Arboriculture & Urban Forestry 45(6): November 2019 predictable in terms of compaction, soil texture, or organic matter. So as much as the literature has identified soil com- paction as a “litmus test” for the extent of tree root systems, the reality of using compaction alone as a predictor or a site factor for tree failures during loading events is tenuous at best. The same caution can be extended to soil texture. Although it is logical and evidence suggests that soil tex- tures do in fact correlate to tree stability (Koisumi et al. 2007), the heterogeneous nature of urbanized soils does not favor this as a predictable site factor. In retrospect, there are three factors that could have either been modified or added to make this data and subse- quent conclusions even more robust. First, tracking the infrastructure repair was arbitrarily set at any activities within five years of the storm failures. That may or may not have been an accurate or best practices decision. Perhaps a closer look at more frequent intervals would reveal other conclusions, or the same could be said for extending the interval to perhaps ten years. The second factor is percolation rate: the rate that soil moisture drains vertically. There is good documentation that saturated soils leave trees more vulnerable to wind throws, either partial or full (Day 1950; Munishi and Chamshama 1994; Steil et al. 2009). In the case of the June 13, 2013 event study, it would have been most advantageous to have run the percolation tests immediately after the rain event that accompanied the winds. It is a time-consuming proce- dure but could reveal even more conclusions from events such as those that combine significant rainfalls with wind loading events. The third factor is determining what other construction or reconstruction activities had taken place below the land- scape surface and out of sight. As generous as the Department of Public Works was with this study, data for the timing and extent of buried utilities is overwhelming and compli- cated and not always readily available to assess. However, it is logical to assume that at least some of those activities took place within the five previous years before the storm struck and involved the loss of structural roots to some degree. Alternatives to Falling Trees As noted in the beginning of this manuscript, the value of collected and interpreted data in urban forestry research is to develop predictive management tools that will moderate losses during these events. To that end, there are some management recommendations that could be considered, based on this interpretation of these two studies. Wider growing spaces are better. Larger trees provide more valuable canopy and require more substantial foot- prints for stability. For boulevards to successfully support those larger trees, a minimum width of 8 to 10 feet (2.4 to 3 m) should be provided. Make better decisions on tree preservation during infra- structure repair projects. Not all trees deserve to be preserved, 267 especially those that are large and will certainly suffer inevitable root loss during the repair of infrastructure, or those trees that are in poor condition and are already posing unacceptable levels of risk. Those trees are hazardous and further expose those sites with unacceptable levels of risk. There are creative measures to resolve unacceptable infrastructure situations, such as lifted sidewalks that cre- ate tripping hazards, requiring the replacement of sidewalk panels. The problem is not the tree or the pavement; the problem is the trip hazard. Trip hazards can be remedied by ramping sidewalk panels with asphalt or other materials that eliminate the hazards that lifted sidewalks present. Panels can be lifted by hydraulic pressure to line up surfaces and the void can be filled with materials to support the panel or panels in place. Sidewalk shaving is a technique where irregularities in panel heights can be eliminated by grinding down the high points, thereby eliminating the trip hazards. Redirect new sidewalk panels well away from tree roots or trunk flares at ground line. This may involve curving the sidewalk out several feet to avoid damaging structural roots, but most communities have sufficient rights-of-way areas where the sidewalks are located to use this as an alternative for preserving valuable, large trees. LITERATURE CITED Alberty, C.A., H.M. Pellett, and D.H. Taylor. 1984. Characteri- zation of soil compaction at construction sites and woody plant response. Journal of Environmental Horticulture 2(2): 48-53. Costello, L.R, J.D. MacDonald, and K.A. Jacobs. 1991. Soil aer- ation and tree health: Correlating soil oxygen measurements with the decline of established Oaks. pp. 295-299. In: R. B. Standiford (Tech coord.). Proceedings of the Symposium on Oaks, Woodlands, and Hardwood Rangeland Management; October 31—November 2, 1990; Davis, California. Gen. Tech. Rep. PSW-GTR—126. Berkeley, CA; Pacific Southwest Research Station, Forest Service, US Department of Agriculture. Day, W.R. 1950. The soil conditions which determine windthrow in forests. Forestry 23: 90-95. Day, S.D., J.R. Seiler, and N. Persaud. 2000. A comparison of root growth dynamics of silver maple and flowering dogwood in compacted soil at differing soil water contents. Tree Physiology 20(4): 257-263. Day, S.D., P.E. Wiseman, S.B. Dickinson, and J.R. Harris. 2010. Contemporary concepts of root system architecture of urban trees. Arboriculture & Urban Forestry 36(4): 149-156. Gilman, E.F. 1990. Tree root growth and development. I. Form, spread, depth and periodicity. Journal of Environmental Hor- ticulture 8(4): 215-220. Harris, R.W, J.R. Clark, and N.P. Matheny. 2004. Arboriculture: Integrated Management of Landscape Trees, Shrubs, and Vines (4th Edition). Prentice Hall, Upper Saddle River, New Jersey, U.S.A. Kamimura, K., K. Kitagawa, S. Saito, H. Yazawa, T. Kajikawa, and H. Mizunaga. 2009. Root anchorage under the combined condition of wind pressure and intensive rainfall: Tree-pulling experiments with controlled soil water content. pp. 149-152. ©2019 International Society of Arboriculture
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