Social resilience and its scale effects along the historical Tea-Horse Road

The historical THR was a network of trade roads across the Hengduan Mountains in southwest China, which extended from inland China to Tibet and farther to ancient India (Forbes and Henley 2011) (figure 1). It was constructed over 1000 years ago. Although it is globally far less known than the Silk Road, the THR is a similar road network with special historical and geographical implications. It partly originated in the early Qin dynasty (221–207 BCE) and thrived in the Tang (618–907 BCE) and Song (960–1279 BCE) dynasties through to the Republic of China (the mainland period, 1911–1949). In the road network, two major roads connected Tibet with Sichuan in the north and Yunnan in the south (Zhou et al 2010, Forbes and Henley 2011). The north road starts from the region of Chengdu and Ya'an and continues up the mountains to Ganze, Markom, and Chamdo. The south road links the traditional tea-producing area of Pu'er, Dali, and Kunming to the north through Lijiang and Diqing (Zhou et al 2010, Xu et al 2019). Both roads join at the region of Markom and Chamdo and then run westward to Nyingchi, Lhasa, and beyond. It must be pointed out that the above routes are only the main trunk lines of THR that are conventionally recognized (figure 1), and the road locations and directions may vary in certain areas. In addition to the main lines, THR also includes many branch lines that are not well-known or not recognized yet.

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The central area of the THR network is the southeast margin of the Tibetan Plateau, where many ethnic minorities live in remote and steep mountainous areas (Gu 2004). As shown in figure 1, the large THR region typically covers low plain lands in the Sichuan Basin, the middle mountains in Yunan, and the high mountains in Tibet. Accordingly, the major land area is characterized by alpine grasslands, meadows, and shrubs due to high elevation and low air temperature, while farming areas are concentrated mainly on low areas and valleys with a mild climate and fertile soils. The climate in the region varies, largely depending on local topography, but is generally dominated by the transition and combination of both the Indian summer monsoon and the western North Pacific summer monsoon (Wang and Lin 2002, Hao et al 2020), with an average annual temperature of 15 °C to 20 °C and overlapping rainy and hot seasons. The average annual rainfall ranges from 1100 to 1400 mm but is concentrated from May to September (Li 1987, Zhang 1989). With their dense river networks and sharp changes of terrain and elevation (from over 7000 m to approximately 600 m at Chengdu), areas along the THR particularly face more frequent and intense natural disasters; e.g. drought, flood, debris flow, landslides, cold waves, and earthquakes, while the human activities of deforestation, slope cultivation, road construction, and mining increase the risks (Zhou 2014). However, overall, as with a typical mountain society, the THR region people kept living with such disasters and developed continuously through their long history.

This study collected data from various historical gazetteers, chronicles, compiled datasets, and recent research results, which provided information on meteorological and hydrological records, flood events, disaster impacts, and disaster relief, as well as elevation data from digital elevation models (DEMs), and modern social–geographical research findings. Note that because local materials and records of the THR region before the central China power managed the region in the Ming dynasty (1368) are relatively rare, mainly the available literature from the Ming (1368–1644) and Qing dynasties (1636–1912) was examined. Meanwhile, there are much less historical data on the Tibet areas than on Yunnan and Sichuan provinces. Therefore, some published materials in recent times were referred to and reanalysed based on alternative sources.

Specifically, various local gazetteers in the THR region were reviewed to get specific information on hazard events and impacts. Another source of adopted materials was the chronicles of the Ming and Qing dynasties, from which disaster information was collected for this study. It is not possible to list all the gazetteers and chronicles here because sometimes only one sentence or a few words of a chronicle are referred to, and several materials had no clear source. Several thematic datasets compiled in the 20th century further supported essential information at a larger scale with comparable information on other areas of China. Also, the adopted materials came from sources of research publications in recent times, most of which are referred to in the bibliography. The DEM data in figure 1 were accessed from the ASTER GDEM V2 (www.gscloud.cn) by CIAT (Jarvis et al 2008). Details of the major material sources are listed in the supplementary table 1 (available online at stacks.iop.org/ERL/16/045001/mmedia).

