Arboriculture & Urban Forestry 35(1): January 2009 The depth of the barrier and its interior surface configura- tion (smooth or vertical ribs) are less important than its diam- eter, because the #5 container barrier failed because it was too small in diameter, whereas the #15 container and DeepRoot barriers performed equally well even though the #15 container was 18 cm (7 in) shorter and smooth inside. Although the #5 container barrier reduced the number of surface roots that developed dur- ing the term of our study, our results also indicate that root barri- ers should be constructed of durable material and installed with the top edge exposed, similar to conclusions of Peper and Barker (1993). Also, they must be large enough in diameter [36 cm (14 in) minimum] to accommodate the roots and root crowns of mature trees to maximize the effect on surface root development. Although others concluded that root barriers may be most effec- tive at reducing surface root development in sites with well-drained, noncompacted soil (Wagar 1985; Urban 1994; Barker 1995a, 1995b; Gilman 1996), we found that tree roots grew below a barrier and returned to the surface soil a very short distance beyond the bar- riers even when soil texture, bulk density, and water content were near optimum for root growth at the bottom of the barrier. The uni- form coarse sandy loam soil in our experimental plot was main- tained at well-watered conditions with irrigation throughout the study, and its bulk density from the surface to the depth of the barrier was 1.5 g/cm –3 or less, which is not limiting to root growth in sandy loam soil (Veihmeyer and Hendrickson 1948; Zisa et al. 1980). These findings are supported by Gilman (2006) in which barriers in well-drained soil did not produce deeper root systems. The tree growth results establish that circular root barriers can reduce growth in some tree species, although the reductions doc- umented in the study were not viewed to be enough to reduce the trees’ aesthetic value. Data from the study also provide doc- umentation of surface root system distribution for Ficus and Liquidambar . LITERATURE CITED Barker, P.A. 1995a. Managed development of tree roots I. Ultra-deep root ball and root ball casing effects on European hackberry. Journal of Arboriculture 21:202–208. ———. 1995b. Managed development of tree roots II. Ultra-deep root ball and root ball casing effects on southwestern black cherry. Journal of Arboriculture 21:251–259. Costello, L.R., C.L. Elmore, and S. Steinmaus. 1997. Tree response to circling root barriers. Journal of Arboriculture 23:211–218. Costello, L.R., and K.S. Jones. 2003. Reducing Infrastructure Damage by Tree Roots: A. Compendium of Strategies. Western Chapter of the International Society of Arboriculture, Cohasset, CA. Gilman, E.F. 1996. Root barriers affect root distribution. Journal of Arboriculture 22:151–154. ———. 2006. Deflecting roots near sidewalks. Arboriculture and Urban Forestry 32:18–23. McPherson, E.G. 2000. Expenditures associated with conflicts between street tree root growth and hardscape in California, United States. Journal of Arboriculture 26:289–297. Peper, P.J. 1998. Comparison of root barriers installed at two depths for reduction of white mulberry roots in the soil surface, pp. 82–93. 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, 5–8 March 1998, San Francisco, CA. International Society of Arboriculture, Champagne, IL. Peper, P.J., and P.A. Barker. 1993. A buyer’s technical guide to root barri- ers, pp.186–193. In: Watson, G.W., and D. Neely (Eds.). The Landscape Résumé. L’influence de quatre barrière racinaires circulai- res sur le développement des racines de surface et sur la crois- sance de l’arbre a été évalué sur le Liquidambar styraciflua et le Ficus macrocarpa dans le Sud de la Californie. Six ans après leur installation, les barrières racinaires ont réduit le nombre total de racines poussant dans les 15 premiers centimètres de sol et ont pratiquement éliminé les grosses racines de surface (5 cm et plus de diamètre) dans un rayon de 120 cm autour du tronc. Divers types et dimensions de barrières non commerciales à faibles coûts – incluant les pots servant à la production des arbres en pépinière dont on avait enlevé le fond – se sont aussi avérés efficaces pour diminuer le nombre de grosses racines de surface. Cependant, les racines ont poussé sous les barrières et sont par la suite revenu à la surface, et ce même si la texture du sol, sa densité et son contenu en eau étaient à peu près à un degré optimum pour la croissance des racines sous le niveau de la bar- rière. Plusieurs petites racines de 1,25 à 2,5 cm de diamètre ont été observées qui poussaient dans les 15 premiers centimètres de sol juste au-delà de la barrière soit à 18 à 60 cm de distance du tronc. Aucun type de barrière ne parvient à réduire le nom- bre de petites racines de chacune de ces deux espèces au-delà d’un rayon de 60 cm du tronc. Maintenir les surfaces pavées à une distance minimale de 120 cm de l’arbre devrait être aussi ©2009 International Society of Arboriculture 45 Below Ground: Proceedings of an International Workshop on Tree Root Development in Urban Soils, 1 Sept. and 1 Oct. 1993, Lisle, IL. International Society of Arboriculture, Savoy, IL. Peper, P.J., and S. Mori. 1999. Root barriers and extension casing effects on Chinese hackberry. Journal of Arboriculture 25:1–8. Randrup, T.B., E.G. McPherson, and L.R. Costello. 2001. A review of tree root conflicts with sidewalks, curbs, and roads. Urban Ecosystems 5:209–225. Smiley, E. T., A. Key, and C. Greco. 2000. Root barriers and windthrow potential. Journal of Arboriculture 26:213–217. Smiley, E.T. 2005. Root growth near vertical root barriers. Journal of Arboriculture 31:150–152. Urban, J. 1994. Root barriers: An evaluation. Landscape Architecture 84:28–30. Veihmeyer, F.J., and A.H. Hendrickson. 1948. Soil density and root pen- etration. Soil Science 65:487–493. Wagar, J.A. 1985. Reducing surface rooting of trees with control planters and wells. Journal of Arboriculture 11:165–171. Warriner, W. 1999. The Ficus tree. Western Arborist 24:48–50. Wilson, B.F. 1967. Root growth around barriers. Botanical Gazette 128:79–82. Zisa, R.P., H.G. Halverson, and B.J. Stout. 1980. Establishment and early growth of urban conifers on compact soils. USDA Forest Service Research Paper NE-451. Dennis Pittenger (corresponding author) University of California Cooperative Extension, Central Coast & South Region Department of Botany & Plant Sciences University of California, Riverside 4114 Batchelor Hall Riverside, CA 92521, U.S.
[email protected] Donald Hodel University of California Cooperative Extension, Los Angeles County P. O. Box 22255 Los Angeles, CA 90022
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