Introduction

Although paddy rice fields are primarily used as agricultural land for staple crop production, they also harbor a diversity of organisms. Wild plants, as well as insects, birds, and aquatic animals, are important components of the paddy field ecosystem (Heckman 1974). However, modern agronomy defines wild plants as weeds, and improved agricultural practices have eliminated various wild plants from paddy fields through advances in drainage and irrigation systems, reinforced concrete levees and banks, and the use of herbicides (Shimoda 2003). As a result, in countries such as Japan, many of the wetland plants that previously grew in paddy fields have become endangered species and are now recognized as indicators of environmentally benign agricultural practices. Conversely, a variety of wild plants found in paddy fields are utilized for food, forage, or household purposes in many Asian countries, such as India (Datta and Banerjee 1978), Indonesia (Soerjani et al. 1987), and Thailand (Cruz-Garcia and Price 2011).

This study was conducted in paddy rice farming communities in the Lao People’s Democratic Republic (the Lao PDR), where the rural population does not use herbicides and gathers various wild edible herbs in paddy fields for daily use. Diverse wild herbaceous plants, including rare species, grow in these paddy fields and some are used as food, medicine, or materials for handicrafts in the central Lao PDR (Kosaka et al. 2006). In northeast Thailand, where the vegetation and local culture are similar to those in the central Lao PDR, wild plants found in paddy fields accounted for a large proportion of the edible wild greens that were gathered by farmers (Moreno-Black et al. 1996; Price 1997). Importantly, these plants were not only consumed by the people who harvested them, but they were also traded in the local markets (Moreno-Black and Price 1993). Our preliminary field observations in the Lao PDR revealed that the paddy field vegetation in the north of the country differed from that in the central region, and that a mixture of wild edible herbs was sold at the local markets in the north of the country.

This study was conducted in the northern Lao PDR to answer the following key questions: (1) what kinds of wild herbs are grown in paddy fields, (2) how does the rural population gather, use, and identify the wild edible herbs that grow in their paddy fields, and (3) why are mixtures of wild edible herbs sold at markets? Specifically, we sought to examine the agrodiversity (Brookfield et al. 2003) of paddy fields, which is a significant component of human-plant relationships that differs from the traditional dependence of humans on monocultures of staple crops.

Study Site and Methods

Study Site

The Lao People’s Democratic Republic is a landlocked country in mainland Southeast Asia with an area of 236,800 km2 (Fig. 1). Of the approximately 5.3 million people in the country, 80% live in rural areas (ADB and UNEP 2004). A variety of wild plant resources are used daily for food, medicine, fuel, fiber, oil and resins, and as materials for handicrafts (Greijmans et al. 2007). Field surveys were conducted in Xamneua District, the provincial capital of the Houaphan Province, which has an area of 16,500 km2 and a population of 322,200, and is located in the northeastern Lao PDR adjacent to Vietnam (Fig. 1). The total area under rice cultivation in the province was 26,323 ha in 2005, which consisted of 11,485 ha (43.6%) of paddy rice fields (lowland rice fields in the monsoon season), 13,570 ha (51.6%) of swidden fields (upland rice fields), and 1,268 ha (4.8%) of lowland rice fields irrigated in the summer season (MAF 2006). The Xamneua District is located in the central part of the province and is home to 46,800 people, including the Lao, Tai Leu, Tai Dam, Tai Deng, Khmu, and Hmong communities. Paddy rice fields are located in valleys, whereas swidden cultivation is conducted on the mountain slope. There are three seasons per year: a warm and dry summer (March–April), a monsoon season (May–October), and a cool and dry winter (November–February). About 85% of annual rainfall (930 mm) occurs during the monsoon season. The mean annual maximum temperature is 19.8 °C and the mean annual minimum temperature is 12.1 °C (World Weather Online 2013).

Fig. 1
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Location of Xamneua city (a) and map of the study site (b) in Houaphan Province in the Lao PDR. Solid white dots on map (b) indicate the location of field plots.

