Dead trees are fascinating because they provide us with a permanent record of their lives. The annual rings that record the tree's experience with drought, nutrients and temperature are familiar to most of us. Somewhat less familiar, but easy to see, is the record of all the insults, accidents and stresses of life as a giant, long-lived organism.
The photos in on this page are from a very old blue ash (Fraxinus quadrangulata) that died and fell over a few years ago. As the bark decayed and sloughed off, the wood was exposed, showing us the most recent wood made by the vascular cambium. The cambium is an amazing tissue. It is a continuous sheath of stem cells that lie between the wood (xylem) and inner bark (phloem). All the wood in the world is made by the cambium of trees. The stem cells of the cambium are constantly adjusting their cell division to make the appropriate amount of wood and phloem, and to react to changes in their environment, including gravity, wind and wounds.
I think that the cambium serves as a pressure sensor - the pressures created by the wood to the inside and phloem to the outside, combined with pressure changes due to wind and gravity, determine what kind of cells the cambium produces.
The wood just below branch junctions is heavily wrinkled. This represents the combined influence of gravity and wind. The branch constantly flexes in response to wind, and the cambium below constantly adjusts to stabilize the main stem. On the main stem, the ripples in the wood are also probably due to wind and gravity, but are more subtle. These surface ripples are reflected deeper in the wood, and lead to the figure so highly prized by woodworkers.
Once the tree is dead and down, and the cambium is dead, other processes begin. Wood is under tremendous stress and strain in a living tree. As soon as the tree dies, the wood begins to crack as its moisture content changes. The cracks provide easy entry for decay fungi, protists and insects. The holes in the wood are made by beetles that feed on dead wood. Many of these beetles carry fungi with them that accelerate decay. Over time, insects, bacteria, protists and fungi gradually take the tree apart, converting the wood to energy and biomass. How fast this happens depends on temperature and moisture: in moist tropical forests, a tree may disappear completely in a few months. In drier or colder forests, complete decay may take centuries or may never happen.
The process of death, decay and recycling is critically important to forest health and stability. Without decay, nutrients would not be returned to the soil and be made available to the next generation of trees. Wood decay enriches the soil.