Biatoridium monasteriense

Biatoridium monasteriense has a comparatively narrow natural range centred on western and central Europe. It was first described from Münster in Germany and remains scarce throughout that region, with records scattered from the Low Countries through Denmark and southern Scandinavia to the Alps and the British Isles. Recent work confirms its extreme rarity and ongoing decline in continental Europe – for instance, it has disappeared from parts of the Eifel and is red-listed in several national assessments. A single collection from ash coppice in the province of Utrecht published in 2016 represents the first Dutch record and lies near the geographical centre of the species' European range. Outside Europe it is known only from disjunct occurrences in the Pacific Northwest of North America. The lichen is an obligate epiphyte on deciduous trees with chemically neutral bark. Most European collections come from field-maple (Acer campestre), elm (Ulmus spp.), elder (Sambucus nigra) and, especially, old coppiced ash (Fraxinus excelsior). It favours deeply fissured, permanently moist bark on ancient stools where shaded crevices, overhanging buttresses and water-retentive sponge-like cortex provide a stable microclimate. …