Assessment Notes

Justification

Phaeocollybia scatesiae occurs in very disjunct subpopulations in California from Santa Cruz, Mendocino and Humboldt Counties. It is widespread in the Pacific Northwest; mostly along the coast and Coast Range forests in Oregon, with a few scattered sites in the Cascade Range and the Olympic Peninsula in Washington State. This species may be restricted to mature and old growth forests. Suitable habitat is now rare and fragmented, and continuing to decline in both quality and geographic extent. The scarcity of remnant old growth and mature forests in the Coast Range and Cascade foothills, and their fragmented condition are a major cause for concern; it is unknown if this species is capable of colonising and persisting in younger forests. Steel et al. (2023) calculated that over 30% of the conifer forests in the southern Sierra Nevada Mountains transitioned to non-forest vegetation between 2011-2020 and that between 50-85% of mature forests significantly declined in quality during this time. Extrapolating these data to more northern California into Washington State yields a conservative estimate of a loss / decline of at least 30% of habitat for this species over the past three generations and continuing into the future. It is suspected that the species' population will be declining at a similar rate. It is assessed as Vulnerable A2c+3c+4c.

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Geographic range

This species is known from very disjunct subpopulations in California from Santa Cruz, Mendocino, and Humboldt Counties. It is more widespread in the Pacific Northwest; mostly along the coast and Coast Range forests in Oregon, with a few scattered sites in the Cascade Range, and the Olympic Peninsula in Washington State.

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Population and Trends

The population is widespread, but highly disjunct. This species may be restricted to mature and old growth forests. Suitable habitat is now rare and fragmented, and continuing to decline in both quality and geographic extent. The scarcity of remnant old growth and mature forests in the Coast Range and Cascade foothills, and their fragmented condition are a major cause for concern; it is unknown if this species is capable of colonising and persisting in younger forests. Steel et al. (2023) calculated that over 30% of the conifer forests in the southern Sierra Nevada Mountains transitioned to non-forest vegetation between 2011-2020 and that between 50-85% of mature forests significantly declined in quality during this time. Extrapolating these data to more northern California into Washington State yields a conservative estimate of a loss / decline of at least 30% of habitat for this species over the past three generations (50 years) and continuing into the future. It is suspected that the population of this species will be declining at a similar rate.

Population Trend: Decreasing

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Habitat and Ecology

It is ectomycorrhizal with conifers; especially Douglas-fir (Pseudotsuga menziesii), Grand Fir (Abies grandis) and Western Hemlock (Tsuga heterophylla), and possibly Tanoak (Notholithocarpus densiflorus). It may be restricted to mature and old growth forests where it fruits in fall, forming small to large clusters in soil, typically in areas with thick duff.

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Threats

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 brethren is habitat destruction, via the logging of old-growth forests to which it appears confined to and an increase in fire frequency and intensity. 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). 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

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 (Castellano et al. 1999). 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 sites to better understand population trends and habitat restraints and requirements; i.e. is it restricted to old growth forests?

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Use and Trade

No use/trade is known.

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Source and Citation

Mueller, G.M. 2025. Phaeocollybia scatesiae. The IUCN Red List of Threatened Species 2025: e.T273227366A273246743. Accessed on 22 November 2025.

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• Country occurrence