It’s that time of year again where the leaves on the trees change color into shades of yellow, red, and brown. But how do the leaves change color anyway and how do they know when to change? The answer comes from a combination of the pigments within the leaves and the length of the night.

There are three main pigments that are involved in the color of leaves. Chlorophyll is the most famous for giving leaves a greenish color and helping in photosynthesis. Carotenoids are another pigment that exists year-round within leaves that are responsible for making some leaves appear yellow, orange, and brown. They are overpowered by the amount of green chlorophyl in leaves from spring through summer. To provide the colors of red, purple, and crimson, anthocyanin starts to be produced in the fall.

It is a common misconception that leaves change color based on the decreasing temperature, but that is only part of the story. The production of chlorophyl slows down gradually as the nights get longer until production is stopped. With no chlorophyl in the leaves, carotenoid and anthocyanin pigments become much more pronounced. The veins carrying the fluids to the leaf progressively close off, drying them off as well and promoting the production of anthocyanin.

Leaves need to dry up during the fall because the watery sap within them freezes readily, so it is not beneficial to continue photosynthesizing through the winter.  The shortened hours of sunlight also give a less ideal means of producing sugars. However, the fallen leaves drop to decompose and stock soil nutrients for the surrounding plants and soil organisms.

With a smooth succession of warmer to cooler nights, the colors in the leaves become more vibrant. Once the sugars are made during the day, they are prevented from leaving at night, leading to more vibrant colors. So, when there is a warmer period during the fall, the intensity of the leaves is lowered.



The variety of tree species (and the pigments mentioned above) at Peace Valley Nature Center ensures a colorful autumn palette. Be sure to visit soon before it disappears!