Oakmoss

Oakmoss (Evernia prunastri) is a fruticose lichen of the family Parmeliaceae, despite its name having nothing to do with true mosses. It forms tufted, bushy clumps of flattened branches that fork repeatedly like miniature antlers, usually 3-10 cm long, hanging or spreading from twigs and trunks.

The defining feature of the genus is the two-toned strap: unlike most fruticose lichens, Evernia branches are dorsiventral, meaning they have a distinct upper and lower face. The top is greenish-grey to yellow-green and the underside is conspicuously paler — chalky white to very pale cream — so a twisted branch flashes white.

Oakmoss is soft and pliable when damp and brittle when dry, and it is one of the most abundant and widely collected epiphytic lichens in temperate Europe, where it has long been harvested for the perfume industry as a source of fixative aromatic compounds.

Thallus form

The thallus is fruticose and tufted, attached at a single narrow holdfast, with branches that are strap-shaped and flattened rather than round. Branching is repeatedly dichotomous, giving an antler or stag's-horn outline, and branch width tapers steadily toward the tips.

Upper and lower surfaces

The upper surface is greenish-grey, yellow-green or olive, often finely wrinkled or channelled, and frequently peppered with granular white soredia along the margins and on the face. The lower surface is markedly paler — dull white to pale cream, sometimes faintly pitted or grooved — and this two-tone contrast is the single most useful field character.

Texture

Branches are soft, leathery and flexible when moist, becoming stiff and brittle when dry. There is no tough central cord inside; a broken branch shows a loose white medulla filling a flattened tube.

Fruiting bodies

Apothecia are rare. When they occur they are stalked brown discs on branch tips, but almost all field material spreads instead by soredia and detached branch fragments.

Oakmoss grows on the bark of deciduous trees, most abundantly oak, but also on ash, birch, blackthorn, hawthorn, apple, willow and other broadleaves, and it will readily colonise conifers, wooden fence rails, gate posts and old walls. It prefers well-lit situations — hedgerow trees, wood edges, orchards, parkland and open canopy — rather than deep shade.

The species is widespread through temperate Europe, North Africa, parts of Asia and North America, and is moderately tolerant of general atmospheric conditions, making it one of the commoner large epiphytes in agricultural landscapes. It is perennial and can be examined in any month, though it is softest and greenest during wet weather.

Evernia prunastri is an epiphyte living entirely on the outside of bark, gathering water and dissolved nutrients directly from rain, mist and stemflow, and manufacturing carbohydrates through its green algal partner. It takes nothing from the tree's living tissues and is equally content on dead wood and fence timber.

Reproduction is overwhelmingly vegetative: soredia are shed as fine granules that establish on adjacent bark, and whole branch fragments broken off by wind and birds start new colonies. Fallen tufts contribute organic matter to woodland floors and are frequently gathered by birds as nest lining, which also helps to move fragments around a landscape.

Ramalina farinacea is the most frequent confusion — it is similarly strap-shaped, tufted and greenish, but it is the same pale greenish colour on both faces, whereas Evernia prunastri is distinctly white beneath. Ramalina branches also tend to be stiffer and more sharply channelled, with soredia in neat oval patches along the margins.

Pseudevernia furfuracea shares the flattened antler form and two-toned construction, but its upper surface is a colder grey-white with scattered isidia and its lower surface is black or dark brown rather than chalky white. Foliose Parmelia species can look similar at a distance but form flat sheets pressed to the bark rather than free tufts, and have black rhizine-bearing undersides.