Tree Health and Physiology in a Changing Environment

A tree is a large, long-lived, perennial, compartmented, woody, shedding, walling plant. This definition is based on new tree biology concepts (Shigo, 1986a,b, 1991) and explains much about how mature trees function through their unique structure. When the tree begins its life, it is mostly leaf in mass (Fig. 7.1a). As a tree grows in stature, it becomes mostly stem in mass and the foliage represents only a few percent of the total mass. Roots remain relatively constant at about one-fifth the total mass as a tree grows from a small sapling to a mature standard in the forest canopy. Branches represent only a small fraction of total mass, which decreases over time, as older branches are shed. Also shed are leaves, roots, and outer bark. However, aging wood cannot be shed, but dies internally as sapwood is transformed into a core of protection wood, often called “heartwood” (Fig. 7.1b,c; Table 7.1).