Very odd Xanthoria

[JennyS]

A Xanthoria from Dorset yesterday in the axil of a shaded Birch twig, no rhizines but (among the lower side grot) very occasional hapters. X. polycarpa seems closest but substrate too acidic and the thallus is very nodulose with few apothecia.

Thoughts and comments appreciated as I'm hanging onto the specimen and just calling it Xanthoria sp. for now!

[Fay Newbery]

It certainly is odd. The apothecia don't seem to be particularly stalked.

[Neil Sanderson]

What about Xanthoria calcicola? Looks like the coarse globose isidia of this species on the thallus surface. LGBI 2 says:

Lowlands, especially on calcareous, nutrient-rich stonework, brickwork, tiles, monuments, rare on bark and wood

I have never seen it on bark, but it seems it can grow in this habitat

Neil

[JennyS]

Thanks for the suggestion Neil - but wouldn't Birch twigs be too acidic a substrate and surely the outer lobes are wrong for a corticolous X. calcicola?
Forgot to mention in first post but the thallus is only 1.5 cm

[Neil Sanderson]

Xanthoria calcicola is not a strongly held option, but it would, I suppose be stunted growing on a Birch! Seems like the classic young Cladonia problem; too small/under developed to be identifiable. Otherwise out of ideas.

Neil

[Glos lichens]

I wasn't sure whether to post this at all, but given Mark Powell's comment elsewhere on this site that Xanthoria polycarpa is a species "gone out of fashion" and several discussions I've had recently with people that either never see it or have trouble differentiating it from parietina, this seems the right time and place.
I come across what I believe is Xanthoria polycarpa quite often, particularly on urban twigs, but these are two recent rural finds, from birch in a garden in north Gloucestershire and an apricot tree in my garden in central Gloucestershire.