As New Englanders we are particularly in tune with the changing colors of deciduous tree leaves during autumn. It is one of those seasonal cues that warns us of the approaching winter, reminds us of a new school year, and makes us think of jack-o-lanterns, apple-picking, and fall harvests.
Scientists have long know how the leaves change colors. As leaves prepare to fall off the tree (an adaptation for the colder temperatures approaching), they stop producing chlorophyll, the pigment that makes leaves green. Without the green pigmentation, other pigments are produced, and thus, other colors "shine through". We even know the molecular makeup of these pigments. However, biologists have yet to figure out why trees bother to produce these other colors. Why spend energy on creating a red or yellow or orange pigment?
Carl Zimmer has a post on his blog that discusses some experiments that investigate changing leaf color. There are some intriguing ideas there, but, in the end, we still do not know why leaves change color.