So…why did a tree fall?

s-tree-risk-assessment-qualified-copy.png (155×235)by Ronald E. Rothhaas Jr.

Arbor Doctor LLC

 

So…why did a tree fall over on a car in Cincinnati killing the person inside? This may seem like an easy question but the answer is complex.

Why did a tree fall?  Obviously, gravity caused it to fall.  Gravity acts upon every tree but obviously millions of trees don’t fall over in the presence of gravity.  Why was this one different?

  • Tree failure is not as rare as you may think.  The US Forest Service maintains the International Tree Failure Database.  As of 1/6/2015 there were 7,008 Tree Failure Reports in the ITFD Database. Scientists and arborists study these failures in the hopes of preventing future mishaps.
  • This tree was an urban street tree. Urban street trees are subject to soil compaction, vehicle accidents, construction damage, air pollution, salt pollution, drought, and severe microclimates which are hot and drying in summer and variable in winter.
  • This tree had a sidewalk on one side and a street on the other. It is likely that construction impacted the tree over the years.  Scarring of roots or trunks allow entry of pathogens which, after many years, can result in failures such as this tragic incident.
  • The tree was growing at a street side location which typically contains dense and compacted soil. Such soils are much different than forest soils.  Organic matter and biological activity crucial for healthy roots are lacking.  Root systems often languish and eventually decline, leading ultimately to failures such as this. These negative pressures on tree health can often be moderated with proactive root health care measures.
  • This was reportedly a pin oak (Quercus palustris).  It is my experience that pin oaks and red oaks (Quercus rubra) suffer catastrophic failure considerably more often as they age than most other trees. About 20 years ago, a violent straight line wind storm destroyed 500 trees in Spring Grove Cemetery and Arboretum.  Of these 500 trees, approximately 272 were pin and red oaks.  The decay and failures observed were similar to the Bond Hill failure. This failure was not an exception but actually fairly common in trees such as this.
  • This tree appeared to have root rot as shown in this photo here>>>.  It is unclear in the photo the extent to which this flaw would have been detectable pre-failure.
  • This tree was previously inventoried and evaluated by city of Cincinnati Urban Forestry personnel.  At that time it was reportedly deemed worthy of removal.  Here is what officials said:”The trunk and roots of the tree had no visible evidence of damage or illness,” City Manager Harry Black said in a statement Monday afternoon. “The canopy of the tree did demonstrate minor deadwood which led to the tree being marked for inclusion in the preventative removal list.”An investigation Sunday indicated the tree suffered from invisible root rot, Black added.”Unfortunately, in this case, there was no visible above-ground evidence that the tree suffered from root decay,” he said. “(It) did not show any signs of immediate need for attention.”
  • It is doubtful that a tree with only “minor deadwood” would be marked for removal.  It is more likely that there was at least moderate die back indicating tree decline and the need for removal.
  • It is conceivable that this tree did not show itself to be an emergency.  About 10% of tree defects are not detectable.  Tree failure causes are complex.
  • A former urban forestry manager told me they had thousands of trees to maintain and inventory.  Even when trees are identified as requiring removal, most of them must go through a time consuming city bid process before they are removed. Emergency trees can be removed quickly but determining which trees are emergencies is complex.
  • Trees fail in complex manners which are only partially understood.

So, while people prefer to find a smoking gun, this case is complex.  A thorough investigation of the pre-failure expression of symptoms would be required before any conclusion could be rendered as to whether any negligence might have been involved.

Note:  The purpose of this article is to present general information to educate the reader about tree failure and tree care. I have not personally examined the Bond Hill tree.  I have no opinion on the legal liabilities which may present themselves in this case.  The information in this posting represents information gleaned from news reports.  I am an ISA Certified Arborist with a Tree Risk Assessment Qualification.  I am a member of the American Society of Consulting Arborists.  I am not a lawyer and I don’t play one on TV! 

Identifying and managing the risks associated with trees is a subjective process. Since the nature of tree failures remains largely unknown, our ability to predict which trees will fail and in what fashion is limited. As currently practiced tree evaluation involves examining a tree for structural defects, associating those defects with a known pattern of failure and rating the degree of risk.  The tree hazard assessment involves three components: 1) a tree with the potential to fail, 2) an environment that may contribute to that failure, and 3) a person or object that would be injured or damaged (i.e. the target). By definition, a hazard situation requires both a defective tree and a target.  Unless a target is present, a tree cannot be hazardous. As a result, assessing hazard is not limited to evaluating failure potential. Hazard evaluation must consider the potential presence of a target.