Patt Morrison

<em>Patt Morrison</em> is known for its innovative discussions of local politics and culture, as well as its presentation of the effects of national and world news on Southern California. Hosted by

Arborist explains why some trees fell, others survived

by Patt Morrison

A boy whose school was closed climbs fallen trees on Green Street on December 1, 2011 in Pasadena, California. David McNew/Getty Images

The most recent Santa Ana winds took down trees that were hundreds of years old and had previously withstood devastating earthquakes and winds.

Frank McDonough is the botanical information consultant at the Los Angeles County Arboretum, where as many as ten percent of its trees cannot be salvaged. He said that righting some common misconceptions about trees can prevent more of them from toppling in the future.

According to McDonough, how big a tree is and what kind it is, whether native or non-native, have little effect on whether it will surrender to high winds. Rather, the kind of care a tree gets dictates its longevity.

While many Angelenos pay to cut branches off their trees on the assumption that it makes trees healthier, "lacing" or pruning can actually make a tree weaker. McDonough said the tree's complex branching structure in its thick canopy helps keep a tree sturdy when the wind blows; too much lacing makes it a pushover in high winds.

Too much trimming can also damage root structure. "What makes food for the tree? The leaves," he said. "Fewer leaves, less food for the tree ... It kind of conserves, pulls back, sloughs off roots underground so the root structure gets smaller and smaller. Well this is a big tree, lots of mass in its wood. That's just not good at all."

The arborist said that as he inspected fallen trees near the arboretum in Arcadia, he noticed that the trees more prone to falling over were the ones that had been over-trimmed or that had been constructed around, thus damaging their roots.

"Trees' roots work like an antennae tower, put up by guide wires," he said. "[A plane] may be able to hit one of the wires, but if it hits two or three, that antennae's coming down." The problem, said McDonough, is knowing which root to cut – it might be the one that determines whether the tree lives or dies.

Construction around a tree – yard, sprinkler or sidewalk work, or any digging, can affect a tree's health. The tree's root structure should be about the same size as the tree's canopy.

It's also counterintuitive, he said, but it's a misconception that trees need a lot of water because Los Angeles is drier. Overwatered trees are susceptible to a root fungus that can weaken the root structure. McDonough said that heavily watering a tree once a week is enough.

"The bottom line is all trees are disasters waiting to happen, because they have finite life spans and they pretty much expire when they're at their greatest mass." McDonough suggests that owners who plant large trees next to property should prepare exit plans for aging trees.


What made certain trees more vulnerable or durable than others? Is there anything you should know when replanting downed trees in your neighborhood? Patt checks in with a local tree specialist who’s been surveying the damage around town.


Frank McDonough, botanical information consultant, Los Angeles County Arboretum and Botanic Gardens

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