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The local market at the center of Xamneua city (Fig. 1) opens both in the morning (6:30–9:00) and in the evening (15:30–18:00). The largest area of the market is occupied by a perishable foods section where more than 50 vendors deal with meat from domestic animals, wild and cultivated freshwater fish and shellfish, seafood imported from Vietnam and Thailand, wild and cultivated leafy vegetables, fruits, and spices, wild edible insects, ready-made dishes and sweets, and natural medicines. The vendors rented space for LAK 250,000–300,000 (Laotian kip) per year per space (concrete or wooden table of about 1 m2) and an additional LAK 2,000 per selling day (USD 1 = LAK 8,005 on July 24, 2012). Some vendors specialized in retailing, whereas others gathered, harvested, or cooked foodstuffs by themselves. The perishable foods section was surrounded by other sections selling clothes and miscellaneous daily goods, and small restaurants.

Xamneua village and Nasakang village, located about 1 km from the Xamneua market at an altitude of 1,200 m, were selected for the following field surveys (Fig. 1). Both villages were established in the 1970s by the local Lao people and Lao, Tai Dam, Khmu, and Hmong immigrants. About 70% of the households were engaged in paddy rice cultivation, but some new immigrants did not have their own fields. Paddy rice fields in this area were located in the riverside lowland and on the river terrace (Fig. 1) and were irrigated through a small-scale canal system from a dam upstream. The most popular rice variety cultivated was “Khao Kai Nooi Leuang,” a member of the photo-sensitive sticky rice variety group called “Khao Kai Nooi” for monsoon-season cropping. Seeding was conducted in May, transplanting in June, and harvesting in November. The average yield of paddy rice was 2–3 t/ha. Overgrown wild plants on levees were cut using a long-handled machete called a “pha” 2–3 times a cropping season. Unwanted plants in fields were manually removed and either buried in the field or piled up on the levee when they were too numerous. Herbicides, pesticides, and chemical fertilizers were not applied. Wild fish, snails, crabs, shrimp, and water insects were also gathered in the fields. Some people grew tilapia fish (Oreochromis niloticus) by releasing young individuals in the fields.

Methods

The local market at the center of Xamneua city was visited a total of ten times from the first visit in August 2005 to the last in July 2012. Useful plants sold at the market were recorded for three days per visit in the summer (March), monsoon season (May, June, July, August), and winter (November). The local name, domestication status (cultivated, transplanted, or wild), habitat, use, and price for respective plants were documented in the morning and evening, and specimens were collected. More than ten vendors were interviewed per day. This paper focuses on the wild edible herbs gathered in paddy fields and sold in a mixture. The species composition of the mixture of herbs they sold was recorded on March 2–4, March 25–27, May 2–4, July 2–4, July 27–29, and November 16–18. The relative abundance of each species in the mixture was determined by visual assessment of its relative volume (%) with 5–10 replicates in each survey period. To evaluate seasonal differences in mixtures, a non-metric multidimensional scaling ordination was conducted for the species composition data using the Bray-Curtis distance index by PC-ORD software package (McCune and Mefford 2006).

The paddy field vegetation was surveyed in the summer (March 25–26, 2012) and monsoon season (July 27–28, 2012) at Xamneua village and Nasakang village (Fig. 1). The survey method differed with different habitats of the paddy fields (Fig. 2). Twenty fields in both villages were selected as field plots to cover different vegetation types according to preliminary observations (Fig. 1). Plot areas ranged from about 25 to 400 m2. The quadrat method was used to determine the composition and coverage (%) of wild herbaceous plants (spermatophytes and pteridophytes) in each field plot. Four quadrats (1 m by 1 m) per plot were established, and mean species’ coverage was calculated per plot. The importance of each species was estimated using an index of specific value (ISV; Pinder and Rosso 1998); species with a value > 0.2 were considered dominant species (Kosaka et al. 2006; Tomita et al. 2003). For wild herbaceous plants (spermatophytes and pteridophytes) on levees and banks, the structure of the various microhabitats (i.e., shore, flat part, and grassland as defined by Yamaguchi and Umemoto 1996) prevented us from establishing quadrats (Fig. 2; Kosaka et al. 2006). Therefore, transects were established for the levees of the field plots (each transect was 5–20 m long and 0.2–0.4 m wide, with four transects per field plot) and at the banks of the irrigation ditch (each transect was 5 m long and 0.3–1 m wide, with two transects per bank and near the respective field plots). Then the abundance of wild plants was recorded using a qualitative scale (Pauchard and Alaback 2004): A, abundant (present along > 25% of the transect length); C, common (10–25%); I, intermittent (< 10%); and R, rare, when only one group of individuals was present. The nomenclature for the sampled plants followed Harada et al. (1993), Ho (1999–2000), Santisuk and Larsen (1997–2012), and Smitinand and Larsen (1970–1996). Voucher specimens are lodged in the herbarium of the Faculty of Forestry, the National University of the Lao PDR.

