TREE BIOLOGY

For those customers who would like to know more about tree biology, please look through the following section of material which will explain not only tree biology, but also help you to self diagnose possible stress issues brought on by extraneous circumstance which may directly be affecting the mechanisms described here.

Trees are made up of 3 major structural areas. Roots, Trunk, and Crown/Canopy.

Lets start with the hidden part of the tree that is often the location of stress on many urban dwelling trees. The ROOTS.

Roots

Roots

Growth
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.

Form

Form

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:
1.Tap root (hickory, walnut, butternut, white oak, hornbeam)
2.Heart root (red oak, honey locust, basswood, sycamore, pines)
3.Flat root (birch, fir, spruce, sugar maple, cottonwood, silver maple, hackberry)

Most of the tree species we have here in Colorado are in the last category of FLAT ROOT structure.

That being said, root damage, root drying, heavy wind and ice, and cold temperatures can be extremely stressful to trees with shallow flat roots which is why we suggest erosion control, mulching over the root zone ever couple of years with good quality organic mulch, and paying attention to roots as not to damage shallow surface roots and noting how much water the root zone is receiving.

Mycorrhizal Association

Mycorrhizal Association

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.

Trunk

Trunk

Growth in the diameter of plants is due to the cell divisions in the cambium, an extremely thin cylinder of meristermatic tissue found just under the bark. New cells are formed on both sides of the cambium each year. Those to the inside make up the xylem, which conducts water and nutrients; and those to the outside make up the phloem, which transports sugars, amino acids, vitamins, hormones, and stored food. In the xylem, the fibers provide strength and the vessels allow water and nutrient flow to the leaves.

The annual rings found in tree stems are a result of variations in growth rate and in the type of wood produced early and late in the growing season. Within each ring, the lighter wood is springwood, formed early in the season with larger, thin walled cells; the darker, thick walled cells of the summerwood are formed later in the year. When counting the rings to determine the age of a tree, both of these bands are included in one year. The environmental conditions of an individual tree, most notably the amount of moisture and light available, are recorded each year in its rings. The width of these rings may be used as a measure of the health and vigor of the tree.

A cross-section of a tree stem reveals differences in its basic structure. Heartwood is found at the center of the tree. It is composed of old xylem tissue that is no longer living, but still retains structural strength and infection resisting ability. Sapwood is the living xylem inside the cambial layer that is actively involved in fluid transport. Researchers have found that the number of annual rings still living at any time is highly variable, ranging from one to 20 rings depending on the species. The living phloem cells just to the outside of the cambium, the inner bark, provide nutrient transport. The outer bark is composed of dead phloem cells that are pushed to the outside, and sloughed off by the tree over time.


Damage to trunks from cars, lawnmowers, string trimmers, hand tools, vandals, or disease vectors like bacteria, fungus, or bugs are them main causes of trees becoming weak or beginning to decay on the main stem.

Inspecting your trees once per year or having an arborist inspect your trees every couple of years is a good preventative measure agains possible

Crown

Like roots, trunks and branches grow in length from apical meristems found in buds, which are essentially telescoped shoots, leaves, and/or flowers. Buds containing all of the above are referred to as mixed, while those containing one or the other are referred to as either leaf buds or shoot buds. The terminal bud, located at the apex of the main stem, forms the trunk of the tree over time. Lateral buds, formed at the leaf axils and nodes along the trunk, grow into branched and flowers.

Within the bud, two growth habits are possible, fixed growth and free growth. Fixed growth occurs in species such as pines, hickory, and oaks, where the buds contain a preformed shoot. All of the components of next year's shoot are contained in the bud formed this year; the number of leaves and nodes is predetermined by this year's environmental conditions. The length between leaves and nodes is influenced by the environmental conditions the tree encounters next year. Free growth, in species such as cottonwood, willow, and silver maple, occurs when buds contain shoots with some preformed leaves, but which are also capable of forming additional leaves. These species can continue to grow as long as environmental conditions are favorable. Recurrently flushing growth occurs on many shrubs. These species produce a series of buds at the tip of the same elongating shoot in waves or flushes. Some fixed growers, under favorable growing conditions, are also capable of a second flush of growth in one season.

• If the main apical leader is damaged or cut, hormones will shift, and the limbs nearest the top will begin competing for the dominant top. It is possible for a tree to become a single lead again, but often codominant stems arise.

If you would like to know more about tree biology specificities in reference to your trees or trees you would like to plant at your property, please contact us for an appointment. Estimates are free and we would be happy to share our knowledge base with you.