Warmer weather this spring also means we are now entering severe weather season. Thunderstorms, high winds and tornados can have a big impact on trees. When trees are dormant, most of the high winds can pass right through them, but when trees have leaves, the situation can change dramatically. Leaf surfaces can catch the wind, causing a sail effect that can cause extreme tension on trees. Trees are biologically designed to sustain average wind loads. Trees grow in response to wind also. Trees sense structural stress and attempts to minimize failures through reactive growth. (Extra wood grows in areas that are most weak for the tree.)
Trees must withstand wind to survive. Wind and gravity both impact trees in storms, but the dominant load is from wind. Storm winds push on tree crowns and stems causing root plates to rock. The result of storm winds are forces twisting and bending tree parts causing either the part to fail or the supporting soil to fail. The weakest parts of the tree are most likely to fail. This can be compounded by saturated soil that is weakened during heavy rain events.
A structurally sound tree has numerous advantages over other trees when high winds occur. The crown of the tree is well balanced. There is not an excessive amount of interior growth (causing the sail effect). The large supporting limbs are well spaced and attached securely to the main trunk. Typically these branches originate off the trunk in rather wide angles which allow strong wood to grow completely around the limb.
The trunk of a structurally sound tree becomes narrower the higher up you go. This growth pattern disperses wind much better than a tree trunk that maintains the same width throughout the tree. Structurally solid trees also have strong root systems that firmly anchor the tree into the ground.
Warning signs that indicate a tree susceptible to failure in a wind storm:
Root rot- The tree does not have a solid foundation, and may tip over, uprooting the tree as it falls over. Look out for signs of decay such as mushrooms at the base of the tree, cavities, or insect damage.
Excessive end weight- Trees that have limbs that extend out horizontally are fighting both gravity and wind loads. These type of limbs are more prone to failure. Prune these type of limbs back to an acceptable lateral branch, or cut them back to the main trunk as necessary.
Clustered canopies- Trees that are excessively thick with internal branches and leaves tend to create a larger sail affect and may break in high winds. Branches that are rubbing on each other become weakened over time and also are candidates for failure.
Poor branch structure- Codominant branches (two limbs of equal size originating from the same union) are often weaker and are prone to splitting at the union. Cut one back to subordinate it to the other one, or remove one all together. If this is not an option, fastening a cable system to hold the two limbs together so they can’t separate in a wind event.
Hoppe Tree Service arborists are well-versed in assessing the structure of trees. If you would like an estimate for pruning, removal, or would like a hazard tree assessment, please contact us.
Certified Arborist WI-0477A