Fig. 2
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Different habitats for wild herbaceous plants in paddy fields in Houaphan Province in the Lao PDR.

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In June 2010 and March and July 2012, more than 20 local farmers were interviewed in the Lao language at paddy fields or field huts regarding the local name, use, and their perception of wild herbaceous plants, rice cropping techniques, and plants harmful to rice production in the Xamneua village and Nasakang village. Cropping and gathering activities as well as the growth of rice plants were also observed during the survey periods.

Results

Floristic Characteristics of Wild Herbs in Paddy Fields

One hundred fifteen wild herbaceous species, representing 86 genera and 37 families, were recorded from fields, levees, and banks of Xamneua village and Nasakang village (Table 1). Among these, 52 wild herbaceous species, representing 42 genera and 25 families, were recorded in fields (Fig. 3A). Rotala rotundifolia (Buch.-Ham. ex Roxb.) Koehne (Lythraceae), Marsilea quadrifolia, Digitaria sp. in the summer, and Marsilea quadrifolia were the dominant species in the monsoon season (ISV > 0.2). Asteraceae (4 spp.) and Poaceae (4 spp.), and Cyperaceae (7 spp.) and Poaceae (7 spp.) were the dominant families in the summer and monsoon season, respectively (Table 1).

Table 1 Wild herbaceous species in paddy fields at Xamneua village and Nasakang village, Houaphan Province in the Lao PDR.
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Fig. 3
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Wild edible herbs growing in paddy fields and sold at the Xamneua market in Houaphan Province in the Lao PDR: Marsilea quadrifolia growing between rice plants in the field (a); Centella asiatica (up-arrows) and Spilanthes oleracea (down-arrows) on the levee (b); children gathering wild herbs in a paddy field near houses (c); vendors selling fresh vegetables at the Xamneua market (d); bunches of mixed wild edible herbs for sale (e). Contents of a bunch of mixed herbs from a vendor; Eryngium foetidum 1, Polygonum sp.2, Monochoria hastata 3, Juncus prismatocarpus 4, Houttuynia cordata 5, Hydrocotyle petelotii 6, Centella asiatica 7, Spilanthes oleracea 8, and Marsilea quadrifolia 9 on March 25 (f; Table 2).

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Ninety-five wild herbaceous species, representing 71 genera and 28 families, were recorded on levees (Fig. 3B). The most abundant species on levees in the summer was Ageratum conyzoides L. (Asteraceae), and in the monsoon season, Ageratum conyzoides, Kyllinga brevifolia Rottb. (Cyperaceae), and Paspalum conjugatum P. J. Bergius (Poaceae) were dominant. Asteraceae (12 spp.) and Poaceae (9 spp.), and Poaceae (15 spp.) and Asteraceae (11 spp.) were the dominant families in the summer and monsoon seasons, respectively (Table 1).

Sixty-three wild herbaceous species, representing 50 genera and 21 families, were recorded on the banks. Bidens pilosa L. (Asteraceae), Leersia hexandra Swartz (Poaceae), and Paspalum conjugatum were the abundant species on banks both in the summer and monsoon seasons. Asteraceae (10 spp.) and Poaceae (7 spp.), and Asteraceae (11 spp.) and Poaceae (10 spp.) were the dominant families in the summer and monsoon season, respectively (Table 1).

Gathering, Use, and Perception of Wild Herbs in Paddy Fields

Gathering of wild edible herbs in paddy fields was mainly conducted by women and children during agricultural activities or on the way to and from the fields. Wild edible herbs were also gathered from paddy fields beside houses before cooking in the morning and evening. Villagers were allowed to gather herbs everywhere and at all times within their village area. Villagers knew the places where wild edible herbs were abundant, especially the perennials such as Centella asiatica, Marsilea quadrifolia, and Houttuynia cordata Thunb. Some edible herbs (Table 1) were gathered for both sale and consumption by the harvesters, while other herbs, such as the leafstalks of Colocasia esculenta (L.) Schott (Araceae, “phak boon”) or the young leaves of Hydrocotyle vulgaris L. (Araliaceae, “phak nook vian”), were only for consumption by the person harvesting the herbs.

