Arboriculture & Urban Forestry 40(2): March 2014 30 mm mark was then confirmed as a reason- able indicator of compacted soil by compari- son with a penetrometer. This, however, remains no more an indicator of likely compaction, as while soils were close to saturated as evidenced by pooling, soil moisture was not quantified. The large number of trees found with dam- aged lateral roots and evidence of the loss of descending roots may not be surprising for urban trees growing in parks, where trees are growing in lawns that are regularly mowed. The mowing regularly shaves parts from the exposed lateral roots and these roots are repeatedly wounded and re-wounded, which may afford entry points for disease- and decay-causing organisms (Ruehle 1973; Shigo 1986). The exposed lateral roots are oſten part of the structural root system, and so if a number of them start to decay, then not only is the tree deprived of significant root absorptive surface area, but also may be more prone to windthrow. The finding that soils are compacted in heavily trafficked areas of a city park is also to be antici- pated. Compaction may be due to pedestrian or vehicular traffic, especially if there is active sport- ing activity, particularly in the summer months, when people congregate under the trees for shade. In parks, compaction occurs when people walk along sight lines, but also when walkers and jog- gers run on turfed and mulched areas of the park, rather than on the harder paved surfaces, to reduce the effects of hard surfaces on ankle and knee joints. Compaction impacts descend- ing and lateral roots by reducing aeration and water penetration. Compaction also increases soil strength, which affects root tip growth. This can lead to reduced root extension and the loss of descending roots, which can affect tree stability. That more than one-third of the wind-thrown trees had fill around their trunks (Table 4) needs further explanation, especially as most of the trees were growing in major metropolitan parks. All of the fallen trees were near footpaths through the parks, and these paths, oſten due to tree root damage to the footpaths, were regularly replaced or maintained for pedestrian safety. The simplest and cheapest way of doing so is to raise the paths and then fill is placed around the tree to maintain site levels. Over the past 100–125 years, Melbourne City Council mainte- nance records reveal that paths have been replaced 63 up to six times, with fill levels exceeding 300–500 mm in some instances. These works not only con- tribute to the waterlogging of soils around trees, by interfering with natural drainage and contours, but they may also alter subterranean water flows. In the storm of 2005, without exception, trees that fell were growing beside roads or pathways in parks and gardens (Figure 8). When the trees were inspected aſter the storm, every tree that fell showed evidence of site works, lateral root dam- age, fill around the base of the tree and loss of descending roots (Figure 9). Most (above 90%) showed evidence of compaction and waterlog- ging, but few showed evidence of canopy die- back or excessive deadwood. Trees growing in the same parks and along the same streets but without root damage or interference may have suffered loss of branches during the storm, but none were wind-thrown. In the parks, many trees of the same species that were growing in garden beds or undisturbed turf remained standing while those growing along paths and near buildings fell. Figure 9. Close-up of the base of one of the fallen elms after the 2005 storm, showing in excess of 300 mm fill around the trunk of the tree. In many situations, there are multiple factors that contribute to root system failures that lead to trees falling (Table 4). The strong wind may be the trigger that initiates windthrow, but there may be other contributing factors to whole-tree failure. It would seem that the failure of trees ascribed to windthrow has as much to do with their history and the management practices to which they had been exposed as it does the strength of the storm winds. This raises questions about the role of tree ©2014 International Society of Arboriculture
March 2014
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