Chapter 12 Basic Anatomy of Tree Growth

_Note: portions of the information on this page comes courtesy the Arbor Day Foundation and US Forest Service, at https://www.fs.usda.gov/learn/trees/anatomy-of-tree_

Tree with roots clasping a boulder in Kiental, Germany. Each of the large aerial roots began as a small feeder root that expanded both lengthwise and in diameter over time. Link to image source.

12.1 Where does growth happen?

In bonsai we often want a branch, root, or trunk to increase in diameter, not just length. How does this happen?

If you put several marks on a woody branch and watch it over a season, you will see the marks on older woody areas do not move; cells in those areas have matured already, and do not contribute to extension. Instead, all extension growth happens near the shoot tip.

The tips of both shoots and roots have masses of precursor cells called primary meristem. During the growing season, these meristem cells divide rapidly. Some remain unspecialized precursor cells. Other meristem cells secrete hormones like auxin and cytokinin. Still other meristem cells begin to change into the cells that form new roots, shoots, and leaves. The same thing happens when new side branches form; each dormant bud on a branch has a bit of primary meristem. Even roots lengthen mainly at their tips.

Trees and shrubs have a second area of growth called secondary meristem that lies just beneath the bark of the trunk, woody branches and larger roots. This is the green layer what we call the cambium. The cambium can be split into two separate functional layers: the vascular cambium, and the cork cambium.

12.2 How does cambium make a trunk thicken?

Typical arrangement of the layers in a woody trunk or branch. Modified from original; link to image source.

A woody trunk or branch is built from concentric layers of two kinds of tubes: xylem, which carries water and soil nutrients up to the leaves; and phloem, which carries sugars and amino acids from the leaves (where they are made) to the rest of the plant (where they are used for growth or stored for later.)

Layers of xylem make up the woody center of a branch or trunk. Older, harder heartwood is found towards the center of the trunk, and is made up of old xylem tubes that no longer carry water. Heartwood is the central, supporting pillar of the tree. Although dead, it will not decay or lose strength while the outer layers are intact. A composite of hollow, needlelike cellulose fibers bound together by a chemical glue called lignin. Heartwoodʼs main function is support and storage of waste materials, so on most trees it can be carved out without interrupting water and mineral flow from the roots to the leaves.

Next is a layer of younger sapwood that surrounds the heartwood, and contains the actively working xylem tubes. These xylem tubes form an unbroken connection between the roots and the leaves, through which water can be drawn up by the leaves. Sapwood is the tree’s pipeline for water moving up to the leaves. Sapwood is new wood. As newer rings of sapwood are laid down, inner cells lose their vitality and turn to heartwood. Unlike heartwood, cutting through sapwood can kill part or all of a tree above the cut.

The next layer outside sapwood is the cambium, which is the growing part of the trunk. It produces new bark and new wood each season in response to auxin hormones that re produced by meristem at the ends of branches and pass down through the phloem with nutrients from the leaves.

The inner portion is the vascular cambium. This single layer of cells multiplies like apical meristem, but the resulting new cells form xylem and phloem only. Each time a new xylem layer is formed, it increases the diameter of the trunk or branch. At the same time some of the oldest, deepest sapwood shuts down and changes into heartwood.

Just outside of the vascular cambium is a layer of phloem tubes and cells that form the inner bark, or cork cambium. Like the xylem, the phloem tubes form an unbroken connection between the leaves and the roots. As the vascular cambium creates new xylem/sapwood, the diameter of the tree increase, which makes the phloem tubes pull apart from one another. To compensate, the vascular cambium layer makes new phloem tube cells, that fill the spaces between the older phloem tubes as a treeʼs diameter increases. This layer lives for only a short time, then dies and turns to cork to become part of the protective outer bark.

The outermost layer is the bark, which is the tree’s protection from the outside world. Continually renewed from within, it has densely packed water-resistant cellulose fibers that help keep out excess moisture in the rain, and prevents the tree from losing moisture when the air is dry. It insulates against cold and heat and wards off insect enemies.

If we could watch a tree in cross section as it grew, we would see that most of the actual increase in diameter occurs because of new layers of xylem added to the sapwood. The next biggest contributor to increasing diameter is thickening of the corky layers of the bark. In contrast, the thickness of the phloem layer and two cambium layers changes very little or not at all.

Cross section of a 4-year old woody stem at 100X magnification. The outer layer, stained green, is the bark and cambium. The 4 layers of open, purplish-stained tissues are the 4 layers of xylem tubes laid down in each of the 4 years of this seedling’s life. The living outer layers are sapwood; as the innermost xylem layers die, they become heartwood. Link to image source.

12.3 Putting tree anatomy to work

Knowing how a woody trunk or branch is arranged makes it easier to understand how and why trees react to injury. For example, a tree will die if it is girdled, that is, cut all the way around its circumference. The cut does not have to be deep, just enough to break the vascular cambium. If a tree is girdled, sapwood still can carry water up but there is no way for nutrients to move down from the leaves to the roots. The roots begin to die, and lose their ability to extract nutrients from the soil. This reduces the flow in xylem, which weakens the leaves further, and eventually causes the tree to die. What you may not know is, if you reconnect the phloem path with a root or branch graft, you may be able to save a girdled tree.

Another common injury happens when training wire is left in place too long. When wire bites through the bark, phloem vessels are exposed to the outside world for long periods. Viruses, bacteria, and fungi gain direct access to open channels that let them move everywhere in the tree.

Woodʼs structure also helps explain the difference in what conifers and deciduous trees will tolerate when styling. Junipers can be styled with large strips of bark removed and the underlying wood painted with lime–sulfur to simulate weathering. This is extremely hard to do with deciduous trees. The reason why has to do with the structure of xylem and phloem vessels. In deciduous trees, xylem and phloem vessels are larger, porous, and often have lateral rays, radiating channels that connect several vessels. When bark is removed, these lateral channels let the tree bleed sap excessively, or take up a toxic dose of lime–sulfur. In contrast, a juniperʼs xylem and phloem vessels are narrow, less porous, and have few lateral ray connections. When bark is removed from junipers, opened vessels seal quickly with resin. The trade-off is that nutrients and water cannot bypass an injury site on a juniper; foliage and branches directly above any break in the cambium usually die. The same break in the cambium has very little effect on the overlying foliage growth of deciduous trees like maples or crabapples.