1. Introduction
Climatic change and the resulting changes in natural environments have repeatedly influenced human activities during the past 2000 years (Zhang et al. 2007; Büntgen et al. 2011; Kathayat et al. 2017; Butler 2018). With the increasing impacts of climate warming on human society, the relationships between historical processes and environmental changes have attracted increasing attention (F. H. Chen et al. 2019; Williams et al. 2020; Yan et al. 2020; Zhang et al. 2020). With the appearance of increasingly high-resolution climate reconstruction data products and climatic simulation data products, increasingly systematic research on the interaction between the natural environment and human society under the background of climate change has emerged (Büntgen et al. 2011; Guillet et al. 2017; Di Cosmo et al. 2021). Theoretical models connecting climate change to international and intrastate conflicts have incorporated case studies to facilitate statistical research (Ide 2017; Koubi 2019). Suboptimal climatic conditions may lead to livelihood losses, economic declines, and increased risks of armed conflict (Haug et al. 2003; Buhaug et al. 2008; Butzer 2012; Di Cosmo et al. 2017), while optimal climatic conditions can increase material and fuel supplies for imperial expansion and human migration (Büntgen et al. 2011; Pederson et al. 2014). Due to the complexity of its role as a contributing factor to conflict, climate change acts as a “threat multiplier” in many cases, and a previous study revealed that even small climate changes can lead to increasing human conflicts (Hsiang et al. 2013; Huntjens and Nachbar 2015; Dodson et al. 2020). In Inner and Southeast Asia, scientists have focused on the rise of empires and the evolution of civilizations under changing climatic conditions as well as on the social impacts associated with these interactions (Buckley et al. 2010; Zheng et al. 2014; Pederson et al. 2014; Yadava et al. 2016; Di Cosmo et al. 2017; Shang et al. 2020). Although many studies have connected civilization rises and declines to climate change (Binford et al. 1997; Buckley et al. 2010; Pederson et al. 2014; Di Cosmo et al. 2017), few have focused on the links among climatic factors, energy-material flows, and the rise of minority regimes in regional East Asian nations. These interactions between climatic and sociopolitical factors undoubtedly critically impacted cultural exchanges at the boundaries of some civilizations.
The Chiefdom of Lijiang was an important local regime in the greater Shangri-la region on the eastern Tibetan Plateau (Goullart 1955; Rock 2014). Following the Mongol invasion in Yunnan, as the chieftain of Lijiang, A-ts’ung A-liang (阿琮阿良) surrendered to the Yuan Empire and began to serve as the local supreme leader (Mushi huanpu; 木氏宦譜). After the Ming Empire conquered Yunnan, the chieftain A-chia A-te (木得) swore allegiance to the Ming Empire, and Hongwu Emperor Yuanzhang Zhu bestowed him with the Chinese surname “Mu” (木). Since then, the chieftains of Lijiang used this Chinese name and were granted the official position of the “Magistrate of Lijiang” (丽江知府) by the Ming Empire. The Lijiang chieftains helped to maintain the rule of the Ming Empire in the region while also seizing territory from surrounding chiefdoms; the Chiefdom of Lijiang ultimately became the most powerful chiefdom in southwestern China during the late Ming Dynasty. The expansion of the Chiefdom of Lijiang influenced the distribution of ethnic groups on the eastern Tibetan Plateau and promoted communication among different ethnic groups, such as that facilitated by the operation of the ancient Tea Horse Road (He 2020; Yang et al. 2021). Although historians believe that climatic factors may have played a role in the evolution of human civilization in northwestern Yunnan (Dearing et al. 2008; Yang et al. 2021), few high-resolution paleoclimatic datasets are available for assessing the role of climatic factors or the associated vegetation productivity and energy conditions as the Chiefdom of Lijiang expanded. Here, we address whether climate change may have contributed, either directly or indirectly, to the expansion of the Chiefdom of Lijiang and, if so, to what extent. We perform a cross-disciplinary evaluation of tree rings and historical evidence to reveal the role of climate change in the expansion of the Chiefdom of Lijiang.
