Just 1% of a tree is actually alive and composed of "living" cells. Nearly all of every tree you see is a formation of non-living cells and very little of a tree's volume is "living" tissue. The major living or growing portion of a tree is a thin film or skin of cells just under the bark (called the cambium). Other living cells are in root tips, the apical meristem, leaves and buds.
This large non-living part of a tree is very important structurally. This support wood is created by cambial-hardened wood cells on the inner cambial layer. Sandwiched between the outer cambial layer and the bark, is the ongoing process of creating sieve tubes which transport food from leaves to roots.
Learn more about a tree and it's parts.
Angel Oak on John's Island, South Carolina - Photo by Steve Nix, Licensed to About.com