Why do some leaves fall in the Fall?

Call me sappy but I think that's very noble.

The leaves on broadleaf trees have cells that are filled with water and sap. If these leaves froze the water inside them would expand and break the cells. When the leaf thawed it could cause the tree to loose valuable nutrients.

In the autumn the day’s become shorter and the sunlight less intense. Trees in the parts of the world where it gets very cold in the winter know the decreasing sunlight is a sign that bad weather is coming. The trees react to this by slowly forming a layer of cells at the base of each leaf. This layer closes off the veins that carry fluids in and out of the leaf. The layer is called the abscission layer. Abscise means “to cut off”. Once the layer is complete the leaf falls. In a few species of trees, like oaks and beeches, this layer doesn’t fully detach all the leaves so, you may see a lot of dead leaves stay on the tree all winter.

There is another good reason for trees to get rid of their leaves. It’s a good way of temporarily getting rid of the insects that eat leaves and lay their eggs on them.