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Thursday, December 22, 2011

The Problem I Warned About on Pruning Pine Trees Just Happened

I said it might happen and it did.
This pine tree has a limb that broke in a
windstorm not too long ago. It is just
at the bottom of the trunk.

Thinning pine trees by removing smaller limbs from larger limbs (thinning the canopy) is NOT a good idea. This has been a relatively recent trend in tree trimming (I do not want to call this arboriculture) is done to reduce the potential that trees will blow over in high winds. Instead, thin the canopy  by removing entire limbs from the trunk to reduce wind damage and blowover . Here is why.

Plants grow both in length (called primary growth) and width or diameter (called secondary growth). When secondary growth occurs along a limb or trunk, progressively, as it get longer, then the limb or trunk exhibits "taper" or a gradual increase in girth along its length. This is good unless you are growing trees to use as telephone poles or for lumber.

A plant develops taper along its trunk or large limbs if the trunk or limb can bend freely in the wind as it is growing. The free movement of the tree trunk or limbs increases the degree of taper. If the trunk or limb is held so that it cannot move (staking so no movement occurs), primary growth increases but its growth in girth (consequently its degree of taper) decreases.

Here is the limb that broke

Taper also inceases if smaller, lower limbs are left attached to a limb or trunk. These smaller limbs, with leaves attached,  send carbohydrates manufactured in the leaves or needles back to the limb or trunk. This helps "feed" secondary growth causing more taper at areas closest to those small limbs. A distribution of smaller limbs along a trunk or limb causes an increase in its degree of taper. On the other side, removing these smaller limbs along a trunk or limb REDUCES the degree of taper.

The limb that broke is in the center of the picture. Notice
how little taper the limbs have. Leaving all the growth
at the ends of the branches also causes the limbs to have
a "weeping" effect.
When a trunk or limb bends, and it is tapered, the stress of the bend (shear) is distributed along a great deal of its length. If there is little taper to a limb or trunk, then the stress is localized at a very small portion of its length. When a limb is not tapered, the stress of bending causes the limb or trunk to "snap" (shear)  at its weakest part or where the majority of the "load" or stress is localized.

Here are three principles to follow to increase taper in a tree:
1. When planting a tree, make sure stakes are located as low on the trunk as possible. Tree stakes should keep the rootball or rootsystem stabilized, not the entire trunk. The trunk and limbs should be free to move in the wind if possible.

2. Remove stakes as soon as possible after the root system has become established. This should normally be one season or less. If it is longer than this then you may have a problem.

3. Leave smaller limbs attached to the trunk for three to five years if they are healthy and vigorous. Try to maintain a ratio of canopy length to pruned trunk of at least 2:1.

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