36 Kristoffersen et al.: Growth of Newly Established Roadside Trees in Response to Weed Control and Pruning ing guides generally recommend minimizing the wound surface. The Danish Tree Pruning Guide recommends removing branches only when the branch diameter is less than one-third stem di- ameter (Dansk Træplejeforening 2000), and German guidelines recommend that branches in conflict with clearance regulations should not exceed a branch diameter of 5 cm (2 in) (FLL 2006). One of the main purposes of these pruning operations is there- fore to reduce diameter growth of lower, temporary branches and thus prepare them for a later removal. This type of pruning af- ter planting is referred to as formative pruning in the following. Thus, weed control and pruning during the critical establish- ment phase have the overall objective to improve the chances of roadside trees to survive the first years after transplanting and de- velop into functioning roadside trees. The aims of this study are to investigate how both maintenance options 1) influence tree growth during establishment, 2) determine whether later pruning opera- tions can be prepared and facilitated by early corrective pruning in order to reduce branch:stem ratio, and 3) provide an opportunity to discuss economic aspects of weed control of newly planted trees. MATERIAL AND METHODS Test Site and Plant Material The investigation was carried out on a newly planted, rural roadside plantation on Hindsgavl Allé (55°30’N, 9°41’E) near the town Mid- delfart, Denmark. Average annual precipitation in the area is 639 mm (25.16 in), based on measurements in the period 1961–1990 (DMI 2005). Precipitation was below the average in 2003 (481 mm, 18.94 in) (DMI 2004), and above the average in 2002 (724 mm, 28.5 in) and 2004 (737 mm, 29 in) (DMI 2003; DMI 2005). Forty Tilia platyphyllos ‘Rubra’ roadside trees were planted in two planting strips on both sides of the road with 20 trees on each side. The trees were planted in the existing soil profile, a sandy loam with good drainage. The trees were delivered from the nursery balled & burlapped with a stem circumference of 12–14 cm (4.7–5.5 in) at 1 m (3.3 ft) stem height. The root ball was positioned in the middle of a 1 m x 1 m x 0.6 m ( 3.3 ft x 3.3 ft x 2 ft) planting hole, stabilized with packed soil and the planting hole was refilled with uncompacted backfill soil. No fertilizers, soil amendments or mulch materials were used. The burlap covering the root ball was decomposable and was not re- moved during planting. After planting, the upper surface of the root ball was just visible. Trees were planted with a distance of 8 m (26.3 ft) between trees and 2.5 m (8.2 ft) to the road. The trees were planted in early spring 2001. All trees were irrigated twice with 100 L (26.4 gal) of water in both 2001 and 2002. Four trees failed to establish in spring 2001. No treat- ments but corrective pruning had been executed at that time: two of the failed trees were pruned, the remaining two unpruned. The replacement trees were excluded from the data analysis. Treatments Weed control Weed control was carried out with a portable blow torch. Weeds were controlled from 2001–2004 such that no weeds were pres- ent on an area of 1 m x 8 m (3.3 ft x 26.3 ft) around the tree during the period of the experiment. Weed control was executed on 20 trees, the remaining 20 trees served as untreated control. Gramineous weeds dominated on the untreated control plots. ©2010 International Society of Arboriculture Pruning Trees received the following pruning treatment combi- nations: 1) corrective pruning at the time of planting, 2) formative pruning two years after planting, 3) both cor- rective and formative pruning, and 4) untreated control. Corrective pruning was executed so that second order lat- eral branches were removed immediately before planting of the trees. Formative pruning was executed once in 2003, two years after planting, and included reduction cuts (cutting to a lateral) of lower lateral branches in order to slow their diameter growth and prepare the trees for later crown raising. None of the prun- ing treatments caused foliage reductions of more than 30%. Experimental Design The experiment comprised 40 trees in total, 20 on each side of the road. The test site was organized in 5 blocks of eight trees each (four trees on each side of the street) in order to account for possible variations of soil conditions along and across the road. The investigation was designed as a split-plot experiment with weed control as whole plot factor and pruning as subplot factor. As weed control and pruning were executed by munici- pal gardeners unacquainted with experimental work, treat- ments were distributed systematically in order to ensure cor- rect treatments. In addition, the distance between individual trees was considered large enough to limit the risk of neighbor effects, and the advantages of a systematic treatment distribu- tion were therefore considered to compensate for drawbacks. In each block, weed control was executed in whole plots on one side of the street (four trees), while the four control trees were in the whole plot on the other side of the street. This pattern alternated from block to block. The four prun- ing treatments were assigned systematically to one tree with- in each whole plot (i.e., a subplot consisted of one tree). Measurements Stem circumference at 1 m above ground was mea- sured after cessation of the growth period in 2002, 2003, and 2005. Branch diameter of the two lowest branch- es was measured at the base of the branch in 2005. Data analysis A regression model was used to describe the development of stem circumference over time in response to weed con- trol, corrective pruning and interaction effects. The analysis was based on a linear mixed model approach (MIXED pro- cedure in SAS 9.1) and included data from 2002, 2003, and 2005. Formative pruning was omitted from the model, as formative pruning treatments started late in the test period in 2003. Block was included in the model as random effect. To attain uniformity of variance, the response variable stem circumference was transformed to its reciprocal value (1/cm). For the purpose of graphical presentation, the reciprocal value was then transformed back into the original scale (Figure 1). In a second model, the effects of weed control and pruning treatments on branch diameter and branch:stem ratio in 2005 were analyzed, and a third model analyzed pruning effects on stem diameter in 2005. Both models included 2005 data only and were based on a mixed linear model approach (MIXED procedure
January 2010
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