Arboriculture & Urban Forestry 40(2): March 2014 Urban construction activities that compact or deposit fill around the base of trees can alter soil aeration, organic matter content, nutrient availability, and water penetration, all of which can have a profound negative affect on tree root systems (Day 1999). Other construction prac- tices that compact the lower soil horizons can make descending root penetration difficult and diminish both the extent and mass of the root plate. Furthermore management practices that alter soil water flows, thereby creating water- logged conditions, can restrict root develop- ment to depths below 200 mm (Coutts 1982; Coutts 1986; Nielsen 2009). The loss of soil strength from greatly increased soil moisture levels further increases the risks of windthrow. Tree protection on development and con- struction sites often has the protection of the structural root zone as an aim, but the more extensive root protection zone protects not only the structural roots, but the lateral and descending roots farther from the trunk (Matheny and Clark 1998: Anonymous 2009). However, while these are admirable attempts at protection they do not guarantee that the root system and root plate will remain intact or retain the stability of the tree. Furthermore, standard protection systems cannot deal with the nuances of every tree and the root systems that develop in response to particular envi- ronments. Many attempts have been made to generalize classification systems describinroot system architecture, but the effects of soil type, soil conditions, and the levels of environmental stress on the development of tree root systems mean that generalizations rarely apply to trees growing in stressful urban sites (Stone and Kalisz 1991; Tobin et al. 2007; Nielsen 2009). Storm events, DATA FROM SITE INSPECTIONS OF WIND-THROWN URBAN TREES especially those involving heavy rain and strong winds, oſten prove to be very busy times for arborists, with the cleanup of wind- thrown and damaged trees. There is also an in- crease in service calls for routine maintenance and other arboricultural interventions, as public awareness of the trees in their cities rises in the aſtermath of a storm event. These occasions also Figure 7. A fallen elm with a shallow root plate growing near a pathway; windward roots pulled from the soil, lack of descending roots, and a pool of water under the base of the tree two days after the major storm event of 2005. The tree has hinged close to the trunk on the leeward side. 61 provide opportunities to observe wind-thrown trees that have been growing in different loca- tions and soil types, to record information relevant to the trees and the sites, and to discern patterns that might be common to the fallen specimens. Data on 80 large wind-thrown trees from eight different genera were collected from site inspec- tions across the City of Melbourne, Victoria, Australia, over 20 years. Most of the specimens were mature, but none had been characterized as senescent before they failed (Moore 1998; Moore 2004). A set of seven criteria were devel- oped to assess trees after failure (Table 3) that allowed for data collection by rapid qualita- tive visual assessments. The data include 30 windthrown specimens when Melbourne suffered a 1 in 150-year storm event in February 2005. After a thirteen-year period of below-average rainfall, 120 mm of rain fell in less than 30 hours, most of it in a 10-hour period overnight with very strong, gusty winds. There was significant property and infrastructure damage, and sev- eral hundred mature trees were wind thrown in parks, gardens, and along streets across the city (Figure 7; Figure 8). The media coverage— radio, TV, and newspaper—was quick to report that the significant loss of trees was due to an act of God (insurance companies accepted claims on that basis) or the fury of Nature. The heavy rain had reduced soil strength and the winds were strong; however, the pattern of wind-thrown trees across the city suggested that other fac- tors contributed to the failure of these trees. ©2014 International Society of Arboriculture
March 2014
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