The consequences of tree failure are a heavy-duty occurrence to deal with. With all that is at stake, every reasonable action should be taken to prevent it. In many cases, the failure is due to the presence of decay of the inner tissues. When heartwood, sapwood, cambium and bark are compromised as a result of decay fungi, they become softened and no longer retain the capacity to lend structural support.
Decay organisms are generally considered to be present in sufficient quantity to cause damage at all times. In short, they’re ubiquitous. Being ever-present on the outside of a tree has its advantages — if you’re a fungal organism, that is. When a break in the bark occurs as the result of a lightning strike, mower injury or a simple limb removal, they enter and begin the degradation process. As arborists and tree service workers, it’s prudent to be able to spot decay and fully understand its consequences for customers.
One of the main reasons there are major concerns about the presence of extensive decay is the proximity of trees to targets. Simply stated, targets are objects of importance that a tree or tree part could fall on, namely people and property. Just as a tree may be infected with a minor, moderate or major amount of decay, a target can be categorized as high, medium or low danger. Generally, the factors that help determine which category a particular space falls into involves the proximity of the tree to valuable structures, such as residential houses or business locations; the awareness level of humans to the potential failure of a tree; and the frequency of exposure — how often people are present near a particular tree. Further explanation is as follows:
High – The space near the potentially problematic tree is constantly visited by humans and contains several high-value pieces of property. Shopping malls, college campuses and child care centers are representative of high target environments.
Medium – The space is used frequently, but is occasionally not occupied by humans. Buildings and other pieces of property are valuable, but not considered to be high dollar items. Examples of medium targets include city parks, most urban cemeteries and residential properties without children.
Low – The space is hardly ever occupied by humans and the objects are of limited value. Pastures and roadside ditches are examples. Unless you’re an obsessive carnivore, there’s not much consequence to a tree falling in the middle of a cow field.
Look under the streetlamp
When looking for a lost or unknown item, the old tale of looking under the streetlamp because the light is better there is somewhat instructive for decay inspection. While any damage to the bark can allow decay organisms to enter and start the decomposition process, inspection should begin at the most likely locations for decay to occur.
- Branch removals larger than 3 inches. Especially on trees that don’t close wounds quickly (silver maple, willow, poplar), removal of the inner tissues and exposure to the elements creates a condition where decay development is a likely occurrence. Trees that have high decay resistance (black locust, Osage orange, walnut) can still develop decay at the site of a branch removal, but it’s less likely.
- Cracks in the trunk and scaffold limbs. Cracks in the bark on the trunk or scaffold limbs are openings, and that’s all a decay spore needs to take up residence in the sapwood. Frost, lightning strikes and sunscald are common causes of cracks that decay organisms take advantage of.
- Basal plate or bole. Mower injury is a common cause of bark removal and subsequent decay penetration. Poorly trained grounds workers are usually the origin of the problem. A few minutes spent with homeowners and operators of mowing equipment is well worth it in terms of damage prevention. Added soil over the top of the roots or in close contact with the trunk can cause similar injury. This damage to the basal plate is common when the grade changes or when “planter boxes” or “tree surrounds” are installed; these landscape elements are usually a tree’s worst enemy.
Vertical and horizontal
Once decay enters a tree, it has the potential to move up and down and from side to side. This is frequently not evident, much like the tip of an iceberg. Fortunately, trees have mechanisms to limit the upward and downward spread of decay.
Although trees don’t heal, they do develop walls of compartmentalization in an attempt to isolate the injury from the rest of the unaffected inner wood. First described by Dr. Alex Shigo with the US Forest Service, compartmentalization is the process of walling off the soft, decay-infected wood from the sturdy, unaffected wood. Generally, four walls of separation are recognized in the process.
Easy and hard
Sometimes decay is easy to find; it’s right there staring you in the face. These cases are typified by exposed wood in a softened condition or the presence of conks, the fruiting bodies of internal decay infections. In other situations it’s hiding in the center of the heartwood behind the sapwood, cambium and bark. In these cases, the hope is that the punky wood does not comprise a majority of the inner tissues. Trees that are growing in a target-rich location are the ones where the greatest concern exists. If a tree is growing in an area where lots of people are around and valuable pieces of property exist, further examination is called for. It’s time to get out the tools of the trade.
Tools of the trade
Inspection is relatively easy if the tree is less than 30 feet tall and the pocket of decay is close to the ground and exposed on the trunk. However, this is not always the case. Fortunately, several inspection tools are available to improve the chances of finding and determining the extent of existing decay.
Binoculars – Relatively simple and commonplace, a good set of binoculars can bring the faraway close up. I recommend the type of binoculars that bird-watchers use. These tend to be designed to observe objects at a distance of 30 to 70 feet, similar to looking high into a tree. These units cost about $100, but are well worth it. The models designed for spectators at football games (where sitting in row 97 of a stadium makes it hard to see the numbers on the jerseys let alone the ball) are more difficult to use.
Tappers – This term applies to any tool that can be used to find hollows or voids in a tree trunk or branch. A hammer, mallet, or the butt of an axe or hatchet are examples of tappers. These tools may confirm the presence of decay, or a “rot pocket,” but should not be used as the only piece of evidence.
Probes – Perhaps the best balance of simple, effective and inexpensive, screwdrivers, metal rods and headless golf clubs can be pushed into an area where decay is suspected. The lesser the resistance, the more likely that decay is a factor.
Drills – Increment borers, normally used by foresters to determine the age of a tree, and resistographs, specifically used to gain an inside look at an otherwise normal looking trunk, are perhaps the best in terms of documenting actual decayed wood. Both are invasive in that they remove bark, sapwood and heartwood, and in so doing open a channel of access to future decay infections.
Sonic tomographs – Tomographs utilize between five and 20 external sonic sensors to help visualize the locations of decay. These sensors are spaced out evenly around the circumference of the trunk. They detect stress waves induced by manual impact and propagated through the wood. The principle that is employed is that sound waves cannot take a direct path through the wood if there is a cavity between the transmitter and receiver. Sonic tomographs create output for the user in a visual form that illustrates the inner pocket of decay.
Your eyes – The tools mentioned above are quite helpful, but perhaps the best tools are the eyes of an experienced arborist. Knowing what to look for, and where to look, makes your eyes the most valuable piece of equipment.
Regular inspection and detection
Detection of decay should not be a “once and done” procedure. The best approach is to set up a regular routine for inspection with clients. In addition to other inspections for insects and diseases, systematic monitoring should be conducted for detection of decay, especially for older tree specimens on a property in high-target locations.
This procedure is like any other, you get what you pay for. Imparting information, advice and recommendations based on knowledge and years of experience is a valuable product. It should be a fee-based service provided as part of a tree care package. Such a monitoring package should be used to provide a thorough plant health care benefit to your clients.