Arboriculture & Urban Forestry 45(6): November 2019 Arboriculture & Urban Forestry 2019. 45(6):279–288 URBAN FORESTRY ARBORICULTURE Scientific Journal of the International Society of Arboriculture & Stability Recovery in London Plane Trees Eight Years After Primary Anchorage Failure By Andreas Detter, Philip J. E. van Wassenaer, and Steffen Rust Abstract. As the intensity and frequency of strong storms increase, the potential for damage to urban trees also increases. So far, the risk of ulti- mate failure for partially uprooted trees and how they may recover their stability is not well understood. This study sets out to explore if and to what extent trees can regain anchoring strength after their root systems have been overloaded. In 2010, ten London Plane (Platanus × acerifo- lia) trees were subjected to destructive winching tests. Two trees were pulled to the ground while eight were loaded until primary anchorage failure occurred and were left standing with inclined stems. In 2013, two trees had failed and six were re-tested nondestructively. By 2018, another tree had failed, and we tested the remaining five again. Rotational stiffness was derived for all trials and served as a nondestructive proxy for anchoring strength (R² = 0.91). After eight years, one tree had regained its original strength, while four had reached between 71 and 82% of their initial rotational stiffness. However, three trees failed during the observation period. The results indicate that partially uprooted trees may re-establish stability over time, but some will not and may fail. In our small data set, it was not possible to identify visual criteria that could provide a reliable indication of tree stability recovery, but our data support the assumption that nondestructive pulling tests can be suc- cessfully employed to determine good vigorous candidates for retention after partial uprooting. Keywords. Partial Uprooting; Pulling Test Method; Restabilization; Storm Damage; Tree Biomechanics. INTRODUCTION The failure of trees with root systems compromised by decay, storm damage, or construction-related damage can pose risk to significant targets and human beings in an urban setting (cf. Smiley 2008; Bergeron et al. 2009; Schmidlin 2009; Smiley et al. 2014; Dahle et al. 2017) and may also pose a risk to those involved in climbing or dis- mantling trees (Detter et al. 2008). Assessing this structural characteristic of a tree is very difficult. In many cases, when a tree is observed to have significant root issues, the recommendation is to remove the tree. This mitigates risk but also removes the stream of valuable social, environ- mental, and economic benefits that a tree provides (Price 2007; Roloff 2016; Kim and Jin 2018). It also prevents arborists and researchers from studying whether such com- promised trees can recover and regain stability over time. Root systems are complex subterranean structures that direct a major portion of the wind load collected by the crown into the ground. Below-ground damage to structural roots can often occur due to root decay or root severance and may also be caused by overloading during storm events, by snow loads, or even by heavy impacts (e.g., during road accidents or avalanches). As the effects of climate change are felt, many predic- tions indicate that the world will experience more variable and more extreme weather. For example, Cheng et al. (2013) predicted that Canada could receive significantly more wind gusts later in this century and that the magni- tude of those gusts would increase. The effect will be stronger for wind gusts over 70 and 90 km/h, and we can expect that more trees will be damaged or destroyed in high wind events. However, not all trees affected by winds experience ultimate failure. It is quite common that after such wind events, some trees are left standing with a lean. While historically many of these trees are removed, there may be alternative management options. Preserving some of these trees may become more important as we strive to increase canopy cover in urban areas for the sake of the benefits provided to those who live among or in close proximity to trees. Static load tests, as introduced by Sinn and Wessolly (1989), can be effectively utilized to inform tree risk assessments on trees with compromised rooting stability (Smiley et al. 2011; Sani et al. 2012). A tree’s rooting char- acteristics can be assessed by applying a moderate nonde- structive load with a winch, measuring the tree’s reactions ©2019 International Society of Arboriculture 279
November 2019
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