- Written by Kevin R. Lee Kevin R. Lee
- Published: 18 November 2019 18 November 2019
Earlier I described the action at the branch tips that is primary, or shoot extension growth, how branches get longer. I now want to talk about how trees increase the diameter of branches and trunks, get thicker, which is secondary growth. Secondary growth is important for continued strength of branches as they get longer. It is also important to increase the internal volume of living tissue used for energy storage. We have all seen the growth rings in the cut end of a branch or a log. They can tell a lot about a tree's history. Trees that have lived for hundreds of years have no choice but to bear witness to fires, severe storms, and other problems that damaged the tree and are recorded in the long story of its life written in the trunk.
Each growth ring is the physical record of that year's life. Trees use a layering system, just like we do when winter comes, and the idea of dressing for outside includes base, mid and outer layers. The tree's growth is like that in a way, each year is another layer.
Think of the bark covering the whole of the tree's body, just as your skin covers yours. Right underneath the bark, and between it and the sap wood, is an thin tissue called the cambium, a layer of cells that I call mother cells. Their job is to continually split off daughter cells that will either be the new growth ring, on the trunk side, the inside, or the complex bark tissue on the outward side.
This yearly growth is initiated at the same time as the buds at the branch tips are starting to expand. If you peel the bark off a branch at this time, what you will find underneath is a slippery substance that seems to have light threads in it. This early jelly-like material is the start of the new growth ring. Wood is mainly composed of two substances, cellulose and lignin; lignin is the "glue" that holds it all together. It holds it together so well that to remove the lignin from the cellulose takes a pulp and paper mill and requires a large input of energy to take apart what trees have carefully built.
This first part of the new growth ring is called early wood. Later, as other layers are added, we have a section of the growth ring called late wood. The whole multicellular complex is being generated as this process continues, as are the new vessels being formed and connected to bring the essential water and nutrients from below. There are many patterns of growth ring construction that a variety of species would show us. Nature's greatest strength is its endless diversity, many patterns, many ways of achieving the same ends.