Arboriculture & Urban Forestry 36(1): January 2010 37 where y denotes stem circumference and x denotes year after planting. Transformed back to original scale, estimated stem circumfer- ence in 2005 corresponds to 28.1 cm (11.1 in) for trees subjected to weed control and to 24.5 cm (9.7 in) for the untreated control trees. Stem circumference difference between the treatments is 3.6 cm (1.4 in), corresponding to 1.2 cm (0.5 in) in stem diameter. Weed control effects on branch diameter were analyzed in a separate model, determining that 2005 branch diam- eter was significantly larger on the trees in weed control plots [38.5 mm (1.5 in)], compared to untreated plots [34.6 mm (1.4 in)] (P = 0.0002), but branch:stem diameter ra- tio was not affected significantly by weed control (P = 0.83). Pruning Figure 1. Growth curves for roadside lime trees (Tilia platyphyllos ‘Rubra’) with and without weed control based on regression equa- tions (1): 1/y = 0.07493-0.00851x, and (2): 1/y = 0.07493-0.00982x. For this figure, the reciprocal response variable stem circumfer- ence was transformed back to its original scale. Single observa- tions are depicted as “o” (weed control) and “+” (without weed control). in SAS 9.1). The response variables were analyzed in response to the whole plot factor weed control, the subplot factor prun- ing and interaction effects. Block was included as random effect. For all models, uniformity of variance was tested by visual in- spection of residual plots produced with the SAS procedure GLM. In order to estimate the market value of trees with differ- ent weed control treatments four years after establishment, 2008 price and size information of three Danish tree nurser- ies was collected to describe the relationship between stem circumference and price. The relationship was analyzed with a nonlinear regression analysis with price as response vari- able and stem circumference as explanatory variable (NLIN procedure in SAS 9.1). Monetary values are quoted in Danish crowns (DKK), Euros (€) and U.S. Dollars (US$), using the an- nual mean exchange course for 2008 provided by the National Bank of Denmark (1 DKK equates 0.134 € and 0.196 US$). Results Weed control Weed control had significant (P < 0.0001) effects on the slope of the reciprocal growth curves and thus on stem diameter growth of the test trees. The growth curves were neither significantly affect- ed by corrective pruning (P = 0.1634) nor by interaction effects (P = 0.6642). No significant differences in estimated start diam- eter [13.3 cm (5.24 in) on the original scale] were determined (P = 0.9504). According to the regression analysis, reciprocal stem circumference for trees subjected to weed control can thus be described by Equation 1, whereas reciprocal stem circumfer- ence for the untreated control trees is described by Equation 2: [Equation 1] [Equation 2] 1/y = 0.07493-0.00851x 1/y = 0.07493-0.00982x ©2010 International Society of Arboriculture All pruning treatments reduced branch diameter signifi- cantly (P = 0.006) and resulted in significantly reduced branch:stem diameter ratio (P = 0.048) four years af- ter planting (Table 1). None of the pruning treatments had significant effects on stem diameter growth (P=0.08). Increase of value In order to quantify the added value gained by weed control, 2008 prices for Tilia platyphyllos ‘Rubra’ of three prominent Danish tree nurseries were plotted against size (described as midpoint of each size class [e.g., 12–14 cm (4.7–5.5 in) is converted to 13 cm (5.1 in)]. A regression analysis showed that the price development of nursery trees can be described by the following equation for the size range 10–12 cm (3.9–4.7 in) to 35–40 cm (13.8–15.7 in): [Equation 3] y = 2.2675x2.2752 where y denotes the price in 2008, and x denotes stem circum- ference size (Figure 2). According to this, the size difference between trees subjected to weed control (28.1 cm, correspond- ing to 4484 DKK, 600.80 €, 878.90 US$) and trees without weed control (24.5 cm, corresponding to 3282 DKK, 439.80 €, 643.30 US$) amounts to a cash value of 1201 DKK (160.90 €, 235.40 US$). If this amount is distributed over the period of four years, it is equivalent to 300 DKK/year (40.20 €, 58.80 US$) or, if the cost of contract weeding is set at 400 DKK per hour (53.60 €, 78.40 US$, standard cost in Denmark), 0.75 working hour per tree per year. Table 1. Estimates of branch diameter and branch:stem ratio of roadside lime trees (Tilia platyphyllos ‘Rubra’) four years after planting in response to corrective and formative pruning treatments (LSMEANS ±standard error). Values dif- fer significantly when followed by different letters (LSD test, P < 0.05). Corrective pruning Formative pruning Branch diameter Branch:stem [mm] - + - + - - + + 40.1(±1.29) a 36.7 (±1.29) b 34.8 (±1.29) b 34.6 (±1.29) b diameter ratio 0.54 (±0.017) a 0.50 (±0.017) b 0.49 (±0.017) b 0.49 (±0.017) b
January 2010
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