: during the period of AD 450 to AD 700 for living trees, standing snags, and archeological wood (separated by dotted lines); black dots show the location of missing rings.

NMS09B

EGS72B

MHG23B

MHG55B

BGT05A

XTT117A

XTT301A

XTT306B

DLH329A

DLH437B

DLH441A

EG041F

EGS61A

EGS75A

NMS11D

WL2S09A

MHG32A

RXM02B

XT101B

ZGR01A

Year AD

BC Year

Rin

g w

idth

(m

m)

Rin

g w

idth

(m

m)

387Shao et al. — Tree-ring chronology of Qilian juniper

there were no missing rings present. Cores with ages younger than AD 1000 from the standing snags were not processed furthermore because the sample depth of the living trees was large enough for the later period. The accuracies of cross-dating and measurements were further checked using the COFECHA program with the default parameters (Holmes 1983). All segments of cores identified by the program as having unusual measurements or significant (p < 0.05) lag- ged correlation with the master chronology were visually rechecked. The calendar dates of the archeological wood were assigned by the COFECHA program using the cross-dated and undated tree-ring series function after deriving the master dating chronology of living trees and standing snags, and the floating dates of the archeological wood samples. The quality of cross-dating among the archeological sites themselves and between the archeological wood, standing snags, and living trees is illustrated in Figure 3, which indicates that the archeological wood, standing snags, and living tree specimens can be cross-dated accurately. In particular, the variation of ring widths in the period 732 BC to 726 BC (Fig. 3A) exhibits an excellent narrow-wide ring pattern for dating the wood of that period. The average 50-year segments intercorrelation from COFECHA is 0.61 for the period 750–701 BC and ranges from 0.59 to 0.67 for the period AD 450–700 (Fig. 3B). From these statistics we conclude that ring-width samples of Qilian juniper can be cross-dated very well in the study area regardless of samples from different groups of trees or from various tree growth limits, upper or lower.

Figure 4. Age profile of selected cross-dated tree-ring series.

Living trees

Standing snags

Archeological wood

-1600 -1200 -800 -400 0 400 800 Year

IAWA Journal, Vol. 30 (4), 2009388

Figure 4 displays the age profile of selected cross-dated tree-ring series for the earlier part of the master chronology. The total time span for the archeological wood is 2373 years from 1580 BC to AD 793. The longest archeological sample covers 1627 years, from 1580 BC to AD 47 with only 20 missing rings (XTT301 in Fig. 4). The total time span of the standing snags is 2563 years covering 769 BC to AD 1794. The longest core of a dead tree covers 1605 years, from AD 58 to 1662 with 18 missing rings (EGS80 in Fig. 4). Some of the cores, such as EGS72, EGS61, and EGS80, form important links between the archeological samples and living trees. The living tree specimens are shortest in total time span, but the earliest ring of the living trees was dated to AD 404, lasting also 1599 years with only 19 missing rings (DLH329 in Fig. 4). The con-sistency in the time span of the longest series among the three types of samples seems to indicate that the maximum longevity of the species in the study area is longer than 1600 years. The maximum overlap is 1562 years between the archeological samples and the standing snags, and 390 years between the archeological samples and living trees. The excellent matching of the age profile of samples (Fig. 4) and strong cross-dating among the three types of specimens enable us to build a precisely dated and well-replicated regional master chronology for the study area.

CONSTRUCTION OF THE MASTER DATING CHRONOLOGY

To ensure the quality of the dating chronology, ring-width series with correlation coeffi-cient less than 0.5 in the output of the COFECHA program (using the default parameters) and series with a percentage of missing rings larger than 2% were excluded in the final chronology development. A total of 1438 series from 713 trees were put together in the COFECHA program for the master dating chronology. The percentage of missing rings is 0.79% in this chronology. The mean length of the series is 555 years, the aver-age mean sensitivity is 0.37, and the series inter-correlation is 0.67. Figure 5 shows the master dating chronology and the sample depth of the entire period. Sample size within the chronology is below ten series before 837 BC and is less than 3 trees before 1458 BC. Therefore, the first 740 years (from 1580 BC to 837 BC) of the chronology should be considered with caution. The sample size in 800 AD is 51 cores. Before AD 800, the maximum sample size is 313 cores around year AD 280; after AD 800, it is 864 cores around the year 1768. Earlier studies on Qilian juniper in Delingha and Dulan have found that the ring widths of the living trees in the study area responded mainly to drought (Zhang et al. 2003; Sheppard et al. 2004; Shao et al. 2005). Inferring from these results, the narrow rings in the dating chronology (Fig. 5) represent dry years. However, climatic inter-pretation of narrow rings before 1000 BC should be cautious due to low sample depth.

