When to worry

Caused by various fungal organisms that degrade heartwood, tree decay is one of the most difficult arboricultural issues to deal with for several reasons, the most important being that it’s hidden. Just like tree root problems, it’s tough to know for sure how much is present and how extensively it has spread.

Assess decay

Regardless of how difficult it may be to determine the extent of the decay, it’s important to make an attempt. There are several methods, all with pros and cons.

• Tap the tree with the butt of a hatchet.

Somewhat like tapping a melon to see if it’s ripe, tapping the bark with a solid object can give a crude sense of how much soft wood is present. If a thud or hollow sound is heard after tapping, it’s likely that some level of decay is present, but what is unknown is the amount. The benefit of tapping is that the location of the pockets of major decay can usually be identified. The downside is the distance that the decay has traveled since the original infection. A plastic or rubber mallet may also be used.

• Use a resistograph.

For several years now, a device known as a resistograph has been used to gain a better sense of the extent of the decay in a tree. It is used by placing the unit against the bark and engaging a fine, motorized drill through the sapwood and heartwood. A printout is produced showing the areas of softened or durable wood; indicated by measuring the resistance to the drill, hence the name, resistograph. The benefit of this device is that a more precise measurement is provided. The negative is the cost of the unit and the time required to operate it.

• X-ray the tree.

A noninvasive device, similar to an X-ray machine for humans, is available. Called acoustic tomography, it provides a visual representation of the inside of a tree. The appearance of the printout is similar to the latest weather forecasting technology for television broadcasts. The pros and cons are the same as for the resistograph, although price and availability may vary.

• Use your experience.

Astute arborists have learned to make observations about trees before and after takedown, and correlating the outer appearance to the presence and extent of decay inside the tree. The best part of this method is that it’s free. The limiting factor is that external symptoms can be misleading relative to the internal condition of the wood.

• Use a probe.

Lots of impervious ground around trees spells trouble.

A sharpened piece of rebar or headless golf club can be effective as well. This method is appropriate only if the decay is visible from the outside of the bark, such as when a wound has failed to close after a limb removal. When pustitle into the tree, a reasonable sense of the extent of the existing decay can be gained. This method is a good one to start with, particularly because it’s quick and easy to perform, and doesn’t call for expensive equipment. The major limitations are that it’s difficult to reach to the end of the decay stream/column, and it only is applicable on outer tree wounds. Other measurement devices include compression meters, stress wave timers and electrical resistance meters.

All of these methods are designed to help calculate the extent or determine how much decay is present, or more importantly, how much punky, soft wood exists compared to sound wood. Some combination is most likely the best course of action. Figuring out the best approach is an individual choice based on all of the above factors.

Fit decay into the formula for tree hazard

Regardless of the extent or amount of decay present, arborists should understand that there are many other important factors that determine the soundness of a tree, as well as the potential liability involved in
leaving it in the landscape. It’s really a matter of perspective, or assessing the overall situation to make a recommendation to the client for reasonable action.

A somewhat closed crack.

Though there are many factors, realize that they should not carry the same weight in evaluating whether to remove a tree from a landscape or take other corrective action. For example, in certain scenarios, the presence of vertical cracks and close proximity to areas with frequent human activity often outweigh the presence of damaging insects and disease infections, especially if they are active exclusively on the foliage.

Each arborist must develop their own formula using various factors, including the following:

• Presence of other tree defects

It’s quite common to observe vertical cracks, co-dominant leaders, previous corrective devices, such as rods, braces and cables, lightning damage, basal damage and other defects when determining the amount and extent of decay.

• Proximity to targets

The distance from swing sets, cars, houses, sidewalks and other human activity locations is a major factor to consider when evaluating how sturdy a specimen may be. If a suspicious tree is located toward the rear of a property, far away from where children frequently play or a family entertains their friends, a greater amount of decay and damage can be allowed before recommending that the tree be removed. Conversely, if a 65-foot-tall specimen is 20 feet away from a residential patio, then it should bear greater scrutiny.

• Decay percentage

The ratio of sound to softened wood is always important. In fact, with some species, such as silver maple, it’s reasonable to assume that if the tree is older than 20 years, decay is present somewhere in the tree; the only real issues are how much and the overall condition of the tree. If the inner fourth of the heartwood is punky, but there are no vertical cracks, co-dominant leaders or significant pests, then the need for action is reduced.

• Sturdiness of wood

As mentioned above, decay is more likely to become a problem in some species than others. The internal conditions and attributes of the wood will greatly influence how rapidly decay organisms will cause decomposition. Trees with low incidence and rate of decay spread include walnut and Osage orange, while willow, cottonwood and silver maple tend to degrade more quickly. Species such as ash, honey locust, linden, birch and beech are intermediate in this regard.

• Location of the decay

The location of the decay is also a valuable consideration. Basal decay is significant in that connections to the root system are lost, as well as the actual degradation of the wood. Pockets of decay that occur near branch connections can eventually cause limbs to drop. Decay that develops in branches that are in the crown and extend away from the balance of the tree is of less consequence, especially if the bracnhes do not hover over areas of human activity.

• Presence of pests

Damaging, infectious diseases and insect pests, such as Dutch elm disease, oak wilt and many borer species, add insult to injury in that when added to the presence of decay, two or more major factors are to be considered instead of just one. This can be thought of as an additive effect, as they are not related, but the presence of both conditions creates a greater concern for the stability of the tree.

• Extent of foliage growth

A thorough assessment of the extent of the shoot growth of a specimen can provide valuable insights into whether other factors besides decay should be considered. The current season’s growth, as well as three to four years previous, should be measured to gain a sense of overall tree health. A decrease in the shoot growth may indicate a problem in the root system such as root girdling or soil hardpan. However, a reduction may also be due to lower than adequate amounts of rainfall in a given year. Again, this is a factor that should be considered with others, not just by itself.

• Area available for rooting

Nearby impervious areas, such as driveways and parking lots, as well as competition from nearby trees pose a threat to the health and vigor of trees. The rooting limitation that these areas cause decreases the ability to exchange gasses and extract sufficient water and nutrients, as well as a reduction in lateral stability.

While tree decay is nothing to be taken lightly, it should not be the solo factor for recommending future action for your client’s trees.

John C. Fech is a horticulturist, certified arborist and frequent contributor located in Omaha, Neb.