Arboriculture & Urban Forestry 36(6): November 2010 Stability has been associated with shallow roots in the wind- ward direction (Stokes 1995), symmetrical lateral roots (Coutts et al. 1999), tap roots (Danjon et al. 2005), leeward sinker root length (Hayfa et al. 2007), and combinations of these and other attributes on various soil types. There appears to be no consen- sus on optimum root structure for tree stability. Mickovski and Ennos (2002) showed that root CSA and trunk DBH together accounted for 52% of overturning moment. Hayfa et al. (2007) found that combinations of root number, root depth, and trunk taper explained up to 80% or more of variability in trunk bending stress to pull trees to a set angle. In the current study, it was found that 97% of overturning moment was accounted for by trunk di- ameter and total root CSA measured just outside original root ball in upper 25 cm of soil (Equation 3). Root CSA and diameter of largest roots, especially in the windward direction, correlated best with trunk bending stress required to pull trees to 10 degrees tilt from unloaded start position (Table 4). Root size (diameter or CSA) on windward and leeward sides was most correlated with maximum bending stress encountered while pulling to 25 degrees (Table 4); however, wind required to push trees to this extent would exceed 45 m·s-1 . Maximizing root CSA and diameter of roots in the upper 25 cm soil profile outside of original root ball appears crucial for encouraging stable live oak in this soil type. The overturning bending moment generated by winching to simulate wind load to uproot trees increases proportion- ately with 1.6 the power of trunk diameter to as high as 2.9 the power of trunk diameter (Crook and Ennos 1998; Stokes 1999; Mickovski and Ennos 2003; Tanaka and Yagisawa 2009). Overturning bending moment for live oak in the current study (Equation 3) scaled to the 3.4 power of trunk diameter (cali- per) but the trunk diameter lower (0.15 m) on the trunk was measured, which could have contributed to the larger expo- nent. In contrast, trunk strength scaled to the third power of trunk diameter (Crook and Ennos 1996; Tanaka and Yagi- sawa 2009). This indicates the trunk on many trees become stronger than the root system as trees age commonly result- ing in root failures instead of trunk failures on mature trees. Roots on trees planted from 57 L containers responded like trees planted from 170 L containers except 57 L trees had larger root diameter/cm2 trunk CSA (Table 1). Relatively large diameter roots may explain the similarity of 57L trees to trees transplanted from the field in bending stress re- quired to tilt trunks to 15 degrees (Figure 4) or more (data not shown). Evidence of this is the importance of large root diameter in resisting maximum overturning (Table 4). Pulling force normalized for trunk CSA (bending stress; Figure 4) required to tilt trees planted from 57 L containers was simi- lar to trees from 170 L containers indicating similar stability three growing seasons after planting. There should be tests for more variation in container size, different species, and allow more time after planting before evaluating stability. This project is limited because it only compared one species grown under one set of conditions in one soil type in one organic substrate in one container type. Production practices including type of container, time in container, irrigation, fertilization, root pruning, and other practices influence root morphology inside the root ball, and these need to be studied in order to bet- ter understand attributes of stable trees in urban environments. Acknowledgments: Thanks go to the TREE Fund and GreatSouthernTreeConference.org for partial funding. 289 LITERATURE CITED Anonymous. 2004. American standard for nursery stock. American Na- tional Standards Institute (ANSI Z60.1). American Association of Nurserymen. Washington, D.C. Blanusa, T., E. Papadogiannakis, R. Tanner, and R.W.F. Cameron. 2007. Root pruning as a means to encourage root growth in two ornamental shrubs, Buddleja davidii ‘Summer Beauty’ and Cistus ‘Snow Fire’. Journal Horticultural Sciences and biotechnology 82:521–528. Burdett, A.N. 1978. Control of root morphologenesis for improved me- chanical stability in container-grown lodgepole pine. Canadian Jour- nal of Forest Research 8:483–486. Coutts, M.P. 1983. Development of the structural root system of Sitka spruce. Forestry 56:1–16. Coutts, M.P. 1986. Components of tree stability in Sitka spruce on peaty gley soil. Forestry 59:173–197. Coutts, M.P., C.C.N. Nielson, and B.C. Nicoll. 1999. The development of symmetry, rigidity, and anchorage in the structural root system of conifers. Plant and Soil 217:1–15. Crook, M.J., and A.R. Ennos. 1998. The increase in anchorage with tree size of the tropical tap-rooted tree Mallotus wrayi, King (Euphorbia- ceae). Annals of Botany 82:291–296. Danjon F., T. Fourcaud, and D. Bert. 2005. Root architecture and wind- firmness of mature Pinus pinaster. New Phytology 168:387–400. Dunn, G.M., J.R. Huth, and M.J. Lewty. 1997. Coating nursery contain- ers with copper carbonate improves root morphology of five native Australian tree species used in agroforestry systems. Agroforestry Systems 37:143–155. Duryea, M.L., E. Kampf, and R.C. Littell. 2007. Hurricanes and the urban forest: I. Effects on southeastern United States coastal plain species. Arboriculture & Urban Forestry 33:83–97. Ennos, A.R. 1995. Development of buttress in rainforest trees: the influ- ence of mechanical stress. In: M.P. Coutts and J. Grace (Eds.). Wind and trees. Cambridge Univ. Press Cambridge. pp. 293–301. Fourcaud, T., J. Ji, Z. Zhang, and A. Stokes. 2007. Understanding the impact of root morphology on overturning mechanisms: A modeling approach. Annals of Botany 100:1093. Fraser, A.I., and J.B.H. Gardiner. 1967. Rooting and stability in Sitka spruce. Forestry Commission Bulletin 40, HMS, London. Gilman, E.F., I.A. Leone, and F.B. Flower. 1987. Effect of soil compac- tion and oxygen content on vertical and horizontal roots distribution. Journal of Environmental Horticulture 5:33–36. Gilman, E.F., and M.E. Kane. 1991. Growth dynamics following plant- ing of cultivars of Juniperus chinensis. Journal American Society Horticulture Science 116:637–641. Gilman, E.F., T.H. Yeager, and D. Weigle. 1996. Fertilizer, irrigation and root ball slicing affects Burford holly growth after planting. Journal of Environmental Horticulture 14:105–110. Gilman, E.F., and C. Harchick. 2008. Planting depth in containers affects root form and tree quality. Journal of Environmental Horticulture 26:129–134. Gilman, E.F., C. Harchick, and C. Wiese. 2008. Pruning roots affects tree quality in container-grown oaks. Journal of Environmental Horticul- ture 27:7–11. ©2010 International Society of Arboriculture
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