Don't stumble over surface tree roots
Contrary to what you might think, tree roots are not coming out to get you. Instead, roots-by design-are simply trying to seek out, gather and control resources in the soil. So when you see roots growing near the surface of the ground, you know a reason must exist for this growth form. And to solve the problem, you must understand what these reasons are.
Roots' reach Trees have three primary forms of roots: the large, woody structural roots near the tree base, the long, woody transport roots and the ephemeral, absorbing roots. The absorbing roots usually do not form woody materials. They recycle (that is, they die and regrow) many times during a single growing season from the woody transport roots. These transport roots carry food to the root tips, as well as carrying water and essential elements to the leaves. The longer a transport root survives, the bigger it becomes.
In trying to determine the reason that tree roots might grow too closely to the soil surface on your site, you must understand that roots grow where soil resources are most plentiful. And if that area is near the soil surface, then that's where the roots will grow.
The importance of O2 and H20 The two resources most important to tree-root growth are oxygen and water. *Oxygen. Roots must have oxygen to survive and grow. Oxygen is available only near the soil surface in large air-filled soil pores. Tremendous competition exists for this oxygen between tree roots, other plant roots, small animals and microbes that decay organic matter.
For tree roots, the search for oxygen is a "good news/bad news" problem. Roots growing too deeply may suffocate. Roots growing too shallowly may dry out and die.
As long as roots have oxygen, they will continue to grow and seek soil resources, including water (described in detail later). Because of the ease with which roots pull water from the soil, roots sense and grow toward areas where plenty of water is available. The quandary is that water supplies must be located in the same soil areas where atmospheric oxygen also is available. Soil organisms quickly use the dissolved oxygen in soil water and, thus, it is not as readily available to tree roots as is atmospheric soil oxygen.
Atmospheric oxygen is essential to tree life. The aboveground portion of a tree has no problem finding oxygen in the air for respiration. (Oxygen content in the atmosphere is about 21 percent.) Even green tissues use oxygen for respiration when photosynthesis is not working. For roots, however, the plight of getting enough oxygen is severe. For unconstrained root growth, the soil atmosphere must contain more than 15 percent oxygen. As soil-oxygen levels fall below 5 percent, root growth stops. Oxygen levels of less than 2 percent lead to root decline and death. The three major problems that cause inadequate soil oxygen are: *Competing organisms *Soil compaction *Water-filled pores (saturation).
All of these problems lead to an oxygen-limited condition of the soil: an anaerobic condition. Under anaerobic soil conditions, different types of microorganisms-primarily bacteria-take over the soil. The anaerobic organisms produce toxins and consume or infect roots. Several tree root rots thrive at low soil-oxygen levels.
Warmer temperatures disrupt oxygen use by tree roots. As soil and air temperatures increase, so does oxygen demand in a tree and in the surrounding soil. For every 18 degrees F increase in temperature, oxygen demand doubles for both tree roots and other soil organisms. Increasing temperatures cause tree roots to respire faster, which uses food and oxygen more quickly.
An additional example of root-oxygen problems occurs on recreation sites where foot and vehicle traffic have compacted the soil, collapsing the soil air pores. To maximize landscape performance, some landscape managers add composted organic matter and nitrogen to the soil, along with continuous irrigation. The result is a rich mess of oxygen-demanding microbes fueled by organic material and nitrogen.
As these organisms use oxygen, and oxygen is not easily replenished because of water filling all available pore spaces, more portions of the site become anaerobic. In anaerobic conditions, microbes can use soil nitrogen, manganese, sulphur and carbon for respiration as oxygen is depleted. Under these same conditions, however, tree roots decline and die.
*Water. Roots' search for water holds similar problems. Too much water and the tree drowns (suffocates). Too little water and the tree starves (desiccates).
Continuous soil saturation or flooded conditions lead to low soil oxygen and, thus, major tree-root problems. Unregulated, poorly adjusted and improperly zoned irrigation all can cause root damage, especially in warm weather. If you have a particular site with heavy saturation problems that you can't remedy, you're probably best off by removing trees from those areas and replacing them with some of the many plants that better tolerate saturated soil conditions. Roots of saturation-tolerant plants will be more effective in using available oxygen than tree roots.
Saturated soils also are prone to mechanical damage, which reduces aeration and thus lessens trees' ability to survive well there. As water fills and occupies all available soil pores, any activity on the soil (walking, driving vehicles or parking, for example) disrupts soil structure. These activities result in rutting, puddling and compacting of the soil, which lead to root injury and death. Soil around new construction sites is a common area where trees fail to thrive. Equipment and workers damage the soil, leading to poor aeration and soil in which it is difficult for roots to grow.
Shallow end of the pool Now that you understand the soil/root environment better, you can predict where tree roots will grow. Roots need to be deep enough in the soil to escape direct sunlight and to gather moisture not lost to evaporation. Roots also must grow shallowly enough to effectively receive oxygen for respiration. A rich growing site can be shielded from root colonization if some type of oxygen-free barrier exists between the tree and the resources. As roots grow, they must have oxygen in every cubic inch of soil through which they extend.
