Assessment Notes

Justification

Butyriboletus autumniregius is an uncommon bolete growing with conifers in California and Oregon, USA. It occurs over a widespread range, but is known from a limited number of records, and not enough data is available to assess trends directly. However, given its broad habitat and geographic range, the species' population is likely relatively stable and no specific threats have been identified. The species is assessed as Least Concern.

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Taxonomic notes

For many years, this species was one of three western North American 'cryptic' species being called Boletus regius Krombholz (Thiers 1975, Arora 1986). Arora and Frank (2014) described three new western North American species, and erected the genus Butyriboletus for the group of closely related 'butter boletes'.

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Geographic range

The species is restricted to far western USA. It is known from Santa Cruz County, California, north into central Oregon, in coastal and coast range forests, and in the Sierra Nevada and Cascade Range foothills from central California into central Oregon. It is uncommon over much of its range with most records coming from Mendocino County north into Oregon, and is rare south of Sonoma County, California.

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Population and Trends

The species occurs over a widespread area associated with a range of potential mycorrhizal hosts, but with a limited number of records (iNaturalist 2021). As it is a recently described species (Arora and Frank 2014) in the 'Boletus regius' complex, pre-2014 records need extra scrutiny and the number of historical records will likely increase. Given its broad habitat and geographic range, the species population is likely relatively stable.

Population Trend: Stable

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Habitat and Ecology

The species is reported from the mixed evergreen coastal/coast range forests of Northern California, Sierra Nevada foothills, Siskiyou mountains in northern California and Oregon and the Willamette Valley and Cascade Range foothills of Oregon. It is ectomycorrhizal but its host associations are still speculative, but believed to be with conifers, likely Douglas-fir (Pseudotsuga menziesii). Other unverified associates include conifers Engelmann Spruce (Picea engelmannii), fir (Abies spp.) pine (Pinus spp.), and the hardwood trees Tanoak (Notholithocarpus densiflorus) and Pacific Madrone (Arbutus menziesii) (Arora and Frank 2014). The species forms sporocarps (mushrooms) in the fall, especially in seasons with substantial early rainfall.

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Threats

This species occurs across a fairly widespread area with conifers; likely with Douglas-fir (Pseudotsuga menziesii) and possibly other conifers and hardwoods. No threats have been directly linked to this species. This species tends to fruit in seasons with early fall rains, so research into trends due to climate change, and the fall rainy season occurring much later in the year should be investigated.

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Conservation Actions

No specific conservation actions have been identified with regards to this species. More data on populations and trends of this species are needed as well as research to identify mycorrhizal host preference.

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Use and Trade

This species is edible, and is often collected for food.

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Source and Citation

Siegel, N. 2022. Butyriboletus autumniregius. The IUCN Red List of Threatened Species 2022: e.T217580637A217581731. Accessed on 22 November 2025.

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• Country occurrence