The lines themselves are caused by an accumulation of fungal and plant chemicals. An interesting feature of these lines is that the accumulation of chemicals causes them to remain visible longer than the structure of the surrounding wood as it decays.
Zone lines can decrease the value of a tree for timber, because they indicate that decay is occurring. However, wood with these lines is often used by woodturners and other craftsmen. Known as spalted wood, it can be turned to produce some interesting decorative pieces and is another example of how the natural processes of decay in a tree can be used for art (see our previous blog on burrs).
It’s unlikely you’ll spot zone lines when out in a wood as you can’t see them until the tree is felled, but next time you come across a stack of felled trees, have a look at the sawn trunks to see if you can spot anything interesting!