Tree roots possess an apical meristem (meristematic tissue found at the tip) that is protected by a root cap. The root cap sloughs off its oldest tissues to provide lubrication as the root is pushed through the soil. As the apical meristem grows, it cuts off new cells through cell division, and a zone of elongation is formed directly behind it. In this area, the new cells are enlarging and differentiating into specialized root tissue.
The rate of root growth is quite variable throughout a growing season. Roots usually begin to grow before the tree top does, although root growth is cyclic and responds to environmental changes such as soil depth, water supply, aeration, mineral supply, and temperature.
Trees' root systems are made up of large, permanent roots (which mainly provide anchorage and transport), and many small, temporary feeder roots and root hairs. It is these small parts of the root system that are the primary water and nutrient absorbers. Many of these small roots function for only one or two years, and then either die or become part of the large root system.
Most tree roots do not penetrate very deeply into the soil. Unless the topsoil is bare or unprotected, trees will concentrate most of their absorbing roots in the top 6 to 18 inched of soil, where water, nutrients, and oxygen can be found.
Tree root systems cover more area than one might expect -- usually extending out in an irregular pattern 2 to 3 times larger than the crown area. However, on a dry weight basis, the "root to shoot" ratio is around 20 to 80%, making the top four to five times heavier than the roots.
The type of roots formed initially is specific to a given species; with age the initial root form is often modified by the growing environment. Such thing as soil hard-pans, water tables, texture, structure, and degree of compaction all influence the mature root form. There are three basic classes of tree root systems:
- Tap root (hickory, walnut, butternut, white oak, hornbeam)
- Heart root (red oak, honey locust, basswood, sycamore, pines)
- Flat root (birch, fir, spruce, sugar maple, cottonwood, silver maple, hackberry)
Roots of most species of trees are invaded by soil fungi to form root-fungus structures called mycorrhizae. The mycorrhizal association is beneficial to both the tree and the fungus. The tree supplies carbohydrates and other growth requirements to the fungus, and the fungus increases water and mineral uptake (particularly phosphorus) of the host tree by increasing the total absorptive area of the root system. There are more than 2500 different fungi which form mycorrhizal relationships with trees; often there are several different fungi associated with an individual tree. The presence of this association is necessary for establishment and growth of many trees; its absence has often reduced the success of new tree plantings, especially on old field sites. Nurseries are now careful to maintain the mycorrhizae populations in the nursery beds.