Altadena Heritage looks at windstorm damage at the Arboretum

Western catalpa at entrance to Kallam garden at the Los Angeles County Arboretum with its trunk completely shattered by the winds of Nov. 30-Dec. 1, 2011.  Photo by Frank McDonough and used with permission.

by Laura Monteros

The Los Angeles County Arboretum in Arcadia lost more than 240 trees and sustained about $2 million in damages in the “once-in-100-years” windstorm last fall.  Thursday evening, Frank McDonough, Botanical Information Consultant at the Arboretum, explained why.  

Rich Benson, engineer and urban lumberjack, introduce McDonough and said the program would show how to “help trees not to fall down.”  This is the second in a series of programs on sustaining the urban forest sponsored by Altadena Heritage. 

“People really love the Arboretum,” McDonough said, citing support and help from county agencies such as the fire department, urban planning, and several others in the cleanup.  “As a result, were able to open up around Christmas,” far earlier than expected.  

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

• Choose wisely.  “What you plant has a lot to do with where you live,” McDonough noted.  Make sure the tree is right for the location. It should be in scale with the garden and house, not be bound by pavement or utility lines, take into account the geography of the property, and be the kind of tree that is good for the area and can stand up to windstorms.
• Maintain properly.  The drip line is the key to much of the maintenance.  Most trees should not have plantings under them past the drip line, and any plantings under the tree should be compatible with the watering needs of the tree. “Overwatering can be as bad as underwatering,” according to McDonough.  Weed whackers can damage the trunk, and ivy puts tendrils into the bark that can crack it, as well as hosting “a lot of bad things”.  Both open the tree to infection.
• Do not overtrim.  Or maybe, don’t trim at all.  Trees have evolved their characteristic shapes to stand up to the environment, McDonough said, so only do as much pruning as is absolutely necessary.  Homeowners should ban people from lacing out trees, because the smaller interlacing branches are needed to balance the harmonics of a tree in a windstorm and keep the trunk from reaching the frequency where it will shatter.  Decreasing the canopy also means decreasing the amount of food the tree needs to grow new roots.  
• Other types of trimming—crowning, to decrease the height of the tree, and pollarding, which cuts all the branches back to the main limbs, can be done, with caveats.  Rather than crowing a tree, it’s better to choose the right scale of tree in the first place, McDonough said.  Pollarding must be done every year from the time the tree is planted, as mature trees that have not been pollarded cannot take it.

For more information on trees and gardening, the Arboretum is a good source, and the upcoming Garden Festival, May 4-6, is a good time to go.  The celebration will unveil the new water harvesting garden as well as having displays, lectures, demonstrations, and plants and merchandise for sale.  Details are on the website: http://www.arboretum.org/index.php/la-garden-show/

Previously: They're urban lumberjacks (and they'r e OK).