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End-weight reduction: thinning the outer canopy 

To test my theory that minor thinning near the end of a branch can significantly reduce the load on that branch, I designed a simple experiment. According to Gilman (2004) thinning is used to reduce limb weight on mature trees in order to reduce stress and strain on long branches, over-extended branches and those with defects. Proper thinning does not change the overall size or shape of the tree. Proper thinning involves removal of relatively small branches in the outer canopy near the ends of the main branches (Gilman 2002). 

Design

Using the family chin-up bar for a fulcrum, I balanced a 5-foot 2x4 stud cross the top of the pipe, ‘teeter-totter’ style and secured it in place with 2 U-shaped pipe clamps so that it could still pivot. At one end of this 2x4, I screwed an eye bolt into the bottom and attached an in-line fish-scale, anchored to the ground. The scale read 0 when the 2x4 was level and un-weighted. On the other end of the 2x4, I secured 2 lengths of plumbers tape to secure a branch. For my study, I used a 12-foot long branch from a Valley oak. The diameter at the cut end was 2 ½-inches. I placed the branch on top of the ‘balance bar’ with the cut end of the branch secured close to the distal end of the 2x4. 

Results

Unpruned, the load that the branch placed on the scale was 55 lbs. After measuring this, I made 8 pruning cuts; one 1-inch diameter cut, one ¾-inch diameter cut, three ½-inch diameter cuts and three ¼-inch diameter cuts. Almost all of these pruning cuts were made in the outer areas of this branch and all cuts were made by cutting back to laterals (reduction cuts). The reduction of the branch load dropped from 55 lbs. to 34 lbs. as a result of this pruning. The total weight of the pruned branches was 5 ½ lbs. Thus, by removing a relatively moderate amount of foliage, making small, easily healed cuts, it appeared that I had affected a 38 percent reduction in branch load. 

Conclusions

One limb tested with rudimentary equipment and a simple design may not prove much. It does, however, suggest that judicious thinning of foliage on small branches, particularly those that are over-extended or where branch strength is compromised by decay or other defect may reduce branch loading enough to prevent failure. I plan to test this theory using additional cut branches to see if I get similar results. I would like to see this approach tested more rigorously under more controlled conditions and with more sophisticated equipment.

 

Steve Garrison

Arborists that Climb

Santa Rosa, California

Western Arborist, Volume 32 Number 4, Winter 2006, page 29.

 

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Randy & Sharon Props ~ Owners

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1312 Ascot Ave ~ Rio Linda, CA 95673

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