Two methods were used to gather herbs: picking the young leaves with a small knife, and uprooting the whole plant manually. M. quadrifolia was gathered in both ways, whereas C. asiatica was often uprooted together with its creeping stems. All of the gathered herbs, regardless of the species, were collected for transport in the same rattan basket on the waist or in a plastic hand basket (Fig. 3C). Ipomoea aquatica Forssk. (Convolvulaceae), one of the most popular and widely cultivated vegetables, was gathered only for consumption by the harvester because it was scarce in the wild at this study site.

Wild edible herbs are referred to using the Lao term “phak,” which also can be used to refer to cultivated vegetables (Table 1). Wild inedible herbs are called “nya,” including the herbs that are used as fodder for livestock. Ludwigia adscendens (L.) H. Hara (Onagraceae, “nya poot”) and Echinochloa crus-galli (L.) P. Beauvoir. (Poaceae, “nya wang”) in fields, and Digitaria spp. and Panicum repens L. (Poaceae, collectively called “nya waay”) and Paspalum conjugatum (“nya feet”) on levees, were regarded as the most harmful species to rice production. In July, Sagittaria trifolia ssp. leucopetala (Miq.) Q.F.Wang (Alismataceae), Cyperus spp., Fimbristylis spp., Scirpus sp., Blyxa aubertii Rich. (Hydrocharitaceae), Utricularia aurea Lour. (Lentibulariaceae), Ludwigia adscendens, Ludwigia hyssopifolia (G.Don) Exell (Onagraceae), Digitaria sp., Echinochloa crus-galli, Panicum repens, Paspalum sp., and Monochoria vaginalis (Brum.f.) Presl (Pontederiaceae) were manually uprooted from the fields and piled up on the levees (Table 1). Some wild edible herbs, such as C. asiatica, M. quadrifolia, M. hastata, M. vaginalis, and H. cordata, were referred to by their common Lao names, and others, such as Crassocephalum crepidioides (Benth.) S. Moore (Asteraceae) and Pratia nummularia (Lam.) A. Br. & Aschers. (Campanulaceae), were referred to by more than one local name (Table 1).

A popular local dish called “sup phak” (boiled vegetables) was cooked by boiling the wild herbs collected in the paddy fields. Ginger and salt were added for flavoring, and chili powder or sesame was added if desired. Many interviewees mentioned that the mixture of several species enhanced flavor. “Sup phak” was taken as a side dish with steamed sticky rice.

Mixed Wild Edible Herbs of Paddy Fields Sold at the Xamneua Market

Wild edible herbs from paddy fields were sold at the Xamneua market, both in the morning and evening (Fig. 3D). The herbs sold in the morning had been gathered the previous evening, and the ones sold in the evening had been gathered that afternoon. They were brought from nearby villages, such as Xamneua, Naliu, Phoxay, Thatmuang, Nasakang, and Nathong (Fig. 1). The herbs were washed in water and the roots were removed before being arranged at the selling place. Vendors sold wild herbs that they had collected themselves, or that they were selling on behalf of neighbors, relatives, or other local gatherers that did not have a selling place in the market. On Sundays, some schoolchildren gathered wild herbs in the afternoon and sold them at their mother’s selling space in the market in the evening. There was a case in which a woman gathered 3 kg of C. asiatica in Xamtai (150 km from Xamneua town) and sold them at the selling place of a relative in the Xamneua market. Wild herbs were arranged among cultivated crops in the same selling place. Vendors brought 0.5–3 kg of marketable wild herbs in plastic bags, often as a mixture of several species, and then arranged these herbs in piles in the selling place (Fig. 3E). They occasionally sprinkled water on the herbs to prevent them from withering on hot days. A single bunch of wild edible herbs (100–200 g) from the paddy fields was sold at the price of LAK 1,000–2,000. There was no difference in price for bunches of herbs containing a mixture of different species. Buyers with large plastic hand baskets often purchased all of the bunches of herbs arranged in the selling space, which amounted to 500 g–1 kg. Buyers were typically city dwellers who did not have paddy fields and who worked as civil servants, schoolteachers, and shopkeepers in the Xamneua city.

