Tree Lungwort Identification Guide

First impression and the wood you are standing in

Tree Lungwort announces itself from several metres away. You are usually in humid, sheltered, long established woodland: a western oakwood dripping after rain, a ravine with a stream running through it, a wood pasture of veteran ash and maple, or an old hazel coppice on a damp north facing slope. Draped over the trunk and lower boughs are broad leafy sheets of lichen, up to 30 cm or more across, hanging loosely and lifting at the edges.

Condition governs colour, and this catches out beginners. Wet, the thallus swells and turns a soft grass green to olive, supple and leathery. Dry, it shrinks to pale grey brown or fawn, papery and brittle, with the lobe tips curling upwards. The same colony can look like two different organisms on two visits, so if the specimen is dry and you are unsure, wet a lobe with a drop of water from a bottle and wait a minute for the green to come back.

Reading the thallus

The growth form is foliose: flat, leaf like lobes attached loosely to the bark so that you can slide a fingernail beneath the margin. Lobes are broad, 1 to 3 cm wide, with rounded tips that are often notched, forked or shallowly antler branched, and the whole thallus builds into overlapping tiers.

The upper surface is the signature. It is strongly reticulate, meaning it carries a network of raised ridges enclosing shallow pits or hollows, so the surface looks quilted, veined, or like the alveolar surface of a lung, which is where both the English and the Latin name come from. Nothing else of this size on British and European tree trunks has quite that regular ridge and pit network. Run a finger over it: the ridges are firm and slightly glossy, the pits smooth and depressed.

Soralia, isidia and where they sit

Along the crests of those ridges and around the lobe margins you will normally find pale, granular to powdery outgrowths. These are soralia producing soredia, small bundles of fungal threads wrapped around algal cells, and they are often mixed with coralloid isidia. Their position matters: on the ridges and margins, not scattered over the whole surface and not down in the pits. With a hand lens they look like fine grey or greenish white grains crumbling off a raised seam.

This vegetative propagation is how the species mostly spreads, since each granule already contains both partners and can start a new thallus if it lands somewhere suitable.

The underside, and the character that settles the genus

Peel a lobe gently away and turn it over. The lower surface is pale buff to cream and brown, and it is tomentose, that is, covered in a short felt of fine hairs. Interrupting the felt are bare, pale, smooth patches that correspond to the pits on the upper side, and scattered among them are darker, tufted rhizines that grip the bark.

What you must not find are pores. Sticta species have cyphellae, distinct round pits with a raised rim opening through the lower cortex, and Pseudocyphellaria species have pseudocyphellae, small pale flecks where the medulla shows through. The absence of both, combined with the felted lower surface and bare pale patches, places a large reticulate foliose lichen firmly in Lobaria.

Fruiting bodies, and why there is no spore print

There is no spore print to take here in the mushroom sense: lichens release ascospores slowly from small cup like apothecia rather than dropping a dense deposit overnight. Instead, examine the surface with a lens for apothecia. In Tree Lungwort they are frequently absent, but when present they are 1 to 4 mm across, chestnut to red brown discs with a thin, slightly rough thalline rim, sitting on the ridges and lobe margins. Under a microscope a squash mount shows colourless, spindle shaped spores with one to three cross walls, in eight spored asci.

This lichen also holds two photosynthetic partners. The main partner is a green alga, but small internal cephalodia contain cyanobacteria capable of fixing atmospheric nitrogen. In section, or as small darker warts, those cephalodia can occasionally be spotted, and they explain the outsized contribution this species makes to nitrogen cycling in old woodland.

Substrate, conditions and what its presence means

Look on the bark of mature and veteran broadleaved trees: oak, ash, maple, beech, hazel, elm and occasionally old conifers, plus mossy boulders in humid ravines. It favours trunks with slightly base rich bark, good light but not full exposure, and above all persistently high humidity. It is present all year, being perennial and slow growing, with colonies potentially decades old.

It is highly sensitive to sulphur dioxide and to ammonia drift from intensive agriculture, and it colonises new trees only slowly, so its presence is widely used as an indicator of both clean air and long ecological continuity. A trunk carrying it usually carries a rich community of other old woodland lichens, which is worth scanning while you are there.

Separating the closest look-alikes

Lobaria virens is smooth and glossy, bright green even when dry, without the strong ridge and pit network and without soredia or isidia, and it usually bears abundant red brown apothecia.

Lobaria scrobiculata is grey to blue grey when dry rather than green brown, is pitted but with a coarser, less regular network, and carries yellowish soralia along ridges and margins; it takes cyanobacteria as its main partner, which accounts for the colder colour.

Lobaria amplissima is pale grey with broad, rounded, smooth lobes lacking the reticulate pattern, and it often bears dark, shrubby cephalodial outgrowths on the surface that look like tiny bushes glued to the thallus.

Pseudocyphellaria species are distinguished by the pale pinprick pseudocyphellae on the underside, and several show a yellow medulla when the surface is scratched.

Sticta species have the unmistakable rimmed cyphellae beneath and characteristically smell strongly when damp.

Peltigera species can look superficially similar in size and lobe form but grow on soil, mossy banks and among grass rather than on trunks, have raised branching veins and long rhizines below, lack the reticulate pits above, and carry their apothecia on upturned lobe tips like small saddles.

Platismatia glauca is thinner, grey, wrinkled and crisped at the margins with sorediate edges, but the surface is puckered rather than genuinely ridged and pitted, and the lobes are far narrower.