Saturday 4 October 2008

Pruning Article Reponse

We have a winner! A few months back I offered a professional pruning multi tool to one reader who sent in a tree pruning question or picture. See the original post here. The randomly selected winer from the many great questions and pictures we received is Mr. Dave P.

The question distilled to its essence is as follows; If you cut down a stand of aspen, Populus tremuloides, there is a technique of damaging the resulting suckers that will eventually prevent the suckers and kill the tree. Is this technique a valid way to prevent regeneration of the aspen or just a tall tale? Dave goes on to describe the method proposed.

Anyone who has removed aspen from their property or seen areas where aspen was harvested in the wild knows that they respond to sever pruning, cutting them down by sprouting hundreds of new shoots. These rapidly growing shoots will choke out the commercially more desirable species like spruce and pine. Typically these shoots are controlled by treating them with chemicals or by cutting them down. The technique Dave describes allows letting the shoot grow to several feet high and then cutting them partially through with a clearing ax , (pic above) at the base of the stem. You must only cut though enough to sever part of the stem and still leave enough to allow the tree to stand up right. Over time these damaged shoots will stop growing and eventually rot and break off at the injury. While they are slowly dying they will prevent the roots from sending up new sprouts and deplete the roots of energy resources.

While I spend most of my days trying to save trees the principles of botany that regulate pruning response could also be used to prevent a tree from re sprouting. Aspens are arguably the oldest living trees although their above ground parts don't last all that long. Aspen roots are know to live thousands of years. When aspens are injured by storms, harvesting, or by browsing the chemicals released by the buds and growing shoots to prevent sprouting and suckering are no longer present. The roots are then free to send up a tremendous flush of new sprouts. This continues until the new shoots are able to generate enough suppressing chemicals to prevent further initiation of shoots.

If you were to partially sever the phloem on the outside of the stem a small portion of the inhibiting chemical would still pass down to the roots and prevent further suckering. Also photosynthate would not travel down to the roots from where it is produced in the leaves. The roots would continue to provide water and nutrients from their stores transporting it upwards in the intact xylem. The roots would slowly starve to death. In the mean time the stems would begin to rot at the site of the injury and would eventually fall over and die as well. It is completely possible that this will work. The mechanisms of the plant regulators are more complicated than this but in this case this simple explanation will work.

I have seen trees with stems girdled by rodents leave out and look fine for a year or two and then just die once their stored energy was depleted. Dave, I hope you enjoy your multi tool. It may be small but you could use it to test this hypothesis! Good luck and thank you all for your questions.

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