Question

Trees can be killed by girdling-the removal of bark and vascular cambium in a ring all the way around the tree. Explain why.

Short answer

Girdling a tree kills it by disrupting the transport of sugars and other organic materials through the phloem, a layer produced by the vascular cambium found just beneath the bark. By removing the bark and vascular cambium in a complete ring around the tree, the essential physiological processes for growth, maintenance, and defense are inhibited, leading to the tree's eventual death.

Step-by-step solution

Understand the structure of the tree's stem

The stem of a tree is composed of different layers. The outermost layer is known as the bark, which provides protection against external factors such as pests, diseases, and mechanical damage. Just beneath the bark is a layer called the vascular cambium, responsible for producing new cells for growth in the tree. It generates xylem (wood) toward the inner part of the stem and phloem (sapwood) towards the outer side.

Role of xylem and phloem

Xylem and phloem have essential functions in the tree's physiology. The xylem is responsible for transporting water and nutrients from the roots to the leaves and other parts of the tree. The phloem is responsible for distributing the energy (sugars) produced during photosynthesis from the leaves to the rest of the tree for growth and maintenance.

Effects of girdling on the tree

When a tree is girdled, both the bark and the vascular cambium are removed in a complete ring around its stem. As a result, the flow of water and nutrients in the xylem is not affected, as it is still connected to the roots and inner parts of the tree. However, the phloem will be severely damaged, which will disrupt the transport of sugars from the leaves to the rest of the tree.

The consequences of disrupted sugar transport

The tree's inability to transport sugars and other organic materials to the roots and other parts of the tree will result in a lack of energy for growth, maintenance, and defense against pests and diseases. Over time, the tree will weaken and eventually die due to the disruption of the essential physiological processes. In conclusion, girdling a tree kills it by removing the bark and vascular cambium in a complete ring around its stem, which disrupts the tree's phloem and the transport of energy to support growth, maintenance, and defense. As a result, the tree weakens and eventually dies.

Key concepts

Girdling: Why It Kills Trees

Girdling refers to the process of removing a strip of bark, along with the underlying vascular cambium, in a complete circle around the stem of a tree. This may sound simple, but it has severe consequences on a tree's physiology. When the girdling process is performed, the tree's ability to transport essential nutrients is disrupted.

• The girdled portion loses the connection through the phloem, which is critical for transporting sugars and other compounds generated during photosynthesis from the leaves to the rest of the tree.
• Unlike the phloem, the xylem below remains intact, continuing to transport water and minerals upward from the roots.

However, without sugar distribution for energy, vital processes decelerate, leading to the tree's decline. With time, the tree weakens due to an energy deficit, making it unable to sustain growth and fend off diseases.

Xylem and Phloem: A Tree's Transport System

In trees, two main types of vascular tissues—xylem and phloem—work as their transport system. Each has its distinct role in maintaining the tree's life functions. The xylem is primarily in charge of carrying water and minerals absorbed from the soil through the roots.

• Xylem vessels act like tubes, channeling water from the roots to various parts of the tree, including the leaves, where photosynthesis occurs.

On the other hand, phloem is responsible for moving sugars and other products of photosynthesis from the leaves to other parts of the tree.

• This movement supports growth, repair, and maintenance as it supplies energy-rich compounds essential for cellular processes.

If either of these systems is compromised, especially the phloem as in girdling, the overall health of the tree is at risk, affecting its ability to grow, reproduce, and protect itself.

Vascular Cambium: The Growth Engine

The vascular cambium plays a crucial role in a tree's growth. This layer of unspecialized cells lies between the xylem and phloem. It is responsible for producing new cells that contribute to the growth of the tree's stem and branches.

• Every year, the vascular cambium generates new xylem cells on the inside and new phloem cells on the outside, thickening the tree each growing season.
• This process not only supports the tree's vertical and radial expansion but also renews its vascular tissues essential for nutrient and water transport.

When girdling occurs, the removal of the vascular cambium interrupts this vital cell generation process. Without new phloem, sugar movement through the phloem becomes impossible, progressively starving the tree of energy. Over time, this lack of energy critically impairs the tree's innate ability to heal and grow, leading to its eventual decline.