Arboriculture & Urban Forestry 45(6): November 2019 265 all. Each variable was considered separately; only the two penetrometer reading terms were significant. Since they gave similar model fits and are known to be correlated, the penetrometer readings at 6 inch (15 cm) depth were chosen for inclusion, as then the additional binary variable was unneeded. The final model included replacement work and genera group, as well as boulevard width, dbh, the pene- trometer reading at 6 inch (15 cm) depth, and their interac- tions with replacement work. Analysis was focused on which variables (e.g., tree spe- cies, boulevard width, sidewalk improvement) were asso- ciated with root failure. A generalized logistics mixed modeling was used with root failure as the response and BSS as a random effect. All calculations were performed in R version 3.0.1 using the lme4 package. RESULTS Wind Loading Event Damage in Metropolitan Minnesota, 1995–2005 Only data relevant to the topic of this study, i.e., tipped or partially tipped trees in boulevard landscapes, is itemized for discussion. Of 1,584 sampled trees, 54% were catego- rized as whole tree losses. Of the whole tree losses, 24% occurred on boulevards, described as the space between the street curb and the public sidewalk. Of the whole tree losses on boulevards, 74% were categorized as tips or par- tial tips. Of the whole tree losses on boulevards, 74% were located on boulevards 4.0 feet (1.2 m) in width or less. Of the whole tree losses on boulevards, 42% were in the dbh ranges of 20 inches (51 cm) and greater; 29% were in the dbh range of 6 to 10 inches(15 to 25 cm). Of the whole tree losses on boulevards, the most common genera were Tilia (18%), Fraxinus (15%), and Picea (12%). Wind Loading Event in Minneapolis, Figure 7. An example of a date stamp on sidewalk replacement/ repair. Date is 07-30-12. parsimonious model was chosen to explore further. First, the primary predictors—replacement work, boulevard width, dbh, and genera group—were considered, along with all two-way interactions. Nonsignificant terms were excluded, and the resulting model compared using AIC (Akaike information criterion) with the full model. Sec- ondary predictors, including penetrometer readings (at both 6 and 12 inch [15 and 30.5 cm] depths), and percent organic matter, sand, silt, and clay, were then added to this model, along with interactions with genus group and replacement work done. For penetrometer readings at 12 inch (30.5 cm) depth, an additional binary variable was included to indicate if the reading was able to be made at Minnesota, June 21, 2013 A total of 3,076 trees representing 18 genera and 27 species were surveyed, 367 (11.9%) of which were identified as tips or partial tips. Only genera were used in this evalua- tion, since each genera was often represented by a mix of species and cultivars. Only when these species and culti- vars were grouped into their respective genera were there sufficient numbers to conduct statistical analyses. A total of eight genera had representative trees that tipped or partially tipped during the wind loading event, namely: Abies, Acer, Celtis, Fraxinus, Picea, Quercus, Tilia, and Ulmus. The major finding is that having sidewalk replacement work done increased the odds of root failure by 2.24 times (95% CI: 1.77, 2.83; P < 0.0001). For illustration, when no replacement work was done, the average Tilia had a 10.6% chance of root failure, the precursor to tips or partial tips; this increased to 21.0% when replacement work was done. Tilia is used as the baseline genus for reference throughout ©2019 International Society of Arboriculture
November 2019
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