Once the winds calm down, many Oregonians will get busy surveying trees around their homes and businesses trying to assess damages and the need for tree care.

Photo courtesy of Michael Brewin

City of Tigard public works crew cutting and removing fallen tree blocking a street on Monday afternoon, December 3rd.

Usually, following any major storm event, many homeowners are anxious to have tree work completed. However, according to Paul D. Ries, a certified arborist with the Oregon Department of Forestry, "it's quite possible that as many trees will be damaged as a result of clean-up as were directly damaged by the storm."

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How to find an accredited arborist

That's because many times it's the improper pruning done to trees by inexperienced homeowners or unqualified tree trimmers that causes the greatest damage.

Topping, the practice of removing large branches and tops of trees, actually creates future hazards rather than eliminating them. A previously topped tree is much more likely to break or uproot in a storm than a tree that has a normal branch structure, according to the forestry department.

"Homeowners should use caution when selecting a tree service company," warned Ries, who manages the state's urban and community forestry program. He recommends waiting for an available certified arborist to prune trees, even if that means waiting longer for service.

Why do some trees remain intact, while others topple over?

It's sometimes hard to predict what will happen to trees in landscapes during storms, but the most common tree hazards occur when trees are unhealthy or stressed. One common reason for a tree to fall is because of some type of root problem. Two of the more common causes are laminated root rot, and armillaria root rot.

The first is fairly common and attacks Douglas-fir, hemlock, noble and grand fir throughout the Pacific Northwest. It's hard to detect but easiest to observe - especially with professional help - in the roots of fallen trees. It's spread by root-to-root contact and can infect other trees nearby.

Armillaria root rot can attack the same tree species, as well as western red cedar, and signs include oozing of sap from the lower trunk, as well as the presence of small, black, stringy growths called "rhisomorphs" around the roots. Roots that have decayed from these diseases may no longer do an adequate job anchoring a tree - especially when high winds are present.

Other causes of trees falling over? Once soils are saturated with water, trees may be rocked over by wind, and on slopes, trees are susceptible to slippage from debris flows.

Why branches break

Sometimes branches break at the "branch collar" - where the branch attaches to the main trunk. This is a part of normal self-pruning, but it's often caused by decay that has gone unnoticed by the home or property owner.

A tree's larger limbs can sometimes become weakened by rubbing or past, unrepaired storm damage. These wounds are weak spots that sometimes snap under the pressure of wind.

Large, horizontal limbs sometimes put unreasonable demands on a tree and supporting trunk tissue under the branch gives way in a storm.

Standing trees with damaged branches should be evaluated by a certified arborist before a removal decision is made. A certified arborist is someone who has passed a certification exam administered by the International Society of Arboriculture, the professional society for arborists. The PNW-ISA maintains a list of certified arborists for hire on their website at www.pnwisa.org