The black lines visible in this stump of a street tree (species unknown but very recently cut down) are from a fungal infection and are known as "zone lines".
Sinclair and Lyon (page 201) say, "Biscogniauxia species produce distinctive plates (pseudosclerotial plates) composed of black hyphae and stained host cells within decaying wood. These structures, appearing as black lines on cut on broken wood surfaces, are commonly called zone lines."
Here is an attempt, a couple days later, to record the tree species:
https://www.inaturalist.org/observations/36230726
And this duplicate observation of that more recent observation, shows the fungus on the outside of the tree stump:
black coating on the inside of a hollow stump, with lines extending out into the wood. Lines were not superficial and extended into the wood when chopped at (last picture)
Spalting of wood?
BHI-F0866
spalted dead wood piece
spalted wood - not sure what species might be involved
Spalting in hardwood (white ash) due to white rot fungus. Tentatively identifying this as White-Rot Fungus.
Per Wikipedia --One of the trickier aspects to spalting is that some fungi cannot colonize wood alone; they require other fungi to have preceded them to create favorable conditions. Fungi progress in waves of primary and secondary colonizers,[4] where primary colonizers initially capture and control resources, change the pH of the wood and its structure, and then must defend against secondary colonizers that then have the ability to colonize the substrate.[4][20]
4-Rayner, A.D.M., and Boddy, L. (1988). Fungal Decomposition of Wood. Its biology and Ecology. John Wiley and Sons: New York.