Not just minor wild edible forest products: consumption of pteridophytes in sub-Saharan Africa

A total of 24 taxa belonging to 14 genera and 11 families are used in sub-Saharan Africa as fodder and human food (Table 1). Leaves of Dryopteris wallichiana (Spreng) Hyl., Nephrolepis biserrata (Sw.) Schott and Ophioglossum grande L. are used as fodder for goats, sheep and other small ruminants in Nigeria [15, 21], while the rest of the species are used as human food. All reported species are indigenous to the region, with Pteridium aquilinum (L.) Kuhn as the most widespread fern, occurring mainly as a cosmopolitan weed with an almost worldwide distribution apart from mountainous, desert and arctic areas [22]. Plant families with the highest number of food plants are Ophioglossaceae, represented by seven species, followed by Athyraceae represented by three species; and Davalliaceae, Dennsteadtiaceae, Marsileaceae, Pteridaceae and Thelypteridaceae represented by two species each. Athyraceae, Dennsteadtiaceae, Ophioglossaceae, Pteridaceae and Thelypteridaceae are among the most species-rich pteridophyte families, represented by at least 80 species [2, 23]. The rest of the families are represented by one species each, as shown in Table 1. The genera with highest number of species are Ophioglossum represented by six species, followed by Diplazium represented by three species; and Ceratopteris, Marsilea and Nephrolepsis represented by two species each. Ceratopteris, Diplazium, Marsilea, Nephrolepsis and Ophioglossum have the highest diversity of species probably because these are large genera worldwide, characterized by at least five species each [23].

Edible pteridophytes have been recorded from east, central, south and west Africa (Figure 1). Most of the ethnobotanical data on edible pteridophytes have been reported in DRC and Nigeria (Table 1). In Nigeria, seven species: Botrychium lanuginosum Wall. ex Hook et Grev, Diplazium esculentum (Retz.) Sw., Diplazium sammatii (Kuhn) C.Chr., Nephrolepis cordifolia (L.) Presl., Ophioglossum vulgatum L. and Pteridium aquilinum (L.) Kuhn are utilized as human food [15, 30, 38]. Seven edible taxa have been reported in DRC, which include Blotiella glabra (Bory) R.M.Tryon, Christella dentata (Forssk.) Brownsey & Jermy, Cyathea manniana Hook., Diplazium sammatii, Lomariopsis sp., Nephrolepis biserrata (Sw.) Schott and Pteridium aquilinum[24, 37]; and five species among them, Ceratopteris thalictroides (L.) Brongn., Diplazium proliferum (Lam.) Thouars, Ophioglossum ovatum Bory, Pteridium aquilinum and Stenochlaena tenuifolia (Desv.) T. Moore have been reported in Madagascar [26, 29, 32, 40]. Three edible species have been reported in South Africa, which include Ophioglossum polyphyllum A. Braun [6, 34], Ophioglossum reticulatum L. [35] and Pteridium aquilinum[39]. Three edible species have been reported in Swaziland, which include Ceratopteris thalictroides, Ophioglossum lusoafricanum Prantl and Ophioglossum reticulatum[27]. Cyclosorus gongylodes (Schkuhr) Link and Marsilea minuta L., are the only two edible species reported in Gambia [28, 31], Pteridium aquilinum reported in Angola, Cameroon and Gabon [32], Ceratopteris cornuta (P. Beauv.) Lepr. reported in Liberia [14, 25], Ophioglossum polyphyllum A. Braun reported in Namibia [33], Marsilea minuta L. reported in Senegal [31] and Ophioglossum reticulatum reported in Zanzibar [36].Pteridophytes occur throughout sub-Saharan Africa, particularly in rainforests and afromontane forests of east, central, south and west Africa, making the plant group effective and reliable source of human food. However, due to lack of direct evidence for pteridophyte use from other countries, particularly in north and central Africa, the importance of pteridophytes can only be inferred from ethnobotanical studies done in Angola, Cameroon, DRC, Gabon, Gambia, Liberia, Madagascar, Namibia, Nigeria, Senegal, South Africa, Swaziland and Zanzibar (Figure 1). Although findings from these countries about edible pteridophytes are diverse in terms of utilized species, the findings may not apply directly to all countries in the sub-Saharan African region. It is clear that more detailed national level studies should be undertaken, particularly in north and central Africa where documentation on pteridophyte use is scanty or missing.

