When is decay most likely to enter the trunk of a tree?

Decay is most likely to enter the trunk of a tree when a large codominant stem is removed. This action creates a significant wound that can expose the inner tissues of the tree to pathogens such as fungi and bacteria. Codominant stems are two or more stems arising from the same point on the trunk, and their removal can result in a large area of exposed wood, especially if proper pruning techniques are not followed.

Such large cuts may not heal as effectively as smaller wounds, leading to a higher chance of decay developing in the exposed area. Additionally, the tree's natural defense mechanisms are often more challenged in the case of larger wounds, making it easier for decay organisms to establish themselves in these areas.

In contrast, while factors like extreme weather conditions, improper fertilization, and the removal of small branches can also affect tree health, they do not present the same direct and immediate opportunity for decay to enter the trunk as the removal of a large codominant stem does, where the size and nature of the wound play a critical role in vulnerability.