Basics of Tree Risk

Large, old bur oak with open multiple open cavities

Arborists have adopted the language of 'risk' when assessing trees over the last 10-15 years ago. Before that, we referred to trees as 'hazard trees' or not. We found describing trees as hazardous was too definite, too black and white. If a tree was labeled as a hazard, that meant it posed an imminent threat when that maybe wasn't what was intended. The new best practices for tree assessment are centered around risk, allowing for a continuum from low risk to extreme risk.

Our formula is basically Likelihood of Failure + Likelihood of Impact + Consequence of Failure = Total Risk. This allows for much more situational flexibility and nuance, and arborists can better advise the owner/manager of a tree. Unless there is an imminent threat (a true hazard), arborists may not advise whether to remove a tree or not.

Instead, they may leave that up to the owner/manager of a tree, the person who has to live with it and deal with the consequences. We will describe the situation as it exists and the current level of risk, discuss potential solutions to reduce or eliminate risk (e.g., pruning, removal), and then ask the homeowner 'how much risk are you willing to live with?' Or maybe 'will you lose sleep over this?' As expert consults, we guide the tree owner/manager's decision making process.