Trees in winter
We all know that trees that drop their leaves in winter are called 'deciduous', but just why do some trees drop their leaves?
For many trees, keeping their leaves throughout winter would be a real liability. As the weather becomes colder, wetter and generally windier, trees with a lots of leaves for the wind to catch could suffer from being blown over. Losing leaves means there is less area for the wind to catch; it can blow through the bare branches leaving the tree relatively unscathed.
Deciduous tree leaves tend to be broad and flat with relatively soft tops and underneaths. In cold weather the liquid sap carrying food to the cells inside would simply freeze, killing the leaf. If too many leaves on one tree were affected, this could kill the whole tree.
As winter days are short, there is not enough sunlight for the leaves to effectively 'capture' sufficient energy from the sun for photosynthesis (the process that trees and all green plants use to make energy for growth).
It makes good sense for a deciduous tree to halt growth and drop leaves until the weather warms up and days start to lengthen in the Spring, when they can grow a fresh set.
The leaf drop process starts in autumn, when days start to get shorter. The trees react to this by starting to grow a special layer of cells at the base of the leaf stalk (called the abscission layer). As this is happening, the chlorophyll (the stuff that makes leaves green and traps the sun's energy in photosynthesis) is withdrawn into the trunk to store for next year. You can see this is happening when the leaves turn colour. Pigments such as xanthrophyll (yellow) and carotene (carrot colour) become visible. These too are then withdrawn and stored in the trunk. The abscission zone finally seals off the now dead leaf, which falls to the ground.
Fallen leaves in a woodland can help insulate root systems from the cold weather. They will rot down in the coming years, putting much goodness back into the soil.
How is it then that evergreen trees don't need to drop their leaves in winter? That's for next time . . .
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