Xanthoria ucrainica (images of British lichens)

Description, based on Kondratyuk (1997):
Foliose, minute (1–2 mm in height), foliose, lobes ascending or pendant, overlapping (imbricated), greenish- to bright yellow, lobes tiny but expanded towards the tip, becoming fan-shaped, 0.7–1.2mm in width, margins and undersurface budding rounded blastidia, 35–65 µm in diameter (microscope), not agglomerated into larger 'conblastidia', rhizines absent; apothecia not seen by Kondratyuk but claimed for British material, spermatia (in orange, laminar pycnidia) ellipsoid. Widespread and becoming common, though sometimes ephemeral, on nutrient-enriched bark and also on man-made substrates, a recently described species very closely related to X. candelaria, under which name it was previously recorded.

Refs: Kondratyuk (1997); Smith et al. (2009), 971; Dobson (2005), 462 (excluding illustration, which is ?X. candelaria); Dobson (2011), 472 (excluding illustration, which is ?X. parietina). In Dobson (2011), the description states "apothecia unknown", but numerous apothecia are shown in the supposed photograph. The accompanying distribution map shows a huge number of records made between 1960 and 1999, even though this species was first published in 1997!

The status of this species, both as a taxonomic entity and perhaps with reference to some of its records in Britain, seems to me to be a little suspect. The tree on which the material of the lower photograph grew also had true X. candelaria (both species det. B.J. Coppins), differing from X. ucrainica in its distinctly and densely tufted habit with erect and narrower lobes, though very similar in size and colour. There were convincingly two entities growing in proximity under near identical conditions. However, it seems very difficult to grasp clear-cut characters that reliably separate the two species, with most supposedly diagnostic characters (notably lobe colour and width) subject to environmental control and seemingly continuous variation. Differences in the size and nature of blastidia are deemed to offer a distinction, but maybe more biometric data are needed?
Kondratyuk described X. ucrainica as material failing to fit well into the existing taxonomy of the group that included the "widespread" X. candelaria, and known from a "few scattered localities in Europe and Asia" (including Britain). This would suggest it should not necessarily be treated as a common species in comparison with X. candelaria, though maybe it is more oceanic, despite initially being described from Eastern Europe, and it is undoubtedly increasing in frequency, as is the case for certain other Xanthoria species. The increase in X. ucrainica does not mean that X. candelaria has gone away!

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Images of British Lichens