Abscission

Abscission is when leaves fall off trees. The reason leaves fall is because of a hormone called auxin in all plants. Auxin helps plants grow and it also forms a layer towards the bottom of each leaf, which is called the abscission layer and it helps support the leaf. The cold winter temperatures slow down the production of auxin, this breaks the abscission layer, causing the leaves to fall from the trees. The fallen leaves also help to give nutrients to the soil.