Leaves - Why do Trees Lose Their Leaves in Winter?

 When leaves fall off the trees, this is called abscission.  Plants contain a hormone called auxin which helps them grow. It’s also auxin that causes plants to turn and face the sun, so that they can absorb more light. In spring, a layer (called the abscission layer) forms at the base of each leaf. Winter temperatures cause the production of auxin to slow down and this breaks the abscission layer, causing the leaves to detach from the trees. Losing leaves helps the tree to retain water during in the winter and means that it needs less energy to stay alive. The fallen leaves help to add nutrients to the soil.