Monday, July 9, 2007

ISA HK/China --- New Criteria in Tree hazard Assessment by USFS

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Dear Station Members,

When ISA arborists are called upon to do a Hazard Assessment (HA) for trees, frequently we would use the guidelines from the following ISA publication:

A Photographic Guide to the Evaluation of Hazard Trees in Urban Areas by Matheny & Clark (1994) which works on a 12 point system for Hazard Analysis.

The Station Manager used to refer to this guide in many of his Tree Inspection & Tree Assessment in the past.

However, the Arborist Profession is an ever advancing profession. New ideas & new criteria frequently come up for review. Below is one of such cases which belongs to another powerful research institute in the world, the US Dept of Agriculture Forest Service (USFS,

"In May 2004, more than 80 professionals from throughout the Mid-Atlantic participated in a second Urban Tree Risk Training Workshop hosted by the University of Maryland, Department of Natural Resource Science & Landscape Architecture. Tod Ericson, Urban and Community Forestry, Maryland DNR Forest Service, took the lead in developing the workshop, which built on a similar workshop hosted in the Midwest last September. Speakers from throughout the Mid-Atlantic States and FS Forest Health Protection program supported the workshop. Jill Pokorny, USFS, Plant Pathologist and Coordinating author of the recently published, " Urban Tree Management: A Community Guide to Program Design and Implementation", presented an historic overview of urban tree risk management and a systematic approach to designing community programs.

A diversity of professionals working in all aspects of urban and community forestry were represented, including state urban forestry and forestry professionals, landscape architects, county and municipal commissions representatives, communities, green industry, utilities, scientists, managers and others. More than half of the participant's surveyed hope to implement risk management programs and provide training on risk management."

After this landmark meeting, several new criteria about HA for trees have emerged. They include:

1. Critical Root Radius (CRR) --- Defined as 1.5 x DBH in feet. It is larger than the Tree Protection Zone (TPZ) of 1 inch DBH : 6 feet TPZ as defined in the ISA Arborists Certification Study Guide. If 40% of the roots in CRR are damaged, the tree is regarded to be in high risk of failure.

2. % of Stem Circumference Decayed / Damaged / Cankered --- If more than 40% of the circumference of any stem, branch, or root column is decayed / damaged / cankered, the tree is regarded to be in high risk of failure.

3. Ratio of Shell Thickness versus Stem Diameter --- The ratio of shell thickness (defined as the undecayed layer surrounding compartmentalized decayed column) versus Stem Diameter is 1 : 6 for Closed Shell (where the decay is enclosed) & 1 : 3 for Open Shell (when the cavity is exposed on the bark area) for acceptable stability in a tree with internal or external decay / damage / canker. Any ratio below this is regarded as having a high risk of failure.

4. % of Stem Constriction by Girdling Root --- If more than 40% of the tree's circumference is constricted by Girdling Root, the tree is regarded with high risk of failure.

5. % of Leaning --- If any tree, healthy or not, leans greater than 40 degree from vertical, the tree is regarded as having a high risk of failure.

6. Proximity of Target --- Any distance within 1.5 x Tree's Height is regarded as a Danger Zone to hit any target (people or object) within the accidental fall of the Hazard Tree.

7. Mis-conception in common Tree Assessment --- USFS experts reckoned that a lot of Tree Assessment were done by incorrectly evaluating the foliage colour & bloom of a tree for safety in the past. They reckoned that just because the crown is full & green, it doesn't necessary mean that the tree is structurally sound. Health & Vigour are related to energy supply. On the other hand, Structural Soundness is related to the condition of stem wood, branch attachment & anchoring roots.

Besides the above, Defects in trees were classified into 7 categories during Tree Inspection:

1. Decayed Wood --- Look for decay at above-ground & below-ground tree parts

2. Cracks --- Look for Shear Crack, Inrolled Crack, Ribbed Crack & horizontal Crack.

3. Root problem --- Check roots in CRR & for Girdling Roots.

4. Weak branch unions --- Look for Weak Union with crack, canker or decay. Check for large Epicormic Branches on decaying stem.

5. Cankers --- Look for area of infection to be over 40% of tree's circumference.

6. Poor Tree Architecture --- Look for 40% or more Leaning, Included Bark & other Weak Branch Attachments.

7. Dead Trees, Tops or Branches --- Look out for any dead tree, diebacks & lodged branches in canopy.

It was regarded by the USFS experts that all trees will fail some time in their life, but trees with defects would fail at the point of defect much more readily & predictably than trees without defect. It is therefore important to avoid causing defects in a tree by sound cultural practices & maintenance. Otherwise, trees will become a liability rather than an asset for the pubic. Outlines for good cultural practices were described by USFS website which were nearly identical to ISA practices.

For carrying out any Tree Inspection or HA, it is very important that the arborist can refer to certain guidelines or Industry Standards because every report the aborist would write may end up as Court evidence in the end no matter how unwillingly. ISA guidelines & ANSI's have been the references in the past. Now we begin to have the USFS guidelines as an extra reference.

Since the USFS guidelines are issued by a Federal Agency of the United States Govt & have legal status, the information so provided will be deemed highly credible in Court presentation in our territory as well as for public acknowledgement. This Station has noticed that some of the research were actually taking reference from ISA publications. USFS is a leading Forestry research organization in the world & United States is as big a country as China with climate, species & terrain diversities as vast as anyone would imagine. Their research does not just confine to temperate trees of a few hundred species like in some European countries. The professionalism & reliability of USFS can be almost accepted without doubt anywhere in the world.

Now with these new criteria in Hazard Assessment provided by USFS, ISA Arborists will have an extra source of reference for HA in their Tee Inspection & Tree Hazard Analysis. By putting together USFS & ISA together, ISA Arborists will be getting the best of both worlds with information & references unmatched by any single nation.

The Station Manager will begin to pioneer some of these new criteria in his next Tree Inspection.

best regards,

Sammy Au
Station Manager

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