Assessment Notes

Justification

Russula xerophila is a poorly known species, known only from two locations in the Sierra Nevada foothills oak zone. Much of this habitat has been lost to urban development and farmland and is susceptible to elevated levels of nitrogen deposition from agriculture in the Central Valley, and oaks have suffered from drought stress. The current direct impacts on the two sites are uncertain, but these remain plausible threats. Although similar habitat exists on the on the western (drier) side of the valley, no surveys have been conducted in the oak zone of this area. As this species is only known from two locations, with plausible threats that could quickly drive the species to Critically Endangered or Extinct, this species is assessed as Vulnerable D2.

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Taxonomic notes

This species was described as Gymnomyces xerophilus from a collection made in Yuba County, California, USA (Smith et al. 2006). Phylogenetic research has shown that these hypogeous lineages have evolved multiple times, therefore they have since been transferred into the mostly epigeous genus Russula (Elliott and Trappe 2018). These hypogeous Russula (Gymnomyces) are notoriously hard to identify to species, requiring diligent microscopic examination, and occasionally, the genetic fingerprint. A key to oak associated "Gymnomyces" can be found in Smith et al. (2006).

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Geographic range

Currently this species is known from two locations in California, USA, in the northern Sierra Nevada foothills ‘oak zone’. Although similar habitat exists on the on the western (drier) side of the valley, no surveys have been conducted in the oak zone of this area (Siegel et al. 2019). There are two records in GBIF (2024) from Mexico labelled as Russula aff. xerophila. Due to the distance from well documented sites, these two records are not included in this assessment

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Population and Trends

Russula xerophila is a poorly known species, occurring in the Sierra Nevada foothills oak zone. Much of this habitat has been lost to urban development and farmland and is susceptible to elevated levels of nitrogen deposition from agriculture in the Central Valley, and oaks have suffered from drought stress.

Population Trend: Decreasing

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Habitat and Ecology

Sporocarps are formed in the spring. They are hypogeous, solitary or scattered, and buried in duff or soil. The species forms ectomycorrhizal associations with Blue Oak (Quercus douglasii), Interior Live Oak (Q. wislizeni) and California Black Oak (Q. kelloggii), and possibly with other Quercus species. It is dependent on mycophagy (primarily eaten by small mammals) for spore dispersal.

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Threats

It has lost habitat due to urban development and expansion of agricultural land. Prolonged droughts have stressed and killed many oaks in the southern and central Sierra Nevada Foothills. Drought also suppresses fruitbody formation. Drift from artificial (nitrogen) fertilizers in agriculture lands in the Central Valley changes the composition of ectomycorrhizal fungi on the roots, and can suppresses fruitbody formation.

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Conservation Actions

Protection of the two known sites and mitigation of threats to potential suitable habitat are needed. Targeted surveys for this species, especially from the southern Sierra Nevada foothills, and on the west side of the Central Valley on the eastern slopes of the Coast Range are also required.

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Use and Trade

No use/trade is known.

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Source and Citation

Mueller, G.M. 2025. Russula xerophila. The IUCN Red List of Threatened Species 2025: e.T273196462A273244327. Accessed on 23 November 2025.

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• Country occurrence