Wild edible ectomycorrhizal fungi: an underutilized food resource from the rainforests of Tshopo province (Democratic Republic of the Congo)

Background Ectomycorrhizal (EcM) fungi constitute a source of income as well as proper food with considerable nutritional value. Although edible EcM fungi are highly diverse and expected to host considerable nutritional attributes, only few studies focus on their use and promotion in the province of Tshopo (DR Congo). This study provides original ethnomycological and diversity data on edible ectomycorrhizal rainforest fungi from the Man-and-Biosphere reserve of Yangambi and the reserve of Yoko. Methods The list of edible fungi follows the current taxonomy. Taxa were collected in plots situated in different types of rainforests. Each taxon is supported by herbarium reference specimens. Ethnomycological data on locally consumed EcM fungi were collected from randomly selected people living near the Man-and-Biosphere reserve of Yangambi and the Yoko reserve. People were interviewed using a semi-structured questionnaire. The interview campaign involved 160 informants, all randomly selected from 6 different ethnic communities. Results The results reveal that rainforests from the Yangambi Biosphere reserve and Yoko forest reserve provide a relatively high number of edible fungi, more than local people actually use. Mixed forest stands hold the highest diversity in saprotrophic edible fungi (p value < 0.001) while no significant difference (p value > 0.05) was observed in the number of saprotrophic and EcM fungi within monodominant forests. In spite of being accessible, this renewable natural resource is underexploited. Although a wide array of EcM fungi is available in primary forests dominated by ectomycorrhizal trees, local people’s major interest goes to the saprotrophic fungi from areas with degraded mixed forests. Conclusion The lack of local interest for EcM fungi is probably related to the considerable distance people have to cover to collect them. As a result, the edible EcM fungi from the Tshopo area represent a potentially interesting but underutilized resource.


Introduction
In many regions of the world, including tropical Africa, edible fungi often constitute a source of income and food, with a non-negligible nutritional value [1][2][3][4][5][6]. According to De Kesel et al. [4] and Degreef et al. [7], documenting the relative importance and potential of locally used EcM fungi remains vital for improving strategies for their conservation and promoting sustainable use. Edible EcM fungi provide high-quality and important amount of crude protein, minerals and carbohydrates, fats, etc. [2,3,5,6]. The amount of fungal crude protein for instance is generally ranked between 19 and 35% of the dry mass [2]. Furthermore, many species of edible EcM fungi provide higher amount of unsaturated fatty acids than saturated ones [2,5]. Unlike saturated fatty acids (found in high amount in animal fats), unsaturated fatty acids are suggested to be essential in the human diet [2]. The high proportion of unsaturated fatty acids and high percentage of linoleic acid found in edible EcM mushrooms lead to consider them as a healthy food [2]. In addition, the proteins of most of edible EcM taxa contain all nine essential amino acids (leucine, isoleucine, valine, tryptophan, lysine, threonine, phenylalanine, methionine, and histidine) used in humans' nutrition [2,5].
EcM fungi constitute one of the most important groups of edible fungi worldwide. In tropical Africa, EcM fungi are massively consumed in the Zambezian region [4,5,[8][9][10][11][12][13][14], and in the savanna woodlands and open forest areas of the Soudanian region [15][16][17]. Within the Zambezian geographical zone, local people collect, sell, and consume impressive quantities of EcM taxa. The socio-economic interest for EcM fungi is considered a major incentive for obtaining strong and broadly supported forest conservation programs [4].
Within the rainforests of Tshopo province (Democratic Republic of the Congo), many species of edible EcM fungi are available. In this area, EcM fungi are typically found within primary forests dominated by EcM partner trees such as Gilbertiodendron dewevrei (De Wild.) J. Léonard, Brachystegia laurentii (De Wild.) Louis, Julbernardia seretii (De Wild.) Troupin, Uapaca guineensis Müll. Arg, and U. heudelotii Baillon [18,19]. This study aims to assess the distribution and diversity of edible fungi within various types of rainforests found in the Man-and-Biosphere reserve of Yangambi and the Yoko reserve. We show how access and availability, as well as trophic level of fungi, affect the interest of local people, and how this in turn creates or annihilates opportunities for forest conservation.
The fungi were collected between 2013 and 2016, mostly within plots situated in the rainforests of the Biosphere reserve of Yangambi (0°51′ 01.62″ N; 24°31′ 43.53″ E) and Yoko reserve (0°17′ 34.9″ N; 25°18′ 27.4″ E) (Fig. 1). Monitoring of plots follows Lodge et al. [25], and fungal sampling was performed within forests dominated by Gilbertiodendron dewevrei, Brachystegia laurentii, Julbernardia seretii, Uapaca heudelotii, and Uapaca guineensis and in mixed forests. Three plots of 100 × 100 m were demarcated in each type of forest; each of them divided in a 20 × 20 m grid. Due to the elongated shape of Uapaca heudelotii-dominated forests, more stretched plots were installed in this forest type. In each plot, the aboveground fruiting bodies of all EcM fungi were collected by walking parallel bands covering the entire plot [26]. Unidentifiable fruit bodies were photographed in situ and dried after notes were taken from their macromorphological features (following [18]). Voucher specimens were dried using a field drier [27] and deposited at the herbarium of Meise Botanic Garden (Belgium).

