Partner trees

Mycorrhiza: an invisible underground alliance

The most important type of partnership between a fungus and a tree is mycorrhiza (literally "fungus-root"). M.Ya. Zerova, in her «Key to the Fungi of Ukraine», explains this phenomenon as a mutually beneficial symbiosis: the mycelium densely wraps around or even penetrates the tree's fine rootlets. The fungus works like a giant pump, supplying the tree with water and dissolved minerals from a huge area the root itself can't reach. In return, the tree gives the fungus simple carbohydrates (sugars) that it produces through photosynthesis.

David Arora stresses in «Mushrooms Demystified» that the vast majority of the most prized edible mushrooms (porcini, truffles, chanterelles, slippery jacks, red-capped scaber stalks) and the deadliest ones (death cap, destroying angel) are mycorrhizal. They can't be grown in a greenhouse on a substrate, because without a living partner tree their fruiting body will never form.

The mycorrhizal bond is often remarkably specific. Roger Phillips notes in his guide «Mushrooms» that the larch bolete (Suillus grevillei) grows only under larches, and the red-capped scaber stalk (Leccinum aurantiacum) under aspens. Once you identify the tree species, you automatically rule out dozens of unsuitable species in the field guide.

Saprotrophs and parasites

Not all fungi form mycorrhiza. Two other large groups use trees in a different way:

- Parasites: They grow on living trees, gradually draining their sap and destroying them. This group includes some honey fungus species (for example, honey fungus — Armillaria mellea), which can wipe out entire forest stands, as well as many bracket fungi (for example, chaga).

- Saprotrophs: They grow on already dead wood (stumps, brushwood, fallen trunks), breaking down lignin and cellulose. This group includes oyster mushrooms (Pleurotus), shiitake (Lentinula) and velvet shanks (Flammulina). They are often "tied" to a particular type of dead wood (conifer or broadleaf).

For a forager, understanding these links has real practical value. In a pure pine forest there's no point looking for mushrooms that form mycorrhiza only with oak, and if you know where old birches grow, you can head straight for brown birch boletes.