Red Alder Alnus rubra

Red alder is the largest American alder. Mature trees are typically 80 to 100 feet tall, growing in lowland forests, along streams, and on disturbed ground from southeast Alaska to southern California (USFS FEIS). For mushroom hunters it matters for two reasons: the fungi that partner with its living roots, and the wood it leaves behind when it dies.

Red alder fixes atmospheric nitrogen through a symbiosis with Frankia bacteria in its root nodules, and soils under alder stands hold more nitrogen than soils under neighboring conifers (FEIS). Living alder also forms ectomycorrhizae, but with a specialist guild of fungi, such as Alnicola (Naucoria), Lactarius occidentalis and Alpova diplophloeus, rather than the chanterelles and boletes of the conifers. None of these is a sought-after edible. Dead alder is a different story: fallen logs and snags are substrate for wood-decay fungi. Oyster mushrooms, for example, grow mostly on hardwoods, especially alder (Siegel & Schwarz, Mushrooms of Cascadia).

Red alder is a pioneer species. With Douglas-fir it is a principal pioneer tree of low- and middle-elevation forests, colonizing landslides, clearcuts, stream banks, and other disturbed sites (FEIS). The thin, smooth bark is gray-whitish and mottled, often covered with moss, and never becomes thick and ridged like conifer bark. The leaves are oval and serrated, bright green above and paler below. Small woody cones stay on the branches through winter.

Red alder generally grows below 2,500 feet elevation. It is short-lived, with a maximum age of about 100 years (FEIS). The wood is used for furniture and cabinetry, and red alder coals are used in the Northwest to smoke salmon (FEIS).

Associated Mushrooms

Living red alder has its own specialist ectomycorrhizal partners (see above). The mushrooms below are wood-decay fungi that grow on dead or dying alder or on other hardwood wood. Here are some of the most notable.

Oyster Mushroom

Pleurotus ostreatus

Oysters grow on dead wood, mostly hardwoods and especially alder (Siegel & Schwarz), though also on conifers. Two main fruitings, spring and fall, with smaller numbers whenever it is wet enough. Check downed alder after rain.

Saprotrophic Edible

Deer Mushroom

Pluteus cervinus group

Decays the wood of both hardwood and conifer stumps and logs, so alder is just one of many hosts. Free gills that turn pink with age and a pink spore print mark the genus; telling this group from Pluteus exilis can take DNA.

Saprotrophic Caution

Artist’s Bracket

Ganoderma applanatum

Perennial woody bracket on lower trunks, stumps and logs, often of hardwoods but also conifers; alder is one possible host. The white pore surface stains brown when scratched. Not edible.

Saprotrophic

Late Fall Oyster

Sarcomyxa serotina

Grows on decaying logs and dead trees, almost always red alder or bigleaf maple (Siegel & Schwarz), and fruits more in cold weather. The viscid olive cap and orangish gills set it apart from true oysters. Edible but not highly regarded.

Saprotrophic Edible

Honey Mushroom

Armillaria species

Armillaria nabsnona is the honey mushroom that prefers hardwoods, especially red alder (D. Miller, PNW Pictorial Key): slender, orange-brown, few cap scales, spring or fall. Armillaria ostoyae, shown here, mainly attacks conifers. Fruits in clusters from wood and roots. Cook thoroughly, which often lowers the chance of a reaction.

Parasitic Saprotrophic Caution

Photos: Red Alder — Drew Meyer (CC0) · Late Fall Oyster — cgmayers (CC0) · Honey Mushroom — © Sharon Squazzo, CC BY.

Sources: Mushrooms of Cascadia (Siegel & Schwarz, 2024); MycoMatch (© Ian Gibson), a compilation of published descriptions through which most other field-guide authors named here are quoted; Danny’s DNA Discoveries and the PNW Pictorial Key (© Danny Miller).