Arboriculture & Urban Forestry 35(6): November 2009 ties may increase in the watersprouts at some point. Selection criteria for watersprout removal during restoration may be those with high slenderness ratios, although more research is needed to ascertain when slenderness is considered critically high in water- sprouts. Arborists may also choose to use techniques like subor- dination to reduce slenderness ratios in waterspouts or branches. Researchers may wish to investigate if slenderness ratios can be used as predictor of instability in watersprouts or lateral branches. CONCLUSION It seems that Sullivan’s (1896) observation applies to trees: form does indeed follow function. As trees grow a shift appears to occur in the balance between the functions of hydraulics and mechanics. Initially growth takes place with a greater investment in hydrau- lics. As such, young stems and branches are flexible and easily bend during loading events. Mechanics become more important over time, as implied by allocation of more energy in developing cell walls to provide increased mechanical support. The shift in function may coincide with a shift in plant form. The shift in form explains, in part, why the pipe model works well with smaller branches, but is less robust in larger branches. Fractal dimension- ing also appears to be less robust when modeling tree canopies, yet the inclusion of adaptive fractals provide a promising avenue of research. The three similarity models put forth by McMahon (1975) and Niklas’(1992) suggestion that branches and stems tran- sition between the models appear to best explain how tree stems and branches modify the relationship between lateral elongation and radial growth over time. Whether this shift is due to a tradeoff or a balance between hydraulics and mechanics has not been satis- factorily answered. Further research is needed to fully understand how form is influenced by hydraulic and mechanical functions. A better understanding between the form and function should help the arboricultural community improve maintenance prun- ing standards as well as devise guidelines for canopy restoration. Over the past decade, arboricultural researchers have begun to model how tree response to moderate and extreme wind loading events is altered depending on pruning regimes (Smiley and Kane 2006; Gilman et al. 2008a; Gilman et al. 2008b; Pavlis et al. 2008). It is hoped that the information in this review, could be integrated with knowledge gained from such wind loading experiments in order to better understand how canopies are built to withstand the elements. The time is near when arboricultural researchers should consider utilizing the knowledge of branch form and mechanical structure to build a computer model of a representative amenity tree. This simulated tree could be integrated with data gained from on-going wind studies and then subjected to simulated loading events, such as hurricane force winds or ice loads. The simulated events could prove useful in developing predictions of canopy locations with inherent weakness. Researchers could then investi- gate how pruning techniques could be employed to increase can- opy stability, both immediately and over time, as growth is added. Acknowledgments. We would like to thank Drs. Peter Smouse, George Zimmermann, Ming Xu, and Edward Gilman for assistance during the preparation of this literature review. Dr. Brian Kane and a series of anon- ymous reviewers provided critical advice which strengthened this manu- script. Kim Sorvig for a robust conversation regarding adaptive fractals. This effort was funded by the John & Eleanor Kuser Faculty Scholar Endowment. LITERATURE CITED Allen, M.T., P. Prusinkiewicz, and T.M. DeJong. 2005. Using L-systems for modeling source-sink interactions, architecture and physiology of growing trees: the L-PEACH model. New Phytologist 166:869–880. Alméras, T., A. Thibaut, and J. Gril. 2005. Effect of circumferential het- erogeneity of wood maturation strain, modulus of elasticity and radial growth on the regulation of stem orientation in trees. Trees: Structure Function 19:457–467. American National Standards Institute. 2008. American National Stan- dards for Tree Care Operations-Tree, Shrub, and Other Woody Plant Maintenance-Standard Practices (Pruning) (A300 Part 1). Tree Care Industry Association, Manchester, NH. American Society for Testing and Materials. 2000. Standard test methods for small clear speciments of timber (D143–95, Reapproved 2000). ASTM International, West Conshohocken PA. Ancelin, P., B. Courbaud, and T. Fourcaud, 2004. Development of an in- dividual tree based mechanical model to predict wind damage within forest stands. Forest Ecology Management 203:102–121. Barefoot, M.W. 1965. Influence of cellulose, lignin and density on tough- ness of yellow poplar. Forest Products Journal 15:46–49. Berezovskava, F.S., G.P. Karev, O.S. Kisliuk, R.G. Khlebopros, and Y.L. Tsel’niker. 1997. A fractal approach to computer-analytical modeling of tree crowns. Trees: Structure and Function 11:323–327. Berninger, F., and E. Nikinmaa. 1997. Implications of varying pipe model relationships on Scots pine growth in different climates. Func- tional Ecology 11:146–156. Bertram, J.E.A. 1989. Size-dependent differential scaling in branches: the mechanical design of trees revisited. Trees: Structure and Func- tion 4:241–253. Brüchert, F., and B. Gardiner. 2006. The effect of wind exposure on the tree aerial architecture and biomechanics of Sitka spruce (Picea sitch- ensis, Pinaceae). American Journal of Botany 93:1512–1521. Brüchert, F., F. Becker, and T. Speck. 2000. The mechanics of Norway spruce [Picea abies (L.) Karst]: mechanical properties of standing trees from different thinning regimes. Forest Ecology and Manage- ment 135:45–62. Burgert, I. 2006. Exploring the micromechanical design of plant cell walls. American Journal of Botany 93:1391–1401. Casella, E., and H. Sinoquet. 2003. A method for describing the canopy architecture of coppice poplar with allometric relationships. Tree Physiology 23:1153–1170. Chiba, Y. 1998. Architectural analysis of relationship between biomass and basal area based on pipe model theory. Ecological Modelling 108:219–225. Chiu, S.T., and F.W. Ewers. 1992. Xylem structure and water transport in a twiner, a scrambler, and a shrub of Lonicera (Caprifoliaceae). Trees: Structure and Function 6:216–224 Choat, B., T.W. Brodie, A.R. Cobb, M.A. Zwieniecki, and N.M. Hol- brook. 2006. Direct measurements of intervessel pit membrane hy- draulic resistance in two angiosperm tree species. American Journal of Botany 93:993–1000. Choat, B., A.R. Cobb, and S. Jansen. 2008. Structure and function of bordered pits: new discoveries and impacts on whole-plant hydraulic function. New Phytologist 177:608–626. Clair, B., R. Almeras, and J. Sugiyama. 2006. Compression stress in opposite wood of angiosperms: observations in chestnut, mani and polar. Annals of Forest Science 63:507–510. Côté, W.A., Jr., A.C. Day, and T.E. Timell. 1968. Distribution of lignin in normal and compression wood of tamarack. Wood Science and Technology 2:13–37. ©2009 International Society of Arboriculture 317
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