Trees are long lived organisms living in hostile environments that are full of wood rotting fungi and bacteria. It may come as a surprise that trees are essentially composed of sugar! Cellulose, the basic building material for all woody plants, is composed of long strings of sugar joined together.
Bacteria and fungi have the ability to break down the complex sugars in cellulose and digest it. Trees have developed the ability to resist this process through the development of additional chemicals that resist the initial infection and colonization of wood tissue by these pathogens. These resistive chemicals are mostly specialized oils and aromatic hydrocarbons called turpenes and phenols. The tree creates these chemicals in response to the presence of fungi or bacteria and they clog the living cells and passageways in the wood preventing the spread of these wood digesters into new tissue. The resulting process is called compartmentalization.
The process of compartmentalizing the damaged tissue results in it being walled off with an impervious protective layer. Much like the unfortunate crew member that was on the wrong side of a water tight barrier in a sinking ship, living cells on the other side of this protective wall are sacrificed.
If you want an example that is closer to home, look in your wood pile and you can see areas of discolored wood surrounding dead branches or cavities in your split wood. These discolored areas are the chemical barriers that formed the walls of the compartment that contained the spread of the fungi or bacteria in the tree. Wood is a complicated matrix of tissues that have connections upwards, downwards, laterally and towards the center of the wood. Boundaries must be set up in all these directions to prevent the fungi and bacteria from spreading. This process is very energy intensive. Creating the chemical barriers takes energy and the lost energy resources behind the barrier are sacrificed. These resources are lost forever and will be converted to simple sugars and digested by rot organisms. If the tree is successful, it keeps living and grows around the compartmentalized area with new tissue. If it doesn’t the rot will overwhelm the tree and it will fall to pieces. Once the tree is no longer living, no new boundaries are set up and the decay organisms have the advantage. The only thing that stops rapid decay is the lack of a suitable environment and the resistive chemicals left by the living tree.
The decayed tree falls back to the forest floor and is eventually converted into nutrients that will feed the future forest.