A distinctive leafy lichen, with a shining slivery thallus given a spiky appearance by frequent isidia, which can form a dense crust in the centre of the thallus. Commoner to the north and sometimes frequent on old weather dry lignum in old woods and deer parks but also found on acid bark and on worked timber, especially untreated post and rail fences.
Thallus 1–3 (–5) cm diam., often forming rosettes or scattered unorientated lobes, closely appressed to the substratum, the lobes (0.5–) 1–3 mm wide, linear to sublinear, indented or sometimes branching towards the apices, divergent, contiguous or overlapping; upper surface grey-white, ± shining, older parts often obscured by a ± continuous crust of crowded, concolorous or slightly darker cylindrical isidia which may become eroded; lower surface whitish to pale brown with unbranched rhizines. Apothecia infrequent, to 3 mm diam., sessile to semi-stalked; disc red-brown, flat and undulating; thalline margin isidiate and disappearing; paraphyses unbranched, apices not or little enlarged. Ascospores 6–9 × 5–6 µm. Conidia 3.5–4.5 × ca 1 µm, bifusiform. Cortex K+ yellow; medulla C–, K+ yellow, KC–, Pd+ yellow-orange, UV– (atranorin and thamnolic acids).
At times eroded isidia have been mistaken for the soredia of Parmeliopsis hyperopta, which can grow in the same habitats, but this may be distinguished by its truly sorediate upper surface and UV+ glaucous medulla (divaricatic acid). I. aleurites superficially resembles Physcia clementei but that species is Pd–, has narrower lobes, and occurs in a different habitat.
On acid bark and wood, typically old weathered lignum, especially conifer and Birch and also on Oak lignum and split rail Chestnut fencing to the south, rarely directly on peat and siliceous rocks. Can be frequent on lignum standing dead and fallen hulks of Pine in native pinewoods in the north but also more locally to the south on lignum on similar large standing dead or fallen Oak trees in deer parks and pasture woodland. Also found on bark and lignum in younger woods and plantations to the north and on old split Chestnut fencing in the Weald. Possibly in decline in the latter habitat, likely due to increased ammonia pollution and replacement of untreated post and rail fences by sawn treated timber.
It has been described as having been formerly restricted to ancient Caledonian pine woodlands in Scotland and as having spread south, but there is scatter of early 19th century records from the south (Leighton, 1879) which suggests this is a misunderstanding. It does appear to have been lost to sulphur dioxide pollution in most of England since these early records but the large population in the New Forest has every appearance of being a relic of the pre-pollution English population, not a recent colonisation, as are others in England and Wales.
Frequent in the Scottish Highlands, except the far north, occasional to rare to the south, but with some larger populations in old deer parks and pasture woodlands. Very rare in Ireland.
Cannon, P., Divakar, P., Yahr, R., Aptroot, A., Clerc, P., Coppins, B., Fryday, A., Sanderson, N. & Simkin, J. (2023). Lecanorales: Parmeliaceae, including the genera Alectoria, Allantoparmelia, Arctoparmelia, Brodoa, Bryoria, Cetraria, Cetrariella, Cetrelia, Cornicularia, Evernia, Flavocetraria, Flavoparmelia, Hypogymnia, Hypotrachyna, Imshaugia, Melanelia, Melanelixia, Melanohalea, Menegazzia, Montanelia, Nesolechia, Parmelia, Parmelina, Parmeliopsis, Parmotrema, Platismatia, Pleurosticta, Protoparmelia, Pseudephebe, Pseudevernia, Punctelia, Raesaenenia, Tuckermannopsis, Usnea, Vulpicida and Xanthoparmelia. Revisions of British and Irish Lichens 33: 1-98.
Leighton, W. A. (1879) The Lichen Flora of Great Britain, Ireland, and the Channel Islands, third edition. Shewsbury: Leighton
Text by Neil A Sanderson based on Cannon et al (2023)