Monpa, memory, and change: an ethnobotanical study of plant use in Mêdog County, South-east Tibet, China

Background Due to their relative isolation, the previous studies of Monpa plant use were only conducted in north-east India. In October 2013, Mêdog County was no longer remote, thanks to completion of a highway into the county. This study of plant species used by the Monpa had three research objectives. These were (i) to identify and record local names and uses of plants in Mêdog County, (ii) to assess which of these were uses of endemic or near-endemic species within this part of the Indo-Burma biodiversity hotspot, and (iii) to assess how plant uses reflect socio-economic change in Mêdog County? Methods Field surveys were conducted in 12 villages of four townships in Mêdog County, Tibet, China. Two field visits were made. The first field trip was in November 2017 and the second field trip was in May 2018. We interviewed 64 key informants between 21 and 84 years old. Most of them were the village leaders and other local people who are knowledgeable about plants. After transect walks with knowledgeable local people, we used free listing, key informant interviews, and semi-structured interviews during the field work. Plants traditionally used by the Monpa were documented. Utilization frequency was used to assess the significance of each species, and the Cultural Importance index was used to estimate the cultural significance of the species in common. We also used the informant consensus factor (FIC) to determine the homogeneity of the informants’ knowledge of medicinal plants. Results One hundred ninety-four plant species belonging to 82 families and 158 genera were recorded and collected. One hundred twenty-two species, primarily fruits, were food plants. Forty-five species were used as traditional medicines. This included highly valued species collected in alpine areas (Paris polyphylla) and brought to villages in Mêdog, which are at a lower altitude (between 728 and 1759 m a.s.l). Seven edible plant species were also used as herbal medicines. We also recorded 39 species used for other purposes in Monpa daily life. These included nine species that were used to make agricultural tools, five species for dyes and mordants, four species for timber, three species for fuelwood, four species for religious ritual use, three species for washing, two species for incense, two species for thatching, two species for fiber (rope and paper), two “calendar plants” were used to indicate seasons for agricultural purposes, two fish poison plant species, and one species were used as a tobacco substitute. Based on taxonomic insights and from studies elsewhere, we suggested that fiber species were under-reported (c. 14 species were used vs. one species reported used). Even though these plant species are rich and diverse, the use of endemic or near-endemic species was rarely recorded in previous studies. These species included Arenga micrantha (used for starch), Hornstedtia tibetica (fruits), Castanopsis clarkei (edible nuts) and Gnetum pendulum (edible nuts), Ophiorrhiza medogensis (vegetables), Derris scabricaulis (fish poison), Radermachera yunnanensis (agricultural tools), Litsea tibetana (seed oil), Dendrocalamus tibeticus (wine strainers and implements for administering medicine), Zanthoxylum motuoense (spices), Cinnamomum contractum (tobacco substitutes), Morus wittiorum (medicines), and Garcinia nujiangensis (funeral rituals). Despite the absence of roads until 2013 and the impression of “isolation,” Monpa knowledge of plant use reflects three categories of change. Firstly, oral histories of plants used in Bhutan were also encountered by Monpa people after their migration from Bhutan to south-eastern Tibet. Secondly, a “slow change” through centuries of exchange of knowledge (for example of Chinese and Tibetan medical systems), seeds of introduced crops (finger millet (indigenous to Africa), maize (from Meso-America)), and experimentation and use of introduced medicinal plants (such as Datura stramonium, which originates from North America). Thirdly, “fast change” over the past decade. This is reflected in changes in traditional architecture and in rising commercial trade in selected plant resources such as Dendrobium orchid stems and Paris polyphylla rhizomes which are in demand in China’s Traditional Chinese Medicine (TCM) markets). Conclusions Monpa people in the south-eastern Tibet have detailed knowledge of the diverse plant resources. But that traditional knowledge is now faced with a crisis because of the modern socio-economic change. In addition, Monpa knowledge of plants reflects slower changes in knowledge as well. For example, Monpa ethnomedicine has been influenced by traditional Tibetan and Chinese medicine over a longer period in time. Overall, this study provides a deeper understanding of the Monpa peoples’ knowledge on wild plants, including endemic and near-endemic species whose uses have not been previously recorded. Several of these narrowly distributed species, such as the fish poison Derris scabricaulis, could be the focus of further studies. Some wild edible plants may also have interesting dietary constituents which need in-depth studies. These detailed studies could enable the Monpa people to benefit from the use of their traditional plant-derived culture and therefore support the biodiversity conservation.


