Arboriculture & Urban Forestry 38(6): November 2012 Arboriculture & Urban Forestry 2012. 38(6): 287–292 287 Sapwood Cuts and Their Impact on Tree Stability E. Thomas Smiley, Brian Kane, Wesley R. Autio, and Liza Holmes Abstract. Sapwood may be lost due to wood decay fungi or mechanical damage. Assessing the impact of sapwood loss on the likeli- hood of tree failure has not been empirically tested. The purpose of this research was to determine the effect of the loss of sapwood on the flex- ural stiffness of tree trunks for different species and trunk sizes. Three tree species (Acer rubrum, Liquidambar styraciflua, and Quercus acu- tissima) were tested at two sites using pull testing techniques. A portion of the stem was mechanically removed and the trees were again pull tested. As the percent reduction in cross-sectional area increased, the percent reduction in stress to deflect trunks decreased linearly, regard- less of species. Stress from sapwood loss was compared to an equivalent calculated loss in heartwood with the same cross-sectional area. The cal- culated loss of heartwood to cause an equivalent magnitude of stress was almost twice as large as cut area of sapwood. Trees were also test- ed by pulling in opposite directions with respect to sapwood loss. The percentage reduction in stress was greater for trees tested in compression. Key Words. Acer rubrum; Decay; Likelihood of Impact; Liquidambar styraciflua; Notch Cuts; Quercus acutissima; Sapwood Loss; Strength Loss; Tree Risk Assessment. Tree risk assessment is an area of concern to many arborists. Most risk assessment is currently being conducted using a basic visual assessment of the tree. Advanced assessment techniques have been developed that relate heartwood loss with the percent strength loss or loss in moment of inertia of the stem (Wagener 1963; Smiley & Fraedrich 1992; Mattheck and Breloer 1994; Kane et al. 2001; Kane and Ryan 2004). The percent strength loss has been used as a surrogate for likelihood of failure, but it does not account for other variables that would affect likeli- hood of failure, such as the wind-induced bending moment on the stem. Wagener (1963) suggested the maximum allowable loss of one third of the initial strength, which corresponded to a heartwood loss of 70% (measured by the diameter of decay). This threshold was supported by Smiley and Fraedrich’s (1992) measurement of failed and standing trees after Hurricane Hugo. However, when trees are exposed to severe wind (>93 km/hr), even trees with no strength loss can fail (Smiley et al. 2011). There is less research on the effects of the loss of sapwood or wood in the outer stem, which may be lost due to wood de- cay fungi or mechanical damage. Damage may occur from ve- hicle collisions, fire, animals (e.g., beavers), or other causes that remove xylem from the outer stem. Luley and Kane (2009) presented a simple theoretical approach to how loss of sapwood would affect a tree’s strength, but did not conduct any tests. The parallel axis theorem can be used to show that for a given shape and area of removed wood, the moment of inertia of the stem will be reduced by a greater amount if the wood is removed from the sapwood rather than the heartwood. Tree winching and breaking studies have been conducted on forest trees for many years (Peltola 2006), but fewer studies have considered open grown deciduous trees (Kane and Clous- ton 2008). Most of this work involves applying a single point load to the tree until it fails. Alternatively, trees can be loaded in the same way, except that the load is limited to induce trunk strains that remain in the elastic range of the wood. This ap- proach has been used to assess the probability of failure in stand- ing trees (Wessolly 1995; Brudi and van Wassenaer 2001) and the effect of the progressive removal of roots (Smiley 2008). Testing in this way reduces the number of trees needed to dem- onstrate an effect. The purpose of this research was to empiri- cally determine the effect of the loss of sapwood on the flexur- al stiffness of tree trunks for different species and trunk sizes. MATERIALS AND METHODS Three tree species [red maple (Acer rubrum), sweetgum (Liq- uidambar styraciflua), and sawtooth oak (Quercus acutissi- ma)] were tested at two United States sites (Table 1). Smaller diameter trees were tested at the Bartlett Tree Research Labo- ratories in Charlotte, North Carolina, and larger diameter red maples were tested at Davey Research Farm in Shalersville, Ohio. The soil type on which the trees were growing in NC was a Cecil sandy clay loam, and in Ohio, a Ravenna silt loam. On all trees, a digital level (Sears Craftsman model 48293, 60 cm long, accurate to >0.1 degrees) was attached vertically to the trunk with the lower end of the level 45 cm above grade (Smi- ley 2008) (Figure 1). A polyester rope 12 mm in diameter (small trees) or a wire rope 6 mm (large trees) or cable was attached to the trunk at a standard height (3.6 m on large trees and 1.5 m on small trees). Trees were pulled until the level measured an angle of one degree from vertical. A small angle was chosen to ensure that axial strains remained in the elastic range of the wood. A dy- namometer (Dillon ED-200+, Fairmont, Minnesota, U.S.) mea- sured the maximum tension in the rope while the tree was pulled. ©2012 International Society of Arboriculture
November 2012
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