By Corrine Johnston
Have you ever wondered how the leaves can change color in autumn?
Plants contain chlorophyll (the main pigment involved in photosynthesis). Leaf tissue is green because chlorophyll absorbs light in the blue and red regions of the visible spectrum and green light is reflected – which is what we see.
Plants also contain accessory photosynthetic pigments called carotenoids which are yellow (carotenes) and orange (xanthophylls). Xanthophyll for example is what makes carrots orange. These pigments are always present in the leaf but are hidden by the chlorophyll during spring and summer.
In temperate forests such as in Michigan, most woody plants (such as trees) with broad leaves (opposite of a conifer) shed their leaves to survive the low temperatures of winter. In winter, a plant’s metabolism and rate of photosynthesis slows down or may even temporarily stop so there is little reason for a plant to keep its leaves.
As the climate changes from summer to fall, this triggers chemical changes in the plant. Chlorophyll is no longer needed as the plant begins to slow down its metabolism and the chlorophyll breaks down as a result. This allows the carotenoids to be seen. Other chemical changes can result in red and purple fall colors in addition to the yellow and orange carotenoids. Temperature, light, and the amount of water supplied are all factors that influence the degree and duration of fall color.
Listen to David Attenborough and watch an awesome two minute video of the leaves changing color here. In this time-lapse you can see the colors changing in the U.P. of Michigan all the way from space!