Arboriculture & Urban Forestry 34(2): March 2008 Given the variation in soil environments in urban areas, the choice of barrier type, if any, will need to be site-specific. To paraphrase the comments of other authors, barriers appear to be effective where they are least necessary, in loose, well- aggregated, well-aerated soils (Wagar 1985; Urban 1995; Gil- man 2006). FUTURE RESEARCH Studies have been successful in identifying future areas of re- search. In terms of effectiveness, the main question that needs to be answered is whether barriers can be effective under true urban conditions. It is difficult to qualify the true nature of urban conditions, but to be sure, they are characterized by suboptimal soils, extreme environmental conditions, and heterogeneous natural and artificial ground surface types. In stark contrast to these conditions, all but one experiment has been conducted on nonlimiting soils, which generally differ greatly from urban soils. Most experimental sites were characterized as well-drained sites with loamy soil. The only exception contrasted root barrier performance on well-drained and poorly drained soils and found that root barriers function more adequately in well-drained soils (Gilman 2006). Furthermore, given the temporal limitations of the studies, most trees were irrigated and fertilized, maintenance that is not routinely provided for street trees. Finally, the trees in a great majority of experiments were surrounded by natural sur- faces such as bare soil, mulch, or turf rather than concrete or other impervious surfaces that would typically border street trees. Future research is needed to contrast barrier effectiveness under difficult urban conditions. An often overlooked area of barrier research is to determine optimal root barrier depths. This idea has been discussed since the early 1990s (Barker 1994), but actual quantification has not yet been undertaken. In fact, nearly all studies used barriers installed to approximately 30 cm (12 in) in depth. The only exceptions were Wagar (1985) and Peper (1998) who found that 60 cm (24 in) deep barriers significantly decreased mean root number, decreased mean root diameter, and increased mean root depth. There is no obvious scientific reason for the 30 cm (12 in) depth to have become the standard, except perhaps that the vast majority of roots are found in the top 20 cm (8 in) of soil (Vogt et al. 1981; Perry 1982). So theoretically, a barrier depth of 30 cm (12 in) should influence the majority of any tree’s roots. However, many experiments have shown that barriers extending to this depth are ineffective and if “barriers installed at a given depth are not effective in good soil conditions, there is little value to installing them in poorer soils” (Peper 1998). Clearly, there is a need to approach barrier design, and in particular depth, from a scientific perspective. Another area that requires study is determining the effect of barrier use on tree health. Given that root barriers are used to inhibit root growth or displace roots into deeper, poorer soils, it is conceivable that their use results in stress, which when com- pounded with difficult urban planting sites, could compromise tree establishment and survival. Many studies have measured stem diameter, but no comprehensive aboveground measure- ments have accompanied root distribution measurements and so no authors have been able to comment on potential negative effects to trees. Finally, all future research should consider the meaning of “significant” reductions in root growth. Some authors have af- firmed that although their results were statistically significant, 87 they doubt whether the actual differences would be sufficient to curb conflicts. For example, Gilman (2006) reported statistically deeper root depths for trees treated by Biobarrier contrasted against trees grown without a barrier in poorly drained soils, but noted that the actual difference was only 16mm(0.64 in). On the other hand, Peper and Mori (1999) found that barriers signifi- cantly increased root depth 66 cm (26.4 in) from the bole as compared with controls, an actual mean depth difference of ap- proximately 5 cm (2 in). The disparity between statistical sig- nificance and practical implications calls for clear definitions of what goals are expected to be achieved through the installation of root barriers. In summary, root barriers are an invention bred out of neces- sity. Street trees inherently conflict with their surroundings and the results are highly undesirable. Over 30 years ago, barriers were first introduced and experimental research is continually shaping their design and application. Perhaps the most important finding to date has been the variability of root response to bar- riers seemingly influenced by soil type, environmental condi- tions, and species selection. Despite these challenges, future re- search must continue to work toward a solution that will mini- mize conflicts while optimizing the soil environment for root growth. LITERATURE CITED Achinelli, F.G., J.L. Marquina, and R.M. Marlats. 1997. Exploratory study of the relationships between tree growth, site conditions, and maintenance practices in street plantings of Fraxinus pennsylvanica marshall of La Plata City, Argentina. Arboricultural Journal 21: 305–315. Barker, P.A. 1994. Root barriers for controlling damage to sidewalks. In Watson, G., and D. Neely (Eds.). The Landscape Below Ground. Proceedings of the International Workshop on Tree Root Develop- ment in Urban Soils. International Society of Arboriculture, Cham- paign, IL. pp. 179–185. ———. 1995a. Managed development of tree roots. I. Ultra-deep root- ball and root barrier effects on European hackberry. Journal of Ar- boriculture 21:202–208. ———. 1995b. Managed development of tree roots II. Ultra-deep root- ball and root barrier effects on Southwestern black cherry. Journal of Arboriculture 21:251–258. Barker, P.A., and P.J. Peper. 1995. Strategies to prevent damage to sidewalks by tree roots. Arboricultural Journal 19:295–309. Benavides Meza, H.M. 1992. Current situation of the urban forest in Mexico City. Journal of Arboriculture 18:33–36. Coder, K. 1998. Root growth control: Managing perceptions and reali- ties. In Neely, D., and G.W. Watson (Eds.). The Landscape Below Ground II: Proceedings of an International Workshop on Tree Root Development in Urban Soils. International Society of Arboriculture, Champaign, IL. pp. 51–81. Costello, L.R., C.L. Elmore, and S. Steinmaus. 1997. Tree root response to circling root barriers. Journal of Arboriculture 23:211–218. Craul, P.J. 1992. Urban Soil in Landscape Design. John Wiley & Sons, Inc., New York, NY. 416 pp. D’Amato, N.E., T.D. Sydnor, R. Hunt, and B. Bishop. 2002a. Root growth beneath sidewalks near trees of four genera. Journal of Ar- boriculture 28:283–290. D’Amato, N.E., T.D. Sydnor, M. Knee, R. Hunt, and B. Bishop. 2002b. Which comes first, the root or the crack? Journal of Arboriculture 28:277–282. Francis, J.K., B.R. Parresol, and J.M. De Patino. 1996. Probability of damage to sidewalks and curbs by street trees in the tropics. Journal of Arboriculture 22:193–197. Gilman, E.F. 1995. Root barriers affect root distribution. In Watson, G., and D. Neely (Eds.). Trees and Building Sites: Proceedings of an ©2008 International Society of Arboriculture
March 2008
| Title Name |
Pages |
Delete |
Url |
| Empty |
Search Text Block
Page #page_num
#doc_title
Hi $receivername|$receiveremail,
$sendername|$senderemail wrote these comments for you:
$message
$sendername|$senderemail would like for you to view the following digital edition.
Please click on the page below to be directed to the digital edition:
$thumbnail$pagenum
$link$pagenum
Your form submission was a success. You will be contacted by Washington Gas with follow-up information regarding your request.
This process might take longer please wait