Ethnomycological data acquisition and treatment
Ethnomycological data on locally consumed fungi were collected from local people living around the Man-and-Biosphere reserve of Yangambi and the Yoko reserve. Data were collected using open, semi-structured interviews (paper fill-in questionnaires). The interviews involved mainly the head of the family, sometimes assisted by other family members. The questions focused basically on the informant's knowledge concerning the different locally consumed edible fungi. Interviews were obtained from 160 informants, all randomly selected, but belonging to one of 6 ethnic communities (Bakumu, Turumbu, Topoke, Lokele, Ngelema, and Ngando). The entire pool of informants counted 88 men (55%) and 72 women (45%), ranging from 16 to 72 years old. The interviewed communities live in four villages in the vicinity of the Man-and-Biosphere reserve of Yangambi (Yakako, Yalungu, Lyoli, and Lobiloto), 4 suburbs of Yangambi city (IFA, Lusambila, Ekuchu, and Manzikala), and 4 villages surrounding the Yoko forest reserve (Babogombe, Biaro, PK 48, and PK 25).
With an average of 600 households living in the studied area, the average number of studied households revolves around 25 per village (4.2% of the entire pool). Within the 4 suburbs of Yangambi, the distribution of households was 150 (25%), 120 (20%), 80 (13.3%), and 50 (8.3%) respectively reported from Ekuchu, Lusambila, Manzikala, and IFA. According to Gumucio et al. [37], the sample size (n) of the interviewed households should be calculated as follows: n = N 1þNÂe 2 , where N is the total number of available households and e is the level of precision. With a precision level of ± 7%, the sample size was calculated by the following formula: n = 600 1þðNÂ0:07 2 Þ ≈ 152, households that were fitted to 160 informants. Referring to the mean distribution of households per sampling site (villages and suburbs of Yangambi), 7 households were interviewed from each of the 4 villages while 38 from Ekuchu, 30 from Lusambila, 20 from Manzikala, and 12 households from IFA. In each sampling site, all households were numbered. The first numbers referring to the considered sample size were selected randomly using the function "rand.between" of the Excel software.
The analyzed data only refer to locally eaten fungi and allow to present (per species) information on the edible mushrooms' cultural significance (EMCS). According to Pieroni [38], the edible mushrooms' cultural significance index refers to the importance or the role that a given fungal taxon or group of fungi plays in the social life of a group of people or a community. Using pictures or fresh sporocarps of edible fungi, the edible mushrooms' cultural significance for a fungal species corresponds with the sum of the scores for "edibility status" given by all informants, divided by the total number of informants. Edibility status scores or frequencies of mention (FM) were assigned through informants' answers to the following question: Do you eat this mushroom? (yes = 1, no = 0). A Kruskal-Wallis analysis was used to test the role of ethnicity on the edibility status score, and how this changes according to the different trophic groups.

