Assessment Notes

Justification

Phaeocollybia fallax is a widespread species in western North America. Currently it is known from ~100 localities (Norvell and Exeter 2009, MyCoPortal 2024), however, some of these records are historic, and many of these sites have been subjected to repeated timber harvest. Most collections come from mid to late seral stage and old growth forests, but this species appears to be able to colonise forest at a younger age than most Phaeocollybia species. Based on the number of known sites, and recent reports, this species appears to be locally common where suitable habitat exist, but overall its habitat has declined, and continues to do so. However, here it is considered common enough, and has enough of a habitat range to qualify as a Least Concern.

---

Geographic range

This species is widespread in western North America, from the Santa Cruz Mountains in the southern portion of the range, north through coast and Coast Range forests into southern British Columbia, with scattered sites in the Cascade Range in Oregon and Washington, and a single record from Idaho (MyCoPortal 2024).

---

Population and Trends

The population is widespread, currently known from ~100 sites (although about a quarter of these are historic). Although it has a preference for late seral stage and old growth forests, it is not restricted to such sites, and appears to be able to colonise forest at a younger age than most Phaeocollybia species. Based on recent reports, this species appears to be locally common, and stable in protected old growth habitats, but absent or declining from unprotected sites.

Population Trend: Decreasing

---

Habitat and Ecology

It is ectomycorrhizal with conifers and possibly Tanoak; it has a preference for late seral stage and old growth forests. Fruit bodies are usually scattered or in small groups, and often are intermingled with other Phaeocollybia species. It typically occurs in undisturbed forests, in areas with thick duff or moss. Fruiting is in fall and early winter.

---

Threats

This is an ectomycorrhizal fungus species dependent on living host trees for viability. The major threat to this species and its co-occurring co-generic taxa is habitat destruction via the logging of old-growth forests to which this species has a preference for. The extent of old growth forest in the Pacific Northwest of North America has declined 90% in the last century (Society of American Foresters 1984, Haynes 1986). Climate change and droughts, along with forest management practices has made western forests highly susceptible to stand replacing forest fires. Fire is big threat to this species’ population. A stand replacing fire could severely degrade and/or diminish its current range.

---

Conservation Actions

This species is included on the United States Forest Service Northwest Forest Plan Survey and Manage list of rare/old growth forests dependent fungi, and has been actively surveyed for since the late 1990’s. Logging and machine clearing of understory vegetation should be limited in mature and old growth forest in areas where this species might occur. Continued surveys for existing subpopulations are needed to better understand its habitat restraints and requirements; e.g. if it is restricted to old growth forests.

---

Use and Trade

No use/trade is known.

---

Source and Citation

Siegel, N. & Mueller, G.M. 2025. Phaeocollybia fallax. The IUCN Red List of Threatened Species 2025: e.T198479983A198490388. Accessed on 22 November 2025.

---

• Country occurrence