When greens explode into yellows, oranges and reds

By Becca Blond

Have you ever wondered why leaves change color with the cooler temperatures each fall? They put on a magnificent display in Boulder County and the Colorado high country between September and October of each year, when greens explode into yellows, oranges and reds, but how and why does it happen? We sat down with Dave Sutherland, interpretive naturalist with the City of Boulder Open Space and Mountain Parks (OSMP), who shared a few fascinating facts behind why deciduous trees shed their leaves each autumn.

Boulder Magazine: Why do leaves change colors?
Dave Sutherland: If you’re a deciduous tree, leaves are a great thing to have during the summer; they allow you to make lots and lots of sugar through photosynthesis, and they help you pull water up through your trunk when you need it. But during wintertime, leaves become a bad thing to have when you are trying to save water. So you want to dump your leaves in the fall so you don’t have to maintain them over the winter. Besides using too much water, leaves are also a hindrance when it snows because they make your branches heavier, which can lead to them ripping off.

BMag: So you want to lose leaves before the snow, but is there some kind of science behind the color change? Why don’t the green leaves just fall off the tree?
DS: There is a lot of science behind it. When the trees sacrifice their leaves in fall they have to do it gently so it isn’t too traumatic. So first the tree moves its sap and sugar down into its trunk and root system and then stops making food in the leaves. The changes in temperature and daylight also send a signal to the tree that basically says: ‘Yo dude, you have to get rid of these leaves.’ At which point the tree begins to build a barrier at the base of the leaf, like a little wall, that keeps the nutrients from producing in the leaf and eventually causes the ‘amputation’ of the leaf from the tree. Before this happens, though, the leaves change colors because the green color in the leaf, which is the chlorophyll, begins to break down and go away, and suddenly you get all these hidden secret colors showing up. The yellows and oranges and browns you see in the changing leaves have been there waiting the entire time, but you just can’t see them until the chlorophyll breaks down and they shine through.

The CU Boulder
campus has hundreds of mature trees, including some rare ones, for fall leaf collecting. (photo by Carol Brock)

BMag: What about purple and reds? Is that the same process?
DS: The purple colors are actually a different mechanism. Basically, even though the leaf has created this barrier to keep more nutrients from coming in, the chlorophyll in the leaf itself keeps cranking out sugar. But now that sugar can’t escape the leaf anymore because the barrier is keeping it from moving down into the trunk. So all the sugar is trapped in the leaf and it breaks down into a pigment called anthocyanin, which turns the leaves these colors. This is the same pigment that makes strawberries red and blueberries blue; it is the same stuff that makes these fruits stain clothing. The actual color difference between say red and purple leaves has to do with the environment the tree is in—if it’s an acidic environment the leaves look red, but when in an alkaloid environment they turn more purple. And that is how you get those beautiful scarlet maples.

BMag: When is the best time to see leaf colors around Boulder and where should people go?
DS: The best time for leaf colors here is usually the first two weeks of October. I have a couple of favorite spots. A really great place is Sawhill Ponds. This area is really beautiful because there are a lot of cottonwood trees that turn a brilliant golden color. Go in the early morning when the ponds are glass-smooth and you can take these incredible pictures of the blue skies we get in fall and the yellow leaves reflected in the ponds.
Another great place is to start at the Bobolink Trailhead and then follow the South Boulder Creek Trail for 3 miles. It’s an in-and-out trail and it makes for a wonderful fall bike ride with kids. Go in the first weeks of October and just meander along the creek with the bright yellow trees. It is beautiful.

Also check out the South Mesa Trailhead near Eldorado Springs. Here you’ll find more of the cottonwoods along the creek, but if you walk a little farther west you get up into shrub-colored hillsides and a patch of sumac that turns brilliant scarlet and covers the hills.

(Editor’s Note: Another lovely spot is the University of Colorado Boulder campus, which boasts hundreds of mature trees of many different species, as well as rare and imported trees.)

BMag: These are great suggestions. One more: Where would you go for a day trip?
DS: Either the Peak-to-Peak Highway or I’d drive up somewhere around Nederland, like the Caribou Ranch hiking trail and the trail around Mud Lake. Leaves in the high country change a bit earlier, so I recommend going at the end of September to catch them at their peak. One really nice place is around Eldora and the Hessie Trailhead. Drive up there and do a day hike. The aspen trees are just gorgeous!