1. a structural flaw that will cause part or all of the tree to fail.
2. a target of some value at risk of being damaged. This risk may be heightened if environmental conditions do not coincide (site is prone to heavy wind, wet soil, unstable soil structure, construction)
Trees are always at risk due to gravity. Courts generally accept that there is a level of acceptable risk associated with trees. This risk is considered as “reasonable” risk as opposed to “unreasonable” risk.
Reasonable Risk –healthy tree, few defects, in general tree is in good health with good branch habits, vigour, etc.
Unreasonable Risk – fungus, decay, cracks, structural defects deadwood etc.
– Tree owners decide how much risk they are willing to accept. Depending on tree condition, how much they value the tree, how much they value the surrounding targets.
– Having an arborist or urban forester asses your tree can greatly reduce unnecessary removals by pruning, crown cleaning / dead wooding, cabling and bracing, crown reduction / thinning, or if appropriate removal.
Inspection Process – it is important not to confuse tree health with tree structure. Trees may appear health which can be misleading and be hazardous due to poor branch structure, cavities etc.
– tree risk assessors follow a check list which includes: site conditions, species, shape, roots, trunk flare, trunk, branch unions and scaffolding limbs.
Site Conditions – wind, soil, other trees as protection, etc.
Species – strength of trees are species specific, some handle loads better than others. Some species are prone to weaknesses like co-dominate stems or shallow root systems etc.
Shape – inspect for dead limbs, branches that extend beyond the rest of the crown, branch structure, tree lean, symmetry
Trunk Flare – ensure proper flare, soil is not mounded or graded elevated
Trunk – cavities, cankers, stem decay, horizontal cracks etc.
Branch Unions and Scaffolding Limbs – is there branch bark ridges, included bark, holes, cracks, cavities, moisture, split, decay, extent of dead limbs, and proper branch structure.
Trees with high risk of failure include: target canker, soil mounding, shearing cracks, odd shaped bends or crooks in branches, leaning trees, and included bark.
After assessing these risks steps can be taken to reduce the risk a tree may pose if owner is willing to accept.
Assessing a tree and taking steps to reduce the risk or not, does not necessarily mean that it is classified as safe. There are many hazards that may go undetected due to limited access or the inability to view the inside of the tree.