Leaning Trees

Most trees lean to some extent. Phototropism (growing toward light) can cause a natural lean, which may or may not be a problem in later years. The key question is, is the lean natural or is the tree failing? When leaning trees have preliminary signs of failure, further inspection is essential because failure may be imminent (Figure 1). Arborists should be familiar with these signs.

• At ground level, look for soil lifting, movement, or mounding associated with root or root plate disruption, or cracks in the soil near the base. Leaning trees with recent soil lifting, movement, or mounding could indicate the tree is failing.
• At the base, look for compressed or buckling fibers on the lower or compression side and horizontal tension cracks (perpendicular to the stem) on the upper or tension side. Tension cracks are rare and require immediate action-tree removal.
• When roots are severed, there can be a significant loss of root anchoring support, especially for leaning trees. It is the small-diameter, lateral roots under tension that provide the greatest anchoring support for the tree. The tensile (tension-pulling strength) of a 2-inch-diameter root is dramatic and equivalent to as much as 4 tons of holding power.

Cracks

The most common cracks observed in trees are radial cracks. Radial cracks are wood fiber separations along the rays in the axial plane (up and down). Open cracks are physical separations of the wood fibers. Open cracks indicate that part of the tree has failed or is failing. All cracks arise from load imbalances (mechanical stress) and are in most instances predisposed by natural flaws in tree anatomy (natural weak points). Internal cracks may or may not be visible from the outside. Cracks are evaluated for the extent of compromise to the branch or stem cross section, as well as their location and their association with other defects.

Wounds and Cankers

Wounds can lead to cankers, which are localized diseased areas on stem tissue that may be shrunken and discolored. Wounds associated with canker tissue usually fail to compartmentalize completely and often become more serious defects. Wounds must be evaluated for the extent to which they compromise the strength and integrity of a branch or stem. If decay is present, the severity of the defect is greater.

Deadwood

Dead trees and dead branches can fail at any time. Dead and decayed wood fail at different rates depending on species, material size and weight, and resistance to decay. Dead branches or dead tops that have already broken off and lodged (hangers) are especially high risk.

Decay

Understanding the process of tree decay is vital to evaluating trees for decay. The symptoms and signs of decay include cavities, holes, cankers, branch stubs, fruiting structures (mushrooms and conks) of decay organisms, stem bulge, and stem swelling (Figure 2). Tree roots typically decay from the bottom up.

The key to understanding compartmentalization is that decay is confined to the wood present at the time of wounding unless it breaks through the barrier zone. The extent of the internal decay is not the primary issue. Most important is the amount of sound wood present in the stem or remaining wall. (Periodic reinspection is required because some decay organisms can cross the barrier zone into wood tissue that was laid down after initial wounding.)

The amount of sound wood is determined based on stem and branch diameters or cross-sections. Trees can have decay and still be stable; in fact, most large trees do contain decay. Generally, a full-crown tree without a lean can be up to 70 percent hollow-a level of risk that can be tolerated (in the absence of cavities or other factors). Trees such as this may be within safety margins but must be fully evaluated for possible mitigation.