Why trees fell in January ice storm; steps to take before this winter begins

With a hot summer very definitely upon us, and the potential of another cold winter with more extreme weather possible, it’s not too early to start thinking about what you might want to do to try to prevent tree damage similar to last January’s in the winter ahead.

Southeast Portland area residents were shocked by the large number of big trees that fell during the ten-day arctic storm that first struck on late Friday evening, this past January 12th. Well over 1,000 trees fell in all three Portland Metro counties, and possibly a lot more. Weather experts called the storm “historic”, for its combination of low temperatures and high winds – and especially for its week-long duration.

Many falling trees knocked down electric lines, creating widespread power outages. All the MAX trains were halted by debris on the tracks, and damage to overhead power lines. Some trees damaged and even crushed cars and houses. Two people were killed when trees fell on their homes – a man in Lake Oswego; and a woman in Southeast Portland, who was staying in an RV with its owner and another person, getting heat from a stove. The falling tree smashed down into the stove, and the RV burned.

The extent of the damage caused by falling trees stunned our First Responders.

“Trees have also fallen in many previous storms we’ve seen. In this ice storm event, there were a larger number of total tree failures than average,” observed Mark Ross, spokesperson for Portland Parks & Recreation, where the city arborists are employed in the Urban Forestry department.

We wondered just why so many large trees fell during this storm, and talked to several experts to find out. Among them was Brittany Oxford, a community assistance forester in the Urban and Community Forestry program at the Oregon Department of Forestry.

Perhaps surprisingly, Oxford first remarked on how many trees in the region did not fall. For example, Portland alone has 220,000 street trees, 1.2 million park trees, and about 2.9 million trees on private property.

“I would like to point out that while it is easy to focus on the trees that failed, we should also remember that the vast majority of trees in the city withstood this severe weather event in good shape. It is common after events like this for property owners to unnecessarily remove healthy trees, just out of fear. I would caution against doing this, and I ask that tree owners have a qualified arborist assess any risks they think trees on their property might pose, and then make suggestions about maintaining their trees,” Oxford said.

Oxford also told us that there was no single reason why any particular tree fell. The storm included sustained high winds, especially in East Multnomah County, and also at higher elevations – such as the Mountain Park area of Lake Oswego, where so many homes were damaged. But Oxford said those trees’ roots were likely compromised well before the storm.

“Generally, when entire trees fall in winter storms, it is often the result of the combination of saturated soils and high winds. Heavy rain in compacted, poor-draining urban soils can become saturated quickly, making it difficult for existing root structures to stabilize trees. When high winds interact with these conditions, it can result in trees falling.

“If roots have been cut during construction – say, of a driveway, an auxiliary dwelling unit, or a new water or sewer line – that can also make a tree less stable. And some trees have been planted with poor techniques, and have girdling roots that don’t anchor them into the soil very well. In a case like that, 20 or 30 years later when the tree is large, it can fall over from poor anchoring roots, which actually results from how it was originally planted,” Oxford explained.

Other factors cited by experts include tree height, and “isolation” – which increases swaying in high winds.

Although summers in the Portland region are certainly getting warmer, Oxford said that heat alone is unlikely to weaken a tree enough to make it fall in a storm. But droughts are another matter.

“It is important to understand the difference between heat stress, and stress as a result of prolonged drought. Higher temperatures can kill foliage and branches, but would not be lethal by itself to a tree. However, drought stress – linked to hotter and dryer summers – can play a more significant role in stressing trees over a prolonged period. This can also hinder a tree’s ability to produce secondary growth that can protect them from high winds,” Oxford said, reminding that just like other plants, trees need to be watered at the roots during our increasingly hot, dry summers.

Finally, Oxford repeated the importance of not overreacting to January’s unusual ice storm. She urged concerned property owners to regularly have an ISA-certified arborist inspect their trees for risk management and maintenance needs, in order to keep as many as possible.

“Our urban forest provides many irreplaceable benefits within our communities – including cooler temperatures, cleaner air, reduced stormwater runoff, captured carbon, and the promotion of community health and safety. A properly maintained and managed urban forest results in healthier and safer communities,” reminded Oxford, who also praised Portland Parks & Recreation’s Urban Forestry department, which employs the arborists who cleared downed trees from city property.

“Through hiring highly qualified staff, cultivating community engagement, and proper planning, the city of Portland’s Urban Forestry department has been a leader in urban forestry management in the United States,” she said.