32 concomitantly, current season photosynthate (Zimmerman 1974). Perhaps, those species which exhibit a ring porous xylem struc- ture and pre-leaf flowering (such as many Ulmus sp.) should be treated with particular caution early in the growth season. Hoch et al. (2003) also revealed, in the ten temperate forest trees studied, that no particular shortage in non-structural carbohydrates was found during the course of the year despite some fluctuations in levels of NSC. This suggests, at least in healthy trees, that the carbohydrate status of the tree should not be a guiding principle informing the timing of arboricultural operations such as pruning. Examining the provenance of the Askenasy Potential Energy Curve reminds us that it is crucial to treat generic models on tree performance with caution, and emphasizes the need to understand the diverse characteristics that contrasting species exhibit. The functional diversity of trees is such that no model will apply sat- isfactorily to a broad range of species in multiple environments. Arboricultural professionals must be aware of this and should be prepared to interrogate models so that they are applied only to those situations in which they have relevance. Careful and ac- Editorial Commentary curate citations supporting models are central to enabling this. In addition, those publishing models have the responsibility of alert- ing the reader to potential limitations. Indeed, it should be noted the Hoch et al. (2003) used mature trees growing on a high quality forest site for their research. The data may, therefore, contrast with trees that are limited by less optimal conditions frequently found in urban environments. Furthermore, although ten species of tree where used in Hoch et al.’s (2003) research, this clearly represents a fraction of the genetic diversity found across temperate trees. It is hoped that the evidence presented in this short commen- tary will be useful for informing revisions in arboricultural cur- ricula, act as a reminder that generic models should be evalu- ated before they are applied, and contribute toward the general discussion on the optimal timing of arboricultural operations. Acknowledgments. I would like to acknowledge David Elwell for tech- nical support in producing the figures. Thanks must also go to the Biodi- versity Heritage Library for making the historical texts referred to in this commentary available for research. REFERENCES Askenasy, E. 1877a. Ueber die jahrliche Periode der Knospen. Bota- nische Zeitung. 35(50):793–816. [German] Askenasy, E. 1877b. Ueber die jahrliche Periode der Knospen (Forts). Botanische Zeitung. 35(51):817–832. [German] Askenasy, E. 1877c. Ueber die jahrliche Periode der Knospen (Schluss). Botanische Zeitung. 35(52):833–848. [German] Barbaroux, C., N. Breda, and E. Dufrene. 2003. Distribution of above- ground and below-ground carbohydrate reserves in adult trees of two contrasting broad-leaved species (Quercus petraea and Fagus sylvat- ica). New Phytologist 157:605–615. BSI. 2010. BS 3998: 2010 Tree work – Recommendations. British Stan- dards Institute. 70 pp. Harris, R.W., J.R. Clark, and N.P. Matheny. 2004. Arboriculture: Inte- grated Management of Trees Shrubs and Vines. 4th Edition, Pearson Education, Prentice Hall, New Jersey, U.S. 580 pp. Figure 2. Schematic of mean seasonal change in NSC from the branchwood of six deciduous species (Acer campestre, Carpinus betulus, Fagus sylvatica, Prunus avium, Quercus petraea, Tilia platyphyllos) (redrawn from Hoch et al. 2003). Hoch, G., A. Richter, and C. Körner. 2003. Non-structural carbon com- pounds in temperate forest trees. Plant Cell Environment 26:1067– 1081. Kramer, P.J., and T.T. Kozlowski. 1979. Physiology of Woody Plants. Academic Press. 811 pp. Priestley, C.A. 1970. Carbohydrate Storage and Utilization pp. 113–127. In: L.C. Luckwill and C.V. Cutting (Eds.). Physiology of Tree Crops. Academic Press. 382 pp. Schimper, A.F.W., W.R. Fisher., P. Groom., and I.B. Balfour. 1903. Plant- geography upon a physiological basis. Clarendon Press. Oxford. 839 pp. Shigo, A.L. 1991. Modern Arboriculture. Shigo and Trees Associates. Durham. 423 pp. Zimmermann, M.H. 1974. Storage, mobilization and circulation of as- similates. pp. 307–322. In: M.H. Zimmermann and C.L. Brown (Eds.). Trees – Structure and Function. Springer. 336 pp. Figure 3. Schematic showing the seasonal change in NSC for Prunus avium (redrawn from Hoch et al. 2003). ©2012 International Society of Arboriculture Andrew Hirons Lecturer in Arboriculture Myerscough College Bilsborrow, Preston, Lancashire, PR3 0RY United Kingdom
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