possess an apical meristem (meristematic tissue found at
the tip) that is protected by a root cap. The root cap sloughs
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
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
is cyclic and responds to environmental changes such as soil depth,
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
topsoil is bare or unprotected, trees will concentrate most of their
absorbing roots in the top 6 to 18 inched of soil, where water,
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
area. However, on a dry weight basis, the "root to shoot" ratio is
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;
age the initial root form is often modified by the growing environment.
Such thing as soil hard-pans, water tables, texture, structure, and
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)
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
sites. Nurseries are now careful to maintain the mycorrhizae
in the nursery beds.
trunk - crown
- tree form - Tree
Forestry Extension - ISU
Contact: Paul Wray
Last Update: January 3, 1997