COMPARISON WITH QILIAN JUNIPER CHRONOLOGIES FROM NEARBY REGIONS

As mentioned in the previous sections, there are two versions of the Dulan chronology available in the study area. In a comparison between these two chronologies, Sheppard

389Shao et al. — Tree-ring chronology of Qilian juniper

Figure 5. The dating chronology and its sample depth; marked years indicate narrow rings.

IAWA Journal, Vol. 30 (4), 2009390

et al. (2004) identified one ring located at AD 682 missing in the chronology devel-oped by Zhang et al. (2003). When we compared the Zhang et al. chronology with the Delingha chronology, which is approximately 140 km north of the Dulan chronology and was developed using living trees in our earlier study (Shao et al. 2006), we veri-fied that the Delingha chronology contains two more annual rings than the Zhang et al. Dulan chronology in the period between AD 650 and 900. A year-by-year comparison indicates that two rings dated at AD 711 and 875 are absent from the Zhang et al. Dulan chronology (Fig. 6) instead of one at AD 681. We compared the current dating chronology with the Dulan chronology and found that these two independently devel-oped chronologies could be cross-dated perfectly after rings were added in AD 711 and 875 to the Dulan chronology. This suggests that dendrochronological techniques provide highly reproducible results for Qilian juniper, even though the species has a high percentage of missing rings in the study area. To investigate the spatial extent to which this dating chronology can be applied, we compared it with two other chronologies from the Qilian Mountains (Fig. 1B). One of these in the northern part of the Qilian Mountains is drought sensitive (Liang et al. 2006), while the other, located in the middle section of the Qilian Mountains contains temperature and precipitation signals (Liu et al. 2005). Figure 7 shows the two Qilian chronologies created by the COFECHA program and the dating chronology developed in this study for the period of AD 1800 onward. It is obvious that a number of narrow rings occurred simultaneously among the three chronologies. The narrow ring in 1824 was also present in Qilian juniper chronologies from Anyemaqen Mountains (Peng et al. 2007), which is approximately 250 km southeast of the center of the present study area.

Figure 6. A year-to-year comparison of the Dulan chronology (Zhang et al. 2003) (thin line) with the Delingha chronology (thick line) during the period AD 650–900 for the original series (A) and the adjusted series (B); arrowheads indicate the location of the two missing rings added in the Dulan chronology, one at AD 711 and the other at AD 875.

660 680 700 720 740 760 780 800 820 840 860 880 900 Year

660 680 700 720 740 760 780 800 820 840 860 880 900

Rin

g-w

idth

inde

x

Rin

g-w

idth

inde

x

2

1

0

2

1

0

391Shao et al. — Tree-ring chronology of Qilian juniper

Severe and long-lasting drought events are most likely the result of regional atmospheric conditions. These events may have affected tree growth in a large region simultaneously. Therefore, the dating chronology developed in this study can be used in a region larger than the present study area in representing years with slow growth.

CONCLUSIONS

In this study, a precisely dated and well-replicated long regional tree-ring width dating chronology for Qilian juniper was developed using a total of 1438 series from 713 trees. The sampling effort covered 22 archeological sites, 24 living tree sites, and 5 standing snags sites in eastern and northeastern Qaidam Basin. Cross-dating was performed successfully among different groups of samples and among different sites scattered up to 200 km across. The dating chronology was developed using series with a high mean correlation coefficient (> 0.5) and a low percentage of missing rings (< 2%). The chronology is 3585 years long, which is 1065 years longer than the previous longest chronology in China. It correlates with the Dulan chronology developed by Zhang et al. (2003) after two missing rings were added to the Dulan chronology. Our results indicate that dendrochronological techniques provide highly reliable dating of past events using samples from Qilian juniper in the arid and semi-arid regions, although the samples should be treated with caution for missing rings and difficulties in measuring due to

1800 1850 1900 1950 2000 Year

Rin

g-w

idth

inde

x

2

0

-2

-4

2

0

-2

-4

2

0

-2

-4

QF2

SDL

this study

Figure 7. Comparison of the dating chronology (developed in this study) with two other Qilian juniper dating chronologies for the Qilian Mountain.

IAWA Journal, Vol. 30 (4), 2009392

extremely harsh growth environments. In comparison with two other chronologies of the same species sampled approximately 200 km away, we found that narrow rings generally coincide in time, suggesting that some of the past climatic events may have had a widespread impact in the region that limited the tree growth. Therefore, the dat-ing chronology developed in this study can be used beyond the study area, especially in representing severe drought conditions.

ACKNOWLEDGEMENTS

The authors would like to thank all colleagues and students who participated in the sampling field-work. Thanks are also due to Dr. Qibin Zhang for providing the Dulan chronology. This study was supported by grants from the Chinese National Natural Science Foundation (40671194 & 40371118), the National Basic Research Program of China (2005CB422002) and the program from the China Meteorological Administration (ccsf2007-28). Thanks also to the anonymous reviewer who provided valuable constructive suggestions.

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