You probably learned in school why some leaves change colors in fall. And if you're like me then you probably remember that Chlorophyll is green, but your memory gets sorta' fuzzy from there. If so, then here's a short (and somewhat simplified) family tree for green, yellow, orange, red and brown leaves.Chlorophyll acts like the eldest cousin -- it is hard-working, responsible and constructive. Leaves appear green because Chlorophyll absorbs the red and blue-violet wavelengths while reflecting the blue-green and yellow-green. Chlorophyll uses the energy of absorbed sunlight to build sugars out of water (via the roots) and carbon dioxide (via tiny holes in the leaves), giving off oxygen as a waste byproduct.
When the hours of daylight (the "photoperiod") dip below a certain trigger point, cells in the leaf stem begin dividing rapidly and will eventually cut off the flow of water and minerals. Sap begins flowing back towards the roots. The green Chlorophyll that's left in the leaf gets used up, and this allows the yellow and orange pigments to finally show off.Yellow and orange leaf pigments -- i.e. the Carotenoid twins -- are more like the mysterious, middle cousins. They're always hanging around in the background, but they're overshadowed while Chlorophyll is present, so they're only visible for a few weeks in the fall. In the summer, they absorb sunlight in slightly different wavelengths, but they must pass that energy over to cousin Chlorophyll to do the hard work. They also have a part-time gig protecting Chlorophyll from too much sunlight.
Unlike Chlorophyll and the Carotenoid twins, which are present for the entire growing season, the misunderstood, red Anthocyanin is produced in the fall. This is the red-headed, youngest cousin.
When fall trees and shrubs begin drawing down sap for winter storage, the chemistry changes inside of leaves. The metabolization of stored sugars in the dying leaves now produces these red pigments. But why waste so much sugar? Shouldn't plants withdraw all of the sugars from the leaves before dropping them? One theory is that trees and shrubs might "show off" their health with brilliant red displays to warn potential pests to look for a duller (less healthy) plant to lay their eggs on.
Finally, the brown in leaves are the Tannin cousins that you'd rather not talk about in polite company. They linger in the leftover waste products that the plant discards each fall. The less said the better, but Tannins do play a role in resisting decay and deterring other plants from growing over the family roots.
In short, during the winter there is not enough sunlight or water available for Chlorophyll to do its job. So decidious trees and shrubs pull back their sap, drop their wastes, and rest. If it was a productive summer, they will spend the winter living off the efforts of their hard work -- just like many of us living here in Montana.Behind the lens: I photographed this patch of Birchleaf Spiraea (middle photo) one year after a wildfire had swept this forest floor clean.
