I've been here 26 years. The house is 30 yrs. Never been tapped.
Initially, I also thought old branches .................. but there are too many, and too regular..
They run out to the last few yrs of growth; and there were no branches there since I got here.
Also, there are too many vertically, and regular, to be branches. Those cuts are every 18" apart !
I'm not an expert with this stuff, but could it be from frost cracks or some other introduction of fungi.
I can't really see from the pictures what caused it, but isn't it compartmentalized decay?
"Wood-decay fungi spread through wood as microscopic strands called hyphae, which begin as either germinating spores or bits of hyphae carried to the wound by insects. These strands of wood-decay fungi often secrete oxidizing enzymes that discolor wood and reduce its resistance to decay. "http://www.nrs.fs.fed.us/pubs/gtr/gtr_nrs97.pdf