Limb Drop - Tree branches fracture and fall 4 basic ways: 1) fracture at the position where an upright angle near the trunk bends to a horizontal branch; 2) normal shedding at the branch base; 3) shedding of an epicormic branch (sprout) that has a very weak branch collar, trunk collar attachment; 4) the trunk wood above and below the branch fails and the entire branch and trunk fracture and fall. Each of the 4 basic failure patterns are different. The first failure pattern is often caused by construction or other actions that release new space for the tree. Branches that were growing in an upright position now begin to grow in a horizontal position as new space becomes available. The tree architecture begins to change. Because of the new space, the tips of the branches develop many new twigs and leaves. Fractures usually occur where the branch bends from an upright position to the horizontal position. Old flush cuts of smaller branches on the bending branch add points of weakness. Be on alert for large branches that have an abrupt change in angle from tight to the trunk and then to a horizontal position. The second pattern of branch drop is the normal one that occurs after branch death (death of branch symplast). The decay-causing fungi usually decay downward to the branch protection zone within the branch collar. The weakened wood on the outer side meets the tough wood of the branch protection zone. The branch fractures on the outer side of the protection zone, and falls or sheds out of the socket. Symplastless branches on trees near property, powerlines, and people places should be removed before the advanced stages of decay develop, i.e., by professionals. Be especially on alert for large symplastless branches that are in a horizontal position. Symplastless branch removal is also a health treatment because it removes the food source for the pathogens that could grow through the branch protection zone and on into the trunk of the tree. Proper removal, i.e., back to the branch collar or extended symplast without wounding symplast containing tissues, is a key part of a health treatment of symplastless branch removal. The third pattern of branch or limb drop involves epicormic sprouts. Epicormic sprouts often develop after topping has been done. (An epicormic sprout grows from meristematic points along in the cambial zone.) The important part to know here about epicormic sprouts is that they usually have a very weak attachment because the branch collars are small and the trunk collars may actually force the branches away from the trunk. If the epicormic sprout persists for a long time, then a strong attachment may develop as the trunk collars develop about the branch collars. But, in warm climates, the epicormic sprouts may grow so rapidly that heavy amounts of sprout wood form while there has not been time for a strong trunk collar, branch collar attachment to form. The rapidly growing trunk and the rapidly growing trunk collar may actually "pinch off" the epicormic sprout. This could occur during a calm, warm day. The fourth pattern of limb drop centers more about the trunk than the branch. Trunk decay or many trunk barrier zones from wounds, root rots, top injury, or many old flush pruning cuts weaken the trunk at the position where the branch is attached. The trunk collars do "hold" the branch on the trunk. When the trunk wood is weak, the trunk collars will be weak, and the branch and trunk collars may pull out of the trunk. In this case, it is trunk failure more than branch failure, but the result is the same; a branch falls to the ground. A tree hazard check should be made regularly where trees and people are close together. Things do not just happen in nature. Branches do not just "decide" to drop. There are causes. Old flush pruning, tree topping, release of branches because of construction, and many other actions by man start the processes that could end in limb drop, and sometimes problems for property, power lines and people.
See “Loading Mechanical – Branches”.
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