2. Materials and methods
a. Study region
The study region is located in the Hengduan Mountains on the eastern Tibetan Plateau; the landscape in this region is characterized by large rocks and glaciers, steep gorges, and extensive forests. Many large Asian rivers, including the Lancang River, Yangtze River, and Salween River, originate or flow through this region (Fig. 1a). The Asian summer monsoon brings abundant rainfall to the study region during summer, and most of the winter precipitation is produced by westerly winds in the form of snow. According to the instrumental climate data recorded at the Lijiang weather station (Fig. 1a), the mean annual precipitation total is approximately 957 mm while the mean annual temperature is 12.9°C (the average temperatures of January and June were 6.2° and 18.4°C from 1951 to 2020, respectively). June is the hottest month in the study region (with a mean temperature of 18.4°C), and July is the wettest month (with a mean precipitation total of 246 mm). Lijiang has been a multiethnic region since ancient times; the region comprises 12 ethnic minorities, and the Nakhi people account for 57.7% of the total population. The ancient city of Lijiang was also China’s first established World Heritage Site.
(a) Location of the study area. Also shown are monthly total precipitation (orange bars) and mean temperature (blue symbols) at the Lijiang meteorological station (1951–2020). (b) Locations of the sampling sites and meteorological station (Lijiang) in the eastern Tibetan Plateau.
Citation: Weather, Climate, and Society 14, 4; 10.1175/WCAS-D-21-0144.1
b. Documentary sources
The document sources in our possession all represent the Lijiang Chiefdoms because there is very little systematic documentation of the other nearby chiefdoms, especially historical documentation related to the expansion of the Lijiang Chiefdom. The Mu shi Huanpu (木氏宦谱), which records the genealogy of native officials of the Mu family, and other related Chinese sources contain some wealth of knowledge and represent not just the “Nakhi perspective” but also the position of these figures within the Ming Empire. Comparative analyses of these document sources can provide reliable and mutually verifiable information, and the data utilized in this study are contained in some main compilations, including the Mu shi Huanpu, Huang Ming enlunlu (皇明恩纶录, Sun 2014), and Mufu fengyunlu (木府风云录, Mu 2006). Although there is some overlap among these document sources and significant differences exist in terms of the length and detail of the records contained therein, whenever possible, we used the document source that contained the most detailed historical context relevant for our study. In addition, we obtained historical climate data sources from the Chinese Meteorological Disasters Ceremony (Yunnan Volume) book (Liu and Wen 2006).
c. Tree-ring data and statistical analysis
Some new tree-ring samples were collected from a tree stand site in the Jade Dragon Snow Mountain region (Table 1). The tree-ring samples were obtained from moisture-sensitive Picea likiangensis trees in a closed-canopy subalpine dark coniferous forest; this tree is a dominant species that grows at elevations ranging from 3200 to 3400 m (Bi et al. 2015). The tree-ring samples were air-dried, mounted, measured, and cross dated using traditional processes (Fritts 1976). Then, we used the Cofecha program to assess the quality of the cross-dating results (Holmes 1983). Moisture-sensitive tree-ring data representing other fir sites in Weixi (Fang et al. 2010) and Xiangcheng (Li et al. 2017; F. Chen et al. 2019) were obtained from the National Climatic Data Center (http://www.ncdc.noaa.gov).
Information for the sampling sites and the climate/NDVI grid points.
Site chronologies were constructed using the autoregressive standardization Cook (ARSTAN) program. Some detrending methods (negative exponential and straight-line curve fitting) were applied to remove the biological growth trends contained in the raw tree-ring width series. The biweight robust mean values were then utilized to develop site chronologies with ARSTAN (Cook and Kairiukstis 1990). The reliability of these site chronologies was assessed by the expressed population signal (EPS; Wigley et al. 1984). A standard regional chronology (RC) was developed by averaging all site chronologies, and significant correlations (coefficient r = 0.58–0.65) were obtained between the standard RC chronology and site chronologies over the common period (EPS ≥ 0.85). The standard RC chronology could be considered as reliable after 1400 CE, at which time the tree number was 5 and the EPS value was 0.87 (higher than the widely accepted threshold of 0.85).
