58 Moore: Wind-Thrown Trees: Storms or Management? area, still contribute substantially to tree anchor- age, as they bind closely to the soil and consolidate the root plate, increasing its mass (Tobin et al. 2007) While it is clear that all components of a tree’s root Figure 3. Descending or sinker roots typical of urban tree root systems (modified from Watson and Neely 1994). Depending on the species, soil type, and soil con- ditions, descending roots may be more, or less pro- lific, and it is possible that not all tree species develop them or that some species fail to develop them in certain soil conditions (Nielsen 2009). Descending roots closer to the trunk also have a tendency to grow to greater depths in the soil than descending roots farther from the trunk that are smaller in diameter and shallower in their descent. While the lateral roots are oſten within 200–300 mm of the soil sur- face, descending roots may grow to depths of 1000 mm or more (Jacobs 1955; Kozlowski 1971). They persist for a number of years and at maturity may be 100–150 mm in diameter before they die back and are replaced (Moore 1995; Smith and Moore 1997). Both the root plate and the descending roots appear to be important in tree stability (Moore 2008). Nielsen (2009) notes that arborists and foresters describe root systems differently, and so do botanists. Esau (1965) considers that most trees develop a tap root from which lateral roots branch, and consistent with Tobin et al. (2007), the tap root can be considered the first of a number of orders of roots, with the main lateral roots oſten being second-order roots that then persist for the life of the tree and can be described as structural roots when they become woody (Tobin et al. 2007; Nielsen 2009). Arborists oſten describe roots as coarse and fine, which is oſten associated with their assumed functions of fine roots absorbing and coarse roots providing a mechanical role in transport and anchorage (Tobin et al. 2007). Structural roots are important to tree stability, and it is one of the aims of tree protection regulation for development sites to protect them as part of the critical root zone (Matheny and Clark 1998: Anonymous 2009). However, fine roots, because of their large numbers and surface ©2014 International Society of Arboriculture system contribute to its stability (Tobin et al. 2007), the two major components of the root system that contributed to anchorage are the resistance of lee- ward roots to bending (25%) and the resistance of tap roots and descending roots to uprooting (75%) (Crook and Ennos 1996). About 92% of lateral roots have descending roots in close proximity (within 300 mm) to the trunk, and the depth to which these penetrate depends on soil conditions, but on average they were still about 35 mm in diameter at depths of 230 mm in sandy soils (Crook and Ennos 1996). The most important component of the root system in resisting windthrow is the windward side of the root system, which is pulled upwards during overturning (Coutts 1982; Coutts 1986; Stokes and Mattheck 1996). Tree stability is enhanced when external loading forces are smoothly and rapidly dis- sipated (Stokes and Mattheck 1996), which is best achieved by a large surface area with a higher branch- ing density to which branched descending roots contribute. Tap roots are close to the center of rota- tion when a tree is wind thrown, while descending roots are better orientated than horizontal windward roots to resist uprooting (Crook and Ennos 1996). During a windthrow event, the leeward lateral roots bend and eventually break, oſten close to their base near the root crown; the windward lateral roots are pulled from the soil, oſten with their descending roots, if present, intact and the tap root or one, or more, of the larger descending roots closest to the cen- ter of the tree trunk rotate (Crook and Ennos 1996). If the leeward lateral roots break farther out from the root crown, then the longer fulcrum means that an even greater force has been applied. When inspect- ing wind-thrown trees, the lack of descending roots on the exposed windward side of the root system is oſten an indication that they have not been present (Table 1), which may be important in diagnosing the causes of the failure. The pattern of windthrow is similar in dry and wet soils, but in the latter, failure usually occurs closer to the trunk (Figure 4). Other professions have noted the importance of descending roots in stabilizing slopes, with deeper rooted trees stabilizing slopes only to the depth to which descend- ing roots can penetrate (Gray and Sotir 1996).
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