Introduction
This study was carried out in Mêdog County in the south-eastern Tibet, at the edge of two "biodiversity hotspots" which were the Himalayan and the Indo-Burma biodiversity hotspots. Due to its biogeography, topography, and altitudinal range (a difference of 7500 m over 40 km), south-east area of Tibet has high levels of biodiversity. South-eastern Tibet is also culturally diverse, with Tibetan, Hui, Monpa, Lhoba people developing cultural landscapes through farming, pastoralism, wild plant use, and management. The need to combine traditional ecological knowledge with management strategies to achieve biodiversity conservation in local beliefs and practices is well recognised [1]. In their review of conservation needs for the Indo-Burma biodiversity hotspot, for example, the CEPF (2012) stressed the need for "greatly improved information on status and distribution in Indo-Burma to highlight species for which available information is so limited that it precludes any form of meaningful conservation action". This study is a contribution toward both conservation and development.
Although a few studies have been done on Monpa plant use in north-east India [2], this is the first study of Monpa ethnobotany in this formerly remote part of China. No longer remote, this area is undergoing rapid socio-economic change, which may lead to declining knowledge of local plant uses. In other parts of the region, however, 68 medicinal plant species were recorded as used by Tibetans in Shangri-la, Yunnan, China [3]. Traditional knowledge of 168 wild edible plant species were recorded in Tibetans of Shangri-la region, Yunnan, China [4]. The uses of 81 species of vascular plants were recorded in Tibetans of Zhouqu county, Gansu, China [5]. The uses of 54 species of vascular plants and 22 mushrooms were recorded in Tibetan community of Zhagana in Tewo County, Gansu, China [6].

Study aims and research objectives
The aim of this study was to document the ethnobotanical knowledge of the Monpa people in Mêdog County, south-eastern Tibet. As no previous studies had been done on Monpa plant use in China, we had three research objectives. These were (i) to identify and record local names and uses of plants in Mêdog County, (ii) to assess which of these were uses of endemic or nearendemic species within this part of the Indo-Burma biodiversity hotspot, and (iii) to assess how current plant uses reflect socio-economic change in Mêdog County?

Location of the study sites
Mêdog County is the most remote county in the southeastern Tibet, which covers a total area of approximately 31,395 km 2 , located in the lower river valley of the Yarlung Tsangpo River. It is called "Pe-ma-ko" by the Tibetan Buddhists, meaning "hidden lotus." The average annual temperature in Mêdog County ranges from 16 to 18°C. The lowest temperature is 2°C in January and the highest temperature is 33.8°C in July. The annual average rainfall is 2350 mm, the frost-free period is about 330 days, and the average humidity is over 80%. In the horizontal distance of 40 km, Mêdog County has an altitudinal difference of more than 7500 m, with plant species ranging from alpine plant species to tropical plant species and complex vegetation structures [7,8].
We had selected 12 villages belonging to four townships in the Mêdog County from different altitudes for investigation (Table 1). Monpa people accounted for 99.2% of the total population of the Beibeng Township. Dexing Township is the nearest township to the Mêdog County. It is located on the right bank of the Yarlung Tsangpo River, across the river from Mêdog County. Bangxin Township is situated in the northeast of Mêdog County and "Bangxin" which means "flat land" in Tibetan language. Jiaresa Township is located in the northern part of Mêdog County and is one of the three remote towns in the Mêdog County (Table 1).
The location of the 12 study villages in Mêdog County are shown in Fig. 1

