Nothing’s Ever Sudden In Trees
During the early 1980’s, while working with Dr Alex Shigo on the reason for “Sudden Limb Drop”, we discovered that the cause was an association of brown rot decay and decay induced local moisture stress.
Brown rot is an opportunistic disease that enters natural and unnatural wound sites. When all the conditions are aligned it leads to “Sudden Limb Drop” as a result of the fungi’s ability to preferentially take moisture from adjacent uninfected tissue.
The localised reduction of moisture increases the brittle nature of the wood in the dehydrated area. Wet wood is fellable, ( as a test compare bending a dry and green twig) whereas the dryer wood is stiffer and less flexible. The load on the branch remains the same but it becomes focussed at the boundary between the wet and dry tissue. The rigidity of the dry tissue, which is closer to the tree trunk thus also marginally larger in diameter, creates a focal point just inside the wet wood and cellular rupture as a result of overloading. As some tissues fail the focal load increases until “Suddenly” the limb fails at the focal point.
The broken end of the fallen limb looks quite moist, sound and healthy. This is the reason that people did not recognise the cause. To see the cause you have to examine the remainder of the limb still connected to the tree, below the point of failure. When you do this you will find it colonised by brown rot.
There are two types of tree wounds natural and unnatural.
- A natural wound site is caused through the shedding of organs, no longer required by the tree. Such shed parts as mature seedpods, flower parts, old leaves, unproductive twigs, branches or roots. The natural process of the tree is to compartmentalise the tissue behind these abscission points, it does this by producing a chemical barrier.
- This chemical barrier is sustained by the tree until the wound closes. The tree uses energy to produce these chemicals and sustain their levels over time. This process resists the entry of pathogens. If, however, the tree runs out or is low on energy at this wound site, the pathogen can then enter. The resistance to and pressure from pathogen on trees is analogous to that for any living organism. If resistance is stronger than pathogen pressure, the host wins and visa versa.
- An unnatural wound site is a location where internal wood tissue is exposed as a result of:
- a branch may be torn off the tree by an animal during feeding or climbing,
- Improper pruning cuts leaving tissue that is alive but committed to starving to death that becomes a food sauce for pathogens, thus providing them with the resources to overcome the trees defense systems,
- Storm damage or vandalism where branches are ripped off leaving large wound sites that often see tissue shattered for some distance on the remainder of the still connected branch base.
- It can also be genetically induced by a propensity of some man made selections of a genus and/or species that develop poor structural form. These weaknesses, such as branch unions with included bark, leads to branch failure and large slow closing wounds ideal for pathogen entry.
- Maintenance failure by tree owners where the architectural form of a tree, due to their response to growing condition, generic selection (see previous example) or formative pruning during the establishment or maturity of the tree leads to limb failure.
- Trees are generally abandoned in the landscape. Councils and tree owners plant trees but seldom follow through with formative pruning, root space provision, its plant and forget. Trees in the landscape are PETS and like all pets require support and maintenance. This human failure leads to the loss of many trees just as they become significant in the landscape.
- Poor Nursery production practices sees girdled and unnatural root systems that while not reducing tree growth in the short term will see them fail in the future. Failure results from structural failure, tree falls over at the ground or decline and death and the tangle of roots strangle the vascular tissue leading to starvation. Such stress may not kill the tree but enhances the entry of pathogens that lead directly to its loss.
- a branch may be torn off the tree by an animal during feeding or climbing,
The entry of brown rot pathogen into trees is made easier where the tree is under stress induced decline or dieback. Here we need to understand the difference between decline and dieback so read that paper.