The Biology Behind Pruning Young Trees
Structural training is a type of pruning on young trees to help develop proper branching patterns and structure. Negative impacts of poor structure are not prevalent when trees are young. However, as trees grow and weakly-attached limbs become larger, structural issues can become a point of failure. Understanding the biology of trees can help the process of creating well-structured trees in our urban landscapes.
Let’s keep this simple and focus on FOUR tree biology terms that are commonly discussed for pruning younger trees.
#1 Included Bark
Included bark is a point of a weak branch juncture in which bark is pushed inward. Included bark often occurs where there is a narrow angle of branch attachment, such as the angle between two co-dominant leaders. Branches with included bark are inherently weak and have a higher chance of failure.
The yellow arrow indicates included bark between two weakly attached co-dominant leaders on an Autumn Blaze maple tree.
#2 Co-dominant limbs
Co-dominant limbs are two limbs of equal size and importance (or dominance) competing to be a major limb often having a weak attachment. Often these two limbs (termed co-dominant leaders) are the two main limbs in the center of the tree. Becasue of their potential point of weakness, these limbs have a higher chance of failure as they become larger.
Ash tree with two co-dominant limbs trying to be the central leader of the tree.
#3 Redundant Limbs
Redundant limbs are limbs of equal size growing in the same general area creating an overcrowded canopy. The picture to the below shows 3 limbs of the same size all growing very tightly in same area and direction.
Linden trees with 3 limbs essentially in the exact same growing space creating a redundant situation.
#4 Scaffold Limbs
Scaffold limbs are essentially the main limbs that will remain with the tree for its life. They should be well-attached, well-spaced and high enough for lawn or street clearance. All limbs below the lowest permanent scaffold limb should be considered temporary limbs. All limbs below the black lines are temporary limbs. Short-term pruning may include crown raising limbs up to the red line. However, long-term pruning will eventually need to remove all limbs up to the black lines for street clearance. The location of the tree will obviously have consideration regarding the eventual height of the lowest limb (street vs. lawn vs. park)
These swamp white oaks will have NO permanent scaffold limbs below the black dashes. The eventual lowest limbs will be somewhere in the area indicated by black dashes.
