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Journey to the bottom of a tree.

Compared to trunk and crown, relatively little is known about a tree's roots. The study of root systems requires endless digging. Fiber-optic and video technologies have helped somewhat, but revolutionary procedures are yet to come. Much as ultrasound and isotope tracers have helped doctors to "see" inside the human body, similar techniques are needed to give foresters a window to the underground portions of our forests, particularly the troubled urban forest beneath our sidewalks, streets, and parking lots.

The root system of a healthy tree comprises more than half of the tree's size. Though this part of the tree is out of sight, it should never be out of mind. Roots have a tremendous impact on the health and vitality of trees.

The roots may be the most biologically dynamic and the most delicate of all the tree's organs. Most mature trees have a "root collar" that can be recognized as a bulge on the trunk at the ground line. Four to 11 woody roots originate at the root collar and grow horizontally through the soil, generally extending to an area several times larger than the crown spread. These major roots and their primary branches usually show annual rings just like the tree's trunk and branches. Often called the "transport roots," they comprise the tree's framework and support system.

A complex system of smaller roots advances outward and upward from the basic root framework. Most of the absorption of water and minerals takes place through these roots which branch four or more times to form fans or mats terminating in thousands of fine, short tips. The tips vary in length from less than an eighth of an inch to about a half inch and are about the diameter of a straight pin. These are the roots that account for the majority of the tree's surface area. Symbiotic fungi called mycorrhizae help to enhance absorption by these roots.

Because these root tips are so delicate and occur mostly in the topsoil and litter layers, they are susceptible to drought, extremes in temperature, and frost heaving, not to mention human activities. They also serve as food for nematodes, springtails, rodents, and soil microfauna. Injury and death of these fine roots are frequent, but new roots form rapidly. Altogether, there is more "shedding" and new growth in this part of the tree than in any other, including the leaves. This ephemeral aspect of trees has only recently been recognized.

The soil is what holds the under round forest to ether. Soil is more than just dirt. It has taken centuries to develop and supports a web of plant and animal life, which in turn contributes to the further development of soil. Earthworms, for example, help to aerate the soil, a function from which roots benefit greatly. Roots need oxygen as well as water and nutrients in order to survive. If earthworms could communicate to us, they might provide the eyes we need to better understand that mysterious underground portion of the forest. I will use an earthworm as a guide in this article to help relate what is known about tree roots.

If an earthworm had its druthers, it would likely prefer to be born out in the country, beneath a natural forest. Under natural forest conditions, the soil is spongy and covered with a leafy mulch layer that absorbs moisture, retards erosion, keeps the soil moist, and moderates soil temperatures. New material accumulates each autumn as the lower layers are decomposed and incorporated into the soil by a variety of organisms, including our earthworm guide.

Above, closely spaced trees with small crowns form a continuous canopy that intercepts light and wind, buffering the environment below from extremes. The small leaf area of each tree that receives direct exposure to the elements prevents the tree from losing too much water through a process called transpiration. The transpirational pull of water from the root system up through the tree is therefore regulated.

Shielded from sun and wind, the forest soil is able to make more moisture available to root systems. Tree roots must compete with other forest vegetation for soil moisture and nutrients, but the vegetation types that have evolved in the forest along with trees are not highly competitive. A tree's fine root tips, which absorb most water and nutrients for tree growth, are concentrated in the top few inches of soil, just under the leaf litter layer. Everything is available here for optimal root growth-water, aeration, nutrients. The number of fine roots will decrease with depth, but the root system is likely to be well developed to a depth of two feet, or more, if the soil characteristics allow it.

The contrast between a natural forest and urban/suburban environments is striking, especially where roots are concerned. People like to take trees that have evolved throughout the millennia in our forests and grow them in urban/suburban settings that are alien to them. As adaptable as trees are, cities pose major limitations on tree growth.

Consider this comparison. On a hot summer afternoon, you are walking through the forest. Though the humidity is high, the canopy offers cool shade, and the soil underfoot is spongy and moist. You feel like you could walk for hours before resting or stopping for a cool drink.

Imagine walking out of the forest into a typical urban setting. You squint as the blazing sun beats down on you. More heat is reflected off pavement and buildings nearby. The warm, dry breeze evaporates your perspiration in order to cool you, though you don't seem able to keep cool. Soon you will need a drink to replenish the lost moisture. You notice that the ground you are walking upon is hard and dry.

The environment affects trees and people in much the same way. We can walk away from any environment that makes us feel uncomfortable, but trees are stuck where we plant them.

The urban/suburban yard can be a harsh environment for trees (and earthworms). Downtown areas are even worse. Unlike most forest trees, urban trees are widely spaced and develop large crowns. With a large amount of leaf-surface area exposed to sun and wind, water loss from the leaves is high and a correspondingly high demand is put on the root system to replace the water lost. Add to this the fact that water evaporates four times as fast from this exposed soil as from protected forest soils, and the odds for survival quickly stack up against the urban tree.

