Wart lichen

Pyrenula occidentalis, commonly known as a wart lichen, is a crustose lichen species characterized by a smooth to slightly cracked thallus embedded with small, wart-like perithecia (fruiting bodies) that give the genus its common name. As with other members of Pyrenula, it forms a thin, closely adhered crust on bark surfaces, relying on specific microclimatic conditions of humidity and shade to survive. The species is found in forested habitats along the western coastal regions of North America, where it colonizes the bark of mature trees in old-growth or long-established woodland stands.

Pyrenula occidentalis is currently classified as Critically Endangered (CR) by the IUCN, reflecting severe pressure on its remaining populations. The primary driver of its decline is the loss and fragmentation of old-growth forest habitat, which removes the stable, humid microhabitats this lichen depends on for colonization and reproduction. Logging, land conversion, and the fragmentation of remaining forest patches have isolated populations and reduced the availability of suitable host trees with the bark characteristics this species requires.

As a slow-colonizing, substrate-specific organism, it has limited capacity to recover once habitat is disturbed or canopy cover is altered. Conservation attention for this species is part of broader old-growth forest protection initiatives along its range, where the preservation of mature forest stands indirectly benefits lichen communities, including rare crustose species like this one. Encouragingly, increased botanical survey efforts in recent years have improved documentation of known occurrence sites, providing a clearer foundation for targeted habitat protection and monitoring.

Continued inventory work and collaboration between lichenologists and forest land managers offer a promising pathway toward stabilizing remaining populations and informing site-specific conservation measures.

This species grows on the smooth bark of trees in humid lowland and submontane broadleaf forests, typically within long-undisturbed stands that maintain stable moisture and light conditions.