Pruning cuts - collar, collarless and co-dominant (original) (raw)

Okay, you asked for it. I just hate it when people ask a question that makes me have to think and study, but it does make me learn also.

According to Alex Shigo in his "New Tree Biology", true heartwood is part of the trees natural aging process. Discolored wood is injury altered wood. The level of resistance to invasion is different between the two. Only by understanding the difference can we understand why one area of "heartwood" rots and another does not. At this point I am going to include a link to an old photo I took when I worked at Western Illinois University. As I studied up to write this article, I realized that the pattern that was exhibited here, shows exactly what Shigo addresses in his description. http://165.234.175.12/photos/Arbor/C...zed%20Burr.jpg

First a quick differentiation between people and trees. Human aging is measured as a ratio between cell breakdown - autolysis - to restoration in the body. We can make new parts as they are injured or worn out. Trees only generate tissue, they do not regenerate tissue. Therefore, the ratio of aging is based on the volume of wood that can store energy - dynamic mass - to volume of wood that can no longer store energy. (Here I insert another link to a PPT presentation that talks about energy management in trees on both an annual and lifetime basis -- http://165.234.175.12/photos/Arbor1.ppt )

With no outside factors stressing a tree, it's biggest worry is that it could outgrow itself, it is a generating system. The tree must protect its generating system and its mechanical support system. As a tree makes energy, it needs a place to store this energy. This is in the "living" or dynamic mass of the tree. If too much dynamic mass is created, in relation to energy production, the tree may use more energy keeping this DM alive than is created and starve the tree to death, but it needs enough DM to store energy for growth spurts in spring and to fight invasions of various disorders.

Keep in mind the previous statement that there is a difference between heartwood and discolored wood, we will come back to that. There are many ways that the tree, in its effort to ensure mechanical support, alters wood to prevent breakdown. These include "denser wood, cell arrangements, chemicals within cell contents and cell walls to resist pathogens -- extractives, antimicrobial substances -- alteration of micro environment -- extremes in oxygen, moisture, pH, elements, electrical state --and another 'last ditch' method that is seldom considered: 'invite' some mild pathogen in that does not break down wood but will keep out others that rapidly break down wood -- wetwood." (Quoted from New Tree Biology)

In a tree which is not threatened by a pathogen, the balancing act between dynamic mass and energy production is controlled by the

age altering

of inner growth increments to take them out of the storage process. The tree does need this wood for support, if it breaks it dies, so it alters it to resist pathogens which may try to destroy it. This is a top priority and is a static situation. If an injury does allow a pathogen to enter, the

injury altered

discolored wood, which is formed also takes out some of the storage capability, but not according to nature's plan. This compartmentalized wood has different characteristics, including being dynamic in nature to meet the changing invasions it faces, than the age altered heartwood, even though both are protection wood and dark colored.

Now go back to my picture from WIU. The tree, a Burr Oak, started out in a savanna type area on the prairie. As it aged, the balance of generation of energy and storage were maintained by the tree with the creation of age altered heartwood. The tree, did not need that much storage, but it needed to ensure mechanical support. The area later became a golf course for about 20 years and then part of the college campus. During this time of change, stresses and injuries lead to no doubt numerous disorders. More discolored wood was formed, but this was injury altered and a part of compartmentalization. The disease was better able to invade it than the heartwood at the center, formed under a different system, and the resulting pattern ensued.

Hope this made sense and was not too long or too short, Alex has 40 pages of photos and information on the subject.

The use of bacterial wetwood by trees to prevent rotting is a whole nother story.