Zone lines in spalted wood are barriers erected by fungi when it encounters another fungi of similar strength and sometimes when only one fungus is present. The lines aren't dead fungi but are mycelium...the vegetative state of the fungus. While black lines are the most common, they can be many colors...red, pink, green, yellow.One of the most interesting things I've read about spalting, and I have no idea whether or not it's true, is that black-line spalting is the result of fungi wars. separate grows of fungi want to spread into each others territories and when they encounter each other, they fight to the death, and it's the zillions of dead fungi carcasses that are the black lines. So weird it may even be true.
Live trees cannot spalt except for localized, damaged areas like the stub of a broken limb.
Sealing the end of a log will actually promote spalting as it helps the log retain moisture. The optimum conditions for spalting are 80 degrees F and above 20% MC...warm and moist.