290 Smiley et al.: Sapwood Cuts and Their Impact on Tree Stability Figure 5. Scatterplot and best-fit line for the relationship between the percent reduction in area (DA, abscissa, x-axis) and the per- cent change in stress due to cutting (Ds, ordinate, y-axis). The latter ratio was calculated assuming an uncut cross section. The scatterplot includes data from small red maples measured in ten- sion (<) and compression (=). The relationship (Ds = 1.16 - 1.34 * DA) was significant (P < 0.001), robust (r2 both directions of measurement (p = 0.727). = 0.94), and similar for DISCUSSION Greater stress was required to deflect trunks of large red maples because of the smaller proportion of juvenile wood than for the smaller, younger trees. Juvenile wood is known to be less stiff than mature wood for many hardwood species (Williams and Megraw 1994; Zobel and Sprague 1998). Stiffness of juvenile wood is also known to be quite variable and the change from juvenile to mature wood is not abrupt and can vary among spe- cies. This may have explained why the pre-cut stress of small red maples and sawtooth oaks was similar, even though in- herent stiffness of the wood of sawtooth oak (Zhou et al. 1999) is greater than that of red maple (Kretschmann 2010). In addition to the lack of accounting for the presence of ju- venile wood in stems, two other experimental limitations should be noted. First, assumptions made to calculate stress—for ex- ample, that the cross section was circular and that bark thick- ness was similar across all species—may have introduced error. Although bark thickness was not measured, cross sections visu- ally appeared circular for all species. Second, cutting into trunks would release axial growth stress, which is tensile near the bark and compressive at the pith (the change from tensile to compres- sive stress occurs at approximately one-third the distance from the bark to the pith) (Wilson and Archer 1977). The magnitude of axial growth stress measured in some genera [Eucalyptus, for ex- ample (Archer 1986)] can exceed the pre-cut stress calculated for small trees in the current study. Axial growth stress is inversely proportional to the radius of the trunk (Wilson and Archer 1977). Experimental limitations appear not to have undermined the results, however, because the reduction in post-cut stress as more area was removed was consistent for all species. The robust re- lationship lends some confidence that despite presumed differ- ences in wood properties, geometric considerations are more influential. This was expected because the moment of inertia of the uncut trunk (assuming it is circular) is a function of the fourth power of diameter. This reasoning, however, does not ex- plain why the relationship was similar between small and large red maples, given the disparity in trunk diameter. The similar- ity should be interpreted cautiously because large red maples were not subjected to the same loss of area as small red maples. The effect of geometry was further illustrated by the find- ing that the calculated loss of concentric heartwood to cause an equivalent magnitude of stress was almost twice as large as cut area of sapwood. However, it may not be safe to assume that the magnitude of this disparity would apply to trees with trunk de- cay. Previous work has shown that wood produced in response to wounds can be tougher than normal wood (Kane and Ryan 2003). Cutting the tree and immediately testing it does not allow for this response, nor does it account for an effect on trunk strength due to the axial extent of decay. Both of these effects would like- ly occur over time to a trunk that was mechanically wounded. Results should be applied cautiously in practice because trees were only tested in the range of elastic axial strains. As- suming a non-defective stem, failure typically occurs when axial strains are plastic, and non-linear. Future work should investigate the ability to predict failure by measuring elastic axial strains. It was unclear why the mean pre-cut stress for the subset of small red maples was similar regardless of the direction of pull while the mean post-cut percent reduction in stress was greater for trees tested in compression. Although wood is stronger in tension than compression, when the tree was winched, the neutral axis of the trunk would shift to put proportionally smaller and larger cross-sectional areas of wood in tension and compression, re- spectively. Experimental limitations (juvenile wood and assump- tions used to calculate stress) may have confounded the analysis. CONCLUSION When assessing the likelihood of tree failure, loss of sapwood needs to be carefully considered. On a cross-sectional area basis, sapwood loss can change stress nearly twice as much as an equal amount of heartwood loss. The stress related to loss of sapwood appears largely independent of tree species but is highly dependent on the amount of wood that is miss- ing from a stem. Loss of 10% of the cross-sectional area of a stem will reduce the strength of the stem a very small amount (<2%); however, a loss of a third of the cross-sectional area will reduce strength by a significant amount, about 25%. The likelihood of a failed tree impacting a target is an important factor in tree risk assessment and often hard to determine. All other factors being equal, trees with sap- wood loss or decay on the side of the trunk toward a tar- get may be at a greater likelihood of impacting the target. Information in this study should be used with caution due to the variability in the data and the difficulty in calculating the amount of cross-sectional area that has been lost on trees in the field. It should also be noted that trees increase in strength over time if they are healthy and produce wood in response to wound- ing and in response to compression and tension in the cambium. ©2012 International Society of Arboriculture
November 2012
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