Arboriculture & Urban Forestry 45(1): January 2019 SCIENTIFIC BASIS FOR THE COMPONENTS OF TREE FAILURE Likelihood of Impact and Target Occupancy The presence or absence of one or more targets is considered by some to be the most important factor in a risk assessment (Ellison 2005a). When no target is present, there is no risk (Ellison 2005a; Smiley et al. 2011; Dunster et al. 2013). When evaluating tree risk, it is important to assess each potential target’s level of occupancy, as well as the factors that might affect occupancy (Hayes 2002; Ellison 2005a; Sreetheran et al. 2011). For instance, the presence of pedestrian tar- gets in a park varies greatly depending on the time of day. Similarly, a park bench may attract people to a site and prolong their occupation of an area. When assessing occupancy, it is also important to consider that many outdoor recreational sites may have reduced occupancy levels during storms—the very kinds of conditions that increase a tree’s likelihood of failure (Ellison 2015). The occupancy of a site can be qualified using a standard rating system. Alternatively, it can be quan- tified directly using traffic counters or indirectly by accessing existing traffic count data. In laying out the framework for his tree risk assessment method, Elli- son (2005a) evaluated the occupancy of vehicular tar- gets on different road classes using Great Britain’s 1996 transportation statistics. This method multiplies each traffic count by the time needed for a vehicle to pass by, or come to a complete stop in front of, a given point within the target zone of an assessed tree. For pedestrian count data, Ellison (2005a) suggested multiplying each count by five seconds to gauge occupancy at a given point in a tree’s target zone. In addition to the use of existing traffic data, Elli- son (2005a) noted that target occupancy can be easily quantified with the use of pedestrian and vehicle traf- fic counters. Traffic counters allow the assessor to quantify occupancy rates over time, potentially allow- ing for greater accuracy than visual occupancy assess- ments that are based solely on a short visit to the site (i.e., the time it takes to perform a visual assessment) and professional judgment. In assessing two trees in the United Kingdom, Papastavrou et al. (2010) found estimates of traffic occupancy derived from five-minute surveys were up to three orders of magnitude differ- ent than those derived from the professional judgment of a trained tree assessor. Despite the potential 29 benefits of traffic counters and the availability of pre-existing count data in many municipalities, arbor- ists in North America rely on a quick visual assess- ment of site occupancy when assessing tree risk. These subjective assessments can lead to less accu- rate, biased (given the time of day), and more variable estimates of target occupation (Klein et al. 2016). As an example, Klein et al. (2016), found that about 6 in 10 arborists increased their ratings of target occu- pancy one level when viewing videos of sites filmed during rush hour (as compared to their ratings of the same site filmed during non-peak travel times). Failure Potential Tree failure is defined as the breaking of any root, branch, or stem, or the loss of mechanical support in the roots (Dunster et al. 2013). All trees have some level of failure potential (Brakken 1995; Hayes 2002; Pokorny 2003; James et al. 2006); however, this var- ies by species and the presence or absence of various growth and structural characteristics (Hauer et al. 1993; Meilleur 2006; Kane 2008; Jim and Zhang 2013). Factors that influence failure potential include tree health (Hickman et al. 1995), species (Hauer et al. 1993), growth habit (Hayes 2002), branch attach- ments (Lily and Syndor 1995; Gilman 2003; Meilleur 2006; Kane et al. 2008; Miesbauer et al. 2014), con- dition of roots (Brakken 1995; Smiley et al. 2000; Gilman and Masters 2010), presence of decay (Smi- ley and Fraedrich 1992; Kane et al. 2001; Lonsdale 2007; Smiley 2008), maintenance history (Zhang et al. 2007), adverse weather conditions (Duryea et al. 1996; Duryea and Kampf 2007; Hauer et al. 2011), and changes to a site (Jim and Zhang 2013). The two most common types of tree failures are tipping (i.e., whole-tree failures caused by decayed or severed roots, or defects at the root–soil interface) and frac- tures (i.e., decay and hollows that cause breaking of branches and stems) (Mattheck and Breloer 1999; van Wassenaer and Richardson 2009). Terho and Hallaksela (2005) assessed the poten- tially “hazardous” characteristics of Tilia spp., Betula spp., and Acer spp. in downtown Helsinki City, Fin- land, and found that 50% to 70% of potential failure points in park trees that had been removed were iso- lated to the lower portion of the tree (e.g., roots, root flare, trunk). Similarly, Terho (2009) examined three species of felled trees from Helsinki, Finland, and found that roughly 65% of the trees had decay in the roots and trunk. In looking at volunteer tree failure ©2019 International Society of Arboriculture
January 2019
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