Assessment Notes

Justification

Phaeocollybia olivacea is a widespread species in Oregon and California, into Washington State and Idaho, USA, with ~150 known localities. Although most collections come from mid to late seral stage and old growth forests this species appears to be able to colonise forest at a younger age than most Phaeocollbyia species. Based on the number of known sites, and recent reports, this species appears to be locally common where suitable habitat exists. While overall habitat has declined, and continues to do so it is common enough, and has a large enough habitat range to qualify as Least Concern (LC).

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Taxonomic notes

Phaeocollybia olivacea was described from a type collection made in southwest Oregon, USA (Smith 1957). The name Phaeocollybia olivacea has been misapplied to collections of the similar P. fallax, and P. pseudofestiva. All three can grow in close proximity; some collections identified as P. olivacea contain fruitbodies of two or more of these species.

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Geographic range

This species is known from Santa Cruz County, California, northwards in coastal and Coast Range forests, through the Siskiyou Mountains, Oregon coast and Coast Range, Oregon Cascade foothills into Washington state, and rarely in the Sierra Nevada of California. There is one record from the Washington-Idaho border area.

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Population and Trends

The population is widespread and although there is habitat loss in parts of its range, it overall appears stable. Currently, it is known from 150 sites in California and Oregon into Washington State. 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 the number of known sites, and recent reports, this species appears to be locally common, and stable.

Population Trend: Stable

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Habitat and Ecology

It is ectomycorrhizal with conifers and possibly hardwoods; it has a preference for mid to late seral stage and old growth forests. Often fruiting is in arcs and rings, less often solitary or scattered, typically in areas with thick duff. Fruiting is in fall and early winter.

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Threats

Continued loss of mature and old growth forests, due to logging and fires in northern California and the Pacific Northwest are potential threats. Sudden Oak Death (Phytophthora ramorum) has killed large swaths of Tanoak in coastal California; this may have a long term negative effect of suitable habitat. This is a 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.

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Conservation Actions

Site protection and management of old growth forests is required. Logging and machine clearing of understory vegetation should be limited in mature and old growth forest in areas where this species might occur. No specific research is needed with regard to this species.

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Use and Trade

No use/trade is known.

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Source and Citation

Siegel, N. & Mueller, G.M. 2025. Phaeocollybia olivacea. The IUCN Red List of Threatened Species 2025: e.T198479993A198487315. Accessed on 22 November 2025.

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• Country occurrence