Domestication of mushrooms
from the miombo woodlands:
current status and crucial issues for agroforestry
The miombo woodlands harbour a diversity of tree species producing valuable non-timber forest products. While most of these can be domesticated in man-made ecosystems, others may be specific to natural ecosystems. The depletion of natural forests and woodlands jeopardize the continual existence of many of these non-timber forest products. For instance, some mushrooms that obviously occur in natural woodlands or forests do not normally occur in converted lands. This paper focuses on the status of mushrooms in the miombo woodlands and the current effort in Tanzania towards their domestication through harnessed symbiosis with trees in agroforestry systems. The crucial issues involved are highlighted.
The miombo woodlands constitute the commonest woody vegetation in many eastern, central and southern African countries. In Tanzania, they constitute the largest single vegetation type (Temu 1979) and differ from other woodlands in that they are dominated by the genera Julbernardia and Brachystegia (Lind and Morrison 1974). Trees in these genera together with a few others (table 1) make the miombo woodlands unique in that they are symbiotically associated with a variety of mushroom-producing fungi that make ectomycorrhizal symbiotic associations with tree roots (Munyanziza 1994). In addition there are saprophytic fungi and others that have symbiotic associations with termites, which are important in this ecosystem. Thus there are three broad groups of desirable mushroom-forming fungi:
Table 1. Miombo tree genera known to be associated with ectomycorrhizal mushrooms
Sources: Högberg, & Nylund 1981, Högberg 1982, Alexander & Högberg 1986, Munyanziza & Kuyper 1995
· Ectomycorrhizal fungi, forming symbiotic associations that benefit the plants in terms of increased capacity for nutrient and water uptake. This results from enhanced efficiency by fungal hyphae which increase the volume of soil exploited. The fungus, in exchange, gets energy from its host (Harley & Smith 1983). While some plants can live in the absence of the fungal partner, at least under amended (fertilized) environments, mycorrhizal fungi cannot live in the absence of the host. In poor, dry tropical soils, however, mycorrhizal associations are vital for plant growth and survival (Munyanziza 1994). The normal reproductive cycle of mushroon-producing mycorrhizal fungi involves the following stages: (1) spore germination, (2) mycelium growth, (3) infection of the specific host, and (4) fruiting-body (mushroom) production and sporulation. Without the right host the fungus does not reach stages 3 and 4.
· The saprophytes, which do not need living hosts and colonize dead organic matter that they decompose. They are hence involved in nutrient recycling. Saprophytic mushrooms can thus occur in agricultural land, in pastures, and of course in woodlands and forests.
· The termitocytes, which live in symbiosis with termites. Termites culture the mycelium of the fungus in their nests and feed upon it and spread it. It is known that this relationship is obligate for both partners (Härkönen et al. 1995). Termitocytes constitute a delicacy in eastern and central African countries (Ryvarden et al. 1994).
In addition to these desirable fungi there are also a few undesirable parasitic fungi. These parasitic fungi live the expense of their host. An example is Armillaria mellea, which fructifies on living trees. Parasitic fungi are harmful and therefore not welcome in agricultural or forest plantations. The point is not how to domesticate them but how to control them.
The unique ecosystem of the miombo woodlands is currently undergoing several forms of degradation related to human activity. Such activities include making charcoal, collecting fuelwood for home use, making bricks and especially clearing land for agriculture (Chidumayo 1987a, b; 1989, 1991). The rapid depletion of this ecosystem and its major tree genera has many implications, such as-
· Ecosystem-specific products will be lost
· Tree-specific microorganisms will be lost some time after the removal of their tree partners. Among the most vulnerable organisms are the symbiotic mushroom-producing ectomycorrhizal fungi. When a species is in such danger, there are two possible options: (1) protection in its natural range or (2) domestication and multiplication in human-created ecosystems. Domestication can also be triggered by market opportunities. The aim of this paper is to discuss the status and the crucial issues in the domestication of mushrooms occurring in the miombo woodlands.
Intensive forest management has usually focused on timber as the main or the sole end product of forest operations. Thus all activities tend to be aimed at maximizing wood production and neglecting other forest components (Mikola 1970, 1980). This can lead to a failure to achieve sustainable production. Among the non-timber forest products attracting more and more interest for their economic potential and role in environmental protection are wild edible mushrooms (Pegler & Piearce 1980). In several countries, mushrooms are now included in forest resource inventory for their economic returns as a criterion for environmental health. While collected traditionally by local people in the wild, mushrooms have recently attracted scientists, donors and politicians (Härkönen et al. 1993, Ryvarden et al. 1994). Probably this reflects the fear that current land use is unsustainable.
People in the miombo woodlands have limited alternatives for energy and limited possibilities for income generation. In the process of converting land for agriculture, forestry, and other land uses the original vegetation is usually replaced by few a selected species not generally native to the area. In agriculture, these are maize, tobacco, beans, etc., while in forestry the species dominating plantations are eucalypts, pines and cypress. In agroforestry systems, trees given priority have mostly been those fixing nitrogen and with other multiple uses (Kamara & Maghembe 1994). In the miombo woodland areas the tree species composition of plantations, agroforestry and farms have a bearing on the quantity and species of mushrooms found, since the host-symbiotic relationship determines which species occur. Current interest in the domestication of indigenous miombo fruit trees for agroforestry could have benefits for mushroom production (Maghembe 1994).
Domestication of the saprophytic mushrooms is probably the easiest of the three types described. The main requirements are the right substrate and an adequate physical environment. This group of fungi has been domesticated on a commercial scale in Europe, some Asian countries (Oei 1991) and recently also in some African countries. Mushroom eating in Tanzanian rural areas relies exclusively on collection from the wild. Species commonly eaten belong to a few genera, most of which live in symbiosis with termites or trees (table 2). Most of the mycorrhizal mushrooms eaten in Tanzania grow in mutualistic association with a few tree genera occurring in the miombo woodlands (Table 1).
