Common Greenshield Lichen

Common Greenshield Lichen (Flavoparmelia caperata) is a foliose lichen of the family Parmeliaceae and one of the most frequently noticed lichens on tree trunks in temperate regions. It forms circular to irregular sheets 5-20 cm across, closely appressed to the bark, with a distinctive pale yellow-green to apple-green colour that stands out against grey bark even from a distance.

The genus Flavoparmelia was split from Parmelia on the basis of the yellowish pigment in the cortex, and colour remains the fastest way to place a specimen — true Parmelia species are grey to grey-green, never this warm yellow-green.

What completes the identification is the surface pattern: the outer lobes are broad, rounded and fairly smooth, while the older central portion becomes strongly wrinkled and ridged and breaks out into coarse patches of granular soredia, giving a scurfy, blistered look at the centre of an otherwise neat rosette.

Thallus and lobes

The thallus is foliose and closely to loosely attached, forming rosettes 5-20 cm across that merge into larger patches on favourable trunks. Marginal lobes are broad (3-10 mm), rounded, slightly raised at the tips and smooth-edged, without cilia.

Upper surface

Colour is pale yellow-green to apple-green when dry, greener and more vivid when wet. The outer zone is smooth to faintly wrinkled; the centre is coarsely wrinkled and ridged and bears abundant granular soredia in irregular patches and cracks, sometimes covering much of the middle of the thallus.

Lower surface

The underside is black over most of its area with a distinct pale brown to tan zone around the outer margin, and is furnished with simple black rhizines that anchor it to the bark. Lifting a lobe edge to check for this brown marginal band is a useful confirmation.

Fruiting bodies

Apothecia are uncommon: brown, shallowly cup-shaped discs with a thalline rim, borne on the upper surface. Reproduction is mostly by soredia.

Common Greenshield grows chiefly on the bark of deciduous trees — oak, maple, ash, hickory, apple, beech, birch and many others — and also on conifer bark, old fence rails, wooden gates and, less commonly, on siliceous rock and gravestones. It prefers moderately lit trunks and branches in open woodland, hedgerows, orchards, parkland and gardens.

It is very widely distributed across eastern and western North America, Europe, parts of Asia, Africa, Australasia and South America, and is one of the first large foliose lichens to return to areas where air quality improves. It is perennial, present all year, and thalli persist and expand on the same trunk for many years.

This is an epiphytic lichen with no parasitic relationship to its host; the fungal partner builds the shield-like thallus and anchors it with rhizines, while the green algal partner supplies carbohydrates through photosynthesis. Water, minerals and nitrogen compounds are absorbed straight from rain, mist and bark runoff.

Because it takes everything from the atmosphere, its abundance tracks air quality closely, and it is used in lichen mapping as a moderately sensitive indicator species. Spread is dominated by soredia — clusters of algal cells wrapped in fungal hyphae — that are washed and blown from the wrinkled centre onto adjacent bark, so colonies typically appear as satellites around an older established thallus.

Flavopunctelia flaventior and Flavopunctelia soredica are similar yellow-green foliose lichens but are marked with conspicuous small white pseudocyphellae — pinprick pale dots — on the upper surface, which Flavoparmelia caperata lacks. Flavoparmelia baltimorensis is very close but has isidia (small solid cylindrical outgrowths) rather than powdery soredia in the centre, and often grows on rock.

Parmelia sulcata and other true Parmelia species occupy the same trunks but are grey to grey-green rather than yellow-green, and P. sulcata is additionally marked by a network of white ridges and pits. Punctelia species are grey with white dots. Checking colour first, then surface texture (soredia versus isidia versus white dots), separates the group reliably.