Red is another story, because it’s made on purpose. As some leaves die, they produce chemicals called anthocyanins (also found in the skin of grapes and apples) from built up sugars. These chemicals produce a red pigment that can combine with green pigments left from chlorophyll and display different shades of red.

How bright this red is depends on what species the leaf belongs to, its inherent genetics and the environment around it — including the forest, the tree, and individual leaves, said John Silander, an ecologist and evolutionary biologist at The University of Connecticut.

But with the chemical explanation out of the way, the question still remains: Why do some leaves use precious energy to turn red right before dying? There are a handful of evolutionary explanations, Dr. Silander said, but many of them sound contradictory.

According to a 2007 paper published in The Botanical Review, red and yellow fall leaves could be flashing arrows that attract birds and mammals to a tree’s fruits. Animals that stop by for a bite will then do what animals do, dispersing the seeds as they go, thereby aiding in the species’ survival.

On the other hand, colored leaves could work like the wings of the monarch butterfly, warning others about bad-tasting defensive poisons or chemicals that tend to be in red leaves.

But this could be wrong, too. Pests laying eggs in the fall might prefer drab plants rather than bright ones, leaving the bright ones to survive.

“It may be an ‘I’m super-healthy, don’t bother’ signal to potential insects, pests, or parasites that they should look elsewhere,” Kerissa Battle, a community science educator at Community Greenways Collaborative, wrote in an email.

But then again, she said, the red color could also signal that a leaf is on its way out, and there’s not much healthy stuff left to eat before it drops.

With these and other contradictory hypotheses, what else could explain all that red foliage? Robert Guy, a professor of forestry at the University of British Columbia, suggested it could have something to do with the autumn sun.

While it doesn’t appear to be true in all trees, red probably works like a sunshade in maple trees. Abby van den Berg, an ecologist at the University of Vermont, found that protecting leaves from sun damage lets them do important stuff on their way out, like sending nutrients back into the tree to store for later use. If something goes wrong during this critical process, the whole tree suffers. This is also why new buds are also often red — that’s another critical time when a tree needs to avoid sun damage.

“Protection from bright light (and probably UV light as well) during the process of leaf senescence is the best explanation for why the leaves of some trees turn red in autumn” Dr. Guy wrote in an email.

(http://www.autoadmit.com/thread.php?thread_id=3398865&forum_id=2#31726597)