According to historical records on natural disasters, in its early time the THR area suffered mainly from earthquakes, floods, droughts, and diseases. Since the Yuan (1271–1386) and Ming dynasties (1386–1644), the development of mining and mountain farming increased the frequency of various disasters. Disaster events were increasingly reported and documented, which provided information on debris flows, landslides, cold waves, frost and snow, fires, and wind disasters. Overall, early records of disasters in the study area were relatively scarce and simple but much more detailed than in later times, especially after the Ming and Qing dynasties.

In terms of changes in types of disasters over time, the THR region does not manifest clear periodic or phased changes in general. However, there were a few periods when specific types of disasters were more concentrated than in others. For instance, Yang argued that there were a quasi-3 year cycle and an 11 year cycle of the beginning times of the rainy seasons in Yunnan. Also, a statistical analysis of natural disasters in Yunnan (details in the supplementary section) found that floods and earthquakes were the most frequently reported disasters documented in the past 600 years. There are many fluctuations, but a general increasing trend can be observed in all the various types of disasters.

The THR region, because it largely overlaps the eastern part of the Tibetan Plateau, is geologically developed with fault structures, strong neotectonic activities with complex and diverse strata lithology, much wind erosion, and significant vapour–air exchange. Those conditions increase the region's susceptibility to disasters (details in the supplementary section). Generally, the spatial pattern of disasters over the large THR area has four characteristics:

• (a)  
  Geological disasters such as earthquakes were densely distributed along the seismic fault belts, and the southeast margin of the Tibetan Plateau suffered them most frequently.
• (b)  
  Geomorphic disasters including landslides and rock collapses were mostly distributed in the deep-cut alpine valleys. Such valleys were concentrated in the THR region because many roads were originally constructed along them.
• (c)  
  The uneven spatial–temporal distribution of water and precipitation led to significant horizontal differences of floods and droughts. The lowland THR regions in Sichuan and Yunnan had severe seasonal droughts and floods, as documented in various records. However, the higher-elevation regions had more stable water situations due to available snow and ice resources.
• (d)  
  From the social–economic perspective, although physical disaster events showed certain spatial patterns, they were concentrated mainly in areas with intensive human activities. Therefore, the extent of disaster losses was closely related to the areas' populations, their levels of socio-economic development, and the property types, values, and specific locations.

Disaster events could cause various damages and losses in all aspects, particularly in agricultural production, housing, schools, and infrastructure including roads, dams, and bridges. However, the macro statistics of disaster impacts are often merely a collection of numbers and cannot show the real situations of specific victims. To provide more ground-based information on the natural disasters, in this section a few specific disaster events and their impacts on local communities and households are analysed.

A level M7.5 earthquake occurred at Xichang on 19 March 1536 (明嘉靖十五年二月二十八日), which affected a large area between the Chengdu Plain and central Yunnan (Jiang 2005). The main effects of the earthquake were house collapses and casualties. Because the earthquake occurred at midnight, the number of deaths was large and not precisely counted. A few archives particularly documented that many local government officials and soldiers died in the disaster. Beyond the significant damage to croplands and the living environment, long-lived rumours and whispers had cast an inextricable shadow on the psychology of the local residents. Most people could not distinguish the authenticity of much of the information, which led to long-term social turmoil and panic (details in supplementary table 2).

A 3 year famine in Yunnan from 1815 to 1817 (嘉庆云南大饥荒) was the largest and most severe famine recorded in pre-modern Yunnan. That disaster resulted from the significantly poor harvest of main crops such as rice and buckwheat caused by low summer and autumn temperatures after the eruption of the Tambora volcano in 1815 (Yang et al 2005, Hao et al 2020). The lowest temperatures were in 1816 when the average temperature in August was likely 2.5 °C–3 °C lower than the annual average; this was called the 'year without a summer' (Oppenheimer 2003). Also, cool winds and drought aggravated the weather conditions so much that rice failed to flower and the millet died in the field without maturing (Hao et al 2020). This case indicates that failed harvests result from bad climate conditions; e.g. cold in 1815, cold and drought in 1816, and drought in 1817. Due to the failed harvests in Yunnan, famine affected 29 of the total 88 counties. Many people died or were forced to migrate. In extreme cases, people ate a type of white clay soil (观音土) but it could have led to death because it was indigestible.

