Arboriculture & Urban Forestry 34(4): July 2008 Arboriculture & Urban Forestry 2008. 34(4):207–215. 207 The Effects of Pruning on Drag and Bending Moment of Shade Trees Michael Pavlis, Brian Kane, J. Roger Harris, and John R. Seiler Abstract. Arborists assume that pruning can help reduce the risk of tree failure by reducing the pressure exerted on trunks by wind (drag-induced bending moment), but there are few studies that quantify this effect. We simulated wind by driving trees in the back of a pickup truck from 0 to 24.5 m/s (0 to 55 mph) and measured drag-induced bending moment as well as tree morphometric data for Freeman maple (Acer × freemanii), swamp white oak (Quercus bicolor Willd.), and shingle oak (Quercus imbricaria Michx.). Measurements were taken before and after application of one of three American National Standards Institute A300 pruning types (raising, reduction pruning, thinning). Reduction of drag-induced bending moment differed by pruning type, largely in accordance with the mass of foliage and twigs removed. The effectiveness of pruning types was also species-dependent because crown architecture affected how much mass each pruning type removed. In general, per unit of mass removed, reduction pruning more effectively reduced the drag-induced bending moment than thinning or raising. Reduction pruning reduced the center of pressure height and, presumably, increased crown porosity after pruning. Prediction of the reduction of drag-induced bending moment was not reliable based on reduction in crown area after pruning. We discuss the practical applications of our findings. Key Words. Bending moment; drag; pruning. Arborists commonly prune trees for a variety of reasons, includ- ing risk reduction, maintaining tree health, and aesthetics. His- torically, in the United States, trees were topped and lion-tailed to reduce the risk of tree failure; topping was expected to reduce windthrow by reducing tree height and therefore the leverage the wind could exert on it. Lion’s-tailing was expected to facilitate wind passage through the crown by making it more porous. These pruning practices, however, are no longer recommended (Lilly 2001) nor were their presumed benefits ever rigorously quantified. Instead, for arborists in the United States, the A300 Standard (American National Standards Institute [ANSI] 2001) describes four conventional pruning types and requires arborists who adhere to the standards to develop objectives before pre- scribing a particular pruning type(s). Pruning types and prescrip- tions have been developed mostly without the benefit of empiric tests of shade trees. This is especially true with respect to the effect of pruning on drag reduction, and, consequently, reducing the risk of tree or branch failure. Some studies conducted in wind tunnels have investigated the effect of pruning on drag of coni- fers (Fraser 1962; Mayhead et al. 1975; Rudnicki et al. 2004) and deciduous trees (Vollsinger et al. 2005), but interpretation of re- sults is limited because few replications were used (Fraser 1962; Mayhead et al. 1975) and trees were small (less than 2 m [6.6 ft] tall) (Rudnicki et al. 2004; Vollsinger et al. 2005). Smiley and Kane (2006) examined the effect of pruning on drag reduction, but the study included only a single species (red maple [Acer rubrum L.]) and three wind speeds. Recent efforts in Europe to assess tree risk using a pull test have led to the development of prescriptive pruning to reduce drag consistent with risk reduction (Wessolly 1995). Such efforts are a useful start to address risk reduction issues more quantita- tively, but the pruning prescription, which amounts to a series of crown reductions, is based on a theoretical reduction in drag (D) from the classic equation, D 0.5**A*CD*U2 [1] where is air density, A is crown frontal area, CD is the drag coefficient, and U is wind speed. The fundamental premise is that crown reduction reduces drag consistent with crown area reduction and lowers the center of pressure height of the crown. Crown reduction thus has the effect of reducing bending mo- ment, which is the product of drag and center of pressure height. Crown reduction also reduces the wind speed to which the tree is exposed because wind speed increases logarithmically with distance above the ground (Davenport 1968). Although Wessolly (1995) did not test trees to determine whether the predicted reduction in drag and bending moment was realized, Smiley and Kane’s (2006) results suggest that per unit of crown mass re- moved, reduction pruning more effectively reduced bending mo- ment than thinning and lion’s-tailing. Many questions remain, however, because drag coefficients among species differ widely (Mayhead et al. 1975; Rudnicki et al. 2004; Vollsinger et al. 2005; Kane and Smiley 2006). This suggests that species may respond differently to different prun- ing types. The objectives of this research were to investigate: 1. The effect of pruning on drag and bending moment of shade trees; 2. The effect of wind speed on drag and bending moment of shade trees; 3. Which tree morphometric data best predict the effect of pruning on drag and bending moment of shade trees; and 4. Whether species influenced the effect(s) of pruning and wind speed on drag and bending moment. METHODOLOGY Trees We tested three shade tree species, Freeman maple (Acer × freemanii)—a cross between red and silver (Acer saccharinum ©2008 International Society of Arboriculture
July 2008
| Title Name |
Pages |
Delete |
Url |
| Empty |
Search Text Block
Page #page_num
#doc_title
Hi $receivername|$receiveremail,
$sendername|$senderemail wrote these comments for you:
$message
$sendername|$senderemail would like for you to view the following digital edition.
Please click on the page below to be directed to the digital edition:
$thumbnail$pagenum
$link$pagenum
Your form submission was a success. You will be contacted by Washington Gas with follow-up information regarding your request.
This process might take longer please wait
