The final form of a mature tree is determined by the dominant growth of some buds and shoots at the expense of others, a phenomenon known as apicaldominance. In pines and most conifers, the trunk or main stem grows more each year than the other branches, and the branches attached to the trunk grow more than the secondary branches. Strong apical dominance in these species results in a very orderly growth habit that forms a conical tree; this type of growth habit referred to as excurrent growth.
Most deciduous trees do not show strong apical dominance and therefore typically exhibit less orderly growth. Instead, many shoots grow at the same rate, many branches form, and it sometimes becomes difficult to identify the main stem. These species are referred to as decurrent or deliquescent growers, and usually have large spreading crowns.
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.
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.
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.
Trees can share root systems. If two roots of the same species grow next to each other, as they grow in diameter, they may grow or graft together. This has implications when trees growing next to each other are infected with vascular diseases which may be transmitted through root grafts.
Understanding & Appreciating the Importance of Roots
Trees are an important part of our environment in Iowa communities in the urban forest and in our 2 million acres of natural woodlands. Tree size ranges from a few feet in height and width to more than 100 feet tall and wide for the largest tree in Iowa. Tree tops are visible and appreciated; tree roots are not visible, usually misunderstood, and certainly not appreciated very much. The root system of woody plants are essential for their survival and growth. Often, the general decline of woody plants is directly related to the health and vigor of its root system.
The roots of young seedling trees are often classified as to either having a tap root (most oaks, walnut, hickory) or having a fibrous root system (maples, ash, cottonwood). Only if soil conditions are favorable, may the tap root grow downward vertically for some distance. As the root system continues to enlarge with tree growth, the root type becomes less recognizable. Below ground environmental factors including moisture, oxygen supply, soil texture, below ground obstacles, other roots, and animals determine the ultimate shape and extension of the tree’s root system.
You can visualize what tree form above and below ground looks like by comparing it to a wine glass connected to a wide flat plate.
Most Tree Roots Are Found in Top 18 Inches
Healthy trees have extensive root systems. Most tree roots are shallow; the majority of roots are found in the top 18 inches of soil. Usually, more than 50 percent of a tree’s roots are in the top six inches of soil. Most favorable growing conditions are in the upper profile of soil; moisture and nutrients are first available near the surface in water and nutrient cycling. Roots extend laterally for considerable distances. Root systems may occupy an area four to seven times the surface area occupied by the crown of the tree.
Root Growth Affected by the Soil & Environmental Conditions Present
Root systems are modified by their environment. Soils with higher clay content have less extensive roots than soils which are sandy or better drained. Trees growing in mulches or in a “natural” woodland environment will have more roots closer to the surface than trees growing in turfgrass environments. Roots growing in fertile, moist and well aerated soils will be more numerous, less extensive and closer to the trunk than roots in less desirable soil. Roots in sandy soils will be more extensive, less fibrous and fewer in number.
Root to Shoot Ratio
For trees to function at their optimum, as the above ground portion of the tree grows and expands, so must the root system. The relative relationship of the root to the shoot is expressed as the ration of the weight of the root to the weight of the top. Under normal conditions, this ration is from 1/5 to 1/6, meaning that the top is 5 to 6 times as heavy as the roots. Another way to view the root system is to consider that a seedling may have 2 to 6 feet of root length while the root system of a mature oak tree may be hundreds of miles in length.
At the point of attachment to the trunk of the tree, there are relatively few roots; these divide and redivide, becoming progressively smaller in diameter until reaching the extremely fine rootlets. Roots have no regular branching pattern like the shoots which develop at nodes from buds in the axils of leaves. Lateral roots arise from tissue on the outside of larger roots. The root cap protects each root tip as it is forced through the soil by elongating tissue just behind the tip of the root. Just back from the root tip are the root hairs which are fine outgrowths of single cells. Water and nutrients are absorbed through root hairs and by the tips of these smallest roots. Root hairs are functional for a few weeks to two years and then are sloughed off.
Avoid Damage To Tree Roots
Healthy trees will have a healthy, expanding root system. As we work with landscape plants, avoid practices which will damage or inhibit the growth of a trees root system. Soil compaction and trenching, digging or adding fill dirt within tree root zones often cause health problems for those trees in the future. Healthy roots anchor the tree, provide uptake of nutrients and water for growth and cooling, provide storage for food reserves, and produce organic materials required for tree growth. These root functions are vital to a vigorous and functional tree.