The Vital Cambium

The bark on the outside of the tree protects the living cambium layer that lies just underneath. It is the cambium layer that produces the growth rings inside the tree, adding new layers of phloem and xylem each season (see illustration below).

If mice or rabbits chew away the bark exposing the cambium layer, that layer will dry out and die. If the tree is completely girdled this way, it will likely die since it cannot transport nutrients from the roots up to the leaves.

When grafting it is crucial to unite the scionwood's cambium layer to the recipient stock's cambium layer, and then sealing them to prevent them from drying out.

(See sections on grafting and working with scionwood)

Phloem and Xylem

The cambium is a very thin layer of tissue, sometimes only one cell thick.

It is the living layer that produces phloem towards the outside (eventually becoming bark) and xylem on the inside (becoming sapwood).

The phloem, on the bark side, transports the food manufactured by the tree's leaves down to the roots.

The xylem carries water and other nutrients up to the leaves.

The cambium is a protective barrier.

Dry rot disease (visible as a white fungus) can kill a tree quickly, but if the tree is otherwise healthy, it will build barriers within its wood, compartmentalizing the fungi, keeping them dormant for years.

In healthy, standard size trees the dry rot may take over 25 years to kill them after it enters the wood.