Cultural background: Monpa in Mêdog
The literal meaning of Monpa is "man of the lower country," referring to several ethnically related peoples, which may not be related linguistically [9]. In the early to mid-eighteenth century, due to the hardships and heavy taxes in Bhutan at the time, the local Monpa people heard that there was a sacred place in the southeast of Tibet, called Pe-ma-ko (now Mêdog), where there was the chance for a better life, so they decided to flee from Bhutan to Tibet [10]. The oldest record of mentioning Monpa was the Tibetan epic of King Ling Gesar during the fourth century [11]. The language used by "Monpa" belongs to the Tibeto-Burman language group of the Sino-Tibetan language family [9]. However, the Monpa people do not have their own written characters. Consequently, the history of the Monpa people is known through oral histories and Tibetan literature [12]. In 1964, the Monpa group was officially recognized as an independent ethnic group by the People's Republic of China [13] and approximately 25,000 Monpa currently reside in the low-altitude areas of Tsona, Nyingchi, and Mêdog in the Tibet Autonomous Region of China [14]. In China, the highest population of Monpa speakers lives in Mêdog County, a biologically and culturally diverse region that is 34,000 km 2 in extent.

Field survey
Ethnobotanical fieldwork took place over 45 days spreading between November 2017 and May 2018. We

Data analysis
Ethnobotanical quantitative indices including utilization frequency, informant consensus factor (FIC), and cultural importance index (CI) were adopted. The use frequency of certain species was estimated by utilization frequency: Where N m was the number of certain species mentioned by informant, and N i was the total number of informants. High f values indicated the plant used frequently [15].
FIC was determined using the following formula to evaluate the information of medicinal plants distributed between informants and to determine the homogeneity of informant's knowledge on medicinal plants.
Where N ur was the number of use reports from informants for each ailment category, and N t was the total number of species used by all informants for this ailment category. Values of FIC ranging between 0 and 1. High FIC values (close to one) showed the agreement among the informants about this ailment category. On the contrary, low FIC values (close to zero) showed disagreement among the informants [16].
Each species mentioned by an informant within one use category was a use report (UR). Cultural importance index (CI) was used to indicate the spread of the use (number of informants) of each species as well as to determine diversity of uses.

Results and discussion
In contrast to the staple diet of naked barley, yak meat, mutton, buttered tea, and barley wine of traditional Tibetan, they seldom eat vegetables or fruits, yet plant resources in Mêdog County are more plentiful. Monpa people have a wide variety of vegetables and beverages in their traditional daily diet. The most frequently used part is the fruit (58 species, 47.5%) in this study, which is consistent with the situation in northeast India, which has similar bio-climatic conditions [18]. This demonstrates the rich diversity of wild fruits and vegetables in the region, providing local residents with sustainable economic pillars and livelihood security by targeting wild edible fruits and vegetables that can be developed commercially [18]. Out of 122 wild edible species, seven are also used as herbal medicine. Equisetum ramosissimum, for example, the root of this species is usually eaten directly as a fruit and boiling the aerial part could be used for treating rheumatism. Wild edible plants with high CI values may have peculiar dietary constituent and require further research. In addition, the alcoholic beverage consumed by almost all Nepalese and Tibetans (known as "jnard") which has the same ingredients as this yellow "wine" in Mêdog County [19]. Seasonal fishing and fishing by poison are also great economic activities for many tribal people in the world. Studies in Nepal recorded that four entirely different plant species exploited as fish poison plants [20]. Tsering et al.'s study was focused on higher altitude species used by Monpa people, including the medicinal plants Aconitum heterophyllum, Neopicrorhiza scrophulariiflora, Paris polyphylla, Rhododendron hodgsonii, Swertia chirayita, and Taxus baccata [2]. In our study, one of these higher altitude species was recorded (Paris polyphylla).

Edible fruits and vegetables
Food categories include fruit, vegetable, starch, oil, nut, beverage, condiment, and forage (Fig. 3). The most widely used wild edible species are fruits (42  County, whose ripe fruits can be taken orally and act as medicine for aperient and juice of the tender leaves cures oral ulcers in the district Udhampur, J&K, India [22]. Monpa people have been using stone casseroles as cookware since ancient times [23]. Monpa people like to eat "hot-pot" dishes in the stone casserole including wild vegetables and meat which are a popular food combination in Mêdog County. Wild vegetables with high CI values included Crassocephalum crepidioides (0.78), Pimpinella diversifolia (0.56), Rorippa dubia (0.31), Solanum torvum (0.2), and Gonostegia hirta (0.08). Gonostegia hirta is a leafy vegetable that is also used as a functional food to provide energy for children and elders [24]. It is interesting that while it is used as a medicine in Bhutan [25], Entada rheedii is a popular addition to food by Monpa people. The cooking method for Entada rheedii is time-consuming due to the toxicity of the seeds. The seeds have to be detoxified by leaching and heating. The seeds are boiled with water, which is poured off each time to clear away the toxicity, then refilling the pot with water, repeating the process more than ten times. The Entada rheedii seeds are then cut into pieces and fried with rice.
Other categories were less frequent in use such as forage (13), starch (seven), beverage (seven), oil (six), spice source plant (five), and nuts (two). However, Monpa people have a rich tradition of extracting beverage, starch, and oil from specific plants in the region.

