I went and had a look for yellow lichens on ordinary trees in my parish today. What did I find? Thread. https://twitter.com/obfuscans3/status/1348728335292129281

25 year old apple orchard. Twigs and branches conspicuously covered with yellow lichens. Everything visible here is Xanthoria parietina but there are other yellow species present in the orchard...

On more shaded branches we get a nice comparison of X. parietina growing next to Candelariella reflexa, the other common lichen on these apple trees.

Candelariella reflexa starts as dull yellowish olive green blobs of thallus from which bright yellow soredia form, eventually producing quite large areas of bright yellow powdery patches.

This is the same but with sunlight illumination, X. pari. in foreground and Cand. reflexa behind.

One tiny patch of Candelariella reflexa surrounded by Halecania viridescens (looking very much like algal crust. Algal crust is also present on left side of twig).

To find some different yellow lichens I looked at this fallen dead elm.

Here we have tiny Xanthoria polycarpa, (Lecanora chlarotera), X. parietina and also a load of dull beige yellow crust - the latter is algal. Algal crusts can be difficult to distinguish from sorediate lichens.

Xanthoria polycarpa in centre of image, three main X. parietina. Plus something different in lower right - clusters of minute yellow frilly fans, X. ucrainica (part of the X. candelaria agg.). Can you make out which is which?

Red = Xanthoria polycarpa
Blue = X. ucrainica
Purple = X. parietina
Orange = buff coloured algal crust.

This shows part of the difficulty in recording lichens. The tiny lobes are juvenile X. parietina, same as the large-lobed lichen in upper left.

So the small lobes are just baby X. parietina, a mature one in upper left.

However, there are some tiny yellow apothecia present, not associated with any lobes (ringed in red). These belong to Caloplaca cerinella!

A little cluster of Caloplaca cerinella apothecia just catching the sun. Each apothecium c. 0.2 mm diam. C. cerinella particularly likes growing on ash and elder twigs.

Zooming in on the little patch of C. cerinella apothecia. Note no yellow thallus present (thallus is almost inapparent, mostly immersed in the bark).

That's all the *bright* yellow lichens I could find on twigs and branches in half an hour: Caloplaca cerinella, Candelariella reflexa, X. parietina, polycarpa, ucrainica. Also quite a few potential sources of confusion!