What constitutes a "high-risk tree"?

A "high-risk tree" is defined specifically by its structural integrity and the potential hazards it poses. A tree with significant defects that has a high likelihood of failure possesses characteristics that can lead to a risk of falling or causing damage to people, property, or other trees. This condition could be due to factors such as extensive decay, serious structural weaknesses, or other significant damage that compromises the tree's stability.

Additionally, the determination of risk takes into account the likelihood of failure in relation to targets in its vicinity—meaning if the tree were to fail, it could negatively impact an area down below, which includes homes, cars, and pedestrians. Thus, it is the combination of these defects and the probability of the tree failing that categorizes it as high-risk.

In contrast, a tree with no targets below it does not necessarily pose an immediate risk, as it may not threaten people or property even if it were to fail. An old tree might still be healthy and sturdy, and healthy leaves do not directly correlate to the risk of failure in terms of the tree's structural integrity. Therefore, the best answer clearly encapsulates the essential attributes that define high-risk trees in the context of tree risk assessment.