Two species of lichens are thriving on the trunks of trees in the Asda supermarket car park in Dewsbury but nationally, as sulphur dioxide levels decrease, one of them appears to be gaining ground while the other is becoming rarer.
The yellow lichen with the pale-rimmed orange apothecia (the fruiting bodies of the lichen) is Xanthoria polycarpa, which has spread as levels of sulphur dioxide in the atmosphere decrease.
It’s common on the coast but in urban settings you’ll often find it on twigs, fences and even on painted signs. It can tolerate high levels of nutrients and nitrogen, so it’s common around farms.
What is the source of nutrients or nitrogen here? Do these trees serve as starling roosts or, here in the middle of a car park, does the nitrogen come from vehicle exhausts?
The majority of lichens don’t have common names but the Latin name often includes a succinct description of the species. The genus name Xanthoria means ‘golden yellow’ while the species name polycarpa refers to its many fruiting bodies.
Acid Rain
Between the splodges of yellow, a grey-green lichen, Hypogymnia physodes, covers most of the bark. It grows in similar situations to the Xanthoria and tolerates acidic conditions but, nationally, this lichen has declined as air quality has improved. The introduction of smokeless zones and the phasing out of coal-fired power stations has resulted in less sulphur dioxide in the atmosphere and less acid rain.
It’s only in unpolluted sites that this lichen will produce fruiting bodies; there are three or four of them in my photograph above: red-brown discs on short stalks.
This lichen might be thriving here because it’s growing on the bark of a flowering cherry: cherry bark is highly acidic and in a dried, powdered form, has been used for medicinal purposes. The horizontal linear scar, visible amongst the lichens on my photograph, is characteristic of cherry.
The ‘Naked-below Bellows-like’ Lichen
The genus name of this lichen, Hypogymnia, means ‘naked below’ and refers to this lichen having no rootlike growths – rhizinae – on its undersurface.
The species name, physodes, means ‘bellows-like’ and I guess this refers to the lobes along the fringes of the lichen which are shaped like a pair of bellows.
In places the fan-like margins appear to be sprinkled with flour. These are described as farinose soredia. 
Farinose means ‘flour-like’. The soredia produce powdery granules which contain the fungal and algal cells of the lichen; these become detached and can grow into new lichens.
Leaf-like Lichens
Both these lichens are foliose, which means they can easily be detached from the surface they are growing on. Other lichens can be crustose and can’t easily be detached or fruticose: bushy.
Lichens, an Illustrated Guide
Coming across these in the Asda car park in Dewsbury gives me a chance to test out my new lichen field guides.
The first one that I reach for is the Field Studies Council’s Guide to common urban lichens 1 (on trees and wood). There aren’t many yellow lichens to choose from, so I soon narrow it down to Xanthoria polycarpa, then, taking a good look at my macro photographs of the grey-green lichen, I immediately pick out Hypogymnia physodes from the chart. I need illustrations; I’m not methodical enough to work my way through a key to identify a species.
There’s a brief summary of key features and habitat in the symbols and the tables included in the FSC guide, but it’s useful, now that I’ve got a name to look it up in an index and to go for the more detailed description and notes on distribution in Lichens, An Illustrated Guide to the British and Irish Species, by Frank S. Dobson, which has just been republished in a revised and updated edition.
There are six FSC guides to lichens in particular habitats – such as churchyards, rocky shores and moorland – so if you’re in one of those habitats it’s easier to have the few dozen of the species you’re most likely to come across to choose from, rather than trying to find it amongst the one thousand plus lichens in Frank Dobson’s book. The two publications work well together.
I already had five field guides on my bookshelves which include a selection of lichens but only one of them, The Observer’s Book of Lichens, briefly mentions the particular Xanthoria that I’ve photographed, but it isn’t illustrated and there isn’t enough of a description to enable me to identify it.
If you’re interested in lichens, I’d say it’s worth investing in these two publications (below). I went for the hardback and I’m pleased to say that it’s been given a binding that stays open when you put it down on the desk, which is so useful when you’re looking something up!
Links
FSC Wildlife Pack 20: Lichens at NHBS (Natural History Book Service)
Lichens: An Illustrated Guide to the British and Irish Species at NHBS