Trees in the temperate zone shed their leaves annually. Before falling, the leaves get yellow and orange shades. But why? First of all, why do the trees renounce their foliage?

Contrary to the popular belief, the freeze does not affect the trees during low temperature periods. It just dries the trees. At negative temperatures, trees can't extract water from soil while the large surface of the leaves continues to sweat. So this is an anti-drought mechanism.

Trees that do not shade their leaves (like coniferous) have tinny leaves covered with wax, which sweat very little. When the sun (the daylight) goes down, the leaves stop producing chlorophyll, the green pigment of the photosynthesis. The trees do not respond to temperature, they won't shade the leaves earlier because of a chilly fall or delay the process because of a warm autumn.

Without chlorophyll, the carotenoids, orange and yellow pigments, are exposed. Carotenoids play a minor role in photosynthesis and are abundant in many vegetables and fruits (e.g carrots). They are precursors of A vitamin. "The yellow color has been there all summer, but you don't see it until the green fades away", said Paul Schaberg, U.S. Forest Service plant physiologist. "In trees likes aspens and beech, that's the dominant color change."

But other species like Boston ivy, maples and ashes deliver a brilliant red foliage before losing it. Unlike the carotenoids, the red color comes from anthocyanins produced only in the fall. Anthocyanins pigments present hues from blue to red in accordance with the acidity. They are found in many fruits (like strawberries, red apples, and plums) and flower petals.

Anthocyanins - beneficial antioxidants - act as sunscreen, by blocking out harmful UV radiation and shading the leaf from excess light, but serve also as antifreeze, protecting cells from easily freezing. Stress factors like freezing cold, UV radiation, drought, lack of nutrients and pathogens increase anthocyanins production in trees, so red leaves can also signal stress.

If the leaves turn red early, much before normal foliage falling, the tree is most likely suffering from a fungus or another stress factor. Scientists don't know exactly why the trees invest in producing anthocyanins then they are about to lose the leaves. "People have speculated that maybe it's something that helps leaves deal with stress," Schaberg told.

"If making the anthocyanins helps the leaf stay on the tree a little longer, it may help the tree absorb some of the good things before it falls off. The tree can use those resources for the next growing season."

Anthocyanins could provide an early signaling system about environmental problems.