food forests and other permaculture forests open interesting possibilities about the cultivation of edible mushrooms, which seem not very much explored yet. With actively introducing and coupling edible fungi/mushrooms with trees, shrubs and other edible plants it might be possible not only to grow plants faster, but to have an additional harvest and to exclude inedible ones. Similarly to how some cheese varieties are made by inoculation with the right fungus strain, to exclude bacteria and other fungi.
And there is a good reason to eat mushrooms: they are fruits and intended to be attractive and to be eaten by animals (like us; really, why else the nice colors and smells?). Harvesting doesn't harm them so far, as long as they have a chance to propagate. So, these fungus networks are quite similar to fruit trees...
Most mushrooms are perennial, last for many years as mycelium in the soil, and are therefore ideal to be coupled with perennial plants. Sepp Holzer mentions, that he could grow some mushrooms like Boletus edulis, which nobody else had cultivated before. I'm from Germany, so I use only the Latin names. But I recently noticed, that similar or even the same fungi species grow all over the temperate climate zones, just like the trees they are coupled with.
Edible mycorrhizal fungi would have already two functions, if they are introduced into a permaculture system. One good possibility might be truffles, which are known to be mycorrhizals. Currently they are often grown in fenced off forests, where the only other product is timber.
But also, when they aren't mycorrhizal fungi it might be worthwhile to introduce them. About 70% of the mushrooms (at least in middle Europe) are edible, a wonderful protein source and have lots of different, interesting tastes. Another 25% are inedible (bad taste) or slightly toxic (usually cause digestive problems), only 5% are dangerous. There are several very delicious ones out there, which nobody cultivates yet, and many people don't even know. Some grow together with only a single tree species, for instance Leccinium scabrum with birches.
It may take some time to get to know some of the mushrooms properly, because of inedible or toxic similar species. Here, a local expert and classification books might help. (A local expert is always the best start for learning new species, with books it can be a bit tedious. But with every correct determination the confidence grows and it is more fun to learn new ones.)
Ok, here is a list of edible mushrooms, which grow here, but might also be common in other temperate regions or wherever the same trees grow. The ones which fruit on the ground are probably also mycorrhizals. I marked the ones with '*', that have an exceptionally good or special taste. Just put the names into Google image search or Wikipedia. For most species it is mentioned in Wikipedia, whether they form mycorrhizal partnerships or how they feed otherwise.
Only in deciduous forests: Agaricus lanipes*
most Leccinum* species (e.g. Leccinum aurantiacum and Leccinum scabrum, often associated with only a single tree species, mycorrhiza)
Tuber macrosporum* (a truffle, grows under oaks; all Tuber species are mycorrhiza)
Only in conifer forests: Agaricus sylvaticus*
Hygrophorus olivaceoalbus (under spruces)
Hygrophorus hypothejus (under pines)
most Lactarius species, e.g. Lactarius deliciosus*, all mycorrhiza
Marasmius scorodonius (tastes like garlic, spice mushroom)
Rhizopogon roseolus* (truffles, all Rhizopogon species are mycorrhiza)
most Suillus species, e.g. Suillus luteus, often associated with only a single tree species, obligate mycorrhiza
In mixed forests (deciduous/conifer trees): Agaricus silvicola*
Phallus impudicus (grows in moist areas, saprophagous, edible is only the interior of the egg-like immature fruiting bodies, more a funny experience than an actual food source. Sometimes they grow in mulched gardens or on compost heaps.)
On grassland: Pasture is a good place to grow mushrooms because of the additional nitrogen! All the following mushrooms like or even depend on that.
Coprinus atramentarius* (saprophagous, grows near trees or stumps; very good taste, but toxic together with alcohol until three days after the meal)
Coprinus comatus* (saprophagous, additionally catches and feeds on nematodes; grows sometimes on compost heaps or in gardens)
Stropharia rugosoannulata* (saprophagous, grows on large amounts of decaying matter)
Langermannia gigantea (or Calvatia gigantea), grows on nitrogen-rich soil. This one is exceptional, the mushrooms can grow up to half a meter in size. Probably a good option for growing on pasture.
Saprophytes or endophytes on wood: (some of them have to be boiled shortly to remove tannins before they are edible)
On living trees: Albatrellus confluens (on conifers)
Fistulina hepatica (on old oaks, endangered species)
On living trees and dead wood: Armillaria ostoyae and mellea (saprophagous, sometimes parasitic, often produces large amounts of mushrooms)
Dendropolyporus umbellatus (saprophagous, sometimes parasitic)
Laetiporus sulphureus (parasite mostly on deciduous trees, still grows out of the wood of cut down trees)
Pleurotus ostreatus* (well known culture mushroom)
Sparassis crispa* (at the foot of old pines)
On dead wood: (These three are similar in cultivation. They also look similar, but taste quite differently.)
Flammulina velutipes* (grows over winter, easy to cultivate)
Hypholoma capnoides (on conifer wood)
Kuehneromyces mutabilis* (on wood of deciduous trees)
Fungi growing on wood can be just breaking down the wood, be parasitic on trees, or they can be growing inside a living tree and still be beneficial to the plant.
Paul Stamets shows in his TED talk pictures of an Armillaria ostoyae mycelium, which is several kilometres wide. This is the largest living organism on Earth.
From the above list already available from breeders, at least in Switzerland and Germany, are: Morchella esculenta, Macrolepiota procera, Coprinus comatus, Flammulina velutipes, Hypholoma capnoides, Kuehneromyces mutabilis, Stropharia rugosoannulata, Pleurotus ostreatus, and also Fistulina hepatica. Flammulina velutipes is well known in Japan as Enokitake. It is quite unique with its fruiting regime (starts in November and lasts until January, naturally grown), only Pleurotus ostreatus also grows in Winter (and other Pleurotus species, which are in culture, but not native here). All of these are saprophagous fungi, no mycorrhizals.
Most people worry about toxic mushrooms - here are the 5% really toxic ones in middle Europe:
Amanita muscaria (this one is well known, complex cocktail of neurotoxins)
Amanita pantherina (similar, but stronger than A. muscaria)
Amanita phalloides (deadly, destroys several inner organs)
Cortinarius rubellus (destroys kidney tissue)
Galerina marginata (deadly, same toxin as Amanita phalloides)
Gyromitra esculenta (can be deadly raw, but heating destroys most of the toxin, grows only in spring)
Inocybe erubescens and the whole genus Inocybe (vision disorders, colics, vomiting)
Paxillus involutus (similar to an allergic reaction; less toxic, but increasing symptoms with more meals)
Tricholoma pardalotum (heavy digestive problems, which increase with each new meal, so that the second meal is probably deadly)
So, these ones are definitely good to know, when one intends to collect similar looking edible mushrooms. The classification books usually mention similar mushrooms, so compare them carefully until you are completely certain that you got the right one. The fastest and most fun way to lay a good foundation is, as I mentioned above, one or two hikes in the forest with an expert.
Ok, I hope this little compendium is a good starting point to get to know mushrooms better and consider/actively introduce them more as elements in permaculture.