Hammered Shield Lichen Identification Guide

What you are looking at

Parmelia sulcata is a lichen — a fungus living in partnership with a green alga of the genus Trebouxia, forming a single composite body called a thallus. The fungal partner builds the structure and the alga supplies carbohydrate from photosynthesis. That is why the identification routine is different from a mushroom's: there is no cap, no gills and usually no spore print, and the characters that matter are surface texture, propagules, the underside, and a few drops of reagent.

First impression and setting

Almost any tree in a temperate landscape will do. Look at the trunk and lower branches of an ash, oak, willow, sycamore or apple, at an old wooden fence rail, at a mossy wall top, or at the bark of a roadside conifer. Parmelia sulcata is one of the most frequently encountered foliose lichens across the northern hemisphere and tolerates a fair degree of urban air pollution, so it thrives on street trees and park furniture where fussier species have gone.

The first impression is a grey rosette, three to ten centimetres or more across, lying loosely on the bark with lobed edges that lift slightly at the margin. Neighbouring rosettes merge into extensive grey sheets. Held at arm's length the surface looks finely cracked or beaten, as though the metal had been worked with a small hammer — the character behind the common name.

Lobes and upper surface

Get closer. The lobes are two to five millimetres wide, flat to slightly concave, with squared-off or shallowly notched tips rather than fine finger-like divisions; they overlap one another towards the centre of the thallus and are more separate at the growing edge. Colour is grey to blue-grey or greenish-grey when dry, becoming greener and softer-looking within seconds of a rain shower as the alga rehydrates. Older central parts often turn brownish and become encrusted.

The surface pattern is the key. It is raised into a low network of ridges and shallow depressions, and along the crests of those ridges run pale, chalky white lines. Those white lines are pseudocyphellae — breaks in the outer cortex that allow gas exchange — and in this species they are elongated and reticulate, forming a net rather than scattered dots. That net of white lines over a ridged grey surface is the diagnostic look.

Soredia

Follow the white lines and you will find they eventually rupture. Along the pseudocyphellae and around the lobe margins the cortex splits and produces soredia: fine granular powder made of algal cells wrapped in fungal hyphae, each grain capable of starting a new thallus. In well-developed material they form continuous mealy ridges and marginal fringes, so the whole rosette looks frosted.

The texture test is important. Rub a thumb over the propagules: soredia smear as a powder. This is the character that separates the species from its nearest relative more reliably than anything else.

Lower surface and rhizines

Slide a knife blade under a lobe and lift it. The lower surface is jet black over most of the thallus, sometimes paling to brown at the extreme lobe tips, and it is densely furnished with black rhizines — simple, unbranched, root-like anchoring threads — that reach almost to the lobe margins. Dense black rhizines running right out to the edge are part of the identification and immediately separate the species from several look-alikes with bare margins or pale rhizines.

Interior and spot tests

Scrape or slice through a lobe. Under the thin upper cortex lies a bright green algal layer, then a white cottony medulla, then the black lower cortex. The medulla must be white for this species.

Two reagent tests then confirm it. A drop of potassium hydroxide solution on the upper cortex gives a yellow reaction. A drop of the same solution applied to exposed medulla — scratch a small window first — turns yellow and then rapidly deepens to blood red as salazinic acid reacts. Para-phenylenediamine on the medulla gives orange to red. Sodium hypochlorite gives no reaction. The K+ yellow-turning-red medulla is characteristic and is the standard laboratory confirmation.

Fruiting bodies and spores

Apothecia, the fungal fruiting bodies, are uncommon in this species because it reproduces so effectively by soredia. When they do occur they are disc-shaped, sunken to shortly stalked, three to ten millimetres across, with a red-brown to chestnut concave disc rimmed by a thalline margin of the same grey as the lobes.

There is no spore print in the mushroom sense. To examine spores you cut a thin vertical section of an apothecium and mount it: the asci are club-shaped with eight spores each, and the ascospores are simple, colourless, ellipsoid and roughly twelve to twenty by eight to twelve micrometres. In practice the field identification rests entirely on the vegetative characters, and finding fertile material is a bonus rather than a requirement.

Substrate, season and ecology

It grows on bark of both broadleaved trees and conifers, on twigs, on worked timber such as fence rails, gates and posts, on mossy walls and on siliceous rock, and it can spread onto adjacent moss cushions. It is present year-round; lichens do not have a fruiting season in the way fungi do, and growth is slow, measured in a few millimetres of radial expansion each year.

Ecologically it is a pioneer and a generalist: relatively tolerant of nitrogen enrichment and of sulphur dioxide compared with sensitive lichens, which is why it colonises roadside and urban trees. The thallus is a habitat in its own right, sheltering mites, springtails, bark lice and other small invertebrates, and small birds strip pieces of it to line and camouflage their nests.

Separating it from the closest look-alikes

Parmelia saxatilis is the critical comparison. It shares the ridged, reticulate surface with white pseudocyphellae and black rhizines. The difference is the propagules: it produces isidia, which are solid, cylindrical, coral-like outgrowths with an intact cortex that break off as small pegs, not powder. Rub them — they feel granular and knobbly and do not smear. It is also usually browner and is more often found on rock.

Parmelia sulcata versus Punctelia species: Punctelia has white pseudocyphellae as scattered rounded dots on a smooth surface, not as a raised reticulate net, and its rhizines are usually pale to brown.

Hypogymnia physodes has swollen, hollow, inflated lobes, no rhizines at all on a shiny black underside, and produces soredia from burst lobe tips.

Flavoparmelia caperata is yellow-green rather than grey, with broad wrinkled lobes and no ridge network.

Parmotrema species have much broader lobes, often with marginal cilia, and a broad naked brown zone on the underside near the margin where rhizines are absent.

A field routine

Grey foliose rosette on bark or timber; surface raised into a network of ridges; white pseudocyphellae running as lines along the crests; those lines breaking into powdery soredia that smear under the thumb; underside black with dense black simple rhizines to the margin; medulla white; potassium hydroxide on the medulla turning yellow then red. That sequence identifies Parmelia sulcata anywhere in its range.