I am an amateur naturalist trying to discover everything living in my garden.
I've previously blogged the yellow lichen Xanthoria parietina that grows on the upper, east-facing exterior window sills of my house. On the lower sills there's another yellow algal/fungal partnership taking place (photo 1 - click to enlarge), this time in the form of a crustose, Caloplaca lichen.
I'm not a lichen expert, but having referred to the text books, I'm fairly confident the species here is Caloplaca citrina. The apothecia (the little, yellow spore-producing 'pin cushions') are spread about over a powdery, yellow thallus. Were this C. holocarpa (another common Calopaca with yellow/orange apothecia) the thallus would be grey. The thallus of C. dalmatia is cracked by thin black lines. The lack of any change in lichen texture / colour near the perimeter of the patch also distinguishes it from other various other superficially similar Caloplaca species such as C. decipiens. Identifying lichen is all about noting the tiny details!
There is yet another common possibility: the superficially similar powdery, yellow lichen Candelariella vitellina. In this case however there is an acid test (if you'll forgive the (chemically imprecise!) pun) to tell the two apart: a tiny drop of potassium hydroxide applied to most Caloplacas will turn them red. Candelariellas on the other hand, show no reaction. Photo 2 shows the positive result in my case.
I've read a suggestion that the purpose of yellow colouration in lichens is to provide protection against the harmful UV part of sunlight. I'm happy to accept this but it makes me wonder why only some lichens need to bother (there are plenty that don't: green, grey, white and even black lichens being commonplace - the two Verrucaria lichens I've previously blogged for example) .
Finally, the fact that C. citrina occupies the lower sills or my house and X. parietina the upper ones, makes me wonder whether the latter is more tolerant of low light conditions. Can anyone tell me?