Heavy snow, freezing, and hail are frequent disasters that occurred mainly in alpine areas, especially on the Tibet plateau. Due to its harsh climate conditions, the north and west parts of Tibet had more snowstorm and freezing disasters, whereas the east and south areas suffered more from hail (Zhou 1990). Such disasters may not have directly caused loss of human life, but they affected livestock and crop production, which in turn endangered the living conditions of local farmers and herders (details in supplementary table 2). Some snowstorm events lasted 3–5 months, which caused serious food shortages and livestock deaths from freezing. The loss of livestock production resulted in pastoral families descending into extreme poverty. Many residents had to migrate, and livestock markets closed (Zhou 1990, Sun 1999). Hail disasters had relatively less impact, but they occurred suddenly and were difficult to prepare for. Because hail disasters occurred mostly in summer and autumn in valley areas (e.g. Shannan and Nyingchi) with relatively advanced agricultural production, the major loss from hail disasters was a failure of the crop harvest (Ni and Xie 2007).

Another typical type of disaster in the THR region was geomorphic disasters due to the unique landforms of high mountains and deep valleys with intensive human activities, such as those along roads with much traffic. Most of those disasters were site-specific and based on the local geomorphic environment, and occurred over short durations; i.e. often from a few minutes to hours. The spatial effect of a single disaster was relatively small, and there was rarely an individual geomorphic disaster that caused large-scale damage (and thus resulted in fewer relevant historical records). However, the power of disasters can make them enormously destructive, and they occurred broadly across many sites of the THR region from the past to the present (Shang et al 2003, Wang et al 2019). Geomorphic disasters could result in deaths of humans and animals, damage to roads, buildings, and hydropower stations, and subsequent floods and river siltation.

4.1.1. Central governing system

4.1.2. Local governing system

4.2.1. Agricultural technologies

4.2.2. Infrastructure and engineering technologies

4.2.3. Early warning and forecasting technologies

4.3.1. Social charity and voluntary assistance

4.3.2. Social and family network relief

Due to the lack of scientific and technological knowledge, cultural and religious thought played an important role in the construction of local disaster concepts. The ideological and cultural forms, as well as the belief systems of the specific historical area, also influenced and reflected disaster response models. In the continuous adaptation to natural disasters, many subareas along the THR formed unique disaster cultures, which are reflected in many aspects such as folk beliefs, religious ceremonies, living etiquette, agriculture production, and daily life. For example, in the Yi minority region (彝族地区) at Xichang, the oral culture represented by Erbi (尔比) is rich in local knowledge about disaster reduction with regional characteristics, which reflects the Yi people's perception of climate change and the prediction of and preparation for disasters. Their disaster reduction measures in daily life carry their rich experiences in adapting to the habitat environment; they have been maintained through the family clan system. These unique cultural matters have been passed down from generation to generation and form the local knowledge system related to disaster reduction.

As analysed in section 4, officials, gentry, and ordinary people have all made various efforts to cope with disaster impacts, involving different levels of prevention and relief measures before and after the occurrence of a disaster. Each measure could have a different influence on the resilience of various entities at a certain spatial scale and over a certain period (figure 2), which is expressed synthetically as scale effects. The systematic implementation of these measures ensured the overall resilience of the THR social system.

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The resilience measures were further classified into nine types, which covered multiple dimensions regarding governance, technology, and social and cultural aspects (figure 2). For each type of measure, its functioning suitability at different spatial and temporal scales was identified; i.e. at what scale the measure is effective. In figure 2, the scale effects are illustrated on the x axis (temporal scale) and y axis (spatial scale). In addition, the effects of each measure were qualitatively assessed in terms of their utility and effectiveness in enhancing the resilience to flood disasters. The qualitative assessments were made relative to each other based on statements in the literature cited in this study as well as on some general knowledge. For instance, measures of the central government could recover from post-disaster recovery in a few weeks or months or to systematic and large-scale water conservancy projects in years or decades. Many regions have their own experience, culture, and beliefs regarding disaster management, which could benefit resilience enhancement at all times. However, these measures generally have little actual effect in the practice of disaster relief, thus many black bubbles occupied various scales but with small sizes (figure 2).