Results
Checklist and diversity of wild edible fungi Table 1 presents the checklist of edible fungi recorded from the rainforests of Tshopo. Based on the information found in the literature and ethnomycological inquiries, 78 species of macrofungi are edible.

Trophic groups and distribution of edible fungi
Edible species belong to 3 trophic groups (saprotrophic, ectomycorrhizal, and termite associated). The saprotrophic taxa (wood and litter decaying fungi) represent the most species-rich group (44 species), followed by the EcM fungi (31 species) and termite-associated taxa (3 species, all Termitomyces). Whereas the number of edible fungi significantly differs between trophic groups, strong variation in the composition of edible fungi is observed between forest types (Fig. 2). Figure 2 shows that the mixed forests, on average, hold the highest diversity in saprotrophic edible fungi, while EcM fungi reach their highest species numbers in the monodominant forests. Edible termite-associated fungi (Termitomyces sp.) show the lowest species numbers and seem equally represented in both mixed and monodominant forests. Considering all fungal trophic groups, plots from monodominant and mixed forests showed similar average numbers of edible species, but variations are more important in the monodominant forests. The result is that the highest numbers of edible species were encountered in some of the monodominant forests, especially in the forest dominated by Gilbertiodendron dewevrei.
Within monodominant forests, we observed no significant difference in the number of saprotrophic and EcM fungi (Fig. 3). In contrast, within mixed forests, the number of saprotrophic taxa was significantly higher than in the EcM forests.

Usage of edible fungi
Almost a third (25 species) of the 78 edible species are not used for food by any of the interviewed people (  Table 3). Because of their low numbers (three species), differences in the mean numbers of consumed termiteassociated species are evidently small. In general, the most commonly eaten mushrooms (with the highest edibility index) are saprotrophic or woody-decaying fungi (Table 3), the most appreciated ones being Auricularia spp., Marasmius buzungolo, Lentinus squarrosulus, and Schizophyllum commune (Fig. 4). Only a small fraction of EcM fungi, especially from the genus Cantharellus, are reported as a delicacy by some populations. The scores of EcM fungi are even more lowered, knowing that some local populations (Kumu) systematically reject all edible Russula because of their vivid colors. In terms of diversity and quantity, local markets (Yangambi) offer far more saprotrophic than EcM fungi (De Kesel pers. obs.).

