Arborists are often asked to assess tree hazards on a property and are taught to look at tree health, target, and size. Jesse Milton, in his casual research, reports that in 38% of trees that he has observed to fail, the defect was either not visible or there was no defect. It is no wonder that arborists often recommend to take down healthy-appearing trees near commercial and residential structures. The decision is particularly tough when the tree is a specimen hardwood that is healthy. To complicate matters is the bigger issue of probability; typically, it is more likely for a person to get in a car accident than for the tree to fail. Arborists are left with measuring the risks and rewards of leaving a tree. Due to liability issues, these trees are typically removed. Is this the right course of action or are there alternatives?
Georgia Urban
Forest Council
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4 Comments
38% seems awfully high, in speaking with a consulting arborist in Atlanta, the number of trees he has observed with no sign of defect is closer to 5%. Resistagraph testing might also reduce the percentage of trees where no defect was visible.
Emphasis should be placed on “visual”. To accurately complete the statement: “95% of the trees that fail have some type of defect which results in that failure. Those defects may not be visable in 35% of all failures.”
Decisions to remove trees should be based on the best available science, and application of a hazard rating that considers, the tree part(s), likelihood of failure, and target potential.
Ed
As a clarification, the casual research I performed was in regards to tree failures during the slew of high-wind weather events in April of 2009 only. We were seeing loading speeds of over 50 mph, which is very unusual.
Of those failures that I recorded, 67% had a defect, but only 38% had a defect that was plainly visible prior to failure. Twenty-three percent of the failures did not have an quantifiable defect at all.
In HongKOng, all tree failures are proved to have it’s defects, as the climate here is hot and wet, and the damages by termites is not unusual. All professional guidance are expensive,and lacking the kind of specialist in this small island. So whenever there are typhoons, it counts on your luck whether you are the target or not.