Notes from J.Boyce in Forest Pathology, (1948): "Narrow zone lines, usually black but sometimes dark brown, are a common phenomenon of many decays. They are frequently formed in the white rots, and they are more common in decayed woods of hardwoods than in conifers. At times black zone lines may be fantastic in pattern." Boyce goes on to identify the various fungi, bacteria and parasites responsible for each type of decay and the characteristics and colors typical of each.

In essence, a fungus first invades the wood (incipient decay), establishes itself, and then breaks it down. The zone lines mark the intermittent progress of each type of fungus as it advances into the wood, or the point where the tiny filaments (mycelia) of two competing fungi clash for possession of the tree's tissues. Generally the wood is softer behind each zone line and harder ahead of it. Says Boyce, by way of example, "In aspen, three stages of decay have been recognized. In the incipient stage the wood is faintly colored from light pink to straw-brown; in the intermediate stage it is colored from straw to chocolate-brown, but it is still hard and firm; and in the advanced stage is included all soft, punky wood irrespective of color. Wood in the incipient and intermediate stages is utilized for some purposes but in the advanced stage is always rejected."