Arboriculture & Urban Forestry 32(2): March 2006 75 was defined as a distinct swelling on the branch base where branch wood joined trunk wood. In September 1999, the 12 branches were removed from all 20 trees. The pruning cuts extended from just outside the branch bark ridge on the upper side of the branch to the outside of the branch collar on the lower side. In cases where branch collars were not readily apparent, the cut was made from just outside the branch bark ridge perpendicularly through the branch. In earlier studies, this technique resulted in little cambial dieback on the trunk beneath the cut (Eisner et al. 2002a). The branch unions from 10 trees (12 unions per tree × 10 trees 120 branch unions) were harvested in January 2000 (4 months after pruning), from 5 trees (60 branch unions) in January 2002 (2 years after pruning), and from 5 trees in January 2004 (4 years after pruning) for a total of 240 branch unions. The harvested samples included the branch union with approximately 2.5 cm (1 in) and 10 cm (4 in) of intact trunk above and below the branch, respectively. All samples were then split with a sharpened metal wedge along the me- dial longitudinal plane (i.e., through the pith) and shaved with a razor blade to expose the branch and trunk piths. The amount of discolored (darker than normal) and decayed (darker and softer than normal) wood that formed after branch removal was scanned, measured, and normalized ac- cording to Eisner et al. (2002a). Data were normalized against sample trunk cross-sectional area to account for scaling in- fluences between cut locations and then log10 transformed for data analysis (Eisner et al. 2002a). At the time of dissection, the presence or absence and length of included bark visible on the medial longitudinal surface was recorded. Pruning wound closure after unions were split was categorized into the following categories: to- tally unclosed (woundwood about at the edge of the pruning cut), mostly unclosed (woundwood beginning to grow over the pruning cut), half closed (about half of the pruning cut covered with woundwood), mostly closed (woundwood mostly covering the pruning cut), and completely closed (woundwood completely covering the pruning cut). Lastly, samples were evaluated for the presence or absence of pith connection. Pith was classified as connected if branch and trunk pith were continuous. The stepwise and GLM (SAS Institute Inc., version 8.0) procedures were used to test the impact of all continuous variables (aspect ratio, branch diameter, bark inclusion length, wound closure, time) on the length and area of dis- coloration and decay that developed after branch removal. The t-test was used to compare equality of slopes of linear lines relating decay and discoloration with time. The Tukey studentized range (HSD) contrast of means procedure was used to analyze the impact of class variables (presence of visible branch collars, pith connections) on discolored and decayed area. The frequency procedure was used with chi- square analysis and the Fisher exact test to evaluate relation- ships among class variables. In all statistical analyses, P val- ues less than 0.05 were considered significant unless other- wise specified. RESULTS AND DISCUSSION The model that best described the amount of discoloration was Log10-normalized discolored area 4 years after pruning −0.079 + 0.408 (aspect ratio) + 0.106 (aspect ratio*trunk diameter); R2 75%. This model indicates that the amount of discolored wood that formed after a pruning cut was de- pendent on the diameter of the cut and the ratio of the branch to the trunk diameter (aspect ratio). The area of discolored wood behind the pruning cut increased significantly (P < 0.01) as the aspect ratio increased from 0.25 to 1.0 at 4 months, 2 years, and 4 years after branch removal (Figure 1). An aspect ratio of 1.0 indicated that the removed stem was the same diameter as the trunk. Little discoloration occurred when aspect ratio was less than 0.2. The t-test showed that the slope of this relationship was greater at 2 and 4 years than at 4 months after pruning; the slope 2 years after pruning was similar to the slope 4 years after pruning. This increase in slope indicated that trunk discoloration increased over time at a greater rate when removed branches were large in relation to the trunk compared to those that were small in relation to the trunk. Discoloration progressed very slowly in the 4 years after removal of small branches on larger trunks. There was no relation between the depth of discoloration into the trunk and the aspect ratio or branch diameter, but the depth (length) of discoloration increased with time (Figure 2). Since the depth of penetration was not related to the aspect ratio, the increasing discolored area with aspect ratio de- scribed in the above model suggests a filling in of the dis- colored area. In other words, the discoloration produced ad- vancing thin streaks at small aspect ratios, and discoloration filled in between the streaks as the ratio increased. These results are consistent with observations of large columns of decay after removal or breakage of codominant stems on trees that compartmentalize poorly, especially when the stems are large in diameter (Shigo 1985, 1986). Our results also support the relative absence of such decay in hardwood tree species with branches that remain small compared to the trunk (Toole 1961). Something other than aspect ratio appears to deter- mine the penetration of decay. The area of discoloration 4 months after pruning was sig- nificantly greater for branches with pith connections than for those without (Eisner et al. 2002a), but not 2 and 4 years after pruning (data not shown). Apparently the lack of pith con- nection between the removed branch and the remaining stem played a role in retarding movement of advancing fungi soon after pruning, but its importance diminished with time. These results corroborate past observations that pith connections are associated with poor compartmentalization at the branch ©2006 International Society of Arboriculture
March 2006
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