The relationships between climatic factors and the standard RC chronology were assessed using a correlation analysis over the 1951–2020 period. To obtain regional climate signals, we used gridded climate data, including monthly mean temperature, total precipitation (Harris et al. 2014), self-calibrating Palmer drought severity index (scPDSI; van der Schrier et al. 2013) and mean normalized difference vegetation index (NDVI; Pinzon and Tucker 2014) data, for our analyses. The data from July of the previous year to September of the current year were used for the correlation analysis.
Some seasonal-mean NDVI subsets were screened in the correlation analysis with the standard RC chronology from the previous July to the current September. To indicate past NDVI variations, the observed NDVI record was regressed against the regional ring-width chronology. The leave-one-out method (Michaelsen 1987) was applied to assess the statistical validity of our reconstructed NDVI model since the NDVI dataset containing observations collected from 1981 to 2015 was not sufficiently long to be separated into verification and calibration periods. The statistical analyses included the calculation of correlation coefficients, a sign test, error reductions, and conducting product-means statistical tests (Fritts 1976).
3. Results
Significant negative correlations (P < 0.05) between the standard RC chronology and temperature occurred in May (−0.39) and June (−0.26). Significant positive correlations (P < 0.05) between the tree-ring chronology and precipitation were found for the prior September (0.21) and the current January (0.30) and May (0.45). Much higher positive correlations (P < 0.01) were seen between the standard RC chronology and scPDSI from the previous year to the current growing season (Table 2). These results indicated that the growth of these coniferous trees was limited by moisture, resembling other previous reported findings (Fang et al. 2010; Bi et al. 2015; Li et al. 2017). Meanwhile, the standard RC chronology was positively correlated with April (0.35) and July (0.36) NDVI of the growth year (P < 0.01).
Correlations between the standard RC chronology and monthly total precipitation, monthly mean temperature, scPDSI, and NDVI. Results are shown for the prior July–December and the current January–September. The two asterisks indicate significance at the 95% confidence level or better.
In general, past experience indicates that as seasonally averaged NDVI is more representative than just one single month, we used the seasonally average NDVI for further analysis (Liang et al. 2005; Coulthard et al. 2017). Thus, we also tested different seasonal NDVI combinations with the standard RC chronology to assess the NDVI reconstruction. The strongest correlations between the standard RC chronology and the seasonalized NDVI were found from April–July (r = 0.635; P < 0.001). A significant positive correlation (r = 0.622; P < 0.001) was also found between the April–July NDVI and PDSI. The drought changes during the growing season from April to July may have imparted the signal that was recorded both by the growing-season NDVI and by tree growth in the coniferous forest. In consequence, the tree growth and growing-season NDVI share a similar variation.
To indicate long-term vegetation productivity changes, we developed the NDVI reconstruction based on the moisture-sensitive RC chronology in this study. The final calibration model (NDVI = 0.513 + 0.047RC) explained 40.4% of the variance in the observed NDVI record during the period of instrumental operation, from 1982 to 2015 (Fig. 2), and explained 33.2% of the variance in the leave-one-out cross-validation results. A positive relative error (RE) (0.328) indicated the significant predictive skill of the reconstruction equation. The result of the sign test (28+/6−) was found to be significant at the 99% confidence level, indicating that the reconstructed NDVI data tracked the direction of the observed NDVI data well (Table 3).
(a) Comparison between actual and reconstructed April–July NDVI values from 1982 to 2015. (b) Scatterplot of actual and reconstructed April–July NDVI values, showing a linear relationship. (c) Reconstructed NDVI values from 1400 to 2020 CE. Highlights include an extended wet period with high NDVI values (1460s–1630s CE; blue shading) and a severe and extended twenty-first-century drought (pink shading).
Citation: Weather, Climate, and Society 14, 4; 10.1175/WCAS-D-21-0144.1
Leave-one-out cross-validation statistics for the NDVI reconstruction. Note that r is correlation coefficient from the leave-one-out cross validation, r2 is explained variance from the leave-one-out cross validation, RE is reduction of error, ST is sign test, PMT is product means test. The two asterisks indicate significance at P < 0.01.
