"Autumn is a second spring when every leaf is a flower." - Albert Camu

Fall is a time of magnificent beauty when we are treated to a final burst of colour before the start of winter. But, what is the chemistry behind these colourful creations and why is it significant?

Leaf cells produce pigments which help them make food. The three pigments that colour leaves are chlorophyll (green), carotenes (yellow) and anthocyanins (reds and pinks).
Weather influences the depth of autumn colours in following ways.

Cold nights: The chlorophyll is destroyed with low temperatures causing green leaves turn yellow. With further fall in temperature, anthocyanin production is enhanced and the leaves take on a red colour.
Dry weather: More anthocyanin is produced and consequently leaves are redder.
Bright sunny days: In autumn, the production of new chlorophyll stops. But photosynthesis can still occur on sunny days, using the remaining chlorophyll. As sugar concentration increases, more anthocyanin is produced and the leaves are redder.

The process of leaves dropping is known as abscission. Auxin is a hormone which plays a role in the abscission of leaves. Young leaves produce auxin during the active growing season. As long as auxin levels are steady the leaves remain attached to the stem.However, in autumn, as days shorten and temperatures cool, auxin production in leaves starts to decrease. Thereby causing the leaf to break away from the plant. The leaf is finally blown off by the wind or falls from its own weight.
A tree without leaves requires less energy to remain alive. Leafless trees can tolerate winter storms, snow and strong winds more easily.