Rings and black lines in tree trunks

Whilst we hope you’re not felling too many trees, such work is sometimes necessary and it can provide a fascinating opportunity to look inside the tree and study its features.

You’re probably familiar with the 'normal' configuration of tree rings in a sawn trunk; these concentric growth rings are formed as the tree grows by creating a new layer of cells. Each ring marks one cycle of seasons, or one year. The rings themselves can be used to date trees, and can also be used to study past climates (a science known as dendroclimatology).

Most of us are familiar with the rings inside a tree trunk (Photo: WTML / Richard Becker)
Most of us are familiar with the rings inside a tree trunk (Photo: WTML / Richard Becker)

Black lines in tree trunks

But what does it mean when the tree rings don’t look as you’d expect?

Occasionally when a tree is felled, the tree doesn’t have a clear ring structure; instead black lines are visible throughout the sawn trunk. These black lines appear to interrupt the natural ring structure, and don’t seem to form any particular pattern. The lines are sometimes distinct and sometimes accompanied by wider black staining throughout the wood.

These black lines are known as ‘zone lines’.  They indicate that the dead heartwood in the tree is being decayed by fungi. The black lines themselves are the 'battle fronts' between various fungal colonies; they’re an intentional feature of the fungi to protect themselves from adjacent colonisers so that they don’t have to share resources.

Complex black lines across a sawn trunk (Photo: Keith / Woodland Trust supporter)
Complex black lines across a sawn trunk (Photo: Keith / Woodland Trust supporter)

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! 

Look out for other fascinating features in nature.

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