16 Watson and Hewitt: The Relationship Between Structural Root Depth and Vigor of Urban Trees The range of data collected in this initial study was limit- ed. Information on a wider variety of species is needed. Given the accepted variability of soils on urban sites, more informa- tion on site soil conditions could improve researchers’ under- standing of which factors are most affecting tree performance. Though the reduction of tree performance due to deep roots was modest, the problem may be so widespread that there could still be a significant impact on the urban forest as a whole. When all species are combined, the structural roots were at least 8 cm deep on 26% of the trees. Growth and vig- or is being reduced in some cases by as much as 50%. The po- tential reduction in ecosystem services provided by these un- derperforming trees over their lifetime could be substantial. Acknowledgments. This study was funded in part by a grant from the USDA Forest Service Urban and Community Forestry Program on the recommendation of the National Urban and Com- munity Forestry Advisory Council. Cooperators providing invaluable assistance at the study sites were E. Thomas Smiley, Liza Holmes, and Elizabeth Schulte of Bartlett Tree Research Laboratories; Jim Barbarinas, Urban Forestry Services, Inc; Linda Chalker-Scott and Eric Eulenberg, WSU Puyallup Research and Extension Center; Peggy Drescher, Charles Shonte, and Glenn Willis, Village of Glen Ellyn; and Kristen Vollrath, The Morton Arboretum. LITERATURE CITED Arnold, M.A., G.V. McDonald, and D.L. Bryan. 2005. Planting depth and mulch thickness affect establishment of green ash (Fraxinus pennsylvanica) and bougainvillea goldenrain tree (Koelreuteria bi- pinnata). Journal of Arboriculture 31:163–170. Figure 1. Box plots for species exhibiting a significant relation- ship between root depth and crown vigor rating (1 = vigorous, 7 = replacement needed). deep root system. Trunk diameter growth was reduced on two of the three species (Table 2), possibly reflecting overall low- er performance, though crown appearance was acceptable. At most, root depth could explain less than half of the reduction in crown vigor, trunk condition, or trunk diameter growth. Root depth is only one of several factors affecting tree performance on street tree plantings. Soil conditions, seedling rootstocks, and in- stallation and maintenance procedures on street tree planting sites can be highly variable, even within an apparently uniform planting on the same street. High variability in non-experimental field plots compared to their more controlled counterparts can make poten- tial effects of deep root systems on tree vigor difficult to discern. CONCLUSIONS These results show that deep root systems are not causing widespread decline and losses to urban trees in the first ten years after planting. There was a significant relationship between root depth and tree performance for only some species, explaining less than half of the reduction in any indicator of performance. Most of the species in- cluded in this study were considered at least moderately tolerant of adverse conditions. Deep roots may have a greater effect on more sensitive species. Even a modest reduction in vigor could be seri- ous for species that find it challenging to grow in urban landscapes, and for many moderately sensitive species on very difficult sites. Arnold, M.A., G.V. McDonald, D.L. Bryan, G.C. Denny, W.T. Watson, and L. Lombardini. 2007. Below-grade planting adversely affects survival and growth of tree species from five different families. Arbo- riculture & Urban Forestry 33:64–69. Berrang, P., D.F. Karnosky, and B.J. Stanton. 1985. Environmental fac- tors affecting tree health in New York City. Journal of Arboriculture 11:185–189. Bryan, D.L., M.A. Arnold, A. Volder, W.T. Watson, L. Lombardini, J.J. Sloan, L.A. Valdez-Aguilar, and A.D. Cartmill. 2010. Transplant season, irrigation, and planting depth effects on landscape establish- ment of bald cypress and sycamore. Arboriculture & Urban Forestry 36:57–65. Day, S.D., and J.R. Harris. 2008. Growth, survival, and root system mor- phology of deeply planted Corylus colurna seven years after trans- planting and the effects of root collar excavation. Urban Forestry & Urban Greening 7:119–128. Dirr, M.A. 2009. Manual of Woody Landscape Plants (Sixh Edition). Stipes Publishing L.L.C., Champaign, Illinois, U.S. 1325 pp. Giblin, C, J. Gillman, D. Hanson, G.R. Johnson, and P. Weicherding. 2006. The effects of soil depth on the long-term health and frequency of storm damage to trees in the upper Midwest, pp. 33–39. In: G. Wat- son (Ed.). Proceedings of Trees and Planting: Getting the Roots Right Conference. Gilman, E.F. 1997. Trees for Urban and Suburban Landscapes. Delmar Publishers, Albany, New York, U.S. 662 pp. Harris, J.R., J. Fanelli, A. Niemiera, and R. Wright. 2001. Root pruning pin oak liners affects growth and root morphology. HortTechnology 11:49–52. ©2012 International Society of Arboriculture
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