Why Leaves Change Color in The Fall?

The primary thing that triggers the changing of colors in leaves is the length of day.  However, moisture and temperature play a role as well.  For instance, an extreme drought in the summer can delay somewhat the changing of the colors in fall.  Why this is the case is not wholly understood, but it is possibly from the tree not being able to make as much food to store up for the winter, in the case of the drought, so it might be trying to push the envelope a little in terms of making food for a couple more weeks before needing to shed the leaves.

Temperature also plays a part in the ultimate vividness of the color.  However, as far as the timing goes, seems to play a very small role given that trees of the same species at very high elevations, where it is colder, will have their leaves change color at nearly the exact same time as those of the same species at lower elevations on the same latitude line.

Primarily though, as the length of day shortens, at a certain point which varies by species, some mechanism in the tree will trigger it to begin the process of closing up the veins to the leaves and eventually shedding them, lest they freeze while the veins are still open which can potentially harm the tree.

There are three main things that give leaves their color.  Those are: chlorophyll (green), which is necessary for photosynthesis; carotenoids, such as carotene and xanthophylls, which produce the orange and yellow colors, but whose roles are not entirely understood; and anthocyanins, which give us the shades of red and purple.

In the former two cases of chlorophyll and carotenoids, they are both present in the leaves during the summer, but the chlorophyll more or less covers up the carotenoids, so you see a mostly green leaf, rather than orange or yellow.  The anthocyanins, on the other hand, are primarily produced as a result of glucose that is trapped in the leaves when the veins are closed off.  These sugars then break down as a result of sunlight and produce the red and purple pigments.

During the summer, the plant is continually producing chlorophyll to aid in the production of glucose, which the tree uses for food.  Once the day length decreases sufficiently, the tree gradually starts to decrease the production of chlorophyll and the veins to the leave slowly close off.  When this happens, what you have left is the carotenoids and, depending on the species and environmental factors, possibly the produced anthocyanins.