Table 2 shows the relative abundance of wild edible herbs from paddy fields sold at the Xamneua market. Twenty-two species, representing 21 genera and 16 families, were recorded. Among them, 13 species were annuals and 9 species were perennials; 4 species were hydrophytes, 4 species were hygrophytes, and 14 species were mesophytes. These herbs were mainly gathered on levees (16 species), in fields (8), on banks (5), and in ditches (3) (Fig. 2). M. quadrifolia was the most abundant herb sold every season, while Alternanthera sessilis (L.) R.Br. ex DC. (Amaranthaceae), Laphangium luteoalbum (L.) Tzvelev (Asteraceae), Nasturtium officinale W.T. Aiton (Brassicaceae), Myosoton aquaticum (L.) Moench (Caryophyllaceae), Astragalus sinicus L. (Fabaceae), Houttuynia cordata, Lindernia sp., and Viola sp. were only sold in small amounts.

Table 2 Relative abundance of mixed wild edible herbs gathered in paddy fields sold at the Xamneua market in Houaphan Province, the Lao PDR.
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More species were sold in the summer than in the monsoon and winter seasons. The average number of species in each mixture was eight in the summer, three in the monsoon season, and four in the winter. For example, nine species were sold in a mixture on March 25, 2012 (Fig. 3F). Although there were fewer species in each mixture in the monsoon season, C. asiatica, M. quadrifolia, and M. vaginalis were often sold separately in large amounts. E. foetidum, N. officinale, and H. cordata were often cultivated and were also sold separately in large amounts; however, only those herbs growing in the wild were counted in Table 2. The non-metric multidimensional scaling ordination showed seasonal differences in the species composition of a mixture (Fig. 4). The species composition of the herb mixtures was observed to vary in the summer and during the monsoon. During the summer there was relatively more C. asiatica, Cyanotis axillaris (L.) D.Don ex Sweet (Commelinaceae), or Juncus prismatocarpus R.Br. (Juncaceae), with the latter containing a relatively high abundance of M. quadrifolia or M. vaginalis. The winter composition of the herb mixtures was intermediate between the two.

Fig. 4
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Non-metric multidimensional scaling ordination of species composition data of mixed wild edible herbs sold at the Xamneua market, Houaphan Province in the Lao PDR (stress = 19.82). MarB, species composition recorded in the beginning of March; MarE, end of March; MayB, beginning of May; JulB, beginning of July; JulE, end of July; NovM, middle of November; numbers after the “-” indicate the number of replicates.

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Discussion

Among 115 wild herbaceous species recorded in the paddy fields of this study site, 25 species (22%) were used locally and 22 (19%) were sold at Xamneua market as edible greens (Table 1). The number of wild edible herbs in the paddy fields that were used or sold was larger in this study site than in the surrounding regions; namely, six species were sold at seventeen markets in Vientiane (Fig. 1; Saito et al. 2007), eleven were used and three were sold at two villages in Savannakhet (Fig. 1; Kosaka et al. 2006), and fourteen were used at one village in Kalasin in northeastern Thailand (Fig. 1; Price 1997). M. vaginalis was the most common species used in those regions. The use of typical temperate-zone species, such as L. luteoalbum, P. nummularia, M. aquaticum, J. prismatocarpus, and Viola sp., was characteristic of this study site.

The importance of wild herbs in agricultural fields has been illustrated in several previous studies. Chacon and Gliessman (1982) showed that wild herbs were classified into “buen monte” (good plants) and “mal monte” (bad plants) in the traditional tropical agroecosystems of Mexico, prompting those authors to propose the “non-weed” concept. The “non-weed” concept can be applied to this case study in the northern Lao PDR. Wild edible herbs and cultivated vegetables were both referred to as “phak” (Table 1), which is the Lao word for vegetable or the prefix for vegetable. “Phak” were clearly differentiated from wild inedible herbs called “nya.” They were not only consumed as vegetables in daily meals, but they were also sold at markets after being arranged equally with the cultivated vegetables in the selling place as a source of cash income (Fig. 3D, E).

