Mushrooms and Their Relationships with Trees

By Cate Hughes

Though trees may be the celebrities of the forest, they coexist there with other, smaller organisms, some of which are highly valued: wild mushrooms! These mushrooms, some of which you can forage in Georgia, are not just trees’ neighbors. Rather, beginners’ guides to mushrooming detail which tree species you’re likely to find certain mushrooms on, under, or near. Different mushrooms have different preferences, and grow accordingly — some, harmlessly; others, not so much.

Mycorrhiza describes a symbiotic relationship that forms between fungi and the root system of a vascular plant, such as a tree. As in all symbioses, both fungus and host benefit from the relationship, though in different ways. The fungus colonizes its host’s root tissues, improving the host’s moisture and nutrient absorption capabilities. In exchange, the host plant provides the fungus carbohydrates produced through photosynthesis. Some highly prized varieties of edible mushrooms, such as golden chanterelles (Cantharellus cibarius) and king boletes (Boletus edulis), are mycorrhizal fungi, each with particular preferences regarding its symbiotic partner. You’re most likely to find king boletes under spruce (Picea), pine (Pinus), birch (Betula), and oak trees (Quercus), but other species in the Boletaceae family — many of which are edible — can be a bit pickier. American slippery Jack (Suillus americanus) and aspen boletes (Leccinum insigne), for example, will grow in symbiosis with only Eastern white pines (Pinus strobus) and aspens (Populus), respectively. Golden chanterelles grow in northern Georgia, most often in pine forests or under other conifers.

Another familiar mushroom, colloquially called Hen-of-the-Woods (Grifola frondosa), is less hospitable to its host. The parasitic Hen-of-the-Woods can cause butt rot of hardwoods, particularly oaks. Butt rot weakens the trunk and impedes nutrient transport through the tree. Far more destructive to trees, though, is the honey mushroom (Armillaria gallica), another choice wild mushroom well-loved for its rich flavor. These unassuming ‘shrooms can kill stressed trees, particularly oaks and conifers. As exciting as it may be to come across a haul of tasty honey mushrooms, the trees nearby may not be so thrilled.

Not all non-mycorrhizal fungi is detrimental to tree health, though. Many mushrooms are famously saprotrophic, the processors of dead and decaying organic matter. Mushrooms growing on a stump or felled tree, in a pile of woodchips — these are our saprophytes. You might be familiar with oyster mushrooms (Pleurotus populinus), which resemble, well, a cluster of oysters. Though oyster mushrooms can be parasitic, rotting the sapwood of hardwoods such as aspen, they are mainly saprotrophic. Weirdly, oysters are one of few known carnivorous mushrooms! They eat nematodes, or roundworms, in addition to a diet of decaying matter. “You are what you eat” must not apply here — oyster mushrooms are a delicacy in many cuisines!

Morels (Morchella esculenta), arguably the most famous of these exquisite ‘shrooms, seems to behave saprotrophically, though in some cases morels appear to act symbiotically with trees and other plants. The tree species associated with morels varies considerably, depending on region, morel species, and other factors, but a good place to start looking is near American elms (Ulmus americana), American sycamores (Platanus occidentalis), spruces, and other hardwoods. Saprophytes are hugely important to a forest’s ecosystem, for in breaking down organic matter into inorganic matter, they provide nutrient sources for plants.

(This ought to go without saying, but before foraging your own mushrooms, seek expert advice. When in any doubt, throw the mushroom out!)