Arboriculture & Urban Forestry 40(4): July 2014 Summer 1 and 2 for excurrent trees. Trees in win- ter (leafless) were 70% less damped than summer (in-leaf), which is consistent with reductions in ζ of leafless trees for Bradford pear (75%; Kane and James 2011) and London planetree (Platanus) (90%; Roodbaraky et al. 1994). Sellier and Fourcaud (2005) reported that foliage was the major source of damping of Pinus pinaster saplings. The reduc- tion in damping when trees are leafless may be of little consequence if drag forces are the primary cause of damage to trees because leaf area is strongly correlated to drag forces (Kane and Smiley 2006). Computer modeling showed an inverse rela- tionship between ζ and f for Pinus pinaster (Sellier and Fourcaud 2009). The authors suggest that tree characteristics that would cause a tree to have a lower sway frequency would also cause a tree to be more damped, thereby offsetting the increased risk of wind-induced damage. Based on field mea- surements, Bruchert and Gardiner (2006) reported an inverse relationship between ζ and f for forest- grown Picea sitchensis trees that followed a longitu- dinal gradient from stand edge to mid-stand. The natural frequency of trees near the stand edge (10 m) was higher and the damping ratio was lower than trees farther into the stand (90 m). Unfor- tunately, there is a scarcity of reports from field measurements correlating natural frequency and damping ratios of open-grown deciduous trees. The results of the present study partially support the inverse relationship between ζ and f. When all trees from summer tests were pooled (both forms, before and aſter pruning) by year, ζ in Year 1 (mean = 0.1133) was greater than in Year 2 (mean = 0.1007; SE = 0.003; P < 0.05). Mean f was lower in Summer 1 than Summer 2, although the effect of Year was superseded by the three-way interaction form × prune × year (Figure 3). In other words, ζ increased while f decreased from Year 1 to Year 2, but interactions with other factors prevent definitive conclusions to be drawn. More field measure- ments are needed on various species of open-grown amenity trees to explore this hypothesis further. CONCLUSIONS Excurrent trees had a higher natural frequency than decurrent trees in summer and winter, and the pruning in summer increased the frequency of excurrent trees more than decurrent trees. Damping 227 ratio was not affected by pruning for either crown form, except for Cycle 3 on decurrent trees. Decur- rent trees had a larger percent of their branch mass in the top half of the crown than excurrent trees, which could subject them to larger wind-induced stress on their trunks and increase the risk of fail- ure. Researchers of the current study conclude that structurally pruning to reduce length of upright branches competing with the leader on young red maple (Acer rubrum L. ‘Florida Flame’) trees to maintain an excurrent form reduces the risk of wind-induced tree failure. Although pruning caused frequency to increase in Year 1, trees returned to the same pre-pruned frequency by Year 2, which may suggest a functional biomechanical equilib- rium that trees maintain in their environments. Pull-and-release tests on annually pruned trees over multiple years could help elucidate this further. Acknowledgments. We would like to thank Chris Harchick for technical assistance on this project. Thank you to the TREE Fund and GreatSouthern- TreeConference.org for funding this project. LITERATURE CITED Baker, C.J. 1997. Measurements of the natural frequencies of trees. Journal of Experimental Botany 48:1125–1132. Bruchert, F., and B. Gardiner. 2006. The effect of wind expo- sure on the tree aerial architecture and biomechanics of Sitka spruce (Picea sitchensis, Pinaceae). American Journal of Botany 93:1512–1521. Ciſtci, C., S.F. Brena, B. Kane, and S.R. Arwade. 2013. The effect of crown architecture on dynamic amplification factor of an open- grown sugar maple (Acer saccharum L.). Trees 27:1175–1189. de Langre, E. 2008. Effects of wind on plants. Annual Review of Fluid Mechanics 40:141–168. Duryea, M.L., E. Kampf, and R.C. Littell. 2007. Hurricanes and the urban forest: I. Effects on southeastern United States coastal plain tree species. Arboriculture & Urban Forestry 33:83–97. Francis, J.K. 2000. Comparison of hurricane damage to several spe- cies of urban trees in San Juan, Puerto Rico. Journal of Arbori- culture 26:189–196. Gardiner, B.A., G.R. Stacey, R.E. Belcher, and C.J. Wood. 1997. Field and wind tunnel assessments of the implications of respacing and thinning for tree stability. Forestry 70:233–252. Gilman, E.F. 2003. Branch-to-stem diameter ratio affects strength of attachment. Journal of Arboriculture 29:291–294. Gilman, E.F., and J.C. Grabosky. 2009. Growth partitioning three years following structural pruning of Quercus virginiana. Arbo- riculture & Urban Forestry 35:281–286. Gilman, E.F., and S. Lilly. 2008. Best management practices: Tree pruning (second edition). International Society of Arboricul- ture, Champaign, Illinois, U.S. ©2014 International Society of Arboriculture
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