(Continued from A Year in the Life of Your Tree - 5)
All living organisms need a defence system. It is the nature of life on earth and its continuous energy transfers. Everything feeds on something else, if you can call drinking of sunshine eating, and everything is eaten by something else. This is all good, a master plan deeply ancient in its practice, even the continents are slowly being recycled.
At the most simple level the single celled organism and its cell wall is the first model of defense. Perhaps it also has cilia that can move it towards its food source and away from danger. Plants, trees do not have that option, they stand and endure. Their remarkably complex chemistry has given them some good tools. Most complex lifeforms need chemicals for a variety of processes and defence and they produce them as needed. Think of ear wax. In plants they are called secondary metabolites; alkaloids, terpenoids and phenolics are the major groups, with thousands of different chemicals throughout the plant world.
Ancient humankind used plants for food and later, experimentally, as medicines. In a highly diverse environment like the Amazon jungle, the variety of plants with useful secondary metabolites that could be taken as drugs easily numbers in the hundreds of species. Indeed, up to about 120 years ago plants formed the basis of all pharmacopoeias world wide. In many cases they still do; wander into a Chinese medicine shop sometime.
As we talk about tree defence, another aspect is the fact that unlike animals, plants never replace damaged parts or tissue. Every time you get a little cut on a finger, eventually each damaged or destroyed cell will be replaced by a exact copy in the same place. Not so with the trees; they are generating systems, we are regenerating systems. Just like a deep sliver that never came out of a finger and is encapsulated, tree use a process called, CODIT, Compartmentilization of Decay In Trees. A knot in a table top is a compartmentalized branch core.
The concept of CODIT is Dr. Alex Shigo's words and ideas. He was born May 8, 1930, and worked as a research scientist for the United States Forest Service from 1959 until 1985. He was leader for a pioneering project on the action of decay inside trees. During this time he published over 270 scientific research papers. Afterwards he published a handful of essential books for arborists and foresters. He traveled and lectured widely to disseminate his deep understanding; his ideas changed how tree work is done around the world. Dr. Shigo originated the concept of CODIT. For more on CODIT, see Dr. Shigo's New Tree Biology.
Trees suffer wounds from a number of causes. There is the loss of branches from storms. There is physical damage from animals. There is feeding on leaves by insects and other animals and feeding on tissue by aggressive fungi, bacteria and viruses. And there is tree care; yes, the best of pruning is still seen as a series of wounds from the tree's perspective.
Back to the single cell and its wall for a moment, all living organisms have an inside versus the great outside. Life, a highly ordered circumstance, happens in enclosed sealed bodies that have a very different and complex set of activities inside compared to the outside world. Entropy says that energy always flows from a greater complexity of order to a lower complexity of order. Life is only maintained by the constant input of energy to maintain that high order, and defence is a major part of maintaining that order.
As soon as the bark has been removed by any cause, a complex set of reactions begins. Inside the protected tree's body where its chemistry is highly controlled, any contact with the air and its 20% oxygen content is a serious problem. The tree solves this problem by walling off some tissue to act as a buffer from the outside world. This is a sacrifice, but the leak must be plugged.
It is like accidentally breaking a window during a deep cold spell. You will insulate and put up a plywood temporary boundary until proper repairs are finished. Depending on the state of the tree, greater and lesser amounts of healthy tissue are used to separate and manage the hole in a previously closed system.
What happens is that phenols and turpines, naturally anti-fungal and anti-bacterial compounds, secondary metabolites, are collected around the wound site and deployed in a loose series of "walls". A wall forms to stop any deeper penetration into the trunk. Walls also form to stop decay and oxygen burn from moving any higher or lower into the trunk. Not walls in the sense of a solid partition, but walls in the sense of a concentrated zone of secondary metabolites that usually stops the invasion, seals it off, CODIT. Most of the time this works very well, and costs are kept as low as possible. The costs are the loss of tissue from the actual wound, the loss of tissue used as the barrier between the inner and outer world, and the energy costs of performing the rescue operation. This adds up -- consider hundreds of pruning cuts.
There is a great difference between pruning cuts and other types of wounds. If, pruning cuts are performed properly at natural target sites, especially branch collars, the overall cost of the wounds is as low as possible. If the collar is damaged, a flush cut has been made and the amount of internal trunk tissue taken up to compartmentalize this type of wound is many times greater than if the cut had been made at a natural defense site, a branch collar. Dr. Shigo's book deals with this old problem, flush cuts, in great detail with lots of pictures of the inside damage in the trunk.
Continued in A Year in the Life of Your Tree - 7)