38 Kristoffersen et al.: Growth of Newly Established Roadside Trees in Response to Weed Control and Pruning tibility to wind damage (Gilman et al. 2008) or if it is intended to restore a disturbed root:shoot ratio. The importance of the latter is controversial, as it may be counterproductive to reduce the assimi- lation capacities by removing foliage if water and nutrition can be supplied in sufficient quantities (Kjelgren and Cleveland 1994). However, this may not be the case on the planting site, and here an artificial restoration of the root:shoot ratio may contribute to tree establishment and survival (Ranney et al. 1989; Findlay 1997). The intensity of pruning operations is critical in regard to Figure 2. Nursery prices of lime (Tilia platyphyllos ‘Rubra’) depending on size in Denmark. Reference lines designate size and price of test trees with weed control (solid line) and without weed control (dashed line). DISCUSSION Weed Control The presented results provide evidence of the highly beneficial effect of weed control on tree growth. This is in accordance with a number of studies in arboriculture and forestry (Wat- son 1988; Samyn and De Vos 2002; Groninger et al. 2004; Stewart et al. 2005), and accentuates the importance of weed control to support establishment of roadside trees. Whereas different weed control techniques have proven to aid tree es- tablishment, it may be worthwhile to consider techniques such as mulching that have additional beneficial effects (e.g., chemi- cal and physical soil characteristics) (Chalker-Scott 2007). The capitalized size increase of weed-controlled trees of 300 DKK (40.20 €, 58.80 US$), equivalent to 0.75 h per tree and year, is considered adequate to perform a strict weed control regime under Danish conditions: Assuming a minimum area capacity of portable blow torches of approximately 100 m2 /h (120 yd2 (89.7 yd2 controlled area per tree was 8 m2 (9.57 yd2 /h) (Rannertshauser and Bertram 1999), the area controlled in 0.75 working hours corresponds to 75 m2 hours would enable 9 visits (75 m2 divided by 8 m2 ). For this study, the ), implying that 0.75 ). Earlier in- vestigations by Kristoffersen et al. (2008b) with blow torches for weed control on traffic islands reported a total of eight treatments in the period from April to November sufficed to control weeds ef- ficiently. It is therefore suggested that the cost of providing weed control during the establishment period of roadside lime trees was less than the resulting value increase of the trees during the first four years. Thus weed control provides a positive return on investment. Pruning In this study, neither pruning treatment had significant effects on stem diameter growth. If considered necessary, pruning can thus be applied without fearing negative consequences for tree growth in the establishment period. This can be the case if apparent prob- lems in crown structure can be resolved early, if later pruning op- erations can be prepared and facilitated, in order to reduce suscep- ©2010 International Society of Arboriculture effects on tree growth. As reported by Li et al. (2001), severe pruning (removal of one-third to two-thirds of crown) of newly planted trees will result in reduced stem diameter growth, as as- similation capability is reduced and physiological processes are disturbed. In this study, pruning never even came close to this order of magnitude, and it could be argued the mild pruning per- formed here moved tree growth from undesired locations (lower lateral branches that have to be removed later to comply with clearance regulations), to desired locations (upper crown parts). Even with the mild pruning performed in this study there was some evidence that pruning may have affected stem diameter growth (p = 0.08). Therefore, stronger pruning operations presum- ably would have resulted in reduced stem diameter growth, too. Although a combination of corrective and formative pruning reduced the branch:stem ratio from 0.54 to 0.49, the desired ra- tio (<0.33) described by the Danish Tree Pruning Guide (Dansk Træplejeforening 2000) was not reached. Moreover, the deter- mined effect of pruning was only small. Similar branch:stem ra- tios on young linden trees ranging from 0.46 to 0.50 (two low- est lateral branches) are reported by Uehre and Cleusters (2006). It therefore seems doubtful whether young trees like the test trees can achieve the optimum branch:stem ratio of 0.33 at all. However, the effect of the pruning operations on branch:stem ratio is likely to increase well beyond the test period. As carbohy- drate production and growth is allocated to the upper, growing part of the crown, the lower branches will be shadowed, further reduc- ing their assimilation potential and, in consequence, their growth. Altogether, the lack of pronounced negative conse- quences for tree growth encourages mild pruning opera- tions at or following tree establishment, focusing on prepa- ration of crown raising and removal of structural problems. CONCLUSION Weed control in the first four years after planting had significant positive effects on stem diameter growth of lime trees. The costs of weed control during the establishment period can be balanced with the capitalized size increase achieved by weed control. Cal- culations based on nursery prices in Denmark indicate an increase in value by weed control during four years after planting corre- sponding to 0.75 hours of contract weeding per tree and year. This is supposed to be sufficient to effect strict weed control and even additional minor maintenance operations. Weed control can thus be considered as cost-effective. In addition to the known posi- tive effects of weed control on tree establishment, urban forestry/ greenspace authorities are thus also provided with economic argu- ments in favor of maintenance operations promoting tree growth. Neither pruning treatment had an effect on stem increment, but branch development was retarded by all treatments. Mild pruning operations proving beneficial at a later point in tree development are therefore considered feasible without risking
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