What percentage of a dormant mature tree is biologically living?

Only 1 per cent of a dormant mature tree is biologically living. The rest of the tree is made up of non-living wood cells. Even when not dormant, a large percentage of the tree is not alive. Living cells and tissues are only located in leaves, flower buds, roots, and a layer of cells under the bark called the 'cambium'. During dormancy, leaves and flower buds fall off the tree, lowering the number of living cells considerably.

Younger trees may have a higher percentage of biologically living cells during their dormant period. This is because a sapling contains fewer non-living wood cells and more cells focused on growth. The larger a tree becomes, the less growing it requires, therefore the living cells die off and become part of the non-living section of the tree, such as bark.

A tree needs non-living cells for protection, particularly during the winter. They support the structure of the trunk and branches and protect living cells from harm. When cambium cells die, they harden to form the wood of the tree. A new cambium layer forms beneath the old layer and the cycle continues for the rest of the tree's life. When a tree is chopped down, the old cambium cells are visible as rings. By counting the rings, dendrologists can estimate the age of the tree.