In analyzing the various resilience measures, it was found that the suitability and effects of each measure varied when the targeted spatial scope changed. A certain measure was effective only in some suitable areas. For example, local governing measures can be implemented only in their responsible territories, and social networks can be helpful often only at a physically reachable region where people can help each other in person. Local governing measures may contribute very little if the aim is to improve the resilience of the nation as a whole or a few individual households or individuals. It is therefore argued that the specific spatial scope must be clarified when judging the effectiveness of a resilience measure. Note that the scale effects of these measures in figure 2 were evaluated separately. The bubble size indicates the effect of each individual measure being implemented separately. However, the different measures were generally implemented in parallel and with superimposition and integration, which enabled a comprehensive resilience to disasters at various scales, from individuals to national and global scales. In the case study of the THR area, there had not been any measures to improve resilience at the global scale (figure 2), because the global scale was not involved in this case study, no flood event had affected the globe, and no countermeasures could work at a global scale.

Due to the different spatial scale effects of the resilience measures, the resilience of an entire social system also manifested largely in different spaces. A city is often more resilient to disasters than a village because a disaster event may greatly destroy a village but is unlikely to destroy an entire city (assuming that the city has a larger area, a higher population, more diverse economic sectors, and so forth). Therefore, the major functions of a larger urban system could still operate when parts of it are destroyed. This means that a larger urban system is more resilient than a smaller village system, which could completely lose all functions. Many components in a large, complex system could be substitutable, so the system as a whole can survive and continue after a catastrophic impact on several components. In other words, in the case of several households damaged in a flood event, there could be low or no resilience of those households to the event, but an entire community or city could have high resilience to the event. In this regard, it is generally true that a larger social system with diverse components is more resilient than a simple smaller social system. Thus, it makes significant sense to clarify the spatial scale and the entity when considering the resilience of a social system.

This further indicates that diversity can support a resilient system with a self-regulation mechanism and a bottom-up decision-making process. The resilience of the THR area is due to the combined result of many interactive measures, including empirically wise and scientific coping measures, the motivation to pursue economic and trade benefits, hard work and patience in life, and the strong religious beliefs of the local people. Beyond that, the area also received significant governance, regulations, and assistance from powerful central dynasty authorities.

Similar to the spatial scale effects, the effectiveness of resilience measures performs a dynamic role over different time periods. For instance, a blood-based family network may lose its cohesion after it develops over generations, so the existing relief function among family members will become less effective or even disappear. Meanwhile, the next generation will create its own new family network, which could update or replace the fading network of the old generations. Technologies of disaster prediction and prevention could develop and advance to be more precise and broadcast timely. Others, like agricultural techniques, could be generally adopted and eventually internalized as a normal state of the social system, which may not be considered as a disaster-resistant measure but indeed could enhance social resilience in general. As shown in figure 2, although very few single measures could keep direct effects over periods at century level, many can indeed be repeatedly and continuously implemented and transformed to more advanced measures. Therefore, the effects of resilience measures can actually be accumulated and continued at a century scale and longer.

Accordingly, the overall social resilience is also dynamically strengthened and improved as time evolves. Especially, people learned new knowledge and skills from past disaster relief activities, which enriched the social knowledge system and were transferred to future generations. Here, Friedrich Nietzsche's motto 'What does not kill me makes me stronger' is appropriate, and it could be argued that as long as a social system is not completely destroyed, it could and should develop to be more resilient to a repeated disaster event. This may not apply to very small scales like individual people or households because they are often the direct victims and can be dead or completely destroyed in flood events. Rather, the resilience of larger entities (e.g. villages, towns, cities, nations) can be and was indeed continually developed and strengthened over time.

It is true that the ancient THR has declined in recent times, together with its tradition and culture of transporting goods by human carriers and horseback through the rugged mountain paths. It is also true that many people migrated out, villages were abandoned or merged, very little trace of horseback transport remains, tea and horse trades were significantly reduced, etc. However, note that the decline was caused mainly by the general development of social economy and technologies, not the impacts of natural disasters and environmental stresses. The decline may be better called a transformation; i.e. the upgrading of the THR area to a more advanced and more resilient social system. The livelihood and living conditions of residents as well as local infrastructure and social connections have all largely improved. With these, the human society in the THR area has become much more resilient in the face of disasters that it used to suffer.