Assessment Notes

Justification

Rhizopogon flavofibrillosus is widespread with a disjunct distribution, but has rarely been collected. It is currently known from 10 localities in the Siskiyou mountains in California and Oregon, the Cascade Range in Oregon and Rocky Mountains in Idaho and Montana. Many Rhizopogon species have spores which stay viable in soil for many years; waiting for conditions to be optimal before growing. but it is unknown if R. flavofibrillosus is one of the ‘sporebank’ species. Forest decline throughout its range probably is having a negative impact on the population of this species. As this is a hypogeous species, its detectability is low and there are relatively few people searching for it. Given this, plus the amount of suitable habitat, it is likely that the population size is much larger than the few reports documented. Assuming that it occurs in up to 500 times more localities, each with 2-4 mature individuals gives a conservative estimate of population size of 10,000-20,000 distributed in small, isolated subpopulations. It is assessed as Near Threatened.

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Taxonomic notes

This species was described from a collection made in Idaho, USA (Smith and Zeller 1966). Many Rhizopogon species lack distinctive morphological characters, and can only be reliably identified with genetic sequences. The reported Channel Island, California collection was associated with Bishop Pine; and follow-up studies only found R. vulgaris and R. occidentalis on the islands. We are not considering that collection as part of this concept.

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Geographic range

Rhizopogon flavofibrillosus is widespread with a disjunct distribution, but has rarely been collected. It is known from the Klamath Range in California, north through the Siskiyou Mountains and Cascade Range in Oregon, east into the northern Rocky Mountains.

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Population and Trends

Its population is widespread distributed in disjunct, small subpopulations in conifer forests. It is currently known from 10 localities in the Siskiyou mountains in California and Oregon, the Cascade Range in Oregon and Rocky Mountains in Idaho and Montana. Many Rhizopogon species have spores which stay viable in soil for many years; waiting for conditions to be optimal before growing. It is unknown if R. flavofibrillosus is one of the ‘sporebank’ species. Forest decline throughout its range probably is having a negative impact on the population of this species. As this is a hypogeous species its detectability is low and there are relatively few people searching for it. Given this, plus the amount of suitable habitat it is likely that the population size is much larger than the few reports documented. Assuming that it occurs in up to 500 times more locations, each with 2-4 mature individuals results in a conservative population size estimate of 10,000-20,000 mature individuals distributed in small, isolated subpopulations of c.1,000 mature individuals or fewer.

Population Trend: Decreasing

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Habitat and Ecology

This species is hypogeous, found as solitary or scattered growths, buried in duff or soil. It is ectomycorrhizal and found in forest with Douglas-fir (Pseudotsuga menziesii), Ponderosa Pine (Pinus ponderosa), and other members of Pinaceae. This species is dependent on mycophagy (primarily eaten by small mammals) for spore dispersal.

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Threats

Prolonged droughts and decades of fire suppression have drastically altered western montane forests, leading to thicker, denser, Abies dominated forests. As a result, hotter, stand replacing fires (rather than patchwork and understory burns) are commonplace, altering appropriate habitat drastically, and making it ill-suited for this species. Pine beetle outbreaks due to prolonged droughts and higher temperatures have also killed millions of coniferous trees, in particular pines in the Rocky Mountains and the Sierra Nevada.

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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 (Castellano et al. 1999). Modern taxonomic work on Rhizopogon is needed to resolve taxonomic problems. Field surveys are needed to document population size and trends and better understand its habitat associations.

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Use and Trade

No use/trade is known.

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Source and Citation

Mueller, G.M. 2025. Rhizopogon flavofibrillosus. The IUCN Red List of Threatened Species 2025: e.T273227989A273247384. Accessed on 23 November 2025.

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