Arboriculture & Urban Forestry 36(1): January 2010 Arboriculture & Urban Forestry 2010. 36(1): 35–40 35 Growth of Newly Established Tilia platyphyllos ‘Rubra’ Roadside Trees in Response to Weed Control and Pruning Palle Kristoffersen, Oliver Bühler, Søren Ugilt Larsen, and Thomas Barfoed Randrup Abstract. This tree establishment study investigates the effect of weed control and pruning treatments on stem and branch diameter increment of newly planted broad-leaved lime (Tilia platyphyllos ‘Rubra’) roadside trees. Weed control significantly increased stem circumference four years after establishment by 3.6 cm (1.4 in) from 24.5 cm (9.7 in, untreated control) to 28.1 cm (11.1 in). In terms of Danish nursery sales prices, this corresponds to an increase of tree cash value of 1201 DKK (160.90 €, 235.40 US$) per tree. Cal- culating with 400 DKK (53.60 €, 78.40 US$) as cost for contract weeding per hour, this corresponds to 0.75 hours per tree per year for a period of four years. In addition to weed control treatments, trees were pruned at establishment, two years after establishment, or at both times. None of the pruning treat- ments affected stem diameter growth, but branch diameter and branch:stem diameter ratio were significantly reduced by all pruning treatments. Branch diameter ranged from 40.1 mm (1.6 in) on unpruned trees to 34.6 mm (1.4 in) on trees pruned both times. Branch:stem diameter ratio ranged from 0.54 on unpruned trees to 0.49 on trees pruned both times. In consequence, weed control is recommended as a strong management practice. Mild pruning is also considered advisable, if structural crown prob- lems can be avoided at an early stage, and if the tree has to be prepared for later pruning operations. Key Words. Branch Diameter Growth; Branch:Stem Ratio; Establishment Care; Linden; Stem Diameter Growth; Transplanting; Tree Valuation. The period immediately following planting of roadside trees at their final site is often critical for their successful development. During the production phase in nurseries, trees are irrigated, fertilized, and pruned on a regular basis and competing vegeta- tion is controlled (Krüssmann 1997). On the final site, trees of- ten have to cope with much poorer growth conditions as well as reduced maintenance and care (Harris 2007). In this study, ap- proaches to support tree establishment and development on the final site are investigated in regard to their effect on stem and branch growth: weed control, corrective pruning before plant- ing, and formative pruning during the establishment period. Weed Control Newly transplanted trees often experience growth depressions during the first growth periods at their final sites, to a large ex- tent caused by the loss of root biomass during the transplant- ing process (Solfjeld and Hansen 2004). Although information about the extent of root loss is not entirely consistent and de- pends on the production method, there is little doubt that trees planted bare-rooted or balled & burlapped lose roots during harvest (Watson and Sydnor 1987; Gerhold and Johnson 2003; Anella et al. 2008; Struve 2009). Therefore, newly planted road- side trees are particularly sensitive to competition from other vegetation (Watson 1988; Green 1989; Samyn and De Vos 2002; Groninger et al. 2004; Stewart et al. 2005). Control of compet- ing vegetation is supposed to reduce the effects of the transplant shock and to contribute to a fast and successful establishment. Weed management may consist of several approaches, including applications of herbicides, mechanical or thermal weed control or mulching with different organic or inorganic materials. In Denmark, the use of herbicides in public areas is strictly regu- lated. Municipalities therefore rely on mulching and thermal or mechanical weed control measures (Kristoffersen et al. 2008a). Pruning In European nurseries, trees are normally pruned when they are transplanted during the production process (Krüssmann 1997). The removal of shoots increases the root:shoot ratio and con- sequently reduces the foliage area that has to be supported by the recovering root system (Watson 1991; Findlay 1997). In this manner, pruning is meant to alleviate drought stress and increase transplanting success, especially on difficult sites and/or sites with reduced care and maintenance (Ranney et al. 1989). How- ever, as severe shoot pruning negatively affects tree growth (Li et al. 2001) and transplanted trees respond to root loss with a reduc- tion of leaf area in a self-regulating process (Kjelgren and Cleve- land 1994), pruning of transplanted trees remains controversial. In the following, pruning shortly before or at transplanting is referred to as corrective pruning. The main purpose of later pruning operations is formation of a healthy and sustainable crown structure suitable for the pres- ent location. For roadside trees, this often includes crown-lifting in accordance to traffic requirements. In Denmark, the required clearance over roads is 4.5 m (14.8 ft), implying that many branches of newly transplanted trees are temporary and will have to be removed later. In order to minimize damage, prun- ©2010 International Society of Arboriculture
January 2010
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