January 14, 2007
The title (of) this month’s article pretty well sums it up. If you had called your tree surgeon as few as 20 years ago you may have ended up with this unlikely remedy. In the past it was standard practice to fill cavities in mature trees with solid materials to “support the tree”. Imagine if you will the surprise of a fellow arborist years later when he cuts into a seemingly solid tree to find it is full of concrete or bricks!
Cavities in trees form when wood dies and decays. Animals and insects will use the weaker rotten wood to form nesting sites or galleries for there colonies. The two most common causes of injury in trees are incorrect pruning and storm injury. Both actions can result in extensive areas of internal decay in otherwise healthy trees. These rotten areas once excavated by animals or insects can be quite large.
It is important to remember two principals when looking at these cavities: a hollow pipe is as strong as a solid rod and the tree has already formed barriers that will limit the spread of further decay. The first principal flies in the face of the thought, you must fill this cavity or the tree will be weak and fail. What really happens when you fill a living dynamic biological system, a tree, with a solid inert mass of concrete is quite predictable. The tree is further damaged by the constant abrasion of the concrete against the inside of the tree. The concrete is also chemically very different than the tree and will increase the moisture level in the cavity. It will also have a caustic injurious effect on the living wood of the tree. The second principle is that trees are living and act to limit the spread of decay in their tissues. Tree cavities are isolated from the living wood tissue in a tree by strong chemical barriers created by the tree when it was originally injured. If you have ever split firewood you will notice a dark staining around old dead branches in the wood. This is the chemical barrier the tree has erected to prevent the rot from spreading from the dead branch to the living tree tissue. These barriers are reinforced and redirected whenever the tree is re-injured. This injury process occurs every time a tree with a filled cavity moves in the wind. This process of creating resistant chemical barriers in wood is very energy expensive for the tree. Trees that do not have the energy to erect or maintain these barriers will be overcome by rot. When this happens the tree will die.
Occasionally trees will develop cavities that fill with water. It is not recommended to drill holes to drain this water as the tree has created barriers to prevent the water and rot from spreading. Drilling holes will further injure the tree and increase the size of the cavity.
So what is the preferred process for dealing with tree cavities? Access the structural strength of the tree. Is it able to stand with the sound tissue that remains? Access the proximity of the tree to potential targets should it fail. Examine the cavity and if necessary remove any loose debris that remains in the cavity with out injuring the living tissue. Monitor the cavity and see if the tree is putting on strong growth around the cavity evidenced by new healthy tissue.
If the tree is not posing an immediate danger to people or structures it may be safe to leave it where it is and continue monitoring it. When in doubt have a Certified Arborist evaluate the condition of the tree and give you an appropriate recommendation.