Arboriculture & Urban Forestry 36(3): May 2010 contrasted the costs of routine pruning to those associated with topping, finding the former both less expensive and less dam- aging in the long-term. Campanella et al. (2009) followed this with an assessment of the long-term costs of roundover, resto- ration, and thinning of European linden street trees in Belgium. Nowak (1990) evaluated the results of street tree inventories from 11 tree species in the U.S. He observed strong species- specific results in pruning requirements, suggesting that prun- ing cycle may be species-specific. American elm (Ulmus amer- icana) and boxelder (Acer negundo) had the most urgent need for pruning, with London plane (Platanus × acerifolia) and honeylocust (Gleditsia triacanthos f. inermis) the least urgent. Ehsen (1987) described street tree pruning in Germany with a focus on how pruning needs change over the tree’s life-span, moving from a focus on training in young trees to maintenance (e.g., cleaning and raising) on mature trees to reduction in overmature trees. Balder et al. (1997) summa- rized street tree selection and management in Germany, using Berlin as an example. Mascelli et al. (2008) used street trees in Prato, Italy, as a case study of pruning and management. The research foundation for the range of types or styles of pruning varies widely. In some areas, research is only now catch- ing up with long-time practice. In others, research provided clear direction to practice. Where to make cuts and the need to use wound dressings is but one example. The methods for, and value of, pruning young trees to develop good structure has been well- documented whether pruning involves retaining low branches (Leiser et al. 1972), or selective bud removal (Oleksak et al. 1997). In contrast, other pruning practices have less well-devel- oped foundation. There is no research to suggest crown thin- ning improves either tree health or structural stability. And, while it has been common practice for many years, reduc- tion pruning to a branch at least one-third the diameter of the stem lacked a scientific basis. It was not until Grabosky and Gilman (2007) evaluated reduction cuts on two ma- ture oak species that a tentative basis could be established. The architectural style of pruning is common in France (Stefulesco 1995; Drenou 1999; Drenou 2000). In many ways, research has followed practice, as this pruning tech- nique is quite old. Timing and techniques of architectural pruning have been elucidated by the research of scien- tists such as Bory et al. (1996) and Clair-Maczulajtys et al. (1999) who have focused on carbohydrate storage patterns in trees. Pollards are also a common feature of the Euro- pean landscape. Both Austad and Hauge (2007) and Fer- rini (2006c) discuss their physiology and management. Much of the work with crown-raising has occurred in for- estry, where the objective is to have the lower trunk free of branches. From Slabaugh (1957) to Neilsen and Pinkard (2003), research has documented that removal of up to 50% of the live crown of young trees by lifting does not ad- versely impact growth. In summarizing the results from 8 field studies with Douglas-fir (Pseudotsuga menziesii), O’Hara (1991) suggested 33% crown removal as the limit. Pruning is considered one of the important tools in the prac- tice of plant health care. Svihra (1994) summarized the litera- ture regarding eradicative pruning (i.e., the removal of infested and infected branches). Pruning has been used to manage dis- ease problems such as Dutch elm disease (Gregory and Al- lison 1979) and oak wilt (Appel 1994; Camilli et al. 2007). It 113 is also important in the management of bronze birch borer (Ball 1992) and bark beetles (Barger and Cannon 1987). One of the key results of such work is the knowledge that many insects are attracted to fresh pruning wounds. For this rea- son, pruning should take place when insects are not active. Arborists have long believed that proper pruning reduced the likelihood of damage during storms. Duryea et al. (1996) documented the effects of Hurricane Andrew in Florida, sup- ported this observation for some species. Luley et al. (2002) documented branch failures in sweetgum (Liquidambar styraciflua) trees in Rochester, NY, over an eight-year period. Pruning did not reduce the number of failures (most of which occurred while the tree was in leaf), but did result in fewer service requests. Kane (2008) examined the pattern of tree failure following a severe windstorm in Massachusetts find- ing that pre-storm pruning “had little effect on (tree) failure.” Pruning has been used as a tool in evaluating tree response to wind, particularly related to the affect on trunk movement. Smiley and Kane (2006), Pavlis et al. (2008), and Gilman et al. (2008a; 2008b) simulated wind conditions to evaluate trunk movement of young trees in response to crown thinning, raising and reduc- tion pruning. Both crown reduction and crown thinning reduced trunk movement (Gilman et al. 2008a; Gilman et al. 2008b) and wind load (Smiley and Kane 2006). Essentially, the more crown mass removed, the lower the trunk movement or wind load. Gilman et al. (2008a) noted the response was a complex one, and the authors cautioned against extrapolating to larger trees. Moore and Maguire (2005) examined the effects of crown- raising on movement of 14 m to 20 m Douglas-fir trees. Nat- ural sway frequency increased as pruning level increased, although this was not noticeable until 80% of the canopy had been removed. Changes in sway frequency were related to how crown mass was distributed. They noted that treat- ing branches as a lumped mass may not be appropriate. Standards for pruning are found in the U.S. (ANSI 2008), the United Kingdom (British Standards Institute 1989), and Germa- ny (ZTV-Baumpflege 2001). In each case, the standard provides a common vocabulary and procedures for pruning activities. In the U.S., the International Society of Arboriculture (Gilman and Lilly 2002; Kempter 2004) produced a companion volume to the standard, aimed at defining best practice. In a somewhat similar manner, the European Arboricultural Council (2008) recently up- dated the European Tree Pruning Guide. In Italy, the concept of industry standards remains under discussion (see di Lobis 2003). RESEARCH TOPICS FOR THE FUTURE Arboricultural practice should have a foundation in research. In the area of pruning, a foundation is present to some extent. It seems clear that employing removal and reduction cuts has been documented by experimentation and careful observa- tion. Research by Shigo and more recently by Dujesiefken has supported use of the branch collar, natural target ap- proach to selecting the location of a removal cut. Although less well-defined, the same is true for reduction pruning, pri- marily through work of Ed Gilman and Jason Grabosky. In utility arboriculture, reduction cuts take the form of di- rectional pruning—the effort to use a tree’s natural growth pattern to aid in maintaining clearance. The literature docu- ments the value of directional pruning and the problems as- ©2010 International Society of Arboriculture
May 2010
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