Arboriculture & Urban Forestry 45(6): November 2019 number of trees in this study was very limited, and only one species was studied. The findings may differ for trees with lower vitality, trees growing under adverse condi- tions, larger or smaller trees, or trees of other species. Therefore, additional studies are required to enlarge the empirical basis for quantifying anchorage strength recov- ery after primary anchorage failure. When arborists visually inspect trees with increased leans, they should be able to recognize symptoms of root failure after significant storms and also draw conclusions from signs of growth adaptation in response to earlier events (Dunster et al. 2013; Smiley et al. 2017). The results of this study indicate that some trees are capable of recov- ering their stability over time after primary anchorage fail- ure has occurred. Therefore, some insights into the current stability of leaning trees can be made by assessing current tree vitality, a tree’s self-correcting response, and the for- mation of supporting wood (Detter and Rust 2018). Since the likelihood of ultimate failure is generally higher for partially uprooted trees in urban situations, visual assess- ments alone may not be sufficient to identify which trees are good candidates for retention. Three out of eight trees in this study that were left stand- ing after the initial winching tests subsequently failed within eight years, while five others recovered most of their original stability during this period of time. Tree pull- ing tests can be used to effectively determine a conserva- tive estimate of a tree’s ability to withstand strong wind events. The pulling test results for tree #278 in 2013 showed an exceptionally high loss of rotational stiffness. This loss of stability could have been detected with a pull- ing test during a level three tree risk assessment (Smiley et al. 2011) and mitigation could have been prescribed if a target would be affected by a failure. Since this tree failed between 2013 and 2018, the nondestructive pulling test had correctly identified its weakness. Finally, the study shows that some trees can survive partial uprooting, presumably by correcting their growth direction, formation of supporting wood, regrowing roots, and thus eventually restabilizing after a period of time. The pulling test method can help to measure and quantify this effect nondestructively and could be used in conjunction with preventive guying to help identify and preserve some partially uprooted trees rather than removing them. The remaining trees from treatment group 2 may be retested at a later date and eventually harvested to study the strategies of morphological adaptation within their stems and the actual mechanisms involved in the recovery of anchorage strength. LITERATURE CITED Archer, R.R. 1987. On the origin of growth stresses in trees part 1: Micro mechanics of the developing cambial cell wall. Wood Science and Technology 21(2): 139-154. 285 Archer, R.R. 1989. On the origin of growth stresses in trees part 2: Stresses generated in a tissue of developing cells. Wood Science and Technology 23(4): 311-322. Bergeron, C., J.C. Ruel, J.G. Élie, and S.J. Mitchell. 2009. Root anchorage and stem strength of black spruce (Picea mariana) trees in regular and irregular stands. Forestry 82(1): 29-56. Berthier, S., and A. Stokes. 2006. Righting response of artificially inclined maritime pine (Pinus pinaster) saplings to wind loading. Tree Physiology 26(1): 73-9. Blackwell, P.G., K. Rennolls, and M. Coutts. 1990. A root anchorage model for shallowly rooted Sitka spruce. Forestry 63: 73-91. Bonnesoeur, V., T. Constant, B. Moulia, and M. Fournier. 2016. Forest trees filter chronic wind-signals to acclimate to high winds. The New Phytologist 210(3): 850-860. Brudi, E., and P.J.E. van Wassenaer. 2002. Trees and statics: Nondestructive failure analysis. pp. 53-70. In: Tree Structure and Mechanics Conference Proceedings: How Trees Stand Up and Fall Down. International Society of Arboriculture, Champaign, Illinois, U.S.A. Buza, Á.K., F. Divós. 2016. Root stability evaluation with non- destructive techniques. Acta Silvatica et Lignaria Hungarica 12(2): 125-134. Cheng, C. S., E. Lopes., C. Fu, and Z. Huang. 2013. Possible impacts of climate change on wind gusts under downscaled future climate conditions: Updated for Canada. Journal of Climate 27(3): 1255-1270. Coutand, C., M. Fournier, and B. Moulia. 2007. The gravitropic response of poplar trunks: Key roles of prestressed wood regulation and the relative kinetics of cambial growth versus wood maturation. Plant Physiology 144(2): 1166-80. Coutts, M.P. 1983. Root architecture and tree stability. Plant and Soil 71: 171-188. Crook, M.J., and A.R. Ennos. 1996. The anchorage mechanics of deep rooted larch, Larix europea × Larix japonica. Journal of Experimental Botany 47(303): 1509-1517. Dahle, G., K. James, B. Kane, J. Grabosky, and A. Detter. 2017. A review of factors that affect the static load-bearing capacity of urban trees. Arboriculture & Urban Forestry 43(3): 89-106. Detter, A., and S. Rust. 2013. Aktuelle Untersuchungsergebnisse zu Zugversuchen. pp. 87-100. In: Dujesiefken (Ed.). Jahrbuch der Baumpflege. Braunschweig, Haymarket Media GmbH & Co. KG. Detter, A., and S. Rust. 2018. Grundlagen und Kriterien zur visuellen Beurteilung der Standsicherheit von Bäumen. pp. 145-160 In: Dujesiefken (Ed.). Jahrbuch der Baumpflege. Braunschweig, Haymarket Media GmbH & Co. KG. Detter, A., C. Cowell, L. McKeown, and P. Howard. 2008. Evaluation of Current Rigging and Dismantling Practices Used in Arboriculture (Research Report No. RR668). Norwich, United Kingdom. 361 pp. Du, S., and F. Yamamoto. 2007. An overview of the biology of reaction wood formation. J. Integr.Plant Biol. 49:131-143 Dunster, J.A., E.T. Smiley, N.P. Matheny, S. Lilly, and Interna- tional Society of Arboriculture. 2013. Tree Risk Assessment Manual. International Society of Arboriculture, Champaign, Illinois, U.S.A. ©2019 International Society of Arboriculture
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