Why are Leaves on yellow willow turning yellow?

The 'green' of leaves is the chlorophyll in the leaf that converts carbon dioxide and oxygen into energy for the tree to live.

During Autumn, when deciduous trees shed their leaves the tree is effectively going into a form of hibernation. To do this all the energy made over the year is stored in the tree, and all energy to feed the leaves is withdrawn by the tree.

This means that the leaves start to die, and the leaves go yellow (the natural colour of the leaf) as the amount of chlorophyll in the leaves falls. Eventually when each leaf has died, the stem of the leaf will seal up and start splitting away from the tree, meaning the leaf will fall with the next gust of wind.