Why do leaves change color?

The nights are cooling off, the sun is slower to rise - fall is here. While plants don’t have calendars, they do “know” the equinox has just passed. Their own internal clocks track the length of night and day, and they are already preparing for winter. For all but our evergreen trees and shrubs, this means halting the production of the green chlorophyll in their leaves. Plants are thrifty by nature, so the remaining chlorophyll slowly breaks down and is recycled back into the plant’s stems and roots. With the dominant green pigment out of the picture, other pigments in the leaf are revealed. One group of these pigments are the carotenoids. These are responsible for the oranges and yellows we see, famously so in Aspen. Carotenoids are always present, if not always visible, in leaves where they work synergistically with chlorophyll to help the plant photosynthesize. The carotenoids also act like sunscreen, protecting leaves from UV damage. Because these carotenoids are always present, trees known for their yellow or orange fall colors are reliable from year to year.

If you are more attracted to the deep reds and purples that emerge during the fall season, you can thank anthocyanins. Here in Colorado, Sumacs (Rhus) produce a reliable show of deep reds and purples in the fall. Unlike carotenoids, these pigments are not always present in the leaves of plants. The production of anthocyanins in a tree’s leaves is dependent on environmental factors such as temperature and exposure to light. After the layers of chlorophyll and carotenoids have faded, a tree will produce anthocyanins to pull the most sugar and nutrients possible from its leaves before dropping them to the ground. You may notice patches of foliage that are exposed more to light will have deeper reds and purples, while shaded parts of the plant might not show the same colors. This is because the red and purple anthocyanins act as a UV shield against bright sunlight, giving the leaves more protection as the tree extracts the last bit of food before winter comes.