Tuesday, June 16, 2009

Landscape - Storm Damage to Trees

Recent violent storms across the state have done significant damage to trees in the landscape. The following is a reprints of an article on types of storm damage to trees.

There are six main types of storm damage to trees:
(1) blow-over, (2) stem failure, (3) crown twist, (4) root failure, (5) branch failure and (6) lightning. Each type is the result of a complex and interactive mix of tree problems and climate.


With blow-over, the tree is physically pushed over by high winds. Little biological adjustment is available for a tree (or for people) to make to hurricanes, down-drafts or tornado winds. The wind force on the aerial tree portions is too great for the wood structure. Past tree abuse, poor maintenance, pest problems (like fusiform cankers on pine or root rots on hardwoods) predispose the tree to storm damage by weakening the wood architecture.

Stem Failure

Trees do not heal wounds. Trees can only grow over old wounds and seal them off. This results in a tree carrying in its wood every injury it has ever had. These old injury sites -- and the old and new wood around them -- are structurally weaker than normal solid wood. These damaged areas can quickly fail under a constant wind loading and release. Pest damage, weak wood around old wounds, new wounds and failure of the tree to adjust to wind conditions can lead to stem failure under heavy wind loading and release.

For trees with heavy crowns, abrupt wind gusts and calm periods can lead to stem breakage from release. As the wind load is quickly released, the tree moves back into an upright position. If the mass of the crown moves too quickly when released, the inertia of the moving crown may move too far in the opposite direction leading to stem damage and breakage.

Crown Twist

Tree crowns are the leaves and supporting twigs and branches. Trees are never perfectly symmetrical in every direction. Many trees, through past abuse and poor management, have lopsided crowns. More wind loading on one side of the crown than on another produces a twist (torque) on major branches and the main stem. Over time, the twisting effect can be biologically adjusted from within the new wood. Stem twisting will magnify weaknesses around old injuries and the stem will split or branches collapse.

Root Failure

There are two types of tree roots: fine, absorbing roots and woody, structural roots. As their names imply, ab-sorbing roots have a massive surface area but are weak. Structural roots are woody, have a relatively small sur-face area, but are strong. Both types provide anchorage for a tree. The primary roots growing from the bottom of the stem (root collar) play dominant roles in holding the tree upright while conducting water, essential elements and nutrients. If roots are constrained, diseased or dam-aged by construction or -- as the top of the tree becomes larger -- greater stress is put on the roots. Pulled or snapped roots cause trees to fall or lean.

Branch Failure

Branches are poorly attached to the main stem. A branch is stuck on the side of the stem each year by a small layer of stem wood called the branch collar. The branch collar surrounds the branch base. The woody material from the branch enters the stem and turns downward. This structural arrangement allows the branch to be flexible and disposable. The stem can shut off the branch when the branch becomes a biological liability to the tree.

Heavy loading (as during an ice storm) puts great stress on the branch collar area. Over many years, a tree will adjust to this stress, but ice storms or downbursts that occur only rarely will leave the branches unprepared and susceptible to tearing downward along the stem or snapping. The branch collar area can also be weakened by included bark. This material is bark from both the expanding stem and branch. Where the branch and stem expand against each other, bark can be surrounded and overgrown inside the branch collar area. Included bark leads to weaker structure and a place for pest attack. This is why forks (called co-dominant branches) are structurally weak. These weak areas can easily fail in a storm.


Lightning damage is a life-threatening situation. Lightning either moves in a narrow line down the branches, stems and roots or along a wide pathway encompassing the entire tree cylinder. Lightning directly destroys tree tissues by electrical disruption and heat. Steam explosions down the stem in a wide or narrow band show where the electrical current has moved through the tree.

Massive root damage can remain unseen. Damage caused by lightning leads to extensive water loss that is also life-threatening. Pests quickly attack a lightning weakened and damaged tree. For example, the Southern pine beetle quickly destroys a lightning struck pine.

Information from "Storm Damaged Trees: Prevention and Treatment" by Kim Coder, Professor Silvics/Ecology, Warnell School of Forest Resources, University of Georgia. Go to http://pubs.caes.uga.edu/caespubs/pubcd/C806.htm for the full fact sheet.

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