When are cankers likely to affect tree stability?

Cankers are lesions on the bark of trees caused by factors such as pathogens or environmental stresses that can compromise the tree's health and stability. The stability of a tree is particularly at risk when more than one third of the diameter of the trunk is affected by cankers. This level of damage can significantly weaken the structural integrity of the tree, making it more susceptible to failure.

As the diameter of the affected area increases, the strength and support provided by the healthy tissue around it decrease. When more than one third of the trunk's diameter is impacted, the tree may struggle to maintain its upright position, especially in adverse conditions like strong winds or heavy snow loads. Thus, the amount of tissue lost due to cankers is critical in assessing and determining tree risk stability.

The other scenarios, while they certainly can influence tree health, do not directly speak to the level of risk posed to stability as effectively as the significant diameter loss indicated in the correct answer. For instance, cankers that are small or occur during drought might cause stress but do not inherently compromise the structural strength of the tree as severely as larger, more damaging cankers. Meanwhile, cankers located near the base can be significant, but without the critical threshold of diameter loss,