54 strength, and trees cannot grow in size and main- tain structural integrity beyond these limits. The interest in how trees change as they grow has seen various attempts to model both their form and function. One of the more widely used models focused on the tree as a continuously branching hierarchical network, with a system of ratios that scale constantly across the order of segments, such as the trunk, branches and leaves, or the branched system of roots that makeup the plant (Meinzer et al. 2011). The system developed by West, Brown, and Enquist (1999), oſten referred to as the WBE model, has proved particularly useful for the analyses of vascular transport modeling, fluid flows, and the distribution of nutrients and other resources within the tree (Nygren and Pallardy 2008). However, there have been a number of instances where the scaling and the ratios do not accurately apply even to these aspects of tree form and function (Mencuccini 2002; Coomes and Allen 2007; White et al. 2007; Nygren and Pallardy 2008; Veiga 2008; Capes 2009; Meinzer et al. 2011). Oſten, these failures occur as interspecific differ- ences within species when individuals are growing in less than ideal conditions or where there are specific adaptations to stressful environments (Nygren and Pallardy 2008), perhaps because the model does not recognize the importance of competition for resources and its effects on tree growth (Coomes and Allen 2007; Capes 2009). While the WBE model has a clear application to aspects of vascular transport modeling, despite its deficiencies, it also has implications for the overall mass and surface area distributions of a tree (West et al. 1999). For example, the model provided insight to the extent and surface area of a tree’s root system in relation to the size of its canopy, and the notion that there was as much, if not more, of a tree below the ground as above it (West et al. 1999; Meinzer et al. 2011). The model serves a useful purpose in remind- ing arborists that there has to be sufficient root mass and surface area to supply the aboveground parts of the tree with water and nutrients, and that the anchorage of the tree depends on all of the com- ponents of the root system. However, the work of Peltola (1996), which suggested that no tree species cansurvive storm events without damage when mean wind speeds, over a period of 10 minutes exceed 30 ms-1 near the top of the canopy, is still a useful guide. ©2014 International Society of Arboriculture Moore: Wind-Thrown Trees: Storms or Management? Figure 1. Wind-thrown trees in Kew Gardens (UK) after the 1987 storm. Note the closeness of the tree to the pathway and infrastructure. In this paper, windthrow is taken to mean the failure of a whole tree at the interface of the trunk with the soil, which may involve the liſting of roots, the snapping of roots, or the failure of the trunk at the soil surface (Figure 1). In forestry, the term is oſten used to refer to trees that have trunks fail at or above ground level, or branches broken dur- ing storms, as well as whole-tree failure, but this aboveground breakage is sometimes, and prob- ably better, referred to as wind snap (Allen 1992). Significant economic losses in timber forests and plantations due to windthrow and trunk fail- ure have been widely reported (Peltola 1996; Quine and Gardiner 1998; Peltola et al. 1999; Cucchi and Bert 2003; Grace 2003; Zeng et al. 2007). Forest tree failure due to strong wind has led to significant research into the structural properties of trees and their ability to endure the forces of wind. In cities, tree failure can result in injury and property loss and errors in tree assessment by arborists may lead to costly litigation, especially in the United States (Mortimer and Kane 2004; Barrell 2013). The reli- ance of arborists on visual tree assessment (Mat- theck and Breloer 1994) and the issues of liability and court action have led to conservative tree assess- ments and likely recommendations of tree removal, which in turn have led to many unnecessary remov- als of trees with the potential for longer, useful lives. In considering windthrow of trees, most analyses consider two components of tree structure. The first is the aboveground component of trunk, branches, and foliage, which experience the force of the wind
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