Trees are shedding organisms that periodically cast off parts to provide for growth and to manage disease. This natural function has ensured their continued success as the longest living, large organism on the planet. But the very quality that ensures their survival can make our living with trees a problem. We revel in the shade and beauty that trees provide but are annoyed or threatened by their " habits " People often develop an ambivalent relationship with the trees that surround them.

In extreme conditions, even the healthiest tree can fail. There are genuine reasons to be concerned about the health and stability of trees close to dwellings and recreation areas. There is also a need to moderate our actions and rely on sound information when making decisions that may damage or destroy irreplaceable trees.

This page is designed provide general information about tree inspection methods and techniques. To provide an brief insight into a large and complicated subject. My work in inspecting trees is to try to resolve the concerns that people have about their trees, so that people and the trees that surround them can live together.

Tree Inspection.

An individual tree is very much the product its environment. Looking at the overall picture; the health of the soil, turf, other plants, as well as trees, can reveal the cause of disease, or indicate potential problems. Before a tree inspection commences, a site history must be established. Reviewing the types and levels of change within the trees local environment can guide observations. Changes in a stand, or group of trees, such as: The removal of individual trees, changes in drainage, or wind exposure, can dramatically effect remaining members of the group. By altering drainage and irrigation relationships and changing the dynamics of wind resistance, newly exposed trees face different physical and physiological stresses.

Individual trees can be effected by construction within the root zone. Trenching, paving and soil compaction can damage the root system immediately, or cause damage that may not show symptoms for many years. An assessment of previous impacts along with results of the inspection combine to offer a comprehensive picture of current and potential problems.

The tree inspection begins at ground level. Here, tree species, soil quality, rooting conditions, soil level, irrigation and drainage characteristics can be determined. The level of soil compaction may be tested to evaluate the aeration capacity of the soil. Compact soils can cause problems by restricting the trees ability to discharge the gasses produced as part of the growth cycle. Continual heavy foot traffic and the heavy equipment used in construction can cause compacted soils. Soils with finer particles - clay soils- are more prone to compaction.

A soil test can be made at this time to assess the fertility of the soil. Testing establishes the presence and degree of vital nutrients and the acid balance of the soil. An imbalance of nutrients, overly acidic, or alkaline soil can cause poor vitality and expose the tree to disease. In certain situations a soil moisture analysis may also be necessary to assess the availability of water to the root system.

The presence of fungal fruiting bodies, mushrooms, can indicate decay. Certain decay fungi may destroy support tissues and leave conductive tissues unharmed. The tree appears healthy and continues to grow until the decay outpaces the new growth. Other species of fungus act quickly and destroy the flexibility of wood leaving the tree prone to wind snap. The type of decay and its effect on the stability of the wood depends on the species of fungus involved. The strength loss due to decay may be measured using a Fractometer.

A root crown examination may be necessary if root decay is suspected. By removing the soil at the base of the tree the location, health and condition of the absorbing and support roots can be determined. A mallet is used to test for loose bark, which can indicate the presence of decay in the root crown zone and at the base of the trunk. The mallet may be used to "sound" for decay but this has limited reliability in some species. If decay is suspected, samples may be extracted by core sampling, or test drillings. More accurate and sophisticated equipment includes the Resistograph which uses a constant velocity probe to test structural wood quality. Soil and root tissue samples may be taken to determine the causal agent of disease by laboratory testing.

The inspection continues with above ground observation of the tree, if necessary, climbing into the canopy of the tree where an evaluation of the tree crown is made. An assessment is made of leaf color and size. Trunk, branch and twig color, size and condition, are also assessed. Crown density, past and current growth are combined with an evaluation of the growth habit. Changes in growth rate may indicate prior disease or injury. Previous treatments such as pruning, or cableing are observed. Any Growth Defects are noted, along with: Weak limbs, discolored or missing bark, cavities in branches or trunks and the presence of disease, insects and other stress indications. Trees produce adaptive growth to compensate for the stress related to growth and injury. The shape and formation of limbs and trunks, The Body Language of Trees, can show the ability of the tree to compensate for weakness or indicate internal problems that may lead to failure.

Hazard Trees, a background.

