What makes leaves change color?

Foliage season has officially begun in Park City. Not so long ago, as I was talking to my siblings, back in France, they both asserted that an early freeze was really what triggered the change in colors.

I knew that wasn't totally true and didn't argue until I had time to do some research of my own. While several factors create fall colors (temperature, precipitation, soil moisture), the key agent is light or lack thereof.

As the days grow shorter, less light causes a tight wall to form between the twig and the leaf stalk; as it gets denser the leaf can no longer access nutrients and water and the sugar it contains can't leave it either.

As an immediate result, the green chlorophyll pigment dies. When this happens, two other pigments show their bright faces: Carotene (yellow) and anthocyanin (red); both already exist inside the leaf but are masked by the chlorophyll.

The brown color in oaks is an exception and the result of tannin. In fact, it's the sugar trapped inside the leaves that is largely responsible for those vivid colors. So the next question, of course, is what creates the best fall colors? It seems that a wet growing season followed by a dry autumn filled with sunny days and cool as well as frostless nights produces the most vibrant palette of fall colors.

Unlike many folks believe, freezing temperatures and a hard frost may kill the entire process and lead to poor fall colors. Same thing with drought conditions during late summer and early fall, like we've experienced this year in Park City, that can “shutdown” trees prematurely as they prepare for winter, causing leaves to fall early without reaching their full-color potential.

Now that you know the whole story, make sure to go out and enjoy the show!