The reconstructed and low-pass-filtered mean April–July NDVI data for northwestern Yunnan are shown in Fig. 2c. The NDVI reconstruction shows considerable low-frequency changes over the past 621 years. The average reconstructed NDVI value was 0.558 over the period from 1400 to 2015 CE. Periods of high NDVI values and wet conditions occurred from 1400 to 1412, from 1466 to 1566, from 1576 to 1630, from 1693 to 1703, from 1709 to 1733, from 1781 to 1794, from 1833 to 1858, from 1894 to 1914, from 1927 to 1952, and from 1990 to 2008 CE. Periods of relatively low NDVI values and dry conditions were identified to have occurred from 1413 to 1465, from 1631 to 1692, from 1734 to 1780, from 1795 to 1832, from 1859 to 1893, from 1915 to 1926, from 1953 to 1989, and from 2009 to 2020 CE. Among these periods, the intervals from 1466 to 1566 and from 1576 to 1630 CE witnessed prolonged wet episodes with above-average reconstructed NDVI values in northwestern Yunnan. The rapid expansion of the Chiefdom of Lijiang occurred during these two wet episodes in the middle and late Ming Dynasty, a historical period characterized by many fierce wars between the Chiefdom of Lijiang and the neighboring Tibetan chiefdoms (Mu shi Huanpu; Mu 2006). The wet episode that occurred from 1576 to 1630 was followed by a dry period beginning in the late Ming Dynasty and then a return to wet conditions until the 1720s. The 2009–20 period was controlled by dry conditions interrupted by only a few wet years, and abnormally low NDVI values also occurred in this period in the context of the past 621 years.
Significant positive correlations with scPDSI and NDVI are found with the Hengduan Mountains in the eastern Tibetan Plateau (Fig. 3). As an indicator of the regional productivity of vegetation, the probability distributions of the reconstructed NDVI dataset provide informative comparisons that are useful for investigating the linkages among climatic factors, vegetation productivity, and the expansion of the Chiefdom of Lijiang. The probability of meeting the normal vegetation condition (0.558) computed using the cumulative distribution function (CDF) of the reconstructed NDVI dataset during the instrumental period (52.9%) was significantly similar to that inferred from the observed record (55.8%). In prior centuries, there was a 6.7% lower chance that the NDVI would meet this normal vegetation condition (Fig. 4a). From the range of NDVI values reconstructed from 1466 to 1630, there was a 69.7% chance that the NDVI would meet or exceed 0.558. Thus, in almost 7 of every 10 years, the NDVI would be sufficient to meet normal vegetation conditions if the future climatic conditions returned to their levels recorded during this period. In addition to being wet, the sixteenth-century eastern Tibetan Plateau was relatively warm (Wang et al. 2015). The temperature anomalies reconstructed over the eastern Tibetan Plateau from 1466 to 1630 CE reveal warm (0.18°C) and above-average [from 1000 to 2005 CE (0.0°C)], but not exceptionally warm, temperatures (Fig. 4b). These records show that the expansion of the Chiefdom of Lijiang occurred under consistently wet and warm conditions.
(a) Spatial correlations of the mean April–July scPDSI values (1951–2020) and (b) mean April–July NDVI values (1982–2015).
Citation: Weather, Climate, and Society 14, 4; 10.1175/WCAS-D-21-0144.1
(a) Cumulative distribution functions of the reconstructed NDVI values for different periods: instrumental (1982–2015), full-length (1400–2020), and wet (1466–1630), as well as recent. (b) The 21-yr spline of the NDVI product obtained in this study and the summertime temperature reconstruction of Wang et al. (2015).
