Arboriculture & Urban Forestry 38(1): January 2012 Arboriculture & Urban Forestry 2012. 38(1): 31–32 31 Straightening Out the Askenasy Curve By Andrew Hirons For twenty years the Askenasy Potential Energy Curve (Shigo 1991) has been used by arborists to inform the optimal timing of tree care operations (e.g., pruning). It has influenced arbori- cultural best practices, national standards, and arboricultural curricula (e.g., Harris et al. 2004; BSI 2010). The model es- tablished by Dr. Alex Shigo in Modern Arboriculture describes the modulation of the stored or potential energy within a tree over the course of a year. The model suggests that potential energy rapidly declines during leaf formation and before a pe- riod of high photosynthetic activity rapidly restores the poten- tial energy to levels present in dormancy. A much more gradu- al increase in potential energy continues through summer into autumn, leaving energy levels high at dormancy (Figure 1). trail extending more than 130 years. So how can the evaluation of this literature enhance our understanding of Shigo’s model? It is clear that Shigo’s model accurately reflects the extract presented in Priestley (1970) and Schimper et al. (1903). Howev- er, one of the first things that can be learned from the original text is that Askenasy’s commentary relates to seasonal changes in car- bohydrates in wood, specifically starch and glucose (which Shigo interpreted as “potential energy”). We also learn that the account is based on a single species, Prunus avium. While Shigo would have been clear that this was the case, the omission to note this in his commentary has allowed his readers to make the assump- tion that all temperate trees display the same trend. This may well have been intentional, as other established texts of the time also advocated similar cycles of annual carbohydrate relations in temperate broadleaved trees (e.g., Kramer and Kozlowski 1979). More current literature, however, such as Hoch et al. (2003), Figure 1. Schematic representing the Askenasy Potential Energy Curve (redrawn from Shigo 1991). A broadly accepted application of this model is that me- chanical wounding caused by pruning should be avoided at times where potential energy is low. If pruning is con- ducted during phenophases exhibiting high potential en- ergy, then the tree is much better equipped to respond and is more likely to effectively occlude the wound and com- partmentalize damage. The logic of the model is difficult to dispute, but can an inquiry into the provenance of the ‘Askenasy curve’ provide new insight into its significance? Where did the Askenasy Potential Energy Curve come from? In Modern Arboriculture, Shigo graphically represents a written extract of Askenasy’s work sourced from a paper by Priestley (1970). However, this publication cites the extract as being recorded in Schimper et al. (1903), an English translation of a classic German text on plant geography. This book makes original reference to Askenasy’s 1877 series of papers in the German botanical periodical Botanische Zeitung (Askenasy 1877a; Askenasy 1877b; Askenasy 1877c), creating a literature help provide important resolution on the nature of seasonal car- bohydrate relations in temperate trees. Here, the authors analyze the seasonal variation of non-structural carbohydrates (NSC) in temperate trees. NSC are defined as free, low molecular weight sugar (glucose, fructose, and sucrose) and starch concentrations. The study analyzed leaves, branches, and stems of ten temper- ate tree species; six deciduous broadleaf trees, one deciduous conifer, and three evergreen conifers. When the species data is combined to give a general impression of seasonal variation in NSC (Figure 2), the trend is in contrast to that presented by Shigo [1991 (Figure1)]. In fact, Hoch et al. (2003) show that the NSC pool of branchwood actually increases during spring and early summer before stabilizing through late summer and into autumn. As a result, the validity of the Askenasy curve may have been brought into question had it not been for the fact that by happy coincidence, Prunus avium is one of the species investigated. Hoch et al. (2003) show that at a species level Prunus avium clearly exhibits a decrease in NSC (in branch sapwood) prior to budbreak, which the authors attribute to the prolific pre-leaf flow- ering that occurs in this species (Figure 3). Therefore, it may still be argued that species that exhibit pre-leaf flowering are likely to have a depleted carbohydrate pool at the leaf expansion phe- nophase. Significantly, this implies that the model presented by Shigo (1991), although accurate for the species Askenasy was de- scribing, should not be applied more widely because Prunus avi- um is an atypical species with regard to its seasonal carbohydrate relations. Incidentally, the other group of trees demonstrated to have a marked reduction (although not necessarily statistically significant) in seasonal fluctuation of NSC are those with a ring- porous xylem structure, such as Quercus petraea (Barbaroux et al. 2003; Hoch et al. 2003). This is related to the requirement ring-porous trees have to replace their hydraulic infrastructure (xylem) using reserve carbohydrates prior to leaf initiation and ©2012 International Society of Arboriculture
January 2012
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