Leaf favourites #1

Can't go too far wrong with a maple.

You know why it happens, but here's a refresh courtesy of the Woodland Trust:

The beginnings of leaf drop, also known as abscission, start when a layer of cells is formed between where the leaf stalk joins the stem. This layer, known as the abscission layer, is formed in the spring during active new growth of the leaf.
In autumn, hormones within trees begin to change. The most notable is auxin. During the active growing season, production rates of auxin in the leaves are consistent with the rest of the tree. As long as these rates are steady, the cells of the abscission layer remain connected, which in turn, keeps leaves attached.
But as days shorten and temperatures cool, auxin production in leaves starts to decrease. This triggers cellular elongation within the abscission layer. The elongation of these cells creates fractures, allowing the leaf to break away from the plant. The leaf is finally blown off by the wind or falls from its own weight.