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Trees Pose Peril to Those Below

Pines are especially prone to failure with their combination of "windsail effect" canopy and soft wood.

Aleppo pines, Italian stone pines and eucalypts are among the conifers that most frequently fail, sometimes causing thousands of dollars in damage to homes and vehicles, reports Robert Smaus in the Los Angeles Times.

"Of the hundreds of thousands of trees growing in Southern California, only a minuscule amount topple, but they're very noticeable," said West Hills, Calif. consulting arborist Robert Hansen. For one thing, many of the trees that fell during the recent heavy rains were growing in city parkways, easily reached by news crews.

"Even on TV you can see why these fall," Hansen said. "The roots grow on only one side of the tree because of the curb." Because the biggest roots are usually on the house side of the tree, it tips like it was hinged, right into the building.

A good arborist can spot most problems that eventually bring trees down. "Not all, but most," he said. "Most that fall in a storm or wind were already candidates for failure."

But during the series of storms this month, some trees fell for no discernible reason, said Jan Scow, a consulting arborist in Sherman Oaks who has spent the last few weeks looking at many. On one seemingly cursed residential block in Studio City, several species in different kinds of soil fell within sight of one another.

So how do you know whether big trees on your property might be at risk? Arborists assess risk by the type of tree, the tree's size and the damage it might cause. Evergreen trees, pines in particular, are more likely to come down because their canopy is always present and it's thick and heavy - "a windsail effect," Scow said.

Added Hansen: "I'll bet half the trees that fell in the last storm were Aleppo pines. You won't ever find Bob Hansen napping under one," he said, only half-kidding. "They shouldn't be planted within striking distance of a home."

Another pine that frequently falls is the majestic Italian stone pine. "They can have a spread of 80 feet with roots that fan out for only 10." One beefy specimen was lying on a sidewalk in West Los Angeles after the last series of storms, the latest of several on that street to topple in storms.

"Every tree I saw on the news was either a pine or a blue gum," said Greg Applegate, a consulting arborist in Tustin. Fast-growing trees, including eucalyptus, may topple because of problems that begin while in a nursery container, such as circling or girdling roots, he said. Very old blue gums also get a fungus called sulfur conk, which weakens the base of the trunk and may cause them to fall.

Deciduous trees such as crape myrtle and Modesto ash can be more stable because they lose their leaves before winds and winter storms arrive, but not always. Samuel Knapp, author of the 10-page pamphlet "A Guide for Safe, Healthy and Beautiful Trees" and a consulting arborist in Riverside, said, "Sycamores are very stable, but jacarandas are less so because they're not as adapted to our climate."

Big trees are obviously more of a risk because they are heavy, and those close to parking spots, sidewalks or homes are a bigger risk because they can do damage.

In the recent rains, a huge oak at Occidental College came down but did no damage because it was in a large, open area. Knapp advised leaving it there, on its side, with only a little compensating pruning. "It still looks very handsome," he said.

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June 26, 2019, 11:58 am PDT

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