The traditional healers
Traditional healers are an important aspect of life in rural communities around the world [32-37]. Spiritual healing also includes activities taking place within a formal religious context in rural settings in many countries . The type of healer available for consultation, may be traditional or religious, and they are also part of the individual's culture  as found in Thailand. The traditional healers who were interviewed for this study were all males. In Thailand, traditional healers are predominantly males as reported in many studies from different parts of the country [2-3,12,40]. Thai culture assumes the traditional role of the man in a family as a leader and primary breadwinner, so men are expected to provide for their families . The number of medicinal plant species known was not related to the healer’s age and some herbalists treated only certain ailments such as cancer or postpartum conditions in women as already reported . Herbalists who treat only certain conditions such as cancer, hemorrhoids, or muscular-skeletal disorders may have their knowledge from their parents. Knowledge of traditional healers in Sakornakorn province in northern Thailand was transmitted from their ancestors or parents and was not recorded in written documents . The traditional healers always depended on their memory and familiarity with the plants to use or choose as medicinal plants from the surrounding areas for curing ailments. Some of them had joined training courses provided by the government. It appears that the use of plants in indigenous communities is associated to biological, ecological, and socio-cultural factors, including production techniques and practices, religion, gender, and age [40-41]. Furthermore, Tai Lao healers also used animals and mineral substances for curing ailments, they practiced incantations and held ceremonies as part of the treatments as has also been reported among other cultures [12,38,42]. Medicinal knowledge of healers aims to heal not only physical illness, but also the mind and soul . From the local healers’ point of view lack of successors, erosion of knowledge, conflicts with mainstream knowledge, lack of recognition, restrictive regulations for collecting medicinal materials, lack of adequate intellectual property protection, incompatibility of local ownership values with contemporary laws are some of their concerns .
Numbers of medicinal plants
The best represented plant family among the medicinal plants was Fabaceae as also foud in some other studies [2,12,44]. Fabaceae is a large tropical family and they are common and species rich and much used for medicinal purposes in Thailand . Surprisingly, in this study Poaceae occupied the second most important place among the medicinal plant families of Tai Lao. Normally Poaceae is used for food such as in the case of Bambusa bambos (L.) Voss, Cymbopogon citratus (DC.) Stapf, and Zea mays L. but The Tai Lao healers used this family to treat various ailments such as diabetes, gallstones or jaundice. Some species of Poaceae such as Bambusa vulgaris Schrad. exJ.C.Wendl. was used as a decoration tool, Chrysopogon zizanioides (L.) Roberty was weedy and used as animal feed but the Tai Lao healers used these species as medicine to treat jaundice and diabetes respectively. This traditional knowledge of healers is unique and it is part of their own traditional health care system, which is different and depends on their origin and cultural pattern. A good example of traditional knowledge was the vernacular names for each species given in Additional data 1. They document one aspect of the Tai Lao traditional knowledge. The vernacular names use a language or dialect which is native to a region or country rather than a literary, cultured, or foreign language. As such vernacular names are related to a plant or an animal in the common native speech as distinguished from the Latin nomenclature of scientific classification. Most vernacular names were related to or characteristic of a disease, a place, or a group of plants.
Most used medicinal plants
The medicinal plants most used by the Tai Lao healers were entirely different from the most important medicinal plants used by the related and geographically close Phu Tai  where Rothmannia wittii (Craib)Bremek. had the highest UV and was used to treat jaundice. Similarly, a study among the hill tribes in northern Thailand found differences of traditional knowledge of the Karen and Lawa, who live in adjacent villages within the same habitat and with the same forest resources but they used different species of medicinal plants for the same purposes . This finding resonates with the fact that differences in traditional knowledge is driven by many factors, including cultural, historical and ecological ones [45-46]. The comparison of use of medicinal plants among ethnic groups throughout Thailand found the similarity in the use of plants was not larger for villages inhabited by the same ethnic groups or from the same region than it was for villages inhabited by different ethnic groups or from the different regions. Instead, it appeared that each village had, at least in part, developed its own ethnomedicinal knowledge .
Ailments or disorder treated and Informant Agreement Ratio (IAR)
Globally, digestive system disorders have a substantial effect on morbidity and mortality rates, and especially so in developing countries including in Thailand, where the majority of rural areas lack proper sanitation and awareness of disease prevention. This has led to prevalence of different types of digestive diseases [45,48]. This is true in both southern [2,14] and northern Thailand [45,48] and beyond Thailand, such as reported by Naxi healers for farmers in northwestern Yunnan in China . The medicinal plant species used to treat digestive system disorders are mostly different in each ethnic group such as in different hill tribe in northern Thailand, where Karen and Lua use Leea indica (Burm. f.) Merr. (Leeaceae) for diarrhea  and the Karen use Zingiber montanum (J. König) Link ex A. Dietr. (Zingiberaceae)  to treat the same condition. In Phatthalung in southern Thailand Zigiberaceae is the most used family including species such as Boesenbergia rotunda (L.) Mansf., Kaempferia galanga L., Zingiber officinale Roscoe, and Curcuma longa L. . Furthermore, a total of 37 spp. were used to treat digestive system disorders including Alpinia galanga (L.) Willd. which is used as a carminative, Curcuma longa L. for flatulence, and Clausena wallichii Oliv. and Croton crassifolius Geisel. for stomach disorders. However, different patterns can be found in different areas or among different ethnic groups as demonstrated for the Phu Tai healers , among whom most medicinal plants are used for treatments of tonic. Pholhiamhan et al.  who studied the same ethnic minority group but in a different part of the Nakornpanum province found that most medicinal plants were used for injuries. A study among hill tribes in northern Thailand  found that skin and subcutaneous cellular tissue disorders were the conditions treated by most plants. Srithi et al.  found the use categories with most use-reports were for birth-related conditions. In Jashpur district of Chhattisgarh, India, skin diseases, diabetes, and weakness were the most common disorders reported . In the indigenous knowledge on medicinal plant use by traditional healers in Oshikoto region, Namibia, disorders with the highest number of species being used were mental diseases followed by skin infection and external injuries .
