Closure of Branch Pruning Wounds with Conventional and ‘Shigo’ Cuts

Materials and Methods

The trees used in this study were growing in the Illinois Natural History Survey arboretum 1 mile south of Urbana, Illinois. The trees were in 10O-tree blocks spaced 4 m apart. The pin oaks (Quercus palustris) were planted as seedlings in 1969, the Norway maples (Acer platanoides) in 1970, and the sycamores (Platanus occidentalis) in 1977. Each year for four consecutive years (1983-1986), ten trees of each species were selected for pruning. Four living branches from each tree were removed: two by the ‘Shigo’ method and two by the conventional method. The ‘Shigo’ cuts were an upstroke of the chain saw as A-B in Figure 1. Care was taken not to leave a stub at point A. The conventional cuts were a downstroke of the chain saw beginning at A and angling out from the trunk (not flush with the trunk). Care was taken not to injure the bark on the main stem above the excised branch. A mean of tree diameters 1 m above the ground line is given in Table 1. Branch diameters 10 cm distal to the pruning cut were measured with a diameter tape. Width of exposed xylem was measured horizontally at the widest point immediately after pruning. Branches were excised the first week of May in 1983, the first week of April in 1984 and 1985, and the last week of March in 1986. The width of wood remaining exposed and tree trunk growth were measured annually in late September or early October from 1983 through 1987.

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Figure 1.

Natural target pruning. Excerpted from: Shlgo, Alex L. 1984. Homeowners guide for beautiful, safe, and healthy trees. USDA Forest Service NE-INF-58-84.

Results

The severed branches on the trees averaged 2.5 to 3.4 cm (1 to 1.5 inches) in diameter (range 2.0 to 4.8 cm) and were uniform between treatments within a species (Table 2). The width of wood exposed following pruning was minimally greater than branch diameter on ‘Shigo’-cut wounds but substantially greater with conventional pruning (Table 2). Depending on species, the percentage increase was 4 to 6 percent on ‘Shigo’-cut wounds, and 32 to 51 percent on conventional-cut wounds. The branch collar was larger on maple than on oak and sycamore.

Wound calluses around conventional cuts grew much faster than calluses around ‘Shigo’ cuts, especially the first year (Table 3). After one growing season, the amount of wood exposed on the conventional and the ‘Shigo’ branch wounds were approximately equal even though the conventional cuts were originally much larger. During the second growing season, the callus growth continued to be greater on the conventional cuts. After two growing seasons, more of the conventional cuts than the ‘Shigo’ cuts were fully closed on oak and sycamore and almost as many were closed on maple (Table 3).

On both ‘Shigo’ cuts and conventional cuts on all three species, the amount of wound closure was positively correlated with wound width. The larger wounds contained more callus than the smaller wounds. This was especially evident the second growing season (data not shown, correlation coefficient (R) 0.39 to 0.81). Wound closure was also positively correlated with trunk growth (correlation coefficients (R) 0.02 to 0.34). Wound closure was greater on the faster growing trees.

Discussion

Shigo presents convincing pictorial evidence to support his thesis that natural target pruning will result in reduced discoloration, decay, and internal defects within xylem tissues (5). He also warns the chain saw operator to use great care in the final upstroke cut, especially on large branches. Chain saw kick-back can cause serious injury if the operator does not maintain control at all times.

The conventional cut used in this study is not a flush cut. It is a downstroke with a chain saw that begins outside the branch bark ridge (point A in Figure 1) and proceeds slightly outward (not A to X in Figure 1) but without injury to the stem above the branch (approximately from A to the arrow identifying the branch collar in Fig. 1). In removing a dead branch with a callus collar with the conventional method, much of the callus tissue would be excised to avoid leaving a projection on the trunk.

Aesthetics are especially important in the maintenance of amenity plants. Dead branches, broken branches, branch stubs or knobby growth on trees are not pleasing to the eye. These are also detrimental to the tree because “healing”, i.e., closure of wounds through production of callus tissues, is delayed, prolonged, or slowed (2). Also, minor defects in the interior of the tree have little or no effect on tree vigor, and as long as they do not threaten tree stability should be of minor concern.

In natural target pruning, Shigo exhorts practioners to aim for the targets and try to hit them. “Do not try to ‘be safe’ and leave a stub. A stub gives the pathogen a food source while they grow into the branch corewood.” (5) Nevertheless, with current tools the tendency for those attempting to adhere to the ‘Shigo’ method is to leave a stub. Branch stubs heal very slowly (1). This study provides evidence to show that the pruning wounds remaining after correctly prepared ‘Shigo’ cuts close more slowly than conventional cuts. The question that remains is, which is more serious: 1) the discoloration, decay, and internal defects that may occur following the use of the conventional pruning method, or 2) the aesthetic discomfort of humans and slow closure of wounds on trees resulting from the natural target pruning method?