Does Included Bark Reduce the Strength of Codominant Stems?

MATERIALS AND METHODS

Twenty-six red maple (Acer rubrum L.) trees were harvested between June 1999 and July 2001 at the Bartlett Tree Research Laboratories in Charlotte, North Carolina, U.S. Eighty-four codominant stems were removed from the felled trees, leaving at least 45 cm of stem on either side of the crotch. Crotches were tested within 3 days of harvest to avoid drying of the wood. Stem diameter was measured 30 cm below and above the crotch. Diameters ranged from 4.9 to 23.4 cm.

The crotches were fastened to a large tree trunk using chains 30 cm above and below the crotch (Figure 1). A snatch block was fastened to the nonanchored stem at 30 cm above the crotch. A Dillon 1,818 kg peak reading mechanical dynamometer (Weight-Tronix, Fairmont, MN) was chained to a second tree and served as an anchor point for a steel cable that ran through the snatch block to an electric winch. The cables from the tree to the snatch block and from the snatch block to the winch were nearly parallel and remained the same throughout the trial. The winch was activated until the crotch broke. The peak reading on the dynamometer was recorded and multiplied by two to derive the force required to break the crotch.

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Figure 1.

The crotches were fastened to a large tree trunk using chains 30 cm above and below the crotch. A snatch block was fastened to the nonanchored stem at 30 cm above the crotch. A Dillon 1,818 kg peak reading mechanical dynamometer was chained to a second tree and served as an anchor point for a steel cable that ran through the snatch block to an electric winch; cables were nearly parallel. The winch was activated until the crotch broke.

Regression lines were compared for slope and Y-intercept using the general linear test approach (Neter and Wasserman 1974).

RESULTS

Stem breakage occurred in consistent patterns. The failure occurred at the junction between the codominant stems and separated the two stems evenly (Figure 2). If included bark was present, the break always exposed it. The amount of included bark varied greatly among samples.

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Figure 2.

Failure of codominant stems occurred at the junction between the codominant stems and separated the two stems evenly.

Breaking force varied from 64 to 2,363 kg (Figure 3). The regression line produced from the comparison of stem diameter and force required for breaking the union when there was no included bark was Force = Diameter * 613 – 1388. The r² value was 0.92. When only those unions with included bark were analyzed, the regression line was Force = Diameter * 537 – 1285. The r² value was 0.76. There was a significant difference between the regression lines (p < 0.05).

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Figure 3.

Force required to break codominant stems of different diameters. Diameter was measured 30 cm below the junction.

As an example of the regression, a crotch 10 cm in diameter breaks at 392 and 484 kg for the included bark samples versus the nonincluded bark, respectively. For 15 cm diameter crotches, the break points are 880 and 1,040 kg, respectively; for 25 cm crotches, they are 1,857 and 2,155 kg, respectively.

DISCUSSION

Codominant stems that have bark trapped in the union are significantly weaker than those that do not have bark included. The differences appear to be greater with smaller-diameter stems than with larger stems. Using results from the regression analysis at 10 and 25 cm, the 10 cm stems are almost 20% weaker when bark is present. At 25 cm, included bark stems are only 14% weaker than nonincluded bark unions.

Due to the relatively low reduction in breaking strength at larger diameters, all codominant stem junctions should be considered weak. If trees with codominant stems have a target present that could be damaged if failure occurs, remedial treatments should be applied.