Contemporary systems of tree hazard evaluation are based on the experiences of early arboriculturalists. These people were usually involved in a long term observational relationship with their subjects. Information on tree failure due to defect or decay has been gathered for over a century and many of the valuable insights of pioneering arborists still hold true today. In 1915, Peets offered practical advice on structural support in his book " Tree Repair ". While in 1934, A.D.C. La Sueur, the noted British arboriculturalist included information on Dangerous Trees and Tree Inspection in his work, "The Care and Repair of Ornamental Trees". The piece concludes with a prophetic reference to 'The increasing number of lawsuits'!

Much of the early work was based on identifying diseases and growth defects that may cause tree failure. Methods were initiated to aid in the mechanical support of trees with trunks or limbs weakened by defect or decay. Species that were more prone to failure were identified and many of the pruning practices that lead to tree failure were isolated.
Unfortunately, much of the wisdom gained was passed on verbally and, as such. too often lost through the vagaries of time. The lessons learned, in an earlier age, have had to be repeated, leaving trees damaged, or destroyed in the process. Although we now have a sizeable body of documented scientific research, this cycle continues today. Trees that could have been preserved or identified as hazardous by knowlegeable practitioners have failed, causing damage, injury and death. This is too high a price for experience! In the computer age, we can document our knowledge and experience with ease. Modern systems have simplified the distribution of information. The future for tree care is positive and exciting if we can maintain a willingness to understand and impart our understanding.

Some of the earlier inroads into a systematic approach to hazard evaluation were made by the Parks Service in the USA. In 1963, Wagener, working with the Forest Service in campground recreation areas, related decay concepts to the potential for tree failure. In 1967, Paine, working with the Forest Service in the Pacific Southwest region of the USA, gathered data on tree failure related to species, size and part
[ limb or trunk]. This early work paved the way for the systems that continue to develop today.

In the United States, there are two current tree inspection systems in wide use:


The Resistograph is an instrument, or rather a family of instruments, for detecting decay and defects in trees and timber. The instrument measures the resistance to a needle inserted into the wood under constant drive. The constant drive is provided by either a crank and fly wheel or a battery driven electric motor. The resistance to the needle tip is transferred through an "intelligent" satellite gearbox to a pointer on the top of the instrument that maps the result on a waterproof wax paper printout. Drilling resistance correlates to the mechanical properties of the wood. Defects such as cracks areas of decay, hollows and to a certain extent tree ring structures can be detected and mapped. The resistance is mapped on a 1:1 scale on the wax paper, giving a clear graphical representation of the mechanical properties of the wood. The Resistograph utilizes a 3mm needle tip and a 1.5mm flexible needle that tends to "squeeze" between the fibres of the wood causing very little wounding. The Resistograph is a relatively new instrument and research continues. The high level of accuracy and readily readable results mean that the limitations are only in our ability to intepret the information offered by the Resitograph. The device is manufactured in Germany by IML of Wiesloch. The resistograph is available in a number of forms both mechanical and electronic with differing levels of precision for its various applications. I have begun to use the Resistograph in my practice, both in root, trunk and climbing inspections. I believe this instrument offers a great deal of promise in the diagnosis of defect and decay and in the assessment of adaptive growth as a signal of vitality or of future structural problems.
For More Information.

Treatments to alleviate hazard.
A number of methods of hazard alleviation have evolved over the years, the first and simplest method is to remove the parts of the tree that pose a threat. Pruning to alleviate hazard can be a simple matter of the removal of deadwood or may involve the rigged removal of large limbs with obvious and irreparable defects. However tree topping can never be used as a legitimate form of hazard abatement. The regrowth forced by radical pruning is poorly attached and the energy used in producing such growth can leave the tree depleted and prone to disease.
Cable and rod braces may be used to aid in the support of limbs or trunks with attachments that are deemed weak but not sufficiently hazardous to warrant removal. Lag screws, through bolts or expandable straps are attached to the trunk or limb to be supported. Cables are attached, under minimal tension, between the trunk or limb and a stronger stem within the tree crown. Cables may aid in support but cannot prevent failure.


The fractometer is a device developed by IML industries to test the strength of wood. A core sample is taken and placed in the correct orientation through an aperture on the top of the fractometer. A knurled handle at the base of the instrument is rotated forcing a lever arm on top to apply pressure to the sample. The radial bending fracture strength and radial fracture angle stress required to break the sample is measured. The effect of decay on the strength properties of wood can be determined. The fracture angle in combination with the radial bending fracture strength allows a qualitative analysis of decay.
In the initial stages brown rot fungi digest cellulose and leave the lignin component of wood largely intact, this leaves a stiff but brittle wood. Most early stage white rot fungi prefer to digest lignin leaving wood soft but tough. These physical characteristics can be determined by the Fractometer and the reduction of wood strength and likelihood of failure can be assessed.

