280 with a high-precision inclinometer, and extrapolating those data to determine the minimum strength of the root system (Wessolly 1996; Detter and Rust 2013; Buza and Divós 2016). Estimations of resistance to uprooting are based on comparing this load capacity of the root system with mod- elled wind loading scenarios for a tree at its actual location, as informed by statistical wind data and local wind condi- tions (Brudi and van Wassenaer 2002; van Wassenaer and Richardson 2009; Wessolly and Erb 2016; Esche et al. 2018). The anchorage of trees has been studied in many scien- tific experiments (cf. for an overview Dahle et al. 2017) and was modelled by several authors (e.g., Dupuy et al. 2005; Rahardjo et al. 2014). Tree uprooting is often described as a progressive failure process that occurs in different stages (O’Sullivan and Ritchie 1993), where a number of components play different roles (Coutts 1983; Blackwell et al. 1990; Nielsen 1991). When the change in stem base inclination does not exceed 0.5° during pulling tests, the process is reversible and nondestructive (Coutts 1983; James et al. 2013). As the stem base inclination increases, the maximum resistance of the root system will be overcome at angles between roughly 2° and 7°; after that point the load applied during the pulling test will decrease as the root-soil matrix progressively fails (e.g., Coutts 1983; Wessolly 1996; England et al. 2000; Jonsson et al. 2006; Vanomsen 2006; Lundström et al. 2007). Such excessive root plate tilt is likely to cause damage by bending and breaking roots on the leeward side close to the stem and by lifting the windward side of the rootplate, causing horizontal and vertical cracks in the soil as well as bending and ultimately the breaking of roots in tension (Crook and Ennos 1996). When a severe storm partially uproots a vigorous tree, some roots may still be able to retain their water transport function (Ueda and Shibata 2004). Since living wood is weaker in compression, bend- ing failure is initiated by fibre buckling on the compression side (Niklas and Spatz 2014). This fibre buckling may eventually interrupt water transport. However, the fibres on the tension side of mechanically compromised roots and roots less stressed during such catastrophic events may fully retain their water conductivity. If such a tree is left leaning after the primary anchorage failure, it will usually adapt the orientation of its terminal shoots (Du and Yamamotu 2007) through the formation of tension or compression wood (Archer 1987; Archer 1989). Significant changes in the curvature of the shoot by exten- sion or contraction of the wood tissues has only been observed on stems up to 10 cm in diameter (Berthier and Stokes 2006; Yamashita et al. 2007). It is unlikely that sig- nificant changes in shoot curvature will occur on stems much larger than that due to the rapid rise in flexural stiff- ness with increasing diameter (Fobo and Blum 1985; Coutand et al. 2007). ©2019 International Society of Arboriculture Detter et al.: Tree Stability After Anchorage Failure Furthermore, trees typically respond to a lean by initiat- ing strong increment growth on the side of the stem base in compression from the gravitational loads, which is usually referred to as supporting wood (Götz 2000; Mattheck et al. 2003; Detter and Rust 2018). During wind-induced uproot- ing, the greatest strains will also occur in the area under compression on the leeward stem base (Stokes 1999). Trees are able to increase increment growth in areas with greater strains (Müller et al. 2006; Larjavaara and Muller-Landau 2010). Finite elements modelling has shown that the addition of wood volume on the compres- sion side of the stem base can be most effective at increas- ing stability (Yang et al. 2017). The formation of supporting wood at the stem base may be, among others, one mecha- nism of stability recovery. Adaptive increment growth is stimulated by a change in the loads that trees experience (Bonnesoeur et al. 2016). For example, healthy forest trees have been found to regain their former stability after a thinning cut within five to eight years (Mitchell 2000). Similarly, after the transplant of both small and large trees, the original root system size could be restored within five to thirteen years (Watson 1985) or sooner (Watson 2005). The effect of root sever- ance on tree stability depends on the distance of the dam- age from the stem (Smiley et al. 2014), but young trees can recover their anchoring strength as soon as four years after the root severance occurs (Fini et al. 2012). Our assumption is that trees can recover their anchoring strength within eight to ten years after primary anchorage failure. Experimental data and quantification of stability recovery following overloading of the root system are lacking in the literature. The study presented here provides such data. The degree of root stability recovery after partial uprooting was quantified over a period of eight years. MATERIAL AND METHODS All of the research trials described in this paper were undertaken at the Davey Tree research site in Shalersville Township, Ohio, U.S.A., in a plot with London Plane (Pla- tanus × acerifolia) trees. The trees were planted between 1968 and 1970 on Ravenna silt loam. The trials were undertaken in three separate field seasons in 2010, 2013, and 2018. Table 1 summarizes the trees used in the three test series and Table 2 lists their average diameter and height. The initial research trial was undertaken in 2010. Ten trees with similar diameters at 1 m height, crown shape, and wind exposure were selected for the trial and were pulled until primary anchorage failure occurred. For this project, primary anchorage failure was described as the point during load application (i.e., winching) where the inclination would continue increasing without any further increase in the applied force. This trial could be described as a destructive test since the winching force was applied
November 2019
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