Assessment Notes

Justification

Gastroboletus citrinobrunneus is a rare and poorly known species, recognized by the irregularly convex, bright yellow and brown cap, yellow pores and dark blue staining on all parts. It is known from four or five collections, from two or three localities in the northern Sierra Nevada and southern Cascade Range of California, USA, in mid- to high-elevation Abies forests. Targeted surveys for this species are needed to document its actual distribution and rarity. While it likely occurs in other sites, the fact that this brightly coloured, distinctive species has so infrequently been recorded suggests that it is rare. The population size is very cautiously estimated to be between 1,000-2,000, occurring in scattered subpopulations containing no more than 100-200 mature individuals each. As Sierra Nevada forests have undergone significant changes over the past 40 years due to insect damage, drought, and wildfires, the population of G. citrinobrunneus has declined at a rate suspected to be between 30-49% over its past three generations with declines continuing into the future. Due to its small population size and rapid population decline it is assessed as Endangered C2a(i).

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Taxonomic notes

This species was described from a single collection made near Mineral, in Tehama County, California, USA (Thiers 1979).

The genus Gastroboletus accommodates a number of species with semisequestrate to sequestrate fruitbodies. These morphological forms have evolved multiple times, within different genera of boletes. Some of these species have been transferred to their ancestral genera, while for others, such as G. citrinobrunneus, the closest relatives are not known yet (Siegel et al. 2019).

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Geographic range

This species is known from mid- to high-elevation forests in the northern Sierra Nevada and southern Cascade Mountains in California, USA. Currently it is known from four collections from two voucher-confirmed localities: Tehama Co., near Mineral (the type) and Sierra Co near Yuba Pass; a third from Plumas Co. National Forest, near Lake Davis is unconfirmed.

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Population and Trends

This species is very rare, only known from two or three localities, and four or five collections. More collections, a better understanding of this species, and knowledge of suitable habitat are needed to properly assess population and trends of this species. While it likely occurs in other sites, the fact that this brightly coloured, distinctive species has so infrequently been recorded suggests that it is rare. The population size is very cautiously estimated to be between 1,000-2,000 based on an prediction that the species occurs in 100-200 sites, each with up to 10 mature individuals (Dahlberg and Mueller 2011). It is considered to occur in scattered subpopulations, each containing no more than 100-200 mature individuals.

Sierra Nevada forests have undergone significant changes over the past 40 years. Steel et al. (2023) report that 30% of the region's conifer forest extent was converted into non-forest vegetation during 2011–2020 due to insect damage, drought, and wildfires and the population of Gastrobloetus citrinobrunneus is suspected to have declined between 30-49% over its past three generations (50 years) with declines continuing into the future.

Population Trend: Decreasing

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Habitat and Ecology

It occurs in mid- to high-elevation fir (Abies) forests. Fruit bodies are completely buried to partially exposed in duff or soil. It is ectomycorrhizal, likely associated with White Fir (Abies concolor) and Red Fir (Abies magnifica), and possibly other members of Pinaceae. Fruiting is in summer. This species is likely dependent on mycophagy (primarily eaten by small mammals) for spore dispersal.

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Threats

Sierra Nevada forests have undergone significant changes over the past 40 years and Steel et al. (2023) report that 30% of the region's conifer forest extent transitioned to non-forest vegetation during 2011–2020 due to insect damage, drought and wildfires. Prolonged droughts and decades of fire suppression have drastically altered the high Sierra Nevada 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.

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Conservation Actions

It will be important to protect known sites from management activities, including logging, fuel reduction, or other development and disturbance. It should be added to the USFS sensitive species list. Modern taxonomic work is also needed on this species; to how it may differ from Gastroboletus turbinatus. Historic sites should be revisited and appropriate habitat surveyed for this species.

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Use and Trade

No use/trade is known.

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Source and Citation

Mueller, G.M. & Siegel, N. 2025. Gastroboletus citrinobrunneus. The IUCN Red List of Threatened Species 2025: e.T272184109A272184636. Accessed on 21 November 2025.

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