Inside a tree trunk

Cut a tree trunk across and you see rings of different colour and texture. Those rings are layers with different jobs. From the outside in:

• Outer bark. Dead cells, often corky, that protect the tree from fire, insects, drying out and knocks. A tree sheds or splits the outer bark as the trunk gets wider.
• Phloem (the inner bark). A thin layer of living tubes that carries sugar made in the leaves to the places that need it: roots, buds, flowers and fruit. Sugar travels from where it is made (a source) to where it is used or stored (a sink), so it can move down or up.
• Vascular cambium. A layer only a few cells thick, between the phloem and the wood. Its cells keep dividing. Some of the new cells become wood on the inside; others become phloem on the outside.
• Xylem (sapwood). Tubes and fibres that carry water and dissolved minerals upward from roots to leaves. Most of what we call wood is xylem. Its water-carrying cells are dead and hollow when they work, like pipes.
• Heartwood. Older xylem near the centre that no longer carries water. It is often darker, filled with resins and tannins, and it gives the trunk strength and resistance to rot.

The cambium explains how a trunk gets fatter. Each growing season it adds a new layer of xylem inside and a thinner layer of phloem outside. The wood you see is a stack of old growth layers, and the only living, growing part of the trunk is a thin skin just under the bark, plus some living cells scattered through the sapwood. The biggest giant sequoias weigh well over a thousand tonnes, and nearly all of that is dead wood held up by a thin living layer.

A tree can therefore survive losing its heartwood. Many old trees are hollow and still grow, because the xylem, cambium, phloem and bark around the hole still work. What the tree cannot lose is the living ring near the bark.

Girdling means removing a complete ring of bark (down through the phloem to the cambium) around the trunk. The xylem is untouched, so water still climbs and the leaves stay green for a while. But sugar can no longer travel down past the cut, so the roots slowly starve. When the roots can no longer take up water, the whole tree dies. This is why a rope, a wire or a gnawing animal that goes all the way round a trunk can kill a tree, and why a tree with only a partial wound often survives.