Gary is a very down-to-earth Urban Forester with the University of Minnesota, and he has an entertaining sense of humor. I especially enjoyed his teachings. Intelligent AND entertaining instructors hold a LOT of merit with me!
The following
are notes taken during an one-hour lecture on formative pruning of young trees.
Minnesota
has had several years with straight-line windstorms that created
huge amounts of tree damage. A perfect example is in the Boundary Waters
area in northern Minnesota. Gary and the UofM did many autopsies
on storm damaged trees, and repeatedly throughout the 45 minute
class Gary stressed three major causes of storm damage found in
Minnesota trees:
The first, decay, is often associated with large amounts of damage that take several years to heal over. Obviously, the larger the damage, the longer it takes to heal over, and ultimately, the better chance for old wood decay to set in.
The immediate goal in pruning would be to produce the smallest cut possible, as early as possible in the tree's development after the 1st or 2nd year from planting so that the damage can heal over in the shortest time span possible. On older branches, the absolute minimum diameter cut will prove to be the healthiest for the tree due to healing time and the amount of decay that could set in during healing.
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If you are working for an adamant customer who can't visualize this concept, try to minimize the cut as much as possible.
So, with the minimization of decay discussed, what limbs are considered unhealthy? Assuming all limbs appear to be free of outer damage, bark inclusion is the second leading cause of storm damage according to the autopsies. It has long been falsely believed that narrow crotches are unhealthy. By itself, this concept is false, a narrow crotch can be just as strong as a wide crotch. What makes a weak crotch is a condition known as 'bark inclusion'. This condition is visible where the top of the crotch has a line where the bark appears to be disappearing down inside the crotch of the limb. In effect, the bark from the trunk is competing with the upper bark of the limb, and as much as half of the top of the limb is physically unattached to the trunk. Only the lower half of the limb attachment is supporting the limb weight.
Whether
a narrow or wide crotch, a healthy crotch with strong limb
attachment will have a very noticeable ridge of bark on the upper
surface of the crotch where the bark from the trunk meets the
bark of the limb, continuously pushing upward as strong, healthy
wood grows around the entire point of attachment.
Okay, that takes care of decay and bark inclusion. The third leading cause of storm damage is the condition known as 'co-dominant leaders'. If this term in not immediately obvious, it is a condition were more than one branch on the top of a young tree try to become the main leader straight up. On an ideal tree in the 6-10 foot range there should be one main trunk leading straight up with side branches protruding laterally. Sometimes the central leader will become damaged and two or more of the buds immediately below the injury will sprout and head upwards. If left to fight it out, more than one of these new top shoots will survive and compete for dominance, ultimately weakening the overall sturdiness of the tree.
Ideally (not necessarily realistically) there should be one main trunk reaching to the very top of the tree. Some judgment will need to be exercised here. If a given tree typically reaches 60ft, it would be wise to isolate a central leader up to 15 or 20 feet before letting the tree to its own devices. The theory is that co-dominant leaders allowed to exist below 15 feet create a condition where the biomass on each co-dominant leader puts massive amounts of weight and torque exerted upon the crotch where the co-dominant leaders meet. In a strong winded storm the tree is not only blown back and forth, but twisted around sideways as well, creating extreme pressure on the crotch. (Though not specifically stated in the class, I believe that existance of co-dominant leaders often includes a fair amount of bark inclusion as well.) The end result is that the massive torque and pressure exerted on the crotch results in a split down the center of the tree. This split could kill the tree outright, or leave such massive damage that the tree can't heal over before the exposed heartwood rots, leaving a hollow shell of a trunk to be destroyed in the next storm.
Co-dominant leaders are a condition that need to be corrected as soon as they are noticed, ideally right as the shoots make themselves visible. At this stage the damage will be tiny, easily healed, and the chance for decay or bark inclusion will be eliminated. When corrected early, the isolation of the main leader can be fairly indiscriminate, as the trunk thickens with time there will be no evidence of the pruning and the tree will thicken proportionately to create a 'straight' trunk. The longer co-dominant leaders are allowed to exist, the greater the chance of bark inclusion forming, the longer it will take to heal, and the more lopsided the tree will become when the condition is fixed.
That was the meat of the class, held in the field with actual demonstrations for each situation. There were many questions and discussions that yielded by-notes and discussions, some of which can be read here, http://plantkingdom.com/miscpages/pruning2.htm.
Clickable
Taxonomic Tree of the Plant Kingdom.
PerryPost@plantkingdom.com