But this is not true growth – after all, the tree will shed these structures eventually.
True growth occurs when trees produce new wood. This kind of growth produces tissue that persists from one season to the next.
But even this growth is a very location-specific phenomenon, and it only occurs in a few places throughout the tree.
These places are called meristems.
Apical meristems occur at the tips of branches and roots. They are responsible for a tree’s longitudinal growth along its axis, or, put more plainly, they are the place from which branches and roots grow longer. Arborists and botanists consider growth from the apical meristem to be primary growth.
However, apical meristems also produce the cells that will become the plant’s secondary growth center – the lateral meristems.
Lateral meristems are found in the thin ring of tissue around the circumference of a tree’s trunk, branches and roots. These lateral meristem regions (which exist along the entire length of the branch or root in question) are responsible for increases in girth, rather than length.
Lateral meristems occur in two different tissues of each branch. The innermost ring is called the vascular cambium, while the outer meristem layer is called the cork cambium. Each produces new tissues from both the inner and outer surface of the ring.
More specifically, the cork cambium produces cork cells on its outer surface and phelloderm cells on its inner surface. The cork cells will eventually die and become the outer bark, while the phellogen is a parenchyma-rich layer of cells that serves to store starch for future use.
Further inside the tree, the vascular cambium produces phloem tissue on its outer surface and xylem cells (wood) on the inner surface. Phloem tissue transports starches along the length of the branches, while xylem transports water from the roots to the leaves and supplies support and stability to the tree as a whole.
The Arboricultural Implications of Meristems
With the understanding that trees only grow from these regions, it is important to apply this knowledge to the ways in which we prune trees. There are a number of ways to use this knowledge, but the following include some of the most important:
- Because it removes a large number of a tree’s apical meristems, topping is a destructive practice that should be avoided. Removing a branch’s meristem often results in the branch’s death, and it forces the tree to grow from lateral branches, rather than the branch tip.
- Tree branches do not move higher off the ground with time. Because trees grow from the tips of their branches, and not from the trunk’s base, limbs do not move higher with time. This means that an obstructing limb will remain in the way unless you remove it. Arborists must perform a procedure called a crown raising to deal with these types of limbs.
- Narrowly spaced, co-dominant stems force material to grow inside the wood of the junction. When a tree produces a pair of trunks, the expansion in girth causes the tree to produce wood around the bark material, sandwiched between the two. This makes such junctions weak, as the bark does not attach well to the wood inside the junction.