Fermented beverages
Traditional consumption of alcoholic beverages is an ancient tradition that is still an integral part of Monpa society. The Monpa people have the traditional custom of "three bowls of wine", meaning that guests have to drink three bowls of yellow wine before they enter the door to show their friendship. Seven plant species were used to produce a yellow "wine." The mainly ingredients were rice (Oryza sativa L.), maize (Zea mays L.), Eleusine coracana, and Fagopyrum esculentum. Buddleja asiatica is the most important plant species during the preparation of alcoholic beverage fermentation by Monpa people in Mêdog County. Based on the uses of Buddleja lindleyana and Buddleja officinalis in coloring rice yellow [26] and B. officinalis in indigo fermentation [27], we suggest that B. asiatica is a dual purposeful species, as both a wine colorant and a source of microorganisms that an aid to fermentation (Fig. 4).

Starch sources
Starch in Monpa people diets was supplemented by starch processed from wild species. Cultivated starch sources were from both cereal crops and cultivated tubers ( Table 4 in Appendix). The cultivated cereals were buckwheat (Fagopyrum esculentum and Fagopyrum tataricum), finger millet (Eleusine coracana) rice, and maize. Three taro (Colocasia) species and four yam (Dioscorea) species were cultivated as starch sources ( Table 4 in Appendix). Of these, Dioscorea alata tubers have 80% starch in dry matter [28] and Colocasia esculenta have 70-80% starch in dry matter [29]. Wild species also provided supplementary starch sources (excluding the starch-rich seeds of Entada rheedii mentioned in the previous section). Two of these were palms ( Table 4 in Appendix) in the genus Arenga. As Ellen points out [30], this is one of main starchproducing palm genera used for food in Asia, the other genera being Borassus, Caryota, Corypha, Eugeissona, and Metroxylon. Starch production from Arenga micrantha is poorly known as it is endemic to Mêdog County [8] and is documented in this study (Fig. 5). In contrast, Arenga pinnata is widely distributed in Asia, where it is most commonly served as palm sugar [31], but is also used as a starch source [30]. Other starch sources were Alsophila articulata and Chenopodium album. Today, the uses of Arenga micrantha, Arenga pinnata, and Alsophila articulata are almost abandoned. This decrease in consumption of these wild starch sources has increased the production of cultivated cereal crops in Mêdog County [32].

Seed oils
The Monpa in Mêdog County totally used six wild edibles as the source of oil and fats. One of these species, Perilla frutescens, was also widely used in various tribal groups of the northeast India [33]. Moreover, Perilla frutescens oil is rich in natural compounds that could be developed as nutraceuticals and/or phytomedicine [34].

