368 Slater and Ennos: Assessment of the Remodeling of Bifurcations in Hazel (Corylus avellana L.) to result in a higher breaking stress for this compo- nent (Slater and Ennos 2013), although it is only one factor among many that will affect the breaking stress of any given bifurcation. The mean density of the wood formed at the apices of the braced bifurca- tions was 4% less dense than the wood at the apex of the normally formed bifurcations, suggesting that wood quality had atrophied in response to bracing. Limitations of the Study It is important to acknowledge that this study is based on data collected from semi-mature bifurcations in hazel trees, which gives rise to limitations in the scope of the subsequent findings. This study is part of a series that has examined bifurcations in this par- ticular species to provide anatomical and mechanical models that could then be compared and contrasted to the bifurcations of other woody species by further study. The physiological pathways to this remodeling process were not examined as part of this study and could also be usefully examined in further research. CONCLUSIONS The denser xylem formed at the apex of bifurcations in hazel (and in other tree species) plays a key func- tion in preventing failure at the junction (Slater and Ennos 2013). Although the role of this modified xylem is important in supplying a higher bending strength, its absence does not necessarily result in bifurcation failure: connections formed either side of the bifurcation apex can clearly be adequate to give four year’s longevity or more to the juvenile bifurcations tested in these semi-mature hazel trees. From the pre-drilled bifurcations in this study, it is clear that they can satisfactorily remodel around an induced injury or defect and recover their full bending strength over time. This complements the analysis of Slater and Ennos (2015) that indicates remodeling around included bark can also fully recover the strength of bifurcations in hazel. This process of repair was not uniform among the bifur- cations in this study, and further research could seek to find key factors that relate to the rate of repair of such bifurcations. In contrast, if the hazel bifurca- tion is split at its apex, although it has the potential to remodel, it is much more likely that it will fail completely under further wind-loading due to the initial crack propagating further down the stem. If a ©2016 International Society of Arboriculture rod brace is installed above a hazel bifurcation, then development of the bifurcation will atrophy, identi- fying that thigmomorphogenesis plays an important role in the mechanical development of bifurcations. These findings help to measure the extent and degree of the remodeling of such bifurca- tions with different treatments, and could assist in determining a factor of safety for this compo- nent of a tree’s crown. Further modeling needs to be extended beyond static rupture tests, to investi- gate the movement behavior of bifurcations under dynamic wind loading, which is considered to be a key factor in the impetus for bifurcations to remodel aſter injury or occlude a naturally occur- ring mechanical flaw, such as a bark inclusion. Acknowledgments. We would like to thank the following contrib- utors: Myerscough College, England, for sponsoring this research and for the supply of the hazel bifurcations for testing, and Austin Walmsley metal fabricators of Garstang, England, for the construc- tion of the bespoken metal clamps that were used to attach the bifurcations to the Instron testing machines. LITERATURE CITED Badel, E., F.W. Ewers, H. Cochard, and F.W. Telewski. 2015. Acclimation of mechanical and hydraulic functions in trees: Impact of the thigmomorphogenetic process. Frontiers in Plant Science 6:266. Biro, R.L., E.R. Hunt, Y. Erner, and M.J. Jaffe. 1980. Thigmomor- phogenesis: Changes in cell division and elongation in the inter- nodes of mechanically perturbed or ethrel-treated bean plants. Annals of Botany 45:655–664. Braam, J., and R.W. Davis. 1990. Rain-, wind-, and touch-induced expression of calmodulin and calmodulin-related genes in Arabidopsis. Cell 60:357–364. Coutand, C. 2010. Mechanosensing and thigmomorphogenesis, a physiological and biomechanical point of view. Plant Science 179:168–182. Farnsworth, K.D., and K.J. Niklas. 1995. Theories of optimization, form and function in branching architecture in plants. Func- tional Ecology 9:355–363. Gartner, B.L. 1995. Patterns of xylem variation within a tree and their hydraulic and mechanical consequences. In: G.L. Gartner (Ed.). Plant Stems: Physiological and Functional Morphology. Academic Press, New York, New York, U.S. Gilman, E.F. 2003. Branch to stem diameter affects strength of attachment. Journal of Arboriculture 29:291–294. Grace, J. 1977. Plant Responses to Wind. Academic Press, Lon- don, UK. Hughes, S.W. 2005. Archimedes revisited: A faster, better, cheaper method of accurately measuring the volume of small objects. Physics Education 40(5):468–474. Jaffe, M.J. 1973. Thigmomorphogenesis: The response of plant growth and development to mechanical stimulation. Planta 114:143–157.
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