Arboriculture & Urban Forestry 40(2): March 2014 crown reduction to reduce the exposed canopy area will necessarily reduce the load that wind places on a tree, because depending on which branches are removed, the capacity for mass damping may also be diminished (Milne 1991; Moore and Maguire 2005; Moore and Maguire 2008; James 2010). There is a need to determine whether canopy reduction is efficacious in reducing the risk of windthrow. Trees also fail in different places and in various ways due to the presence of fungal diseases, columns of decay, or hollows (Smiley et al. 1998; Mattheck et al. 2003; Mattheck et al. 2006), which are often undetectable until failure occurs. The probability of failure increased in hollow trees, trees with a high slenderness ratio, and trees with cracks, but trunks can be up to 70% hollow before the probability of failure sud- denly increases (Mattheck et al. 2003; Kane and Ryan 2004). In current arboricultural practices, the aim of using pull tests is to provide data for a diagnosis of probable tree stability while the focus of dynamic analysis is to explain the loads and forces involved in whole-tree failure. THE ROLE OF ROOTS IN TREE STABILITY UNDER WIND LOAD Tree Root Development When a tree seed germinates in undisturbed natu- ral soils, the radicle emerges and usually develops into a tap root (Esau 1965). In many species, it is not uncommon to find a seedling of 20 mm height with a primary root of 150–200 mm in length, which de- velops as a tap root, anchoring the young tree, pro- viding a reservoir of carbohydrate and the necessary water and nutrients to facilitate seedling growth. The tap root provides the framework from which lateral roots develop. In most trees species, however, the tap root could be considered a juvenile character- istic that persists for the early establishment phase of the tree’s life cycle (Moore 2013). The tap root, which oſten descends almost vertically, soon reaches soils that are dense and low in oxygen (Peltola 2006) and nutrients. Furthermore, the nutrients in the rhizosphere surrounding the tap root are soon exhausted. Oxygen levels in soils decrease rapidly with depth, which explains why 95% of the absorb- ing roots are so close to the surface and why tap roots stop extending or die. Oſten, the tap root has 57 then served its purpose and dies leaving the spread- ing lateral roots to perform the roles of absorbing nutrients and water and of anchoring the tree. In many urban trees, the propagation techniques of growing trees from cuttings or growing seedlings in shallow seed trays, and then through succes- sively larger containers, oſten means that there is no tap root, even in young trees, which can affect their anchorage (Khuder et al. 2007; Gilman 2013). MATURE ROOT SYSTEMS: THE ROOT PLATE, LATERAL AND DESCENDING ROOTS The root system of mature trees tends to consist of a relatively shallow, spreading root plate (Figure 2), as opposed to a root ball. The root plate consists of the root crown, structural roots and the network of shallow, spreading, absorbing roots that are located close to the soil surface (300–600 mm deep) and oſten spreading well beyond the drip line of the can- opy (Perry 1982). The root plate of lateral, spreading roots is complemented by the presence of descend- ing (or vertical, sinker, or oblique) roots, which tend to occur within the drip line of the tree and are oſten denser closer to the trunk (Figure 3) but can occur anywhere oxygen is more readily available and where nutrients and organic matter are being actively recycled (Coile 1937; Perry 1982; Nielsen 2009). Descending roots occur across the root plate, but there tends to be more of them concentrated around the base of the trunk where they are oſten confused for tap roots, but they are morphologically and ana- tomically distinct from tap roots (Esau 1965; Moore 2008). The descending roots tend to become more important to trees as they mature, particularly in the development of a heavier root plate (Nielsen 2009). Figure 2. The spread and depth of a typical tree root system (Watson and Neely 1994). ©2014 International Society of Arboriculture
March 2014
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