Citation: Weather, Climate, and Society 14, 4; 10.1175/WCAS-D-21-0144.1
4. Discussion
a. Possible effects of the favorable climate
Most materials and energy exploited by human societies on the ancient Tibetan Plateau were derived from the productivity of dry–hot valleys and subalpine grasslands. The Chiefdom of Lijiang, as an important representative of the Ming Dynasty’s interests in this area, would have required a growing concentration of natural resources to maintain its rule, which could have been sustained only by higher regional grain and pasture productivities. In the barren river valleys and subalpine grasslands, extensive agricultural production activities did not typically supply surplus grain and pasture outputs. Both river-valley agriculture and animal husbandry are closely linked with climate conditions because of their dependence on vegetation productivity, and vegetation productivity on the Tibetan Plateau is directly tied to the availability of moisture during the growing season (Zhang et al. 2013; Luo et al. 2018; Zhang et al. 2018; Hua and Wang 2018). The linkages are reflected in the high positive responses of the regional tree-ring chronology and NDVI data obtained herein for the scPDSI values recorded on the eastern Tibetan Plateau (Fig. 3). Although there is no direct evidence of increase in agricultural output, as the effective index of the food and feed production in southwest China (Huang and Han 2014; Zeng et al. 2019; Li et al. 2019; Cao et al. 2021), the NDVI reconstruction still can provide the background information about high vegetation productivity under appropriate climatic conditions from 1466 to 1630. Meanwhile, as compared with the surrounding chiefdoms, the Lijiang Basin is flat and formed a relatively developed agriculture (He 2020). Since the Ming Dynasty, which brought the establishment of many garrison stations and the arrival of Han immigrants, the agricultural technology of the central plains has spread to the eastern Tibetan Plateau, including the northwestern Yunnan (Pan 2019). Rice, wheat, and highland barley gradually became the staple food crop for the Naxi people, and under favorable climatic conditions, rice cultivation has even extended to Batang during the middle and late Ming period (late sixteenth–mid-seventeenth centuries) (Yang 2014). Although there is no direct evidence (continuous food production records) to support the necessary relation between high vegetation productivity and political and economic power of the Chiefdom of Lijiang, the significant increase in cultivated land [more than 2400 acres (∼970 ha)] and population (+20%) have occurred during this period (Li 2001). Meanwhile, the Lijiang chieftains built many bridges, water conservancy facilities, and Buddhist monasteries and organized an army of more than 10 000 soldiers (Li 2001; Mu et al. 2015). From 1560 to 1625, Lijiang chieftains donated 57 800 taels of silver to the Ming Dynasty; during the reign of A-chai A-ssu (木增; 1587–1646), A-chai A-ssu donated more than 30 000 taels of silver to the Ming Dynasty (Sun 2014) and more than 10 000 taels of silver were spent to build the Xitan temple in the Jizu Mountains. After several generations of rulers in Lijiang, the location became the most prosperous city in the region and an important node on Yunnan’s Tea Horse Road. This indirect evidence implies that the Chiefdom of Lijiang has relatively sufficient manpower and material and financial resources in this wet–warm period, and it suggests that the suitable climate and advanced agricultural production technologies may have increase the carrying capacity of the land, which have supplied ample materials and energy for the Chiefdom of Lijiang.
The wet climatic conditions evidenced following the 1460s occurred synchronously with regional political instability on the eastern Tibetan Plateau, characterized by continuous warfare, the destruction of traditional borders, and the reconstruction of the political order that accompanied the expansion of the Chiefdom of Lijiang on the eastern Tibetan Plateau (Mu shi Huanpu; Sun 2014). In the Lijiang area, the chieftains established powerful armies and began to counter invasions by surrounding chieftain troops. According to relevant historical records (Li 2001), the Chiefdom of Lijiang was invaded by surrounding chieftain troops and suffered serious losses before 1462 CE. Following the Mu Qin reign (1442–85), Lijiang chieftains began to conquer the surrounding areas and extended their spheres of influence to the eastern Tibetan Plateau. The continuous expansion of the Chiefdom of Lijiang over more than 100 years eventually led to changes in the distribution of ethnic groups on the eastern Tibetan Plateau and promoted exchanges among these ethnic groups. The historical causes of the deteriorating relationships between the Lijiang chieftain and other neighboring chieftains have been ascribed to social and political events, specifically the invasions of neighboring chieftains (Pan 1999). However, historical evidence is still relatively lacking, especially evidence from the historical records of neighboring chieftains. Regardless of what caused the initial conflicts, we can surmise that the higher vegetation productivity that occurs under optimal climatic conditions impacts regional political and military power balances under specific historical background conditions. Favorable climate conditions enabled the leaderships to obtain more resources with which the chiefdoms could expand. Thus, favorable climate conditions were a potential factor influencing the successful mobilization of military power during the expansion of the Chiefdom of Lijiang.
