Landscape - Decaying Trees are Hazards

Decay in living trees can weaken their structure and create a situation where injury to people or damage to property could result from falling limbs, trunk breakage, or tree toppling. The following is information on this subject from the University of Kentucky.

A tree normally would not present a hazard in the landscape unless there is a potential target for it to fall on. After all, a falling tree in the middle of a forest is not the hazard that a similar tree would be in a school playground. County Extension Agents, arborists, landscapers, and grounds maintenance personnel should recognize how tree decay disease can create a real hazard in the landscape.

Trees that represent a hazard can often be recognized by evidence or indicators of decay disease or tree structural weaknesses. Some of these will be listed here. An actual evaluation of hazard trees is best done by a professional certified arborist. Knowing that some of these indicators are present should give the tree owner reason to contact a certified arborist.

Visible external evidence. The following is a partial list of tree hazard indicators which, if present, are cause for concern and a stimulus to look more closely at the problem to determine if a hazard really exists.

Tree branch and foliage appearance (indicating a root or lower trunk problem): diminished size, frequency, and health of buds; decreased annual twig growth; reduced canopies; unsatisfactory size, density, and color of foliage; epicormic growth; deadwood and dieback in the crown.

Tree structure: poor crown balance; multiple branch attachments; narrow crotch angles between trunks or between branches and trunk, especially with included bark; long slender non-tapering branches; abnormal crooks in branches; and leaning trees.

Tree diseases and defects: canker diseases; trunk and branch cracks, splits, and bulges; dead bark; bark texture changes; weeping wounds; cavities and hollows; topped trees; branch stubs; flush cuts; dead wood and broken, hanging branches; root decay; deep stem fluting; lack of basal flare; and girdling roots.

Biological indicators: fruiting bodies of decay fungi on buttress roots, trunk, or limbs; fungal mycelial mats or fans; fungal rhizomorphs; insect emergence holes; insect frass; bird or mammal nesting holes; and bee colonies inside the tree.

Site factors: nearby building construction; trenching through the root zone; changes in water drainage patterns; soil compaction; clearing of a densely wooded site leaving remaining trees exposed to wind; soil erosion; changes in grade such as cuts and fills; extremely light soils providing poor root anchorage.

Buried evidence. Less accessible, but very important to tree hazard determination is the partially buried area at the base of the trunk which includes the buttress or flare roots. The root flare area may need to be carefully excavated to reveal decay, fungal mycelium and rhizomorphs, dead bark, injuries, cracks, and other tree defects. For a tree to be safe and healthy, it is necessary for most of the lower trunk and buttress roots to be free from injury and disease.

Evidence inside the tree. Decay of the wood inside the tree can often be foretold by visual evidence such as fungal fruiting bodies, hollows, and cracks. Nevertheless, to survive, grow, and have healthy foliage, trees need only intact bark and a few outer rings of the wood, so a badly decayed tree might not always appear to be a hazard. Therefore, it is important to determine just how much decay is present to determine if the disease has progressed to the point of making the tree hazardous. The presence of decay and a cavity is not necessarily an indication that tree removal is necessary provided that the healthy shell of the trunk surrounding the decayed center is sufficiently thick.

Arborists have tools that can be used to determine what is inside the tree or inside the buttress roots so that a more accurate picture of the tree strength can be obtained. Some of these tools include:

A mallet, used skillfully can help detect a cavity, but it does not tell how much decay is present.

The sound impulse hammer (e.g., Metriguard hammer) or ultrasonic instruments such as the Silvatest and Arborsonic detectors use sound velocity patterns to reveal tree defects.

An increment borer facilitates sampling for study of the annual growth rate of the tree (dendrochronology) and for determining the extent of internal wood decay.

A portable electric drill with a long narrow twist drill bit can be used to determine interior decay and hollow conditions by noting changes in sawdust color, texture, and odor and changes in resistance to penetration at various depths.

A more precise variation of the drill method is provided by the Resistograph F500 decay detection device. A thin, about 1/16 inch diameter probe rotating at high speed is inserted into the tree and penetrates up to a depth of almost 20 inches at constant speed. The changes in power demands on the electric motor resulting from passing through sound wood and decayed wood are printed out on a recording device at the same scale as the distance traversed.

Other detectors employing the concept of wood penetration such as the Densitomat-400, the Decay Detecting Drill (DDD 200), and the Resistograph 1410 operate on the same principle as the Resistograph F500 and also provide a graphic record of the results of drilling.

Electrical resistance measurements inside the tree will show a difference between decayed and healthy wood. The Shigometer is used to detect internal discoloration and decay in the tree and also to provide a relative measure of its vitality.

New non-invasive technology such as thermograpy (measuring a tree's radiant heat), ultrasonic tomography (using sound sensors outside the tree with computer analysis of data), echography (radar), and computed tomography (like the CAT scan used in medicine) are being tried experimentally to detect decay in trees.

As tree decay detection technology advances, it may be possible someday to accurately map out the extent of decay inside the tree. If future technology brings computer-aided 3-dimensional models and species-specific tree strength formulas, it should make hazard tree evaluation even more precise.

Reprinted from "DECAYING TREES CAN BECOME HAZARDS" By John Hartman in the August 6, 2007 edition of the Kentucky Pest News from the Universty of Kentucky, College of Agriculture.