Arboriculture & Urban Forestry 45(6): November 2019 The third factor is probably the most complicated since it includes both the biology and the structure of a living organism, the tree, as well as the location of the failure due to the wind loading event. Failures in the canopy are con- sistently linked to common defects and tree architecture in multiple assessment protocols (Pokorny et al. 2003; Harris et al. 2004; Smiley et al. 2011). Branch attachments, degrees of codominance and location of codominants, dead branches, presence and location of decay, live crown ratio (LCR), and canopy symmetry and density all play roles in a tree’s vulnerability to wind loading events that result in damage to the canopy. Likewise, failures along the tree trunk line are generally associated with decay, cavities and open cav- ities, and more specifically, strength loss. Additionally, those factors that increase the probability of whole tree fail- ures are associated with roots: dysfunctional root systems such as stem-girdling roots (SGRs), confined root systems, severed root plates, atypically shallow root systems, and deeply buried roots, all of which are exacerbated by trees with very dense or asymmetrical canopies, or excessive leans. Tree Root Interactions with Infrastructure Trees are not always innocent victims of tree vs. infrastruc- ture conflicts. Damage to gray infrastructure (sidewalks, streets, curbs, buried utilities) by tree roots can range from moderate to significant in terms of repair costs (McPherson and Peper 1996; Stål and Rolf 1998). In at least one pub- lished seminal research article on the topic (McPherson and Peper 1996), sidewalks bore the brunt of the damage compared to the other utilities. Wang et al. (1988), docu- mented street tree and sidewalk conflicts in Manchester, UK, noting that of the 2,232 street trees observed, 30% were causing unacceptable damage to paved surfaces. The underlying causes for these conflicts, damage to paved sidewalks, and eventual repair to the infrastructure, go beyond a tree root vs. pavement scenario. Boulevard width has been documented as a factor, with boulevards narrower than (approximately) 10 feet (3 m) experiencing significantly more pavement damage than those 10 feet (3 m) or wider (Wagar and Barker 1983; Wang et al. 1988). Specific to tree trunk and root architecture, North et al. (2015) demonstrated that the trunk flare diameter at ground line was critical to predicting whether there would be tree and sidewalk conflicts. Using their formula for establish- ing a minimum boulevard width to accommodate a 20 inch (51 cm) dbh silver maple (Acer saccharinum L.), a width of 10 feet (3 m) would accommodate the trunk flare at ground line and leave an open space of (approximately) 4.0 feet (1.2 m) between the trunk flare and the parallel side- walk and street curb. Additionally, tree species and size have also been revealed as damage to infrastructure factors (Wagar and Barker 1983; Wang et al. 1988; Nicoll and 261 Armstrong 1998; North et al. 2015) although the relative rankings of examined trees were not always in agreement. What was agreed upon was that larger trees, especially faster growing larger trees, were often the main arboreal offenders. The two studies detailed in this analysis of tree failures during wind loading events focused on street tree popula- tions in multiple communities throughout the state of Min- nesota. One study was a multiyear, multicommunity assessment of all types of damage to trees due to wind loading events, categorized as either damage confined to tree canopies, failures along tree trunks, or complete tree failures at or below the ground line. The second study closely detailed one day of wind loading events in one community that resulted in the full or partial windthrow of a significant number of street trees. The intent of both stud- ies was to determine what if any pre-existing factors could be connected to the frequency and type of damage to street trees during wind loading events. MATERIALS AND METHODS Wind Loading Event Damage in Metropolitan Minnesota, 1995–2005 Due to the complexity of various types of wind loading events, this study addressed only those events that were not complicated by multi-directional wind forces (rotations) such as downbursts and derechos (NWS/NOAA 2018). Those wind loading events that were included were thun- derstorms, straight-line winds, and gust fronts; these events comprise the majority of wind loading events that impact urban trees in North America. Data collected on damage to urban trees following wind loading events was confined to communities in the 11 county metropolitan area of Minneapolis/Saint Paul, Min- nesota. Immediately following a wind loading event, the storm type was confirmed with the National Weather Ser- vice Forecast Office—Twin Cities, Chanhassen, Minne- sota. If the wind loading event was identified as one that included wind rotations, the sites were visited by the prin- ciple investigator from the University of Minnesota’s Department of Forest Resources, and the path of the storm was located on a street map. All surveys of damaged trees were conducted beyond the borders of the rotation paths, extending to the points where damage to trees on public properties was no longer evident. Collection of data was conducted by the principle inves- tigator, research fellows in the Urban Forestry Outreach Research and Extension lab, graduate students in the Department of Forest Resources, and trained citizen scien- tists (Tree Care Advisors). All parties conducting the dam- age surveys were trained to observe and collect data in the same manner. Most data collection was conducted by teams ©2019 International Society of Arboriculture
November 2019
| Title Name |
Pages |
Delete |
Url |
| Empty |
Search Text Block
Page #page_num
#doc_title
Hi $receivername|$receiveremail,
$sendername|$senderemail wrote these comments for you:
$message
$sendername|$senderemail would like for you to view the following digital edition.
Please click on the page below to be directed to the digital edition:
$thumbnail$pagenum
$link$pagenum
Your form submission was a success. You will be contacted by Washington Gas with follow-up information regarding your request.
This process might take longer please wait