The quality of the soil is an even bigger problem for urban trees. At best, urban soils can be described as disturbed. Topsoil is scraped off, soil is filled in, it is dug up for utilities, compacted by construction traffic, and altered in just about every way you can imagine prior to the planting of treesor, worse yet, after trees are established. The soils that result are often poorly drained; yet they are like concrete when dry. Root development in these soils is very shallow, with some root systems limited to the upper six inches.

Consider how this compares to natural-forest root systems that commonly penetrate to a depth of two feet-still not as deep as we would imagine. Our earthworm would tell us that the distinction we humans make between shallow-and deep-rooted tree species is not a real one. Like other soil organisms, roots will grow best where water, oxygen, nutrients, and warm temperatures are most available. Typically, this is close to the soil surface. Roots cannot grow where there is no oxygen or where the soil is difficult to penetrate. Root growth, hence tree growth, is restricted by shallow, wet, or dense soils.

Roots would naturally grow downward, but gravity is not strong enough to overcome the response roots have to the favorable environment of the upper soil layers. Some roots actually grow upward, and most roots are quite shallow-just how shallow depends on the souil. A few roots will grow deeper by taking advantage of cracks and openings in the soil-from earthworm channels to sewer lines-as long as oxygen and moisture supplies are adequate.

Ninety-nine percent of a tree's roots are found in the top three feet of soil. In some soils, the important fine absorbing roots may be concentrated in the upper four to eight inches. Root-spread area is often four to seven times the cross-sectional area of the crown or more than twice the height of the tree. Contrary to the way we normally visualize trees, the root system does not mirror the profile of the above-ground portions. A more accurate analogy would be to imagine the complete profile of a tree as a wine glass set on a dinner plate.

Most urban trees do not have tap roots. To confirm this fact, observe city trees that have toppled over. Rather than being anchored in the ground with a tap root, most of them rely on the tensile strength of wood for support. The large horizontal roots, just below the soil surface, serve as cables to anchor the swaying trunk and crown and to counteract the force of the wind. It is this direction in which wood is the strongest.

Just think of a toothpick. Anyone can break one in two by bending it, but try breaking one by pulling on the ends. This anchoring system works best for trees when the soil is dry; when wet, the shallow roots of a tree can be pulled out of the soil by the tipping action of the tree.

Here is another way that forest trees have the upper hand over their urban cousins. In addition to drawing on tensile strength for support, forest trees have the support of intertwined root systems. In pure stands (stands of predominantly the same species), natural root grafts offer additional support.

Soil porosity is another area where urban soils often fall short-from an earthworm's point of view and to other soil organisms, including tree roots. Pores constitute up to 50 percent of a soil's volume and are filled with air and water, both essential for root growth. Pores also provide space for roots to grow. The greater the porosity of the soil, the deeper root development will be. Dense, fine-textured soils resist penetration, while coarse-textured soils are usually very porous and easy for roots to invade.

The amount of clay, silt, and other fine-textured material increases with depth. Organic matter, comprised of decomposing plants, animals, and microorganisms in the soil can act to cement small soil mineral particles, increasing large pore spaces between them while providing nutrients. Frost action and alternate swelling and shrinking of soils between wet and dry periods act to heave, break, and loosen surface layers of soil. Soil organic matter content, soil particle size, frost action, and thus porosity decrease rapidly with depth, making the soil environment less hospitable for roots (and earthworms) until finally biological activity ceases altogether. The depth of this point will vary with soil type.

It is true that the greatest bulk of a tree is above ground, but as already mentioned, the greatest surface area lies underground. There is a limit to the leaf surface (both sides are counted) and root surface that any particular plot of land can sustain. During the height of the growing season in a typical forest or cropland, the maximum leaf surface area is 12 times greater than the soil surface while the root surface area is generally 15 to 28 times greater than the soil surface area. So the root surface area of a tree is likely to be at least twice its leaf area.

Small lots with large houses, driveways, and patios leave precious little space for trees, particularly their roots. Lawn grasses are highly competitive and almost completely prevent root growth in the upper few inches of soil, further reducing the size and capacity of the root system. Typically, the tree root system beneath a backyard lawn is 15 percent of that found in forest soils.

For many urban/suburban trees, the limited root system, large exposed crown, high water loss, and dry soils result in an imbalance between above-and below-ground parts of the trees. This imbalance can lead to stress, especially during hot, dry weather. Such stress can allow secondary insect and disease problems that would otherwise not occur. Many of our tree problems appear to be sudden, when the scenarios have been developing underground for years.

Our earthworm tour guide would tell us-and so would our trees' roots if they could-that the best long-term solution is to turn the situation around. Instead of expecting trees that have adapted to forest environments to live in urban settings, we should make those city settings as close to forest environments as we possibly can. That means giving trees-and especially their roots-more space, and more of the right kind of space, to grow.
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Title Annotation:tree's roots
Author:Hennen, Gary
Publication:American Forests
Date:Sep 1, 1989
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