Arboriculture & Urban Forestry 37(5): September 2011 The tap and heart root systems species appear to have grown larger because of greater root plate depth, while root plate di- ameters approximated the size of smaller DBH surface root system species. Notes of caution are warranted before general- izing from the species classification as surface root system or tap root and heart root systems following Bernatzky (1978), Dirr (2009), and Wray (1997). First, the tendency of an indi- vidual tree to follow the pattern of a root system is modified by the particular growing conditions (Bernatzky 1978). For example, when bedrock limits root depth, the root plate of a tree has a larger diameter and shallower depth than trees grow- ing in conditions without root depth limitations (Coutts et al. 1999). Second, the significant root plate depth correlation with slope could be explained by the water table, which is a limit- ing factor for root depth, being farther from the soil surface in steep slopes than in moderate slopes (Chiatante et al. 2003). A study of colluvium covered slopes in the Cincinnati, Ohio, U.S., area showed that saplings of the tap root system species white ash could stabilize soil as thick as 1 m while sugar maple saplings with a lateral root system could not stabilize soil thicker than 0.5 m (Riestenberg 1994). Tap roots provide greater depth of anchor- ing for the root network; however, tap roots of both broadleaf and conifer species lose importance in regard to tree stability relative to the spread of lateral roots as a tree ages (Chiatante et al. 2003). CONCLUSIONS AND IMPLICATIONS The urban trees in Radnor Lake State Natural Area were not dif- ferentially affected by landslides in regard to species, slide type (soil slide or rockslide), slope steepness, or root system type (sur- face root system versus tap or heart root system), but saplings are lost much less frequently than trees—including fewer saplings lost in rockslides than soil slides. Management of urban areas with hydrogeomorphic landslides should be based on landslide history to identify locales of special concern and to prioritize sites for tree plantings since landslide recurrence at a site is unlikely for decades or at least until a sufficient mass of colluvium rede- velops (Koi et al. 2006). The relationships found for root plate diameter and root plate depth indicate rooting characteristics are a critical aspect to consider during species selection for tree plantings to reduce the risk of landslides in urban areas. Because tap root trees produce more depth in the root cohesion network, tap root trees should be preferred for planting to ameliorate the risk of rockslides or soil slides (Riestenberg 1994). Planting tap root trees is a challenge for arboriculturalists because traditional methods of transplantation do not accommodate fast growing tap roots well. A focus on developing better methods for cultivating species of Carya for preservation of the taproot during transplan- tation (Dirr 2009) could create the means to supply a market for landslide prone areas. In planning for plantings, spacing of no more than 7 m apart is needed for the development of overlapping root networks that provide soil cohesion across an area (Rieste- nberg 1994). If the soil or growing conditions of the site are not uniform, then closer spacing or placement to adjust for subsurface impediments is essential (Riestenberg 1994). In addition to tree density, the extent of root cohesion is dependent upon the species assemblage at a site because of local variability in root cohesion within a stand (Schmidt et al. 2001). Finally, consideration must be given to site conditions such as soil depth, bedrock geology, and water table elevation to increase the likelihood of transplanta- 217 tion success for plantings to reduce landslide risk. When feasible, modifications of a site to eliminate limiting conditions can be done to enhance tree growth and stabilize landslide-prone urban areas. Acknowledgments. The authors acknowledge the contributions of Steve Ward, Park Manager; Jesse Germeraad, Interpretative Ranger and the staff at Radnor Lake State Natural Area; as well as Kent Gallaher, Chair and Bonnie Gallagher, Undergraduate Student of the Depart- ment of Biology at Lipscomb University. We appreciate the review of a draft manuscript provided by Naomi Burnell, Alexandra Gren, Patience McCullough, Antoni Pelc, and Jericha Skuntz of The Pennsylvania State University. LITERATURE CITED Bernatzky, A. 1978. Tree Ecology and Preservation. Elsevier, New York. 357 pp. Braun, E.L. 1950. Deciduous Forests of the Eastern North America. Haf- ner, New York. 596 pp. Chiatante, D., S.G. Scippa, A. Di Iorio, and M. Sarnataro. 2003. The in- fluence of steep slopes on root system development. Journal of Plant Growth Regulation 21:247–260. Coder, K.D. 2010. Root strength and tree anchorage. Warnell School Outreach Monograph WSFNR 10–19. 88 pp. Coutts, M.P., C.C.N. Nielsen, and B.C. Nicoll. 1999. The development of symmetry, rigidity, and anchorage in the structural root system of conifers. Plant & Soil 217:1–15. Cutler, D.F., C.C.N. Nielsen, and M.C. Farmer. 1990. The windblown tree survey: analysis of the results. Arboricultural Journal 14:265–286. Day, S.M., P.E. Wiseman, S.B. Dickinson, and J.R. Harris. 2010. Con- temporary concepts of root system architecture of urban trees. Arbo- riculture & Urban Forestry 36:149–159. Dirr, M. 2009. Manual of Woody Landscape Plants: Their Identification, Ornamental Characteristics, Culture, Propagation and Uses. Stipes Publishing, Champaign, IL. 1325 pp. Duryea, M.L., E. Kampf, and R.C. Littell. 2007. Hurricanes and the ur- ban forest: 1. Effects on southeastern United States coastal plain tree species. Arboriculture & Urban Forestry 33:83–97. Gasson, P.E., and D.F. Cutler. 1990. Tree plate morphology. Arboricul- tural Journal 14:193–264. Gibbs, J.N., and B.J.W. Greig. 1990. Survey of parkland trees after the great storm of October 16, 1987. Arboricultural Journal 14:321–347. Greenway, D.R. 1987. Vegetation and slope stability, pp. 187–230. In: M.G. Anderson and K.S. Richards (Eds.). Slope Stability. John Wiley and Sons, Chichester, UK. Johnson, A.C., and P. Wilcock. 2002. Association between cedar decline and hillslope stability in mountainous regions of southeast Alaska. Geomorphology 46:129–142. Koi, T., N. Hotta, and M. Suzuki. 2006. Sediment yield and vegetation re- covery in a mountainous region with repeated heavy precipitation – A case study in the Omaru River Basin, Miyazaki, Japan, pp. 321–329. In: H. Marui, T. Marutani, N. Watanabe, H. Kawabe, Y. Gonda, M. Kimura, H. Ochiai et al. (Eds.). Disaster Mitigation of Debris Flows, Slope Failures and Landslides. Frontiers of Science Series, v. 47, Uni- versal Academy Press, Tokyo, Japan. Loeb, R.E., J. Germeraad, T. Treece, D. Wakefield, and S. Ward. 2010. Effects of one-year versus annual treatment of Amur honeysuckle in forests. Invasive Plant Science and Management 3:334–339. McNaughton, K.G., and P.G. Jarvis. 1983. Predicting effects of vegetation changes on transpiration and evaporation, pp. 1–47. In: T.T. Kozlowski (Ed.). Water Deficits and Plant Growth, v. 7. Academic Press, New York. ©2011 International Society of Arboriculture
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