Arboriculture & Urban Forestry 40(5): September 2014 The effectiveness of barriers may not be permanent, since pavement damage by 30-year-old sweet cherry (Prunus avium) roots was associated with large roots as deep as 40 cm below the pavement (Nicoll and Armstrong 1997; Nicoll and Armstrong 1998). Depth and installation of the barrier is impor- tant. A 45 cm deep barrier reduced roots under the pavement (Smiley 2008a) while a 30 cm barrier of similar design did not (Gilman 2006). Barriers need to be installed with the uppermost edge above grade. If roots are able to grow over the top of the barrier because of incorrect installation, deteriora- tion of the exposed barrier material, or mulching over the barrier, can result in significant damage to pavements (Smiley 2008a; Tworkoski et al. 1996). Barriers can reduce overall root development of trees (Wagar and Barker 1993; Barker 1995a; Gilman 1996; Smiley et al. 2009), but in most studies, no effect on trunk diameter growth was reported (Barker 1995a; Barker 1995b; Tworkoski et al. 1996; Costello et al. 1997; Peper 1998; Peper and Mori 1999; Gilman 2006; Smiley 2008a). There is no evidence that root barriers will decrease stability. Slightly more force was required to pull over trees within root barriers. The increased stability was attributed to deeper roots (Smiley et al. 2000). The situation may be different if roots are not able to grow under the barrier, such as on sites with very poor soil aeration or very deep barriers. In such a situation, the limited root system on one or more sides could result in increased instability. Other alternatives to root barriers have proven to be effective in preventing roots from grow- ing beneath pavements and causing crack- ing and liſting. Extruded polystyrene foam 10 cm thick installed directly under poured con- crete forced roots to grow under the foam. The expanding roots crushed the foam instead of heaving the pavement (Smiley 2008a). When pavements were laid on a base of coarse gravel or brick rubble, the coarse material was apparently not a suitable environment for root growth between the stones, and the roots grew in the soil underneath it. Thicknesses of 15 cm and 30 cm were somewhat more effective than 10 cm (Kopinga 1994a; Gilman 2006; Smiley 2008a). A 10 cm thick layer of structural soil beneath the pavement is not the intended use of structural soil, but has been used in place of gravel in prac- 257 tice (Smiley 2008a). Whereas the use of gravel dis- couraged root growth, a similar 10 cm deep layer of structural soil allowed vigorous root growth in the soil between the coarse stones, as it is designed to do. Roots in the stone layer resulted in extensive pavement cracking and liſting. When structural soils are used with a minimum depth of 60 cm, or a preferred depth of 90 cm, roots grew to the full depth of the structural soil and were not found exclusively at the surface (Grabosky et. al. 2001). Certain root barrier products that are impregnated with herbicides to reduce root growth can be effec- tive as root barriers, but raise concerns that mycor- rhizae could be affected. Sweetgum (Liquidambar styraciflua, endomycorrhizal) and pin oak (Quercus palustris, ectomycorrhizal) root mycorrhizae col- lected from within 1 cm of a chemically impreg- nated barrier were unaffected in the only reported study (Jacobs et al. 2000). (For an extensive review of root barrier research, see Morgenroth 2008.) Just as disease resistance is the preferred way to control a tree disease, developing trees with deeper root systems would be the best way to reduce pavement damage. Research has shown that root systems of certain tree species that oſten cause sidewalk damage [e.g., shamel ash (Fraxinus uhdei), zelkova (Zelkova serrata), Chinese pistache (Pistacia chinensis)] can be selected for deep rooting patterns. Unfortunately, when these trees were propagated by rooting cuttings; the propa- gated trees did not exhibit the same deep-root- ing characteristic (Burger and Prager 2008). Sewer Pipe Intrusion Tree root intrusion into sewer systems can be a sub- stantial problem. Tree roots rarely damage pipes, but Mattheck and Bethge (2000) hypothesize that when a tree root encircles a pipe, wind loading may result in enough movement to break the pipe, especially when this occurs near material defects. Roots can enter pipes in breaks and loose joints and then proliferate rapidly once inside the moist, nutrient-rich environment. Older pipes have more root intrusions because of age and materials used. Clay and concrete pipes without rubber gaskets in the joints resist root intrusion the least. The most intrusions have been into the smaller dimension pipes, 22.5–40 cm, possibly because the larger pipes are usually deeper in the soil and the roots may not ©2014 International Society of Arboriculture
September 2014
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