Alder Roll-rim
Paxillus rubicundus
The Alder Roll-rim is a mycorrhizal fungus that strictly forms symbiotic relationships with alder trees (Alnus). Its cap reaches diameters of 3 to 10 cm, is yellowish-brown to olive-brown in color, and often features a finely scaly or fibrous surface. The gills are clearly decurrent down the stem and turn dark brown when touched or injured. Like its more famous relative, the Brown Roll-rim, it is poisonous and can cause the dangerous Paxillus syndrome.
Details
Identification
Inrolled cap margin (especially when young), finely scaly cap surface, decurrent gills that brown when pressed, habitat strictly under alder trees.
Diet
Obtains carbohydrates from the host plant (alder) in exchange for water and minerals.
Overwintering
Survival as mycelium in the soil.
Ecology
Ecological role
Ectomycorrhizal fungus; essential for the nutrient supply of alders in moist, nitrogen-rich habitats such as riparian forests.
Natural predators
Insect larvae (fungus gnats), slugs.
Competitor species
Other alder-associated fungi such as specialized Lactarius species.
Ecosystem service
Promotion of tree growth and vitality of riparian vegetation through mycorrhization.
Threats
Destruction of alder carr forests, drainage of wetlands, river channelization.
Scientific profile
Profile
Distinguishing features
Strict association with alder (Alnus spp.), smaller stature compared to Paxillus involutus, distinctly scaly to fibrous cap surface, and more reddish spore print. Microscopically distinguishable by spore size and pileipellis structure.
Habitat
Moist habitats such as alder carrs, riparian forests, riverbanks, and damp parks, always associated with alder trees.
Reproduction
Sexual reproduction via basidiospores; spores are released from the gills on the underside of the cap.
Protection & threats
Status not on standard scale
Main threats
Threatened by the drainage of wetlands, river engineering, and alder dieback (Phytophthora alni).
Conservation measures
Protection and restoration of alder carrs and floodplains; preservation of riparian buffer zones with alder vegetation.