In Dene Park the other day I came across a medium-small standing Birch trunk with lots of these brown to black lumpy fruiting bodies bursting out of its bark. This is the Birch Woodwart, Annulohypoxylon multiforme, aka Jackrogersella multiformis, a fungus in the Pyrenomycete group of Ascomycetes, most commonly noted on Birch although it occurs on a range of deciduous trees. Another species of Dickrogersella exists in two different forms, on Beech and Oak respectively.
The fruiting body is rather formless at first, often an ochre-brown in colour, and as it ages it darkens, and starts to form these multiple black perithechial bumps, with papillate (sometimes in a little depression) ostioles or spore-releasing holes. It would be interesting to do a cross-section to see the perithecia.
http://pyrenomycetes.free.fr/hypoxylon/html/Hypoxylon_multiforme.htm
https://www.nina.no/english/Fields-of-research/Projects/Pyrenomycetes
