The extensive damage included the plantings on Tallac Knoll in the American section, which is shaped like an airplane wing, allowing the wind to shear the top and create turbulence on the downside.  In a section of large deciduous trees, many snapped in half.  The most common losses were due to uprooting, caused by failure of the root plate, McDonough said. 

At the Arboretum, many of the damaged trees have been left in place, whether still standing or lying on the ground.  “We keep them around, because they are part of a collection, and might be the only one of its kind,” McDonough said.  The tree could resprout, and cuttings and seeds can be taken from it and planted.  

The Arboretum sustained so much damage, McDonough explained, because two highs came together over Mt. Wilson and rushed down the mountainside into Arcadia.  Some of this damage, and damage to trees in the surrounding communities, could have been avoided by proper management of the urban forest, he said.

In urban areas, construction, poor choices in trees, and improper maintenance were cited as factors in the widespread damage.  These lead to rot and failure of the root system, trunk, or major limbs.

The root plate, or root shield, is the ellipse of roots in the first 18 inches of soil that feed and stabilize the tree.  Ideally, it should be as extensive as the drip line, wider on the windward side than the leeward, and longer in the other two directions.  Pavement or utility lines that were put in after the tree can cut off the roots and form a “hinge” for the tree to blow over.

Trunk failure can be caused by rot, bifurcated trunks (particularly with an angle more acute than 45 degrees), trunks that are too small in diameter for the crown, and  lack of regular winds to help build up the strength of the trunk.  “We don’t have regular Santa Anas here,” Arcadian McDonough said (a statement some may argue), “so they don’t grow as strong.”  Girdling roots constrict and narrow the trunk at the bottom and cause instability, while above-ground roots that spread out increase diameter and provide stability.

Loss of major limbs can result when trees have been “laced-out”.  Lacing out is a type of trimming that removes all the small branches, often to improve a view or allow light through the canopy.  Without the small branches, the large limbs are subject to breakage.

Rot can occur when trees are damaged through improper pruning, over- or underwatering, and the removal of beneficial organisms that help resist disease from beneath the tree.  Treated lawns and plantings under the tree may kill the very organisms that provide a defense against rot.

Here are some of the key things to preventing damage in trees