b. Expansion process of the Chiefdom of Lijiang under suitable climate background
Although many climate reconstruction studies have suggested that the climate was harsh in the middle and late Ming Dynasty (Zhang et al. 2010; Zheng et al. 2014), tree-ring evidence now indicates that the expansion of the Chiefdom of Lijiang was linked with favorable climate conditions that were conducive to increasing the availability of resources, making the expansion of the Chiefdom of Lijiang possible. Of course, climatic conditions and their associated vegetation productivity are not decisive factors but act as geopolitical multipliers in specific historical contexts. To test the above hypothesis, we classified and selected wars waged between the Lijiang chieftain and other neighboring chieftains extending from 1466 to 1618. We noted the following information: war years, occupied territories, leaders, and some historical context (Table 4 and Fig. 5). During the reign of A-ti A-hsi (木嵚; 1442–1485), the Chiefdom of Lijiang began to conquer the surrounding land in line with the rise in NDVI value. Later, during the reigns of A-hsi A-ya (木泰; 1486–1502), A-ya A-ch’iu (木定; 1503–26), A-ch’iu A-kung (木公; 1527–53), and A-kung A-mu (木高; 1554–68), the conquests of the surrounding chieftains continued while the NDVI also continued to be high; during the reigns of A-hsi A-ya (木泰; 1486–1502), A-ya A-ch’iu (木定; 1503–26), and A-ch’iu A-kung (木公; 1527–53), the frequency of war increased significantly (Table 4). The successful expansion of the Naxi people into the surrounding chiefdoms following the 1460s enabled the development of solid political and military forces tied to the Chiefdom of Lijiang. In its more advanced state of expansion, the chiefdom could not support itself with only internal resources; it also had to exploit the resources of the surrounding conquered areas. In the occupied areas, Lijiang’s chieftains levied taxes, and settlers reclaimed land, establishing new strongholds and mining for minerals such as gold and silver (Pan 1999; He 2020).
Lijiang chieftains’ expansion roadmap in the Ming Dynasty.
Citation: Weather, Climate, and Society 14, 4; 10.1175/WCAS-D-21-0144.1
NDVI reconstruction values and their comparison with those identified in historical accounts (Sun 2014; Mu shi Huanpu, 木氏宦谱; Ming shilu, 明实录).
This kind of regional expansion based on favorable climatic conditions depends on strong support from the central government. As the location represents a barrier on the border of the Ming Dynasty, the chieftains of Lijiang showed strong loyalty to the emperor of the Ming Dynasty, contributing gifts to the emperor of the Ming Dynasty following chiefdom expansions and reporting the details of their battles. Correspondingly, the Ming Dynasty acquiesced in the expansion of the Chiefdom of Lijiang and rewarded the chieftains with official position, honors, and material gifts (Sun 2014; Luo 2019) (Table 4). The Lijiang chieftains were deeply influenced by other cultures, as the expansion led different ethnic groups to travel along the ancient Tea Horse Road in order to reach the eastern Tibetan Plateau, thereby promoting exchanges among various cultures. Lijiang’s chieftains respected Tibetan Buddhism and formed political alliances with some living Buddhas to maintain the stability of the conquered areas and donated a large amount of money to build temples and organize Buddhist scriptures (Pan 1999; Zhao 2001; Liu 2015). All these indicate that the expansion of the Chiefdom of Lijiang was closely related to the impacts of favorable political environment and the suitable climate, through the following pathways: 1) favorable climate → increased agricultural production → population growth and increased resource → increased power of the Chiefdom of Lijiang and 2) favorable climate → increased agricultural production → increased resources → development of good external relations → favorable political environment (Fig. 6). From the end of the Ming Dynasty, the climate began to dry, and the NDVI values also declined. At the same time, as a result of the decline in and fall of the Ming Dynasty, the central government was unable to provide strong support to the Chiefdom of Lijiang, and the land occupied by the expanded chiefdom was gradually occupied by the Khoshut Mongols (Pan 1999). Therefore, the role of climate change in the progress of human history is very complicated, and these changes are not just simple fluctuations between in high and low values. To understand the correlations between climatic factors and human history, we must fully consider the historical background conditions during the studied time period.
