While it is natural for us to look up and perceive the tree from above ground, an even more impressive portion is below ground. Think of the first tree you drew, did it even have any roots? "The roots within the dripline of a tree are estimated to have 2.5 to 4.5 times more surface area than do the leaves (one side) of the tree according to Richard Harris in the Journal of Arboriculture (18:1, January 1992). More recently, it has been discovered that the ideal ratio of root to leaf surface area for photosynthesis is 10 to 1! Because most roots are veiled by a layer of soil we simply cannot properly conceive them in our minds without some serious intention. Upon closer examination we see a massive network of subterranean and superterranean organs that interfaces with the soil and mycorrhizae to absorb water, oxygen (from pore space) and minerals creating the engine for photosynthesis. Most of us know that the mushroom is simply the fleshy fruit of a huge network of roots or mycelium. Similarly, the portion of a tree above ground is the fuel to run the engine. Like us, trees are mostly water, and that water delivery system starts in the soil. A seed starts with the roots that snake through the earth absorbing water and elements to sprout the beginning of the tree above ground. For trees, our understanding of their full structure, including that below, is essential for their health. Too often, particularly in urban areas, roots are trenched, compacted, and removed without the full understanding of its impact on the whole tree. The misunderstanding is acerbated by trees' ability to store energy and live, albeit compromised, for years after the point of contact. While looking up is exhilarating, it is important to remember the heart of the mater is beneath our feet.
The downloadable cue cards are designed to fit in a shirt pocket and can be printed onto 3 1/2 " x 8" card stock and laminated for field use. They are also available in Spanish. Feel free to reproduce them.
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Lately I have been fascinated with the impact of cutting roots on large trees and luckily ran across thisexcellent podcast from Dr. Alessio Fini on the International Society of Arboriculture site. He makes several fascinating points, including that due to the growth of optical fiber and cable, there has been 1000s of miles of new trenching in the world. When urban areas are trenched, urban tree roots are often cut and or mangled. Loss of roots directly correlates to less water absorption which entails a drop in photosynthesis, transpiration, and carbon assimilation. In effect, Dr. Aini compares root severance to drought stress in trees. Moreover the effect of the stress of the trees can take from 3-6 years to manifest. His research also indicates that the side of root severance does not necessarily correspond to damage in that side of the tree but instead has a more general effect on the tree. Typically when an arborist is called on site, it is too late and the damage has been done, unless the arborist was involved at the moment of severance. Techniques to mitigate the damage are irrigation plans, root pruning, mulching, and/or mychorrhizae treatments. There are also tactics to avoid the damage in the first place by tunneling using precision drilling and/or hammer head moles.
Understanding the problem
Tree roots develop and survive where there is adequate oxygen and moisture. Most active tree roots are in the top 3 feet of soil; the majority are in the top 12 inches. The more compacted or poorly drained the soil the closer the roots are to the soil surface. Roots normally grow outward to about three times the branch spread. Only 50 percent of the trees root system occurs between the trunk and the dripline. Roots on one side of the tree normally supply the foliage on the same side of the tree. When the roots on one side of the tree are injured the branches on that side of the tree may die back or die. With some trees, such as maple, the effect may develop anywhere in the tree canopy.
Many homeowners worry about tree roots piercing their pipes creating leaks and clogs. “Typically tree roots exert about 4 pounds per square inch and up to 20psi, which in itself will not create a crack, but is enough to exploit an existing crack,” states Rob Swanson of Specimen Tree in Atlanta, Georgia. Tree roots will infiltrate improperly installed pipes with loose joints or a pipe that has been damaged due to other causes like cold temperatures. It is important that pipes are installed properly and maintained. Often the roots are just exploiting a leak that would have to be addressed at some point regardless of the roots and are accelerating an inevitable problem. Rob Swanson does warn that pipes within the compression plate, the structural root plate that bears the weight of the tree, can exert enough pressure to bust a pipe through the shear leverage they exert as the tree grows, hence the importance of not planting on top or near a pipe. Overall, to minimize conflicts, plant trees away from known pipes and make sure pipes are installed carefully and properly. Also remember that pipes also destroy trees when contractors trench alongside a tree, removing up to 50% of the roots of that tree. Arborists often recommend tunneling as an alternative when you want to spare tree near your home. “Tunneling“ is when you expose the roots using an air compressor and then dig under those exposed roots, bridging the roots over the pipe. Spence Rosenfeld of Arborguard states it best when he says, “pipes and trees co-existing require a delicate balance.”
More cement and asphalt paths are being created through woods in parks as alternative transportation corridors and to view nature. Typically the paths are placed adjacent to mature trees. The damage often shows well after the contractors have left the scene. The cement compresses and damages roots and can prevent water absorption but the most damage occurs during installation when the ground is graded and roots are dug up and/or compressed. The first choice would be to build such a trail outside the critical root zone, however, often this is not an option. I have seen two solutions to the problem. One was to literally bridge the roots with a boarded walkway, piers, anchors, and/or bridge. A promising and less expensive technique called “root bridging” uses expanded slate and geo-textile fabric to prevent soil compaction. The path is then laid on top of slate and fabric. While root bridging is not ideal for the roots under the "bridge", it does provide an opportunity for the roots to move to an area of less compression. This method was recently used in the Atlanta Botanical Gardens path through Storza woods. I have heard claims of cost reductions due to less soil having to be moved around. More importantly who wants a path with a bunch of dead or dying trees along it!
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