The Body Language of Trees.

This book is number four in the series Research for Amenity Trees, published by HMSO books ISBN 0 11 753067 0.

The Body Language of Trees contains extensive insight into the outward manifestations of internal structural problems in growing trees. The 260 pages include numerous illustrations which guide you in the interpretation and evaluation of the symptoms of hazard. This work is the latest in applications of biomechanical theories and practical insight of Claus Mattheck,Physicist and Arborist and Helge Breloer, Arborist, and expert in the law and its applications to trees.

The work is distinctive in its approach to its subject. The bulk of the book is spent in the explanation of the mechanical stresses that relate to trees and the trees response to internal and external stress. Trees respond to these stresses by adaptive growth. The quality of wood changes in relation to the stress in order to maintain ' Uniform Stress '. The multiple external manifestations of this adaptive growth are examined and illustrated.
The book goes on to outline the Visual Tree Assessment method of tree inspection. Where other guides balance three components, Likelyhood of failure, Size of Part and Target, in determining hazard, the V T A method focuses on the tree. The first stage is the visual inspection of the tree for defect symptoms and vitality. If problems are suspected, on the basis of symptoms a thorough examination is carried out. If a defect is confirmed it is measured and the strength of the remaining part of the tree is tested.

The key difference is in the ability to not only detect defects but to determine the strength lost. This is done by structural testing using the Resistograph or another device such as the Fractometer.

Evaluation of Hazard Trees in Urban Areas.

This book is published by the International Society of Arboriculture and is currently in the second edition. ISBN 1-881956-04-0.

This is the second edition of this guide which is comprehensive in the information offered and practical in approach. The many photographic illustrations describe common defects, decay patterns and pruning abuses. A form for the collection of data is provided along with guidance on its completion. Species failure patterns are listed and and formulae are given to calculate the strength loss in trees with cavities.The approach to hazard evaluation is based on the work of Paine and others as described in Hazard Trees a background.

In order for a tree to pose a hazard there must be a likelihood of failure and the risk that, having failed, the tree will do damage. To evaluate the degree of hazard three distinct factors must be assessed.

The factors are.

1/ The likelihood of failure.

2/ The size of the piece most likely to fail. ( from small branch to whole tree )

3/ The presence of a 'target' ( something that would be affected in the event of the failure ). The rating relates to the occupancy or use of the target.

The three factors; Failure potential, Target and Size of part are given a numerical rating, 1 to 4. Failure Potential is rated based on known problems in tree species, existing defect or disease and exposure The three numbers added together give a Hazard Rating with the number 12 being a large part the most likely to fail and cause extensive damage to a significant target.
The hazard rating may be used to prioritize treatment and forms part of an ongoing assessment of tree hazard.

Other resources and links.

International Society of Arboriculture.

Information on Pruning Standards, Publications, and lists of Certified Arborists.

The National Arborist Association.

U.S. trade organization for commercial arboriculture, pruning standards and practices.

The Tree Hazard Web Page

Information on specific defects and disease with photographs.
Maintained by the USDA Forestry Dept. in St Paul.

Tree Care Information Center.

Provides a tree care calendar, Information on tree fertilization, and other maintenance issues. Good links to other pages.
Maintained by Southern Urban Forestry Associates.

How to prune trees.

Good information with illustrations about a variety of pruning techniques. From the USDA forest service.

Shigo and Associates.

Information on Publications produced by Dr. Alex Shigo. Pioneering tree biologist.
A great resource for cutting edge information about tree systems, growth, wound responses and relationships.

About the author.

Robert W.Williams is an Arborist living in the Pacific Northwest of the USA. A resident of the USA for the last sixteen years he is a native of England. His involvement with trees stretches back twenty five years, when, at the age of sixteen, he took a position as an apprentice in the Forestry Department of an estate in Berkshire England. Forestry College and a variety of positions followed, initially in Forestry operations then in Arboriculture with Parks Departments, and Commercial Tree Care Concerns, until he settled with his family in Seattle and formed his own tree care company.
Dedicated to a growing knowlege in the care of trees, Rob is active in Arboriculture on a daily basis and ongoing in his academic education. He is seen here with his patented root crown excavator, Mr. Beezly.

For More Information.

If you are interested I have informational material available on the IML products ordering and prices and can be reached at.

To order the publications described check out the PNWISA home page.