The driving forces and pathway of political, financial, and social factors leading to the expansion of the Chiefdom of Lijiang during the Ming Dynasty.
Citation: Weather, Climate, and Society 14, 4; 10.1175/WCAS-D-21-0144.1
c. Possible influences of the recent warm–dry trend
Our tree-ring record also reveals the climate changes observed since the late twentieth century in terms of the long-term context, corresponding to a period of rapid economic and social development in China. During the 1990s, the regional climate was wetter than any period during the previous 100 years, with 29 years of above-long-term-mean NDVI values. These wet conditions occurred during a period of widespread economic and social transformation (Fan et al. 2007; Chen et al. 2017). The unusually wet decades of the 1990s–2000s were followed by severe drought periods between the 2010s and 2020s (Fig. 2c). Our tree-ring data suggest that the severity of this twenty-first-century drought was matched only by the 1750s–1760s drought, which occurred during a climatic period of relatively cool temperatures (Wang et al. 2015). The average April–July temperature anomaly recorded instrumentally during the twenty-first-century drought (2010–20) was 1.2°C warmer than the average temperature calculated from the twentieth-century record (1960–2009). After 1990, the reconstructed temperatures were 0.51°C higher than the reconstructed mean from 1000 to 2009 on the eastern Tibetan Plateau (Wang et al. 2015). When our NDVI reconstruction was combined with the tree-ring-derived temperature reconstruction on the eastern Tibetan Plateau, the importance of elevated temperatures in exacerbating the 2010–20 drought became clear.
Unusual climate events, such as the wet periods recorded during the Ming Dynasty and the drought experienced in the early twenty-first century, had far-reaching consequences on the Tibetan Plateau, including ethnic integration and widespread social formations. Temperatures are projected to continuously rise on the Tibetan Plateau more than the global average in future decades (Jia et al. 2019; Lun et al. 2021). If future warming induces increased glacial meltwater and rainfall totals (Brun et al. 2017; Yao et al. 2019), warming-induced drought and water resource reductions, as well as their associated political, economic, and social problems, will likely become more common in East and South Asia.
5. Conclusions
A 612-yr perspective on past NDVI variability has been provided by the tree-ring reconstruction for northwestern Yunnan Province, China. We combined the NDVI reconstruction with historical documents to establish the climatic influence on the expansion of the Chiefdom of Lijiang during the Ming Dynasty. Our results show that the climate in Lijiang was relatively warm and humid during the Ming Dynasty. The warm–wet conditions promoted vegetation growth and resulted in relatively high NDVI values. We propose that high NDVI may have favored the political and military power formation of the Chiefdom of Lijiang. At the same time, the external favorable social and political environment were conducive to the expansion of the Chiefdom of Lijiang, spanning the entire boundary between Sichuan and Yunnan. Therefore, climate change may be the key factor of ethnic exchanges and integration in East Asia during this historical period. Additionally, the external social and political environments were beneficial for the expansion of the Chiefdom of Lijiang, which covered the entire boundary area among Sichuan, Yunnan, and Tibet. Therefore, climate change is a possible key factor for the communication and integration of different nationalities in the Tibetan Plateau during the historical period.
Acknowledgments.
This research was supported by the National Social Science Foundation of China (21XMZ077).
Data availability statement.
The tree-ring width data (WXI and MX) are archived at the tree-ring datasets of NOAA’s National Class Data Center (https://www.ncdc.noaa.gov/data-access/paleoclimatology-data/datasets/tree-ring). The YLXS data that support the findings of this study are available from the corresponding author upon justified request.
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