Autumn is my favourite season in eastern Canada. When I was working as a nature interpreter in Toronto last year, I would teach children about why leaves change coulour and fall from their trees. Here's a summary:

Leaf color comes from pigments, which are natural substances produced by leaf cells. The three main pigments in leaves are chlorophyll (green), carotenoid (yellow, orange, and brown), and anthocyanins (red). Chlorophyll and carotenoid are always in leaf cells during spring and summer. But chlorophyll masks carotenoid; this is why leaves are green during the growing season.

In Autumn, trees react to the decreasing amount of sunlight by producing less and less chlorophyll. Gradually, trees stop making chlorophyll. When this occurs, the carotenoid pigment becomes visible. At this point, leaves display their glorious tapestry of fall colours.

In some years, the red fall colors are more vivid than in other years. This is due to temperature and cloud cover vaiances. During the day, leaves produce a lot of sugar, but in the nighttime, low temperatures prevent the sugar sap from flowing through the leaf veins to the trunk and branches. In response, anthocyanins are made as a way to protect the tree by allowing it to save nutrients in the leaves before they fall off.

In winter, the leaves of deciduous trees (aka. trees with leaves instead of needles) will freeze. This is because they are made up of cells filled with water sap. To ensure their survival, trees seal off and shed all plant tissue that can't live through the winter. As sunlight decreases in the fall, the veins that carry sap back and forth through leaves gradually close. A layer of cells, called the separation layer, forms at the base of leaf stems. When this layer is complete, the leaves separate from the tissue that connects them to theif branches, and fall.

-A