Medicinal plant use
According to our survey results, 45 plant species are used as herbal medicines for treating 13 different categories of human ailments. Botanical and ethnobotanical information about these plant species include scientific name, family name, vernacular name, part(s) used, the method of preparation, the ailments treated, and voucher specimen number. Just over half medicinal plants were herbs (51.1%). This agrees with reports from the lower elevation of Bhutan that most of the ethnobotanical plants were herbs [25]. The common use of herbs as sources of medicine found in this study were also indicated by studies conducted elsewhere Fig. 4 The "yellow-wine" production process. a "Cakes" used to start rice "wine" fermentation drying on a bamboo rack. These are made from a mix of species based on a secret recipe. b Preparing the starch base for "yellow wine" from rice (Oryza sativa), maize (Zea mays), Eleusine coracana, and Fagopyrum esculentum. c Finger millet (Eleusine coracana). d Straining the wine using a strainer made from Dendrocalamus tibeticus culms. e Ready for a welcome drink of three cups of yellow wine: cultural and social values underpin the continued production of the yellow wine [35,36]. Leaves (40%) are the most predominantly used parts of these medicinal plants, followed by roots (22.2%), fruits (15.6%), seeds (11.1%), stems (6.7%), whole plant (6.7%), branches (6.7%), and burgeons (4.4%). The preference for leaf has also been recorded among the traditional Tibetan doctors of Mustang district of the north-central part of Nepal [35]. Uses for all illnesses for wild medicinal plants are locally classified into 13 categories (Table 2). These are skin and subcutaneous tissue diseases, circulatory system, immune system, genitourinary ailments, neurological diseases, inflammation, gastrointestinal ailments, endocrine and metabolism disorders, respiratory system disorders, leech bites, snake bites, abortion, musculoskeletal system disorders, and other diseases. FIC results for the 13 illness categories ranged from 0 to 0.75, with the highest for musculoskeletal system disorders (FIC = 0.75; two species, five use-reports), immune system diseases (FIC = 0.67; two species, four use-reports), and respiratory system disorders (FIC = 0.6; three species, six usereports) ( Table 2). One of the important livelihoods of the Monpa is hunting; the highest FIC for musculoskeletal system disorders is related to the damage caused by the accidents.
An empirical observation on the use of medicinal plants by the Monpa people of Mêdog County study area requires cross-validation with published literature on phytochemical and pharmacological properties of medicinal plants reported in this study to verify their effectiveness. Our literature review of 21 medicinal plant species shows that local uses are generally consistent with known pharmacological properties. And based on a literature review, 11 medicinal plant species had partial uses similar with reported pharmacological properties. Literature studies indicated that seven species, namely Artemisia vestita, Coix lacryma-jobi, Equisetum ramosissimum, Oxalis corniculata, Persicaria capitata, Uncaria rhynchophylla, and Uncaria scandens, were used in Tibetan medicine to treat the same ailments [37]. Four other species (Curcuma aromatica, Dendrobium catenatum, Elaeocarpus braceanus, Sambucus williamsii) were possibly the substitutes for Curcuma longa L., Dendrobium nobile Lindl, Terminalia chebula Retz., and Sambucus adnata Wall. in Tibetan medicine. And only one species (Paris polyphylla) was used for different purpose by the Monpa than in Tibetan medicine.
Comparison of the information on traditional medicinal plant use of Monpa ethnic group with ethnobotanical studies conducted in the lower elevation of Bhutan, which has similar bio-climatic conditions [25], shows that only one wild medicinal plant, Datura stramonium, is used in the same for toothache (Table 3).

Natural dyes and mordants
Strobilanthes cusia (Yang-shar-pa), Rubia wallichiana (Lae-nyi), Rubia membranacea (Lae-nyi), Eurya acuminata (Zem-shing), and Curcuma longa (Dgrong) are the commonly used species in traditional dye processes. Fresh stems and leaves of Strobilanthes cusia are a well-known indigo source [78]. Boiling the stems of Rubia membranacea are used as a red dye. Mashed Curcuma longa tubers are used to dye threads yellow, while Eurya acuminata is used to dye threads green. In India, Eurya acuminata is used as a mordant together with Rubia cordifolia [79]. Whether E. acuminata is used in Mêdog as a mordant with Rubia membranacea or Rubia wallichiana is unknown at this stage, but is worth further investigation. This is because the genus Eurya (Pentaphylacaceae) is a known aluminium hyperaccumulator group [80] that are effective as mordants for red dye processes together with anthroquinone rich dye species, including Rubia [81].

Paper making
The Monpa community in Linzhi city is famous for hand-made paper for religious scripts that is made from the stem bark of Edgeworthia gardneri. Peeling the stem bark and removing the outermost layer of the stem bark, the remaining parts are soaked in the water, then stir the solution into a viscous state, pour the solution into a wooden flat mold, and dry it into a paper. Linzhi paper is better than Tibetan paper produced elsewhere.

