Spalting or Pseudosclerotial plates
It’s easy even for fungologists to forget that the objects of our interest are not just inactive and benign things sat on tree trunks.
A newly fallen tree is a massive source of food that has to be fought over; each arriving spore must compete not only with any resident fungi but also with all of the millions upon millions of spores seeking to use the same resource.
By chance our spore settles, germinates and its mycelium explores its new environment, slowly spreading through the wood happily digesting and growing, until it meets the enemy. Is enemy the right word? It implies aggression and hostility even a sense of hatred. Perhaps these are unjustified emotions to hang on a fungus but it deserves to be called more than mere competition for resources.
The tree reveals the story. The black stained battle lines seared into the wood denote the boundary zones of each fungus; each hard fought compartment may be a different species or the same there are no allies.
Mycologists call these lines Pseudosclerotial plates, wood carvers call it spalting.