Spotting danger signs in trees

By Bill Pramuk

A recent trend in the professional world is the concept of risk management. It certainly applies to living with trees.

The idea is that we live in a risky world and there is never a guarantee that we are safe. Those of us making management decisions have a duty to recognize risks and to minimize them to the best of our ability.

As we come into the season of winter storms, tree risks are of increasing concern, since wet soil and windy weather tend to bring out the worst in large, old trees.

Etched in my memory is the image of a large Monterey pine that became unstable in wet and windy conditions a few years ago. The property manager called for advice. As we discussed the logistics of an emergency removal, a couple of strong gusts of wind blew it down before our eyes. Fortunately, only a cyclone fence was crushed; no one was injured.

Arborists are often called upon to evaluate tree risks. Even though numerous researchers have devised methods and formulae for determining strength loss in trees, none are absolutely reliable. Certainly, arborists must base recommendations on careful observations and analysis, but they often are forced to use a gut-feeling to help draw a conclusion. It then falls to the tree owner or managers to make a final decision.

The idea of “a hazard” is a little tricky, because a hazard is comprised of three components. As given in “Evaluation of Hazard Trees in Urban Areas” (Matheny and Clark) they are:

• A tree with the potential to fail

• An environment that may contribute to that failure

• A person or object that would be injured or damaged (i.e. the target)

If any of the three components is missing, there is no hazard.

When comparing a group, or large number of trees, we can do hazard ratings to help with management decisions. ISA (the International Society of Arboriculture) developed a Tree Hazard Evaluation Form for this purpose. The arborist records data about the trees’ characteristics and defects, tree health, site conditions, and potential targets. A score of 1 to 3 is then assigned to each of three categories:

• Failure potential

• Size of the part that is likely to fail

• Target

A high number indicates a high hazard.

That system is helpful for comparing and prioritizing a number of trees. To examine an individual tree, we start with a visual tree analysis. Much can be discovered from simple, careful observation. The practiced eye can spot subtle signs of cracks, decay and instability, that commonly go unnoticed. It is easy to miss big problems too. When lightning blew half of a big redwood tree to smithereens at a winery in Napa Valley a few years ago, the owner told me that people had walked right past the tree into the tasting room and asked where they could see the tree that got hit by lightning. You can’t see until you look.

Here is a simplified checklist for beginning a hazard assessment when you are concerned about tree risk.

• Target: Is there a permanent structure, a road, a play area, a bench or anything that indicates frequent usage of the area. No target, no hazard.

• Size of the tree part that is likely to fail: Is the portion that is likely to fail a 1⁄2 inch diameter dead branch, which might poke someone in the eye if it breaks off and they happen to be in the wrong place at the wrong time? Is it a 4-foot diameter decayed oak tree trunk with enough weight to flatten a car or a house?

• Failure Potential: This is where the practiced eye comes into play. Determining failure potential starts with a visual analysis, which may lead to more thorough physical examination.

Physical examination and evaluation of tree defects can include excavation of the root collar and buttress roots, drilling with a slender bit to feel for decay and cavities, drilling with a Resistograph, which records the resistance of the wood to the bit, or even the use of sonic tomography — a sonogram for trees. All of these require training and practice. None stand alone as a single, definitive method for tree hazard assessment.

Gut-feeling can be an amazing tool. Common sense and our natural sense of beauty and order tend to inform us when something just doesn’t look right.

Visual clues of trouble include a change in the angle of a trunk’s lean, cracks or heaving of soil around the base of a tree, motion in the lower trunk in gusty winds, mushrooms or other fungal fruiting bodies growing on the base of a tree trunk or on limb surfaces, separation at the attachment of multiple trunks or large limbs, and physical defects such as cankers, cracks or splits in limbs.

So, take a close look at your trees and don’t hesitate to call for professional help if something just doesn’t look right.

Bill Pramuk is a registered consulting arborist. Contact him at www.bill pramuk.com, info@billpra muk.com, or 226-2884.

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