Phaeocollybia californica is currently known from ~125 records, from ~60 localities in California and Oregon, USA, although some of these sites are no longer extant. Most of these occurrences are in mature and old growth forests. The species is widespread, but highly disjunct. Suitable habitat is now rare and fragmented, and continuing to decline in both quality and geographic extent due to logging and an increase in fire frequency and intensity. 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 as 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 in the region significantly declined in quality. Extrapolating these data into northern California into Oregon yields a conservative estimate of a loss / decline of at least 30% of habitat for this species over the past three generations (50 years) that will continue into the future. It is suspected that the population will be declining at a similar rate. The species is assessed as Vulnerable.

This species is found in coastal and Coast Range forests from northern California into northern Oregon, and scattered locations in the Siskiyou Range and Cascade Foothills in Oregon.

Phaeocollybia californica is currently known from ~125 records, from ~60 localities (MyCoPortal 2024) in California and Oregon, USA, although some of these sites are no longer extant (Norvell and Exeter 2009). Most of these occurrences are in mature and old growth forests. It is widespread, but highly disjunct. Suitable habitat is now rare and fragmented, and continuing to decline in both quality and geographic extent due to logging and an increase in fire frequency and intensity. 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 in the region significantly declined in quality. Extrapolating these data to more northern sections of forests in California into Oregon yields a conservative estimate of a loss / decline of at least 30% of habitat for this species over the past three generations (50 years) that will continue into the future. It is suspected that the population will be declining at a similar rate.

Phaeocollybia californica is ectomycorrhizal with conifers; especially Douglas-fir (Pseudotsuga menziesii), Grand Fir (Abies grandis) and Western Hemlock (Tsuga heterophylla), and possibly Tanoak (Notholithocarpus densiflorus). This species may be restricted to mature and old growth forests. Fruiting occurs in small to large clusters in soil, typically in areas with thick duff in the fall.

Phaeocollybia californica is an 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. 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 also a big threat to this species’ population. A stand replacing fire could severely degrade and/or diminish its current range.

No use/trade is known.

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 subpopulations are needed to better understanding estimate its population trends and habitat restraints and requirements; i.e. is it restricted to old growth?