Fibers for rope and string
Although Edgeworthia gardneri (Thymelaceae) can also be used for making rope and string, the value of this species for paper making may be a reason why this alternative use was not mentioned by local people

House construction, tools, and utensils
Morus wittiorum, Celastrus glaucophyllus, Terminalia myriocarpa, and Pinus wallichiana are the main timber species that the Monpa used for building their houses, of which Terminalia myriocarpa is the best quality of all timber species but cannot be chopped now because of it is vulnerable species according to the China Red Data Book [82]. Erythrina arborescens, Wendlandia tinctoria, Maesa rugosa, Radermachera yunnanensis, Abroma augusta, Macaranga denticulata, and Phrynium placentarium are used to make agricultural tools or daily-life utensils. For example, Wendlandia tinctoria can be used to make hilts for knives and Erythrina arborescens is used to make carvings for religious rituals. Imperata cylindrica and Themeda villosa are used for thatching, of which the quality of Imperata cylindrica is better than Themeda villosa. Themeda villosa are covered on the roof, paved 5 cm thick, and changed once in 3years, but Imperata cylindrica can be maintained 7 years.
In addition to use of bamboo (mainly Bambusa teres and Dendrocalamus tibeticus) for making household utensils, the rattan Calamus acanthospathus is used to make baskets (Fig. 6). This widespread species is found in China (Tibet, Yunnan) as well as in Bhutan, India, Laos, Myanmar, Nepal, Thailand, and Vietnam which is also used as a source of edible greens (from the young shoots).

Ritual uses of plants
The Monpa belief systems, derived from the pre-Buddhist Bon religion and from Tibetan Buddhism, also link to animistic beliefs, where even the new houses have soul [83]. Laurocerasus undulata seeds oil and liquor are blended together during religious rituals by Monpa people, and then poured near the house to drive away misfortune and malevolent spirits. Nowadays, Monpa culture has been deeply influenced by Tibetan culture and most of the Monpa people believe in Tibetan Buddhism. Cinnamomum iners and Elsholtzia blanda are used as incense sources, which play an important role in the religious rituals of the Monpa. Luculia gratissima locally called "nom-meng" is also used as an offering in religious rituals.
The Monpa in Mêdog County have a unique funeral culture owing to the special geographical environment, cultural background, and religious beliefs. The practice of re-burial occurs when after burial, the bones are dug out for cremation, and the ashes are scattered into the Yarlung Tsangpo River. This cremation is ignited by the leaves of Altingia excelsa locally called "Sang-shing," which was used by the Monpa people to burn the body. In the meantime, yak butter and liquor are periodically added to the fire, along with offerings of rice, maize, and "Konpu" (Eleusine coracana). Secondly, put the leaves and stems of "Sra-gu" (Arundo donax) and the branches of "La-ga-dong-shing" (Garcinia nujiangensis) in turn. The leaves of "Sang-shing" are an essential ignition material for every funeral of the Monpa people.

Fish poisons
Seasonal fishing and hunting are great economic activities of Monpa ethnic community in addition to agriculture. Fishing by poison was well-known throughout the world in historical time [84]. Derris scabricaulis and Hydrocotyle javanica are poisonous plants used by the Monpa people in Mêdog County for fishing. A proper amount of poisonous plants are soaked in the water, the fish will lose consciousness and float to the water within half an hour. However, if you could not catch the fish in time, the fish will wake up. No relevant reports about these two poisonous plants were found. However, the rotenones, saponins, and cyanide are the main active ingredients of these fish poison species [85].

Calendar plants
According to our investigation, Mussaenda pubescens and Meliosma pinnata are used for indicating the time of sowing maize by Monpa people. Mussaenda pubescens blossoms and the weather is fine, then you can sow the seeds in the field, but you cannot sow the seeds after the blooming period of Meliosma pinnata. The flowering period of Mussaenda pubescens is during June to July. The blooming period of Meliosma pinnata is during May to June [86]. The flowering time of these two species is exactly the same as maize planting time.

