ELi on Earth: Why the Leaves Change Colors

It is impossible to look outside and not be mesmerized by the dazzling orange, blazing red, and glittering yellow leaves covering the trees and falling from the sky this time of year. It is a magical time to play outside, and the opportunity to enjoy it is not long lasting.

Ever wonder why leaves change color in the fall? It’s fairly simple, but first you must understand the basics of photosynthesis. Trees absorb water through their roots and carbon dioxide from the air. Simply put, plants use the energy from the sun to convert water and carbon dioxide into oxygen and glucose.

All plants have photosynthetic cells that contain special pigments allowing them to absorb light. The primary pigment used in photosynthesis is chlorophyll, which reflects green light, which is why trees and most plants appear green.

There is not enough water or light for photosynthesis to occur during the winter which is why trees have evolved to shed their leaves. Leaves eventually fall off of trees, not from changing color but because of a special layer of cells called the “abscission” or separation layer. During the growing season these cells are used to carry water to the leaf and food out of the leaf. In the fall however these cells swell, forming a cork-like material, reducing the flow of water and nutrients.

Ultimately glucose and waste products are trapped in the leaf and without water being able to enter, and the chlorophyll disappears giving the leaves their brilliant autumn colors. Fall foliage colors, including the browns, yellows, and oranges have always been present but they were not highly visible due to the chlorophyll.

Conifers, also known as evergreens, keep most of their needles during the wintertime because they are resistant to the cold and moisture loss. On extremely cold nights some needles actually curl to reduce exposure to the cold and dry air.

Three main factors influence the degree and the duration of fall foliage color: temperature, light, and water supply. The most spectacular display of fall colors occurs when the late summer is dry and the early fall has sunny days and cool (low 40s) nights. Cool temperatures, especially at night, promote the production of anthocyanin pigments, which have red and purple coloration. Autumn weather that includes cloudy days and warm nights does not produce as many brightly colored pigments. And of course an early frost is sure to put a quick end to a beautiful fall color display.

This year we have had an especially warm September, and because of this, peak viewing times are approximately one week later than last year. According to The Foliage Network, as of October 22^nd the majority of the lower half of the Lower Peninsula is experiencing “peak color,” so dress warmly and enjoy this magical moment in time because freezing weather will be here before we know it!