Last month I described how the heartwood of Northern Hardwoods can become discolored due to physical injury to the tree. If the wound does not heal, it can also lead to its decay. Basically, there are three stages in this discoloration and decay process in Northern Hardwoods.
The first stage is when the tree is injured and the damaged cells become exposed to the air. Gases and moisture leave the tree and new air and moisture enter which starts the chemical discoloration process. This discoloration does not affect the strength of the wood but it can advance to the pith and travel up or down the stem. A new growth ring will eventually occur that acts as a barrier to the discoloration process and new wood will be healthy and white. (See Northern Hardwoods, Part 1.)
The second stage is when the first micro-organisms arrive at the wounded area. Although these "pioneer" micro-organisms grow on the discolored wood, they are "non-Hymenomycetes"' that is, they do NOT cause decay. Rather, they set the table for decay as the injured cells become more moist and this wood is often referred to as "wetwood", "redheart", or "blackheart".
The third stage begins when the "Hymenomycetes" (decay fungi) get into the act. They begin to eat away at the cell walls and they destroy ONLY the areas that have gone through both Stage 1 and Stage 2. Again, the new healthy growth is not affected by these decay fungi and is actually separated from them by a band of discolored wood. It is in this band that the original "pioneer" micro- organisms remain. As long as the wound is open, the decay continues and many species of micro- organisms compete and interact to completely break down the wood.
Sometimes the different fungi "draw lines" of battle as they compete, forming "spalted wood". If the decaying process continues, the band where the "pioneer" bacteria and fungi have hung out will be digested. Only a hard black rim will remain which separates the healthy wood from the (by now) hollow core.