Magazine article American Forests

Journey to the Bottom of a Tree

Magazine article American Forests

Journey to the Bottom of a Tree

Article excerpt

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. …

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