Kildare nature expert reveals why some trees lose their leaves, while others don’t
Nuala Madigan from the IPCC Bog of Allen Centre explains the science behind the falling leaves
Have you ever wondered why some trees lose their leaves, why other trees stay evergreen, and why the leaves of the beech tree changes colour, but the leaves don’t fall?
A tree that drops its leaves is known as a deciduous tree. All trees grow by the process of photosynthesis. They take sunlight, water and carbon dioxide from their environment. With a natural green pigment known as chlorophyll in the leaves, the plant makes glucose, providing energy for the tree to grow. In winter, as temperatures drop, there is a risk that the water in the cells of leaves will freeze. If they freeze the leaves would die and risk the health of the tree.
To protect the tree and ensuring its survival for the next growing season, deciduous trees allow their leaves to drop, but before this, they will absorb as much of the nutrients stored in the leaves, including the chlorophyll, which results in the leaf colour change.
The tree enters dormancy for the winter ensuring new leaves will emerge next spring, and the tree can continue to grow. So why then do some trees stay evergreen, surely their leaves would also have the potential to freeze? If you look at most evergreen leaves, the shape of their leaves are often needle like.
You might then argue a holly leaf is not needle like, but it is still evergreen and you would be correct. Evergreen trees have a number of adaptations such as the shape of the leaf, or a waxy coating on the surface of the leaf to help minimise water loss, and their cells contain a natural antifreeze, which lowers the freezing point of internal water. These adaptations all help to protect the tree during the winter. Finally the Beech tree leaf changes colour, but does not fall to the ground. This is a process called ‘marcescence’.
There are multiple suggestions for the rational for this. One of them is that the beach holds leaves to help to protect the spring buds.
If you would like help identifying one of your wildlife observations, contact me at the Bog of Allen Nature Centre at [email protected].
- Nuala Madigan, Bog of Allen Nature Centre, Lullymore, Rathangan