Punctelia reddenda?

I am rather hopeful that you have indeed found Punctelia reddenda. Rather than provide absolute confirmation, I will provide opportunity for others to add weight, or cast doubt. Your lichen seems clearly to be a species of Punctelia being a parmelioid with soredia forming from punctiform spots and having the overall Punctelia ‘look’. The negative reaction of the medulla with C ought to distinguish P. reddenda by itself and it appears that you have checked the C reaction carefully. However, I like to use several characters if available and it looks as if your lichen is truly black beneath (ignoring the marginal zone which is brown in all British Punctelia species). The black underside adds considerably to our confidence. The third main character of P. reddenda is the way that the soredia tend to develop into isidiomorphs (which I think I can see). LGBI2 describes P. reddenda as an old woodland indicator but it appears to be spreading into the wider landscape. I have found it on a shaded fence rail in Bedfordshire and on a willow branch in Sutton Park near Birmingham, so don’t be too disconcerted that your occurrence is in an old gravel quarry.

A couple of subsidiary notes.

I am pleased that you checked the C reaction so carefully. So many misidentifications result from using weak or degraded C, and/or testing damp material in the field.

It is not sufficient to see a Punctelia specimen looking a bit isidiate and assuming it to be P. reddenda. I have seen specimens of P. jeckeri which looked rather isidiate but that species has a distinctly pale underside and a C+ red medulla.

Chalkie

Looks OK, the isidiomorphs are a key feature combined with the others and it looks like these are developing, but are best seen on big old thalli anyway.

Concerning its ecology it was always more of an old tree species than an old woodland species, but did also occur on horizontal branches in humid sheltered places as well as on trunks, especially in Devon. In the last couple of decades it has started doing this is the New Forest too and is certainly building up larger populations in the latter habitat, especially on Beech branches in sheltered glades. Spreading more widely would now not be at all surprising. Confusingly Punctelia borreri is getting more frequent exposed edge trees in the New Forest, so they are nearly, but not quite, growing together.

Neil