60 Table 2. Major factors affecting tree stability (after Coder 2010). Factor Soil Windward roots Mass of tree Leeward roots Root plate Attributes to resist windthrow Soil must resist fracture and remain dryer than its plastic limit Longest 2–3 major windward roots must resist pulling out and breaking in tension; they must resist snapping in sheer Weight of the tree, including both aboveground mass and root plate mass, must be sufficiently great Leeward roots must resist buckling or hinging in compression and snapping in sheer Stem base and large roots must provide a wide stiff supporting platform that resists splitting Kozlowski (1971) described Camellia thea as hav- ing most of its feeding roots in the top 900 mm, but with deep roots that ramified through a larger volume of soil. The roots of Jarrah, Eucalyptus marginata, penetrate through a layer of bauxite oſten 5–8 m thick, but in some instances up to 15 m deep (Stone and Kalisz 1991) and then develop a spreading, lateral root system below the bauxite. Eucalypt roots have been observed at depths of 45–60 m (Stone and Kalisz 1991), coming through the ceilings of caves, especially in limestone-based soils. In Banksia prionotes, the typical pattern of root development consists of a persistent and dominant sinker root that penetrates 2–3 m into the sandy soil to extract water, a series of lateral roots that are usu- ally in the top 700 mm of the soil, and fine roots that are dimorphic in both anatomy and function with proteoid roots absorbing nutrients while other fine roots absorb water (Jeschke and Pate 1995). In more typical, natural soil profiles, descending roots pen- etrate to depths of 1.5–3 m (Stone and Kalisz 1991). Trees can be wind-thrown in very strong winds (Table 2), especially when heavy rain has saturated soils, reducing soil strength (Harris 1992; Smiley et al. 1998) (Figure 6). Waterlogged soil may result in the wind throw of a tree, in which the windward root system is exposed more or less intact (Table 1) with descending roots in place as they slip from the weakened soil (Crook and Ennos 1996). Such a situation may see a tree wind thrown even with- out heavy rain, because the soil in the vicinity of the base of the tree has lost strength due to excess water pooling due to poor drainage or altered sub- terranean water flows. The combination of heavy ©2014 International Society of Arboriculture Moore: Wind-Thrown Trees: Storms or Management? rain that saturates soil (reducing the strength of the connection between soil and tree roots) that is followed by strong winds may see trees fail in both urban and forestry situations (Coder 2010). However, even then the wind-thrown tree is usually the exception rather than the rule. In the urban context, both tap and descend- ing root development can be restricted by plant propagation techniques that horizontally cut roots when seedlings are removed from germina- tion trays or pricked out and potted on (Moore 1985; Nielsen 2009). As they mature, such trees may never develop a tap root, and the number of descending roots that these trees develop may be lower than those on forest trees of the same spe- cies (Nielsen 2009). Urban landscape management practices, which damage lateral roots, particu- larly on the windward side of the tree, could leave a tree vulnerable to windthrow, especially if the roots are damaged or severed close to the trunk, which could affect the number of descending roots on the windward side of the tree (Coutts 1982; Coutts 1986; Stokes and Mattheck 1996). Figure 6. A fallen elm (Ulmus spp.) in a prominent Melbourne park with waterlogging, lack of descending roots, shallow root plate, lateral root damage, and paved surface in evidence. Table 3. Criteria used in assessment of wind-thrown trees in Melbourne (modified from Moore 2004). Criteria 1 2 3 4 5 6 7 Evidence of site or trenching works within four meters of trunk Significant damage and/or decay to exposed lateral roots Evidence of the loss of descending (sinker or vertical) roots Evidence of soil compaction in immediate vicinity of the trunk Presence of fill around base of tree Indicators of waterlogging in immediate vicinity of the trunk Canopy dieback and deadwood
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