An experienced ISA-certified arborist with sound technical knowledge and experience can provide tree risk assessments for problem trees to prevent injuries and property damage. Tree risk assessments should not assume every tree defect is a hazard but consider the potential for harm from tree failure. Their job is to quantify and explain the risk so decision-makers can balance safety with the tree’s value. Property owners should also consider legal exposure from unsafe trees. Click here for more information from the International Society of Arboriculture website.
Property owners are responsible for the safety of people and structures near their trees. If the owner, tenant, or neighbor notices an unstable tree, they should call a tree service company that offers tree risk assessment. The tree risk assessor will analyze the setting, consider the risks and benefits, and present a plan for action to prevent or correct any hazard.
Hazard Tree Definition
Trees with structural defects are considered hazardous if people or property are within the falling range. A tree with the same issues in the middle of a forest, away from buildings and people, is not considered dangerous. The arborist finding problems during a risk assessment should see if pruning or cabling can mitigate the risk without sacrificing the entire tree.
Why Do Tree Branches Break?
Below are reasons why trees shed their branches. Some are not linked to illness.
Decay
Branch rubbing, storm damage, and poor pruning cause trees to form barrier zones around each wound. These barrier zones become weak and become susceptible to mechanical breakage.
Branch Collar Breaks
Decay around branch collars is natural and is how trees self-prune.
Supporting Tissue Failure
Large branches allowed to grow without pruning often break from the weight of snow or ice. Pressure from snow or wind causes the supporting tissue around the branch to give way.
Weak Tree Species
Some tree species are more brittle than others. The City of Denver prohibits planting Willows, Box Elders, Cottonwood, Ailanthus, and Silver Maple in public rights of ways because of their brittleness. Large Silver Maples and Cottonwoods are notorious for dropping huge branches, sometimes without warning.
The Tree Risk Assessment Process
Tree History – The assessor should ask the owner if the tree has a history of dropping large branches. They should ask the owner if the tree was previously topped or had storm damage. If not properly pruned, the regrowth from this damage sets the stage for breakage.
Dead or Dying Trees – Property owners need to hire a tree service to remove any dead or dying mature trees near buildings, driveways, sidewalks, or roads.
Dead Limbs – Dead limbs are accidents waiting to happen. Any branch larger than one inch in diameter next to a public area needs prompt attention. Dead branches can fall with the slightest breeze or bump. If the branch is high in a tree, the property owner should hire a tree company with a bucket truck to remove it.
Vigor – Comparing a troubled tree’s leaf cover, size, color, and condition to healthy trees is an excellent way to measure health or vigor. The assessor will recommend the removal of trees in poor physical condition.
Forked Trunk – Pruning one side of the fork when a tree is a sapling avoids this problem. Mature forked trees signal weakness. The assessor should look for bark growth in the crotch and sap oozing. Both denote decay.
Balance – A mature tree suddenly leaning or becoming lopsided indicates root failure and needs immediate removal.
Rot and Decay – Branches and trunks with large cavities, cankers, and fungi growth are clues to internal decay. Sometimes, there are no outward signs of decay. So, arborists hit trees with mallets. The sound differences can detect decay. Some use increment borers and decay-detecting drills to check for rot.
Trunk Wounds and Cracks – Wounds or cracks extending into the ground or on opposite sides of the tree can mean internal circumferential separation of wood. These trees are weak and can become dangerous.
Root Decay – Root decay is hard to detect. Arborists look for mushrooms on or near the base of trees. They may have to dig up some roots to check for rot.
Construction Damage – Severed roots lose their ability to support the tree’s weight. The risk assessment will define the damage and tree viability.
Residential and commercial property owners hire tree service companies to care for their trees. Wise owners authorize removing dangerous trees and retaining as many large old-age trees as possible. Where appropriate, property owners with large tracts of land may leave some dead trees standing for the benefit of wildlife.