Plant parts used
Among the different parts of the medicinal plants, the stem was most frequently used for making medicines, followed by roots and leaves. This agrees with other studies in northern and northeastern Thailand . Traditionally different parts of plants such as stem, roots, fruit, leaves, and seeds are being used for the treatment of different ailments . The therapeutic potential of different plant parts is attributed to differences in concentrations in different plant parts of phytochemical compounds such as tannins, terpenoids, alkaloids, etc. . Dried wood or roots can maintain their bioactive compounds for a longer time after harvesting than leaves can . However, local healers are often concerned about how the use of different plant parts may affect the survival of the medicinal plants’ populations . Traditionally, leaves have been the most favored part used for medicinal purposes in many parts of the world for example in Singapore , India , Turkey , and Ethiopia . This may be because leaves are soft, easy to use, their drug contents can be readily extracted, and they can be harvested in every season . On the other hand, a study conducted in India  showed that roots were the most commonly used part followed by leaves. This discrepancy is probably due to the diversity of plants, weather conditions, and chemical compounds present in the plant parts in different geographic areas. In southern Thailand underground parts were also the most used part of medicinal plants. These differences can be explained by environmental conditions and cultural patterns in the traditional uses of plants by each ethnic minority . However, further and detailed phytochemical screenings of medicinal herbs are required. For traditional knowledge, it is essential to have proper documentation of the plants involved and to know their potential for the improvement of health. Accepting traditional knowledge in a modern context will depend on the scientific backup of the reported effects. Importance should be given to the potential of ethnomedicinal studies as these can be an important base for the discovery of useful medicinally active compounds in plants .
Life forms of medicinal plants
Five life forms of medicinal plants were used by the 14 healers, the most common one being trees (including a tree palm and a climbing tree) followed by herbs, shrubs, vines, and grasses. The same life forms were most commonly represented among medicinal plants used by the the closely related ethnic minority group, the Phu Tai  and in Namibia  but healers in Phatthalung in Peninsular Thailand  most commonly used shrubs as medicinal plants. In China the majority of medicinal plants in Naxi homegardens were herbaceous followed by trees shrubs, and lianas  and in India herbs are commonly used as medicinal plants .
Preparation of medicinal plants
The Tai Lao healers interviewed here described 19 different methods that they used to prepare plant medicines. These results differ from the Phu Tai ethnic group  in total numbers of methods. However, the most common preparation method, which was decoction was also the most common method reported in previous ethnomedicinal studies in Thailand [2,12,45,53] and also in Singapore , Nigeria , and India . The frequent use of oral ingestion of a decoction is most likely because it is an easy way to administer the medicine  and it makes it possible to adjust the taste of the medicine for individual patients. Other methods recorded here and in other studies was inhalation and fermentation. The Tai Lao healers used leaves of Allium ascalonicum L. to treat fevers or Allium cepa L. and Nicotiana tabacum L. to treat nasal polyps by inhalation. This method was commonly used to treat respiratory or circulatory system disorders. The Tai Lao healers also fermented fruit of Phyllanthus emblica L. to treat goiter and mumps. Tinospora crispa (L.) Hook. f. & Thomson was prepared in the same way to treat liver disorders.
Habitats and availability of medicinal plants
Most medicinal plants used by Tai Lao traditional healers were gathered from their home gardens including such species as Allium ascalonicum L. (Amaryllidaceae), Allium sativum L., Alpinia galanga (L.) Willd. (Zingiberaceae), Ananas comosus (L.) Merr. (Bromeliaceae), Carica papaya L. (Caricaceae), Citrus hystrix DC. (Rutaceae), Allium cepa L., Boesenbergia rotunda (L.) Mansf. (Zingiberaceae), and Cymbopogon citratus Stapf (Poaceae). The same species were also used for food by the villagers. For convenience, these medicinal plants were moved and planted in home gardens and used whenever required . This was also the case among the Naxi healers in southern China  where home gardens are ecologically and culturally important systems in which healers cultivate medicinal plants for the wellbeing of farmers. In Naxi only the healers cultivated medicinal plants in their home gardens and they rarely sold them. Plant resources are important to communities where the majority of households rely on integration of traditional knowledge and western medicine for their wellbeing. Medicinal plants maintained in ethnic home gardens are valuable not only for food but also for their economic and ornamental properties . In our study, the majority of medicinal plants were native to Thailand, while, a few exotic plants were used medicinally such as Cymbopogon citratus Stapf (Poaceae), the shrub Pandanus amaryllifolius Roxb. (Pandanaceae), and Plumbago indica L. (Plumbaginaceae) (Additional data 1). In contrast to the dominance of homegarden plants in this study, most medicinal plants used by the Phu Tai ethnic group  and in Phatthalung province, Thailand , were gathered from community forests.