Acarospora piedmontensis

The thallus (lichen body) begins as scattered areoles (small surface units) that develop into small squamules (scale-like lobes), sometimes merging into a more continuous patch as the thallus expands. Squamules are typically about 0.5–1 mm across and less than 1 mm tall, and each develops a stalk (stipe) that is narrower than the squamule itself. The upper surface is brown and matte, and lacks pruina (a powdery coating), with the edges turned downward, often with a black rim. The lower surface is brown but commonly becomes blackened (melanized, darkened by pigment), and it may show deep fissures.^[2]

There is usually one apothecium per squamule. The disc is immersed (sunken into the squamule) and dull brown, becoming red-brown when wet, and can reach about 1 mm in diameter. Under the microscope, the cortex is about 20–30 μm (micrometres) thick, with a dark upper layer and a hyaline (colorless) lower portion, and the algal layer can be thin to uneven, with the photobiont (the photosynthetic partner) often arranged in vertical rows. The hymenium (spore-bearing layer) in mature apothecia is reported as (100–)150–170 μm tall, and the ascospores are single-celled (simple), hyaline, broadly ellipsoid, about 4–5 × 2–2.5 μm. Pycnidia were observed (about 80 μm diameter), with conidia about 3 × 1 μm. No secondary metabolites were detected by thin-layer chromatography; standard lichen spot tests were negative (K−, C−, KC−, P−, UV−).^[2]

In the field, the mature thalli of Acarospora piedmontensis can be recognized by small squamules on a narrow stipe, a darkened underside, and usually a single large, immersed apothecium whose disc turns red-brown when wet. It may be mistaken for A. badiofusca because both can have large apothecia; however, it differs from A. piedmontensis in having stalked squamules and in microscopic characters, including the algal-layer pattern and a taller hymenium. It has also been compared with the western North American A. obnubila, which (as treated there) has an interrupted algal layer, a pale lower surface, and a much thicker cortex, and does not overlap geographically with A. piedmontensis.^[2]

Acarospora piedmontensis occurs in open, sunny settings on granitic outcrops and extensive granitic slabs below about 400 m (about 1,300 ft) elevation. In the type region it occurs with oak (Quercus) species, old-growth junipers, and Pinus taeda.^[2]

It appears to be restricted to the eastern North American Piedmont and is currently known from Georgia and South Carolina. It can form solitary patches 10 cm (about 4 in) across or more, and it may be the dominant crustose lichen in some local granite-outcrop communities.^[2]