Ochrolechia arborea
Ochrolechia arborea is not a very noticeable species, resembling a small form of O. androgyna, until a UV light is shown on to it; then it blazes out in bight yellow fluorescence. To be looked for on weathered wood, both natural and worked in well lit situations. It also occurs occasionally on acidified bark.
Thallus very thin, white to pale grey, often immersed in the substratum, smooth, shiny, little cracked, with a fibrous surface texture like art paper; soralia crater-like, with a thin margin, discrete, <0.4 mm diam., circular, becoming convex and efflorescent; soredia 40–50 μm diam., spherical, dull, pale yellow to green-grey. Apothecia unknown in British material. Soralia C+ red, KC+ red, Pd–, UV+ bright yellow (gyrophoric acid, lecanoric acid (trace), lichexanthone).
The vivid UV+ yellow reaction is unmistakeable. Ochrolechia arborea resembles a small form of O. androgyna but the soralia are much smaller and are densely crowded. Ochrolechia inaequatula is UV– and grows on a wider variety of often dead substrata.
On lignum of acid-barked trees (especially Oak) and worked timber less often on acid bark on broadleaved trees and in, N.E. Scotland, on twigs of Pinus and Calluna stems on coastal shingle.
Certainly under recorded but local with scattered records in the south and a few north to Scotland.
The increasing use of use of strong UV lights in the field has allowed Ochrolechia arborea to be much more easily detected and it is proving to be much more widespread than was realised earlier. It is a local species of interest, but its widely scattered occurrences on worked timber, typically fences and gates in anthropogenic environments, suggests that it may not be Near Threatened as previously assessed.
Britain: Near Threatened
Cannon, P., Kukwa, M., Coppins, B., Fletcher, A., Sanderson, N. & Simkin, J. (2021). Pertusariales: Ochrolechiaceae, including the genera Lepra, Ochrolechia and Varicellaria. Revisions of British and Irish Lichens 5: 1-17.
Text by Neil A Sanderson based on Cannon et al (2021)