Uses of endemic and near-endemic plant species
Uses of endemic and near-endemic species in 12 plant families reflect the plant diversity of this part of the Indo-Burman biodiversity "hot-spot." Uses of these species have been rarely recorded in previous studies. In our field survey, we recorded the uses of narrowly distributed edible plants, for example, Arenga micrantha (used for starch), Hornstedtia tibetica (for fruits), Castanopsis clarkei, and Gnetum pendulum (for edible nuts) (Fig. 7a, d) and Ophiorrhiza medogensis (for vegetables). In contrast to the widespread use of the poisonous Derris trifoliolata (Leguminosae) containing rotenone, which occurs from East Africa to the Western Pacific, Monpa people use Derris scabricaulis which is endemic to Yunnan and Tibet. In terms of farming and equipment used by local households, agricultural tools are made from the high-density wood of Radermachera yunnanensis (Bignoniaceae) while wine strainers and implements for administering medicine are made from Dendrocalamus tibeticus (Poaceae). Litsea tibetana (Lauraceae) is a near endemic seed oil source, as are spices from the endemic Zanthoxylum motuoense (Rutaceae). Additional unusual records are the use of Cinnamomum contractum (Lauraceae) as a tobacco substitute (a species only found in south-east Tibet and NW Yunnan, Morus wittiorum (Moraceae) fruits for medicine and the use of Garcinia nujiangensis (Clusiaceae), a species restricted to south-east Tibet, north-west, and west Yunnan for funeral rituals.

How do plant uses reflect socio-economic change in Mêdog County?
Until October 2013, there were no major roads in Mêdog County and Monpa people practised swidden agriculture, supplemented by hunting and gathering [87]. Despite the absence of roads, Monpa knowledge of plant uses reflect at least three categories of change.
Firstly, through oral history, knowledge of plants that Monpa ancestors would have encountered in Bhutan and Tibet prior to their migration to Mêdog. Secondly, a slow change in knowledge as introduced species were brought to Mêdog along the trade routes. These species include cereal crops from Africa (Eleusine coracana and Sorghum bicolor) and meso-America (Zea mays), cultivated fruits from north-west China (Prunus persica), medicinal plants from Africa (Ricinus communis) and North America (Datura stramonium), fuel from the Mediterranean Basin and the Middle East (Arundo donax), and three South American Solanceae that have come into "traditional" use (Nicotiana tabacum, Solanum americanum and Solanum aculeatissimum). Although oral histories do not indicate when these species were introduced, the fact that several introduced species are used ritually (maize, Eleusine coracana and Arundo donax) is one indication of early introductions. Another indication is the "traditional" medicinal use of introduced medicinal plants such as Datura stramonium (Fig. 8). Although D. stramonium seeds are known to be used for treating toothache elsewhere [88], but the method of preparation and administration used by Monpa people is innovative (Fig. 8a-d). Thirdly, in contrast to these "slow changes," there is "fast While Monpa people still have a wealth of ethnobotanical knowledge that has been passed down orally from generation to generation, the construction of a highway to Mêdog County has stimulated rapid change and possible loss of traditional knowledge. The influence of modernization, social and economic development, and the lack of interests shown by the young generation are seriously threatened to the ethnic culture of no written words [89]. Our research shows that the increasing publicity and availability of Tibetan and Chinese medicines has also affected the indigenous knowledge of the Monpa. There are no exclusive traditional doctors in the villages now and traditional medical knowledge is about to disappear.

Conclusions
Monpa traditional plant-based knowledge are practiced, accumulated, and passed down from generation to generation. The Monpa people in Mêdog County still preserve most of traditional plant-based knowledge. We documented 194 wild plant species belonging to 82 families 158 genera used for traditional medicines, food, dyeing, timber, religion, and other purposes during our ethnobotanical survey. Overall, this study provides a deeper understanding of the Monpa traditional knowledge on wild plants. The study suggests some wild medical plant species might have new active ingredients which are necessitated for further investigation. Since the development of modernization has changed the Monpa lifestyle and production structure, traditional knowledge and biocultural diversity can be essential components to ensure the sustainable development of Monpa community and may play a significant role in the sustainable use and development of Tibetan plant resources. Fig. 8 Experimentation and use of an introduced species in Monpa "traditional" medicine. a Datura stramonium fruits. The seeds are used to treat toothache. b, c Mixing D. stramonium seeds with pig fat. d Creating hot steam by placing a red hot iron in water, on which the Datura seed/fat mixture is placed. e Dendrocalamus tibeticus bamboo culm, sealed using rice around a protruding tube that is placed over the super-heated Datura and pig-fat infused steam to direct the ingredients to the sore tooth and remove the "insect" causing toothache