Structural failures occur when the stresses due to forces acting on a tree exceed the strength of the tree or or the tree/soil connection supporting the tree. Even the structurally strongest tree will fail when a load is applied that exceeds the carrying capacity of one or more of its parts.
Typically, it is a combination of several defects or conditions causing a structural failure. A root or stem decay, or poor structure combined with an unusual loading event, such as a strong wind storm, are frequently associated with tree failures.
While there are several factors involved in a failure, they are characterized as a soil or a root failure.
Root decay, root collar and buttresses rots are often associated with a root failure. Root rots typically begin from wound resulting in an injury. Root decay fungi infect through small diameter roots. Roots that have been stressed due to lack of oxygen due to soil compaction, excessive soil moisture, construction trenching, excavation and cultivation may allow entry of a root decay fungi into the roots. The decay may then progress from the feeder roots into the structural root system. The decay may be located on the underside of larger roots.
When the decay progresses into the heartwood or base of the tree, it is referred to as basal decay, a butt or collar rot. Rots kill water adsorbing feeder roots. This affects the trees ability to adsorb water and nutrients from the soil. This condition may be manifested as dieback within the tree crown. Foliar tip dieback progressing into twig, stem and limb dieback are potential symptoms of a progressive root rot. Tree leans may also be a sign of a compromised root system.
Root rots may be difficult to detect and determine the severity of the infection. Depending on the tree species, location, maintenance practices, root rots may take years before the rot begins to compromise the structural roots. Root rots can directly cause structural root failure or contribute to wind throw.
Tree failure may be due to the soil rather than tree roots. Soils influence root development, therefore it is important to assess how site conditions affect root development. Assessing soil factors such as volume, depth, moisture, compaction and quality, (texture, fertility and pH) directly affect root growth and structural support.
Small soil volume will limit tree root growth and development, especially strong structural roots. This is especially relevant for city urban street trees that are often installed in small planters surrounded by asphalt and concrete streets and sidewalks where space is minimum and roots are often cut and damaged. Underdeveloped root systems may be structurally weak, limit tree growth and health, which reduces the tree’s ability to adapt to the loads it experiences. Construction impacts including grading cuts and fills, trenching, and excavation may severely damage root systems resulting in root rots and or lack of structural support.
Adequate soil depth is required for a well anchored tree. Trees with shallow root systems tend to be more prone to failure than trees with well established deeper root systems. Shallow soils may occur due to underlying bedrock or urban infrastructure. Grading cuts and fills for development may result in soil layering, resulting in saturated soils or over-irrigated soils. Soil type and texture affects drainage, coarse textured sandy soils drain quickly, heavy clay soils retain moisture for long periods of time.
Surface compaction is common in urban development due to pedestrian and vehicular traffic. Compaction and flooding result in reduced oxygen within the soil. Roots require oxygen, as it is depleted within the soil to due compaction, flooding or over-watering, feeder root rots may begin, progressing into larger structural roots.
Assessing root damage is challenging for most lay people. If your tree is in decline, leaning or looks different than it used to, hiring a certified arborist who possesses the specialized training in tree risk assessment is highly recommended.
If your tree has failed, the two most obvious clues as to cause is the appearance of the root ball. If the failed tree has a large portion of the root ball with soil intact surrounding the roots, most likely it was a soil failure. If the tree has nothing but blackened roots sticking out of the root ball with little or no soil clinging to the roots, most likely the tree failed due to a progressive root rot.