Pteridium aquilinum is the most widely used pteridophyte as a source of human food (Figure 2). It is used as food in Angola, Cameroon, DRC, Gabon, Madagascar, Nigeria and South Africa. Ophioglossum reticulatum is utilised as human food in eastern and southern Africa, that is, in South Africa, Swaziland and Zanzibar (Figure 2). Ceratopteris thalictroides is used as human food in Madagascar and Swaziland, Diplazium sammatii is utilized as human food in DRC and Nigeria; and Ophioglossum polyphyllum is used as human food in Namibia and South Africa (both countries in southern Africa) (Figure 2). Despite its worldwide distribution, the range of basic applications of Pteridium aquilinum are similar across its range of distribution, and it is mainly used as a food source [22]. In Japan and Korea, young shoots of Pteridium aquilinum are an important dietary element [22, 41]. The shoots are soaked for a day in water and ashes, then steamed or boiled and eaten as a vegetable or soup [41]. Research by Liu et al. [18] revealed that the leaves and rhizomes of Pteridium aquilinum are consumed by billions of people in the world. The same authors argued that Pteridium aquilinum is part of the diet of many poor people in south Pacific islands to northern and temperate prairies because of its wide distribution and easy accessibility. Ophioglossum are widely eaten in Asia, with Ophioglossum reticulatum used as a vegetable in India and Indonesia [17], while Ophioglossum polyphyllum is consumed as a vegetable in south central Tibet [17]. Research by the same authors, revealed that Ophioglossum polyphyllum is eaten in summer as a vegetable but also dried and stored for later consumption in winter. Ceratopteris thalictroides is commonly eaten throughout south east Asia, for example, in Malaysia and Japan where it is an established luxury vegetable [26]. According to Bassey et al. [30], various parts of Diplazium sammatii are consumed by various people throughout the world. The same authors argued that Diplazium sammatii is not an emergency or primitive food type. This assertion by Bassey et al. [30] correlates strongly with observations made by Pieroni [41] that eating food from the wild is not simply an essential response in times of famine or food shortages or an easy way to obtain primary nutrients, but more often a complex evolutionary process involving different aspects of the relationship between humans and their natural environment.

Young leaves or enrolled fronds, often referred to as crosiers or fiddleheads are the primary pteridophyte food sources in sub-Saharan Africa (70.8%*), followed by leaves (50%*) and rhizomes (16.7%*) (Table 1). (*Some plant parts are reported in more than one plant part category). The majority of pteridophytes are eaten as vegetables or potherbs (66.7%), followed by leaves cooked as leafy vegetables (45.8%), leaves cooked as condiment, leaves eaten raw or as salad, edible rhizomes and leaves fed to livestock (12.5% each), herbal tea and famine food (4.2% each) (Figure 3). When a rhizome is eaten, leaf bases are removed, cleaned and peeled. Pteridophytes consumed as leafy vegetables or potherbs or salads, are usually collected at vegetative stage, when leaves are young and tender. These findings correlates with observations made by Lognay et al. [17] that bundles of freshly uncurled fronds are a common site in the rural markets of the Philippines, Indonesia, Malaysia and New Guinea.

Some pteridophytes used as food sources in sub-Saharan Africa, for example, Diplazium esculentum[42], Diplazium sammatii[30], Nephrolepis biserrata[43, 44], Nephrolepis cordifolia[21] and Ophioglossum polyphyllum[17], have high nutritional value in comparison to the FAO/WHO nutrient standards [45] (see Table 2). These five edible pteridophytes are important sources of macro and micro nutrients which are important for the maintenance of good health and prevention of diseases. The calorific value of 408.5 kcal kg^-1 for Diplazium sammatii and 3413 kcal kg^-1 for Diplazium esculentum (Table 2), compare favourably to 210–1340 kcal kg^-1 reported for common vegetables (broad beans, cabbage, cauliflower, lettuce, spinach) and common fruits (apple, lichi, mango, pawpaw) reported by Seal [42]. The carbohydrate and protein content of the edible pteridophytes shown in Table 2 is also higher than the carbohydrate and protein content of common vegetables and fruits obtained by Seal [42]. All the five edible pteridophytes have high levels of minerals in comparison to mineral content of common food crops, vegetables and fruits available in the published literature. For example, copper and zinc content of the edible pteridophytes shown in Table 2 is higher than the copper and zinc content of common vegetables and fruits obtained by Seal [42]. Therefore, pteridophytes are an important source of macro and micro nutrients and may be consumed by local communities to enhance nutrient intake or diversify the diet of local communities in sub-Saharan Africa.

Pteridophytes can also play an important role to food security in sub-Saharan Africa, can be utilized during drought periods when few crops are available and their year round availability increases their value as an important food plant group. The data documented in this study can be used in determining which pteridophytes can be preferentially utilized to benefit the overall nutrition of communities who inhabit sub-Saharan Africa. There is uniformity in the use of edible pteridophytes in sub-Saharan Africa, regardless of cultural differences in aspects such as language, history and social organization in general (Table 1, Figure 1). Based on the current baseline study, there is need to do further studies in other countries in the African continent aimed at documenting pteridophytes diversity used as food which at the present appear to be more or less homogenous. The observed similarities in pteridophytes use by different ethnic and cultural groups provide an insight into the possibility that regional groups may have been more closely linked in the past. It is probable that pteridophytes were widely consumed by the pastoralists and hunter-gatherers, historically referred to as the bushmen in sub-Saharan Africa. The oral transmission of traditional knowledge on the uses of pteridophytes by indigenous people has largely been eroded over the centuries due to land fragmentation, migration and improved agricultural activities in favour of improved food crops, vegetables and fruits. Detailed studies across national boundaries are also important in order to find out the possible reasons for similarities in pteridophyte uses. Traditional and cultural knowledge about pteridophytes use should vary, as this indigenous knowledge is not formalized. Detailed comparison of pteridophyte use by different ethnic groups will provide us with a clue to understanding pteridophytes use history, cultural diversity and variations of cultural practices of these ethnic groups.