Overview
The Pine Firefungus (Rhizina undulata), commonly known as the ‘doughnut fungus,’ is an extraordinary post-wildfire ascomycete fungus found across the temperate and boreal coniferous forests of the Northern Hemisphere. Colonizing recently burned forest soils and campfire sites, this specialized disc-shaped fungus forms crust-like, undulating cushions of dark chestnut-brown to purplish-black tissue. Functioning biologically as a pioneer post-fire colonizer, it plays a complex dual role in forest ecology: acting as an aggressive root pathogen of newly planted conifer seedlings while actively breaking down organic carbon in fire-sterilized soils.
Biology and Physical Characteristics
The fruiting body (apothecium) of Rhizina undulata develops as an irregular, flattened to convex cushion that becomes lobed, undulating, and convoluted with age. The fertile upper surface is smooth, shiny, and colored in deep dark-brown, chestnut, or purplish-black, edged by a distinctive pale yellowish-buff or cream wavy margin. Unlike typical cup fungi that sit loosely on leaf litter, the underside of the pine firefungus produces hundreds of dense, pale, cord-like rhizomorph structures (rhizoids) that firmly anchor the apothecium deep into charred soil. Microscopically, its long cylindrical asci contain eight large, fusiform, hyaline ascospores featuring distinct pointed apiculi at both poles.
Behavior and Ecology
Rhizina undulata exhibits a specialized ecological strategy known as pyrophily (fire love). Its thick-walled resting ascospores lie dormant in forest soil for decades, unable to germinate under normal environmental conditions; germination is triggered specifically by intense thermal heat shock exceeding 95 to 110 degrees Fahrenheit (35–45°C), such as that produced during wildland forest fires or slash burns. Following fire events, the mycelium rapidly colonizes the heated soil, attacking the root systems of surviving conifers or newly germinated pine and spruce seedlings, causing rhizina root rot. Fruiting occurs within one to two seasons post-fire during summer and autumn, releasing billions of airborne spores before the fungus disappears as competing soil microbes recolonize the site.
Conservation and Human Impact
The species is globally evaluated as Least Concern and is abundant across conifer-growing regions worldwide. In commercial forestry, Rhizina undulata is monitored as a significant seedling disease pathogen in replanted pine and Douglas-fir plantations following prescribed slash burning, prompting foresters to delay replanting burned timber parcels for two seasons until fungal mycelia naturally subside. In native wilderness biomes, the fungus fulfills a vital evolutionary service, accelerating the initial decomposition of charred woody debris and recycling organic matter in early post-fire successional forest ecosystems.
Interesting Factoid
The spores of the Pine Firefungus can lie completely dormant in cool forest soil for decades, springing to life only when intense heat from a forest fire or campfire cooks the soil and triggers their germination.
Last updated: October 2026