Discussion
This study revealed that wild edible fungi from rainforests of Tshopo belong to several trophic groups. They can be either saprotrophic (growing on dead fallen trunks and litter), or termite associated (growing in mutualistic relation with termites), or ectomycorrhizal, i.e., living in a mutualistic relation with the roots of some vascular plants. Within rainforests from the Yangambi Biosphere reserve and Yoko reserve, relatively high numbers of edible fungi are available. A more quantitative approach, similar to the one used by Yorou et al. [41] or De Kesel et al. [4], is needed to ascertain this, but it seems that the EcM fraction of edible fungi is still underexploited. Although a wide array of edible EcM fungi is available in the primary ectomycorrhizal forests, the interest of local people mainly goes to saprotrophic fungi from the degraded mixed forests. Milenge et al. [39] reported that most of these saprotrophic edible taxa typically grow on wood (logs).
Among the reported edible EcM fungi, only species of the genus Cantharellus are considered of some importance in local consumption [39]. As stated by Eyi-Ndong et al. [18], species of the genus Cantharellus are the most important edible EcM fungi found within rainforests from the central African Congo basin. Most species of Cantharellus are known worldwide as good or even excellent edibles. With its 42 African endemic species, the genus is by far best represented on the African continent [9]. In contrast to the mushroom diet chosen by people from Tshopo, Cantharellus species are massively consumed in the Zambezian region [4,5,9,10,13,14,42].
Although other EcM genera, i.e., Amanita, Lactarius, Lactifluus, and Russula, are common in the forests dominated by Gilbertiodendron dewevrei, these edible EcM fungi do not seem to be at all harvested and consumed by   local populations of the Tshopo province. However, in miombo areas of Burundi [7,8,43], and similar woodlands in the Zambezian [4,11] and Guineo-Soudanian ecozone [15][16][17], these genera (and Cantharellus) represent a major source of food and income.
Triggering the interest of local people for nontimber forest products requires science-based information about how much of the resource is available, how much and where it can be obtained in a sustainable way, and what its value may be. Based on productivity data of the 50 most common edible species, 1 ha of Katangese miombo woodland produces an average of 148 kg of fresh fruiting bodies per year [4]. A very similar biomass, i.e., 140.7 kg/ha/year, was obtained from 29 edible species monitored in the savanna woodlands of Benin [17]. From miombo woodlands of Katanga in the Democratic Republic of the Congo for instance, 1 kg of fresh fruiting bodies of Cantharellus spp. costs approximatively 2.8 USD [4]. Local people realize that the yearly revenue obtained from these NTFP goes hand in hand with sustainable use and preservation of host trees and their immediate environment. Losing revenue from not being able to collect EcM fungi is a strong incentive to preserve, or at least make sure, that the forest ecosystem keeps on delivering its services.
In this context, it surely is interesting to monitor natural productions of edible EcM fungi in different types of rainforests of the Democratic Republic of the Congo. So far, this has not been done. Gathering such data is one thing, but convincing local people of changing or widening their diet, i.e., to include more EcM fungi, is another. Several scientists [7,17,27,[44][45][46][47] have reported the role of culture and tradition on the variety of mushrooms consumed by local people. In the context of the province of Tshopo, taboos resting on mushroom consumption create mycophobia and subsequently decrease the interest for some edible EcM species. In spite of their high nutritional value, most Russula species are not eaten by Kumu people because of vivid colors of the cap. Variation in mushroom consumption may also rely on the scarcity of the fruiting bodies [7], and general appreciation is bound to be affected by availability and ease of access to the resource. When some taxa are rare or only available in more remote areas, people may find it difficult to go far to collect them. The result is that mushroom diets can and will shift as a mere result of access and distance to suitable sites [7]. We have no data from eventual hunter-gatherers in the region, but the main reason why people from the Tshopo show a preference for saprotrophic fungi is that these species are found close to the villages and nearby recently cut forests. Because of frequent rains and availability of dead wood, saprotrophic taxa also manage to produce fruit bodies all year round. In contrast, EcM fungi occur abundantly in remote and pristine (or nearly pristine) forests with older growth of Gilbertiodendron dewevrei, Brachystegia laurentii, or Uapaca spp. [19]. With the exception of Cantharellus rufopunctatus, producing fruit bodies at least from April to December [9], most of the edible EcM taxa can only be found during a short period of time.

Conclusion
The results from this study reveal that rainforests from the Yangambi Biosphere reserve and Yoko forest reserve of the Tshopo provide a substantial number of edible fungi.
Although not formally measured, there is little doubt that the local populations collect and use only a small fraction of the naturally produced quantity of edible fungi. Most of the locally used taxa are saprotrophic and easily accessed from either litter or dead wood. It is the availability, abundance, and proximity of this resource that makes saprotrophic fungi from anthropized environments (secondary forests, clearings, fallow, farmland) most attractive and preferred. As EcM fungi are harder to obtain and less frequent, they are a poorly known and underutilized food source. Whereas in the Zambezian region, the major local interest for edible EcM fungi can be used as a strong incentive for forest conservation; our observations indicate that this is not the case in the Tshopo region.
Some locally considered inedible EcM fungal taxa are in fact perfectly edible and used for food elsewhere in Africa. More research needs to be done, but assessing the natural productivity of edible fungi from the rainforests should help us better understand what these fungi can mean as an ecosystem service to local livelihoods. In combination with the measured interest of local people for this resource, one should be able to develop systems to promote the use of this largely untapped resource.