The sale and use of mixtures of paddy field herbs was widely undertaken at this study site (Fig. 3E, F). Bunches of mixed upland field herbs were also sold at Xamneua market (pers. obs.). On the other hand, wild edible herbs were sold and consumed individually in the central Lao PDR (Kosaka et al. 2006). There were three reasons for making mixtures of wild edible herbs. First, wild herbs were inadvertently mixed because they were collected and transported in one basket. Second, vendors and farmers stated that herb mixtures enhanced the flavor of the local dish “sup phak.” Third, the species composition of these mixed herbs differed according to the season (Fig. 4), with the total number of species in a mixed bunch increasing in summer (Table 2). The collection and sale of mixed herbs increased the total amount of edible greens; otherwise, the amount of wild herbs and cultivated vegetables would be relatively small in summer. Moreno-Black et al. (1996) and Moreno-Black and Somnasang (2000) likewise showed that wild plants sold at the local markets in northeastern Thailand were most diverse in the summer when other edible vegetables were scarce. In the past in Japan, wild herbs such as Oenanthe javanica (Bl.) DC. (Apiaceae), Artemisia princeps Pamp. (Asteraceae), Petasites japonicus (Siebold & Zucc.) Maxim. (Asteraceae), Taraxacum spp., Capsella bursa-pastoris (L.) Medik. (Brassicaceae), Cardamine flexuosa With. (Brassicaceae), Stellaria media (L.) Villars (Caryophyllaceae), Equisetum arvense L. (Equisetaceae), Allium macrostemon Bunge (Amaryllidaceae), Plantago asiatica L. (Plantaginaceae), Polygonum sp., and Viola sp. were gathered around agricultural fields and streams, and consumed boiled during winter and early spring when other fresh greens were typically scarce (Arioka 2008). Of these species, Stellaria sp., Polygonum sp., and Viola sp. were common at this study site (Table 2).

Four elements of agrodiversity, namely biophysical diversity, management diversity, agrobiodiversity, and organizational diversity (Brookfield et al. 2003), were examined in line with the findings of this study. Unnecessary wild herbs were selectively removed from paddy fields, which were carefully tilled, leveled, and submerged so that rice could be transplanted to the fields. Levees and banks were regularly repaired and cleaned to keep the field submerged and to serve as walkways. Irrigation and drainage ditches were inevitable for stable rice production, and sizable amounts of money and labor were invested for their construction and maintenance. These incessant farming activities created a variety of micro-environments in and around paddy fields (Fig. 2), which in turn supported a variety of wild herbs (biophysical diversity and agrobiodiversity; Table 1). Gathering wild herbs was not only done for acquiring edible greens: uprooting C. asiatica, M. quadrifolia, and M. vaginalis played a role in thinning the number of wild plants in the fields and levees. Picking only the young leaves of H. cordata allowed its roots to remain intact, which prevented the levees from eroding (management diversity).

Gathered wild herbs were sold at local markets and became a source of income for women (Fig. 3D; Moreno-Black and Price 1993). This trend was promoted by the increasing demands of the nonfarming population for chemical-free wild greens and by the subsequent development of trading activities within and among villages (organizational diversity). Thus, a variety of relationships between people and wild plants were maintained in paddy fields that were originally created for the monoculture of rice.

Conclusion

Since the late 1990s, intensive rice farming has been promoted near urban areas in the Lao PDR through the introduction of high-yield rice varieties, large-scale irrigation systems, and agricultural chemicals and fertilizers. On the other hand, rice for self-consumption has been produced without applying agricultural chemicals and fertilizers to the paddy fields at this study site, where manmade habitats harbored a variety of wild herbaceous plants. The rural population referred to wild edible herbs using the same term as that used to refer to cultivated vegetables and clearly distinguished them from inedible herbs. Wild edible herbs were sold in mixed bunches at the local market, arranged in the same way as cultivated vegetables. The reasons for mixing the herbs were to enhance the flavor of the popular local dish of boiled greens, mixing at the time of collection, and to increase the total amount of edible greens during the summer when the availability of other cultivated vegetables decreases. The relationship between the rural population and wild edible herbs in this study site was a typical case of agrodiversity composed of biophysical diversity, management diversity, agrobiodiversity, and organizational diversity (Brookfield et al. 2003).