266 this analysis because it was the most represented genus in the tipped/partially tipped damage category (52% of all damaged trees). The genus of the tree was also significant (P < 0.0001), even after adjusting for the average dbh of each group. Compared with linden/basswood (Tilia), ash (Fraxinus) had a decrease in odds of root failure of 0.94 times (95% CI: 0.73, 1.21); maple (Acer) of 0.47 (95% CI: 0.29, 0.77); elm (Ulmus) of 0.39 (95% CI: 0.22, 0.69); and “other gen- era” of 0.22 (95% CI: 0.11, 0.46). The influence of genera was most notable when considered in light of sidewalk replacement. For illustration, ash had a 10.0% chance of root failure when no replacement work was done, com- pared with a 20.0% chance when it was done; for maple these were 5.3% and 11.1%, respectively; for elm, 4.4% and 9.4%; and for other genera, 2.6% and 5.6%. It must be noted that Tilia and Fraxinus accounted for 89% of all sur- veyed genera (52% and 37%, respectively). Therefore, overinterpretation of the effects of other genera on root failures and subsequent tips or partial tips should be avoided. Boulevard width was found to have a significant inter- action with replacement work (P = 0.011). When work was done, an increase in boulevard width of 1.42 times (one standard deviation) reduced the odds of root failure by 0.64 (95% CI: 0.49, 0.84; P = 0.001). For illustration, two oth- erwise average Tilia on streets with widths of 4.0 feet (1.2 m) and 8.0 feet (2.4 m) have a 29.4% and 14.6% chance of failure, respectively, when work is done. However, when no replacement work was done, boulevard width was not significant (P = 0.50). Dbh was also found to have a significant interaction with replacement work (P = 0.008). In this case, when no replacement work was done, an increase in dbh of 6.77 inches (17.19 cm) increased the odds of root failure by 1.27 times (95% CI: 1.08, 1.51; P =0.005). So two other- wise average Tilia with dbh of 8.2 inches (20.8 cm) and 21.7 inches (55.1 cm) have 8.5% and 13.2% chance of tip- ping or partially tipping, respectively, when no work was done. When replacement work was done, dbh was not sig- nificant (P = 0.29). Penetrometer measurements at 6.0 inches (15.2 cm) deep also had a significant interaction with replacement work (P = 0.019). So two otherwise average Tilia with pen- etrometer readings of 204 foot-pounds (low) and 516 foot- pounds (moderate to high) have a 9.7% and 11.6% chance of failure when no work was done, but a 24.1% and 18.3% chance of failure when work was done. Percent organic matter (P = 0.94), percent sand (P = 0.55, and percent silt (P = 0.38) were not found to be sig- nificant variables for tipped or partially tipped tree incidences. Johnson et al.: Boulevard Tree Failures DISCUSSION Several factors were found to be consistent with tree tips or partial tips in public landscapes, primarily boulevards during wind loading events. Based on these studies—one being a long-term collection of data from a variety of storms, wind speeds, and site factors, and the other an in-depth assessment of one storm on one day in one city— several common “storm damage triangle” factors were identified. The most common factors that were associated with tree failures were as follows. Roots severed due to sidewalk repair was the most influential tree defect factor related to root failures (P = 0.0001) in the case of the 2013 Minneapolis case study. Regardless of genera, trees were 2.24 times more likely to tip or partially tip compared to those trees not exposed to root losses due to sidewalk repairs. This is consistent with data reported in a similar study in Australia (Moore 2014), where it was observed that root severance due to construc- tion activities (among other activities) was highly coincident to windthrows of mature trees during wind loading events. Trees in narrow boulevards (4 feet [1.2 m] wide com- pared to 8 feet [2.4 m] wide) were more likely to fail when sidewalk repair severed roots. In the 2013 Minneapolis case study, this relationship became significant (P = 0.0001) when combined with sidewalk repair activities that included root severance. Both studies revealed that wider boulevards (greater than 4 feet [1.2 m]) had lower frequencies of wind- throws or whole tree failures (in the case of the 1995— 2005 study). Other variables such as tree size and soil compaction were inconsistently associated with tree failures. In the 1995—2005 study, size did matter, with the majority of the whole tree failures in boulevards, including windthrows, represented by trees with dbh values of 20 inches (50.8 cm) or greater (42% of the incidences). The relationship between size and failures with the June 21, 2013 study was weaker and only significant as a site factor when sidewalk repairs had not taken place. When sidewalk repairs and associated root losses had occurred, dbh was no longer a significant factor. Soil compaction in the upper rhizosphere (top 6.0 in [15.2 cm]) was a factor in the frequency of windthrows only when combined with sidewalk replacement activities. As opposed to soil compaction, though, other soil proper- ties such as organic matter percentage and soil texture were not influential site factors. Soil compaction alone has been noted in several research summaries as an inhibiting factor for “normal” root development, producing root systems that are abnormally shallow and theoretically more vulner- able to damage (Alberty et al. 1984; Gillman 1990; Day et al. 2000; Nielson et al. 2009; Day et al. 2010). However, there have been no similar studies that have shown boule- vard soils or any urban soils to be homogenous and ©2019 International Society of Arboriculture
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