Decay in tree branches is a concern, but requires further investigation to fully evaluate its importance.

A perfect tree is a rare thing indeed. Sure, the majority are well placed, provide many benefits for their owners and contribute to the overall health of the planet. Unfortunately, a sizeable minority are not. Due to weather, pests and human errors, some trees have developed defects that limit their value to the landscape; others are downright hazardous, in that they pose a significant threat to human health and safety.

As described in this article, site assessment is defined to be a process for making observations about the relative health of trees. Site analysis goes a step further, when value judgments and recommendations are made to clients based on the documented observations, training and field experiences of the arborist.

Be thorough

When assessing the health of a tree, it’s important to be thorough. Because trees are rarely the only plants in the landscape, it’s wise to consider all possible interactions that may be present.

Depending on the situation and the needs of the client, site assessment or analysis can be as simple as walking all sides of the tree with the client and making initial observations, or as complicated as using the property survey/plat and walking the neighborhood to gather as much information as is helpful. For example, if the client is selling the property and simply wants to make sure that their specimens don’t pose a liability or hazard for the buyer, then a quick walk and talk may be in order. On the other hand, if a long-term unknown cause has been affecting various trees and longevity and property value is at stake, then a more thorough analysis is called for.

Many factors

In most scenarios, a multitude of factors affect the health and stability of trees in a landscape, whether in a commercial or residential setting. Omitting consideration of any of them will affect the accuracy of the assessment or analysis.

Trees near human activity locations should be closely monitored.

Soil—The various components of soil (drainage, nutrition, pH, organic matter content, compaction, residues from construction) create the rooting and water/nutrient extraction capacity for tree roots. If any of these factors are not optimal, such as pH values that limit the uptake of iron or manganese, or inadequate soil porosity that prevents adequate gas exchange, trees are at risk of failure.

Competition—Other plants in the landscape can limit the amount of water and nutrients available to trees. When placed too closely, shrubs, grasses, ground covers and even nearby trees can compete for necessary inputs, causing stress, inadequate growth and overall unhealthy specimens.

Cracks and codominant leaders are always a concern.

Shade/Sun—Largely a phenomenon of proper placement in the landscape, weakness can occur when a tree that is favored by full sunlight becomes shaded by a new building or growth of nearby trees. This is common when the crowns of adjacent trees become larger. In certain cases, the effects of phototropism become evident, where young shoots begin to orient themselves in the direction of the available sunlight.

Utilities—Whether overhead or buried, utilities can wreak havoc on tree health. The biggest concerns are overhead electrical or communications lines that force topping or side trimming. The weak growth that occurs after cutting is problematic, as is it not organically grown into the heartwood of the trunk. Instead, most of the connective tissue exists in the cambium and sapwood. Trenching operations to install gas or water lines can cause major injury to tree roots. Always be sure to inquire about recent soil excavation, no matter how small.

Root girdling caused by a tree surround.

Construction—If evidence of new hardscaping (benches, concrete slabs, tree
surrounds, patios, brickwork, artwork, etc.) is present, it’s likely that some roots were damaged in the process. In some cases, damage is minimal and trees can recover easily, but more often than not, construction activity leads to a degradation of tree health, especially if combined with other stress factors.

Disease—Diseases, either foliar or systemic, can be a major contributing factor in the demise or success of a tree. Foliar diseases, such as apple scab or tar spot, are not usually serious in the long run. Systemic pathogens, such as Dutch elm disease or oak wilt, can move a tree from the healthy to hazardous category in a couple of years.

Root issues—Although an important concern, root maladies can be quite difficult to observe. In some cases, surface rooting symptoms are obvious and can be noted as part of the assessment. Root girdling or damage from construction, compaction or hardpan are the most common concerns.

Insects—As with diseases, foliage feeders, such as aphids, don’t usually pose as much of a threat as boring insects or ones that vector pathogenic diseases. Current symptoms, as well as previous insect feeding indicators, should be noted.

Cracks—One of the most worrisome observations that can be made is the presence of cracks in the bark and sapwood. Actual separation is a problem waiting to happen, whereas many other factors are simply contributory. When notations of cracks are made, they should be highlighted or followed with an exclamation point.

New concrete slabs and soil disturbance are possible causes of root damage.

Decay—The development of soft and punky heartwood is usually due to the invasion of wood-decomposing fungi after a limb has been removed or a crack in the bark occurs. The most important consideration with decay is to evaluate the relative amount of sound to weak wood present. Quite a bit of inner decay may be present, but if a solid mass of wood surrounds it, it may not be a primary concern.

Soil compaction on high-traffic areas poses cause for concern.

Slope—Any factor that reduces infiltration of water and nutrients is a concern. Measure the degree of slope and consider it in conjunction with compaction and other soil factors.

The human factor

The proximity of a particular tree to human activity is a factor that may trump all others. Weak trees that are present in activity centers, such as campus grounds, office buildings, parking lots, backyards, pedestrian paths and just about anywhere else where people are likely to move about, are red flags in terms of assessment. In addition to the possible presence of humans, property targets, such as fences, outbuildings and cars, are secondary considerations. Although likely to be of lesser concern, even a weak or hazardous tree in a pasture is worth noting.

Report sheet

All of the observations should be compiled into an organized, easy-to-read form that lists each tree assessed, the location on the property, contributing hazardous factors and some sort of rating of current liability. These reports need not be fancy, but should present a message that is understandable to all involved: the client, neighbors and representatives of the legal system.

To develop a form, block off a third (more or less) of the page for a blank box. When used, the property outline and location of important landmarks can be drawn in for reference, along with the healthy and hazardous trees. Identify each tree with a number. Underneath the sketch box, create a notation section for each numbered tree. In this section, make brief comments about your observations of each specimen such as Tree Number 1: “several large cracks in the trunk, codominant leader and presence of previous borer injury.”

Soil covered with asphalt doesn’t allow much chance for water infiltration or gas exchange.

Follow the notation section with a small area for a formula or additive calculation. Each noted hazardous tree should be assigned a rating, taking into account the severity of the factor involved. For example, if a tree is noted to have a deep crack in the trunk that runs from the base to the crown, a “high” rating should be assigned. Another possibility is to assign a numeric value on a scale of 1 to 10. Cracks are perhaps the most serious of factors; a high-rated crack would usually merit a rating of 7 to 9. A foliage disease entry, such as cedar apple rust, would be more likely to have a 2 or 3 assigned to it. Some arborists use a form that tabulates each specimen and assigns an overall rating to the property, while others prefer to address each specimen by itself. The last section should be a listing of your recommendations, with a set of instructions or advice for each specimen, identified with the same numbers as in the observations section. Consider including a photo of the observed trees, at least the ones with high hazard ratings. This will be helpful for your records and when communicating with the client.

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