A landscape soil filled with many competing root systems presents other problems for tree roots. All the root tissues and all the associated organisms in the soil demand large amounts of oxygen. These same plants also demand a lot of water and essential elements. With limited soil resources, and all plants needing roughly the same resources on which to live, root competition is severe. And tree roots may have difficulty competing.
As you can guess, then, tree roots growing on the soil surface are trying to escape bad soil conditions below. Compaction, toxins, low oxygen, excess water, a high water table, excessive mulch or other conditions lead to shallow, woody transport roots.
Floating away You'll sometimes find older, larger roots at the soil surface-not because of poor soil conditions but through root buoyancy and erosion. The density of the materials that make up a tree root is lighter than the mineral components of soil. Over many years, with surface vibrations, frost heaving and compaction, the soil may "buoy" roots onto the soil surface.
Erosion around trees remains one of the scourges of modern development. Erosion carries away productive top soil and essential elements. Erosion also opens the soil surface to reveal surface roots, which then are prone to injury. As water or air moves organic matter, organic layers or soil particles, or as mechanical contact abrades roots, small soil-surface losses can expose many roots.
Natural processes of growth and exposure also lead to some roots appearing on the surface of the soil. A small root that was 0.50 inch below the soil surface-if successful in gathering resources-will grow in diameter. Within a few years, you'll be able to see it on the soil surface. Once at the surface and exposed to the dry air and sunlight, roots can accelerate the formation of bark layers. These exposed roots are easily damaged by machines, climatic events, pests and traffic.
Suffocating roots Mulching with coarse organic materials and adding small amounts of composted organic materials to the soil area at a tree's base can be extremely beneficial. But mulch and compost that, over time, mat down or form impervious layers may prevent oxygen movement into the soil and may shed water.
Some grounds managers add too much mulch around a tree's base. In doing so, they create the anaerobic conditions that drive roots closer to the surface. With heavy mulching (especially when mulches have many fine particles), and too much irrigation, the only aerated area in which tree roots can successfully thrive may be on the soil surface under the mulch. These roots thus become prone to many types of injury, including vertebrate and insect pests, as well as drying-cycle damage.
Species differences Researchers have long cited some tree species as having bad surface-root problems. Most of these species are bottomland/wetland species that normally keep transport roots close to the soil surface and may send down sinker or tap roots. This is because, on wetland sites, the trees can only find their required level of oxygen near or on the surface. On upland sites, that preferred level of oxygen is deeper in the soil. Despite researchers' citations for trees with surface-root problems, these types of species lists are not as important or as accurate as understanding soil attributes.
Treating sites Treatments for surface rooting depend on what is causing the problem. Treating the site for erosion problems when anaerobic soil conditions are at fault, for example, will make problems worse. Therefore, be sure you treat causes, not symptoms. A healthy, well-watered, properly drained and nicely aerated soil can sustain great trees. But soils treated as a bulletin board, where you "stick" landscape plants, will generate many management problems.
Clearly, you must balance water supply, drainage and soil aeration. Procedures that favor one set of circumstances (and one plant group) over others cause and sustain most surface-rooting concerns. If you want great trees and want to minimize surface roots, you must optimize your management practices (see Figure 1, page 38).
Slice and dice The first and last treatment many people use for surface rooting is cutting. Remember that the roots are growing there for a reason, and cutting them out will simply make the tree's plight worse. After all, destroying a large number of surface roots can, at the very least, cause a long-term decline in the tree's health, if not kill it altogether. In addition, some pesticide treatments over the top of newly cut (as well as old) surface roots can be severely damaging. Finally, treatments that initiate callous formation (wound wood), new adventitious shoots or roots, or bark disruption can be great entryways for pests and decay.
If you must cut surface roots, try adding one of the many types of root barriers to minimize severe damage within one event. In doing so, you'll help control further growth and thus only need to cut the roots once, rather than having to follow up with further cutting in the future. Make clean cuts with sharp blades. Minimize bark disruption, root twist and bending (see boxed information, "Tree-root-growth control methods," page 40).
Trees are extremely valuable in landscapes. Clients and customers make many aesthetic, structural, performance and biological demands on landscape trees. Treating surface rooting can be one management event that either adds value while minimizing liability or destroys a tree's ability to survive and thrive. Consider whether your management program is more concerned with helping trees appreciate in value or with simply lopping them off with a saw when they get in the way.
Dr. Kim D. Coder is an associate professor in the School of Forest Resources at the University of Georgia (Athens, Ga.).
A series of publications on controling and redirecting tree-root growth is available on the internet. Go to web site http://www.foresty.uga.edu. Look under the "Community forestry" sections for the "Tree-root-growth control series" and click on one of the six publications. Topics include structural damage by roots, growth requirements, soil constraints, the eight major forms of root control, relevant literature and information sepcifically on root barriers. These are free publications. Other tree-management information is also available at this site.
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