Very little of a tree's volume is actually "living" tissue. Just 1% of a tree is actually alive and composed of living cells. The major living portion of a growing tree is a thin film of cells just under the bark (called the cambium) and can be only one to several cells thick. Other living cells are in root tips, the apical meristem, leaves and buds.
The overwhelming portion of all trees is made up of non-living tissue created by a cambial hardening into non-living wood cells on the inner cambial layer. Sandwiched between the outer cambial layer and the bark is an ongoing process of creating sieve tubes which transport food from leaves to roots.
So, all wood is formed by the inner cambium and all food-conveying cells are formed by the outer cambium.
A Tree's Bud Growth
Tree height and branch lengthening begins with a bud. Tree height growth is caused by the apical meristem whose cells divide and elongate at the base of the bud to create upward growth in trees with a dominant crown tip. There can be more than one if a tree's top is damaged.
Tree branch growth works in a similar way using buds at the apex of each twig. These twigs become the future branches of a trees. Transfer of genetric material in the process will cause these buds to grow at determined rates, creating a tree species' height and form.
Tree trunk growth is coordinated with the increase of tree height and width. When buds begin opening in the early Spring, cells in the trunk and limbs get the signal to increase in girth by dividing and in height by elongating.