Table 2. Fungal genera having mushrooms widely eaten in Tanzania
Härkönen et al. 1995
In Tanzania, there is a growing interest in the domestication of edible mushrooms but there is still a lack of appropriate simple techniques to do it. Tanzania has, however, been chosen to coordinate mushroom cultivation in sub-Saharan Africa.
Mushroom growing in Tanzania is confined at the moment to academic institutions, where trials have been initiated in the laboratory or in the glasshouse. No trials exist in rural areas. The following issues have to be borne in mind when planning the domestication of mushrooms in agroforestry:
· Ectomycorrhizal fungi: The presence of ectomycorrhizal fungal inoculum in agricultural and agroforestry systems does not necessarily mean the production of mushrooms. A number of factors may affect vegetative growth of the fungus and the production of fruiting bodies.
· Termitomyces: These, living in symbiosis with termites, have not yet been domesticated. So far, they cannot be domesticated in isolation from their termite partners. They occur both in forest lands and in agricultural and agroforestry systems. The conversion of forest lands to agriculture or agroforestry is likely to have an effect on Termitomyces.
· Saprophytes: Saprophytic mushrooms can be grown on a wide range of dead organic matter ranging from sawdust to entire logs. The yield, however, will depend upon (1) the nature of the substrate (2) air humidity and temperature (3) moisture of the substrate and (4) other competing organisms. The use of inoculated logs for mushroom cultivation outdoors has been practised in Asian countries with varying success (Oei 1991). Inoculated logs are used in Europe for mushroom cultivation (Y. Claassen, pers. comm.). With the introduction of indoor cultivation, mushroom yield has been significantly improved (Oei 1991).
With declining land productivity and increasing interest in organic farming, the cultivation of edible saprophytic mushrooms offers prospects for using agricultural residues and saw dust. Residues used for mushroom cultivation can be reused in agriculture and agroforestry for soil conditioning, as fertilizers or as animal feed.
· Parasitic mushrooms: The cultivation of edible parasitic mushrooms is a risky venture since one would be building up the disease inoculum, which can shift to crops.
The right host is essential to have symbiosis. Not all trees form ectomycorrhizal associations (Lapeyrie & Högberg 1994); in fact, in the tropics about 95% tree species from endo-mycorrhizas that do not form mushrooms (Mason & Wilson 1994). Clearly in the domestication of mycorrhizal fungi for mushroom production on trees in agroforestry systems, the tree component has to be made of the species forming ectomycorrhizas. A number of exotic species (e.g., Pines and eucalypts) do form ectomycorrhizas (Mikola 1970; Munyanziza & Kuyper 1995), as do many of the indigenous miombo trees.
A further complication is the recognition that ectomycorrhizal fungi develop an ecological succession in forest plantations (Mason et al. 1987; Termoshuizen 1991). Thus mushrooms associated with seedlings (early-stage fungi) are different from those associated with adult trees (late-stage fungi). For increased mushroom diversity and sustained production, the domestication of mycorrhizal mushrooms will mean the creation of an agroforestry system having trees of different species and age groups supplied with the right spectrum of fungal partners.
Mycorrhizal formation and especially mushroom formation is sensitive to soil disturbance (Amaranthus 1992). Soil disturbance in agroforestry systems may take the form of compaction by machinery, weeding by hand, by hoe or by machine. Whatever the form, the mycelial network will be disturbed. Similarly, chemical weeding, which is often done in forests or agroforestry (Amakiri 1977), may have consequences on mycorrhiza fungi and hence on mushroom production (Iloba 1980). The same holds for other chemicals that may be used, for example, to control pests and diseases. Fungicides are obviously undesirable if mushroom production is part of the system. It is also well known that inorganic fertilizers may negatively affect mycorrhizal formation (Arnebrant 1994, Arnebrant & Söderström 1992). Where mushroom cultivation is envisaged chemical fertilizers should be avoided or used with caution.
The amount of organic matter available in forest stands influences the types of mycorrhizae present on tree roots (Harvey et al. 1976). In the Netherlands, Baar (1995) observed that organic matter manipulation (removal or addition of litter) in stands of Scots pines stimulated mushroom production in some fungal species while inhibiting it in others. In the miombo ecozone, farmers increasingly use miombo litter as a mulch in agroforestry systems. Depending upon the amount used, this litter may influence the number and quantity of mushrooms produced in such systems. The domestication of mushrooms in agroforestry will require a clear understanding of the ecology of the intended mushroom species.
Mushrooms constitute an important resource yet to be exploited on a commercial scale to alleviate poverty, food insecurity and environmental degradation in developing countries through their links with agroforestry. Traditionally gathered for household consumption, they have won international attention. However, their future is uncertain given the rate of destruction of their natural habitat. The domestication of edible saprophytic mushrooms in artificial conditions is, however, gathering some momentum in a few African countries. The domestication of edible mushrooms living in symbiosis with other organisms (trees and termites) is yet to start and needs to build on the understanding of the relation between the partners and the environment under which this relation can be optimized. Domesticating edible mushrooms in agroforestry will diversify the farmer's possibilities and increase food security but will require special management of agroforestry systems. This will depend especially on incorporating appropriate tree species in agroforestry systems, practicing limited soil disturbance, understanding carbon dynamics and avoiding excessive use of inorganic fertilizers and chemicals.
I would like to express my gratitude to the International Foundation for Science for financial support and to the Sokoine University of Agriculture for the cooperation of its scientists.
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