Autumn is the favorite season for many people. The days are shorter, the weather cooler, the bright blue skies of summer fade to a softer blue with clouds, and the leaves of some trees start presenting their autumn glory. 

Why do some trees change color and drop their leaves before winter?  And why are there different colors?

Leaves are colored by pigment molecules. Most leaves appear green because they contain an abundance of the pigment chlorophyll. Chlorophyll is the site of photosynthesis where the sun's energy is converted into the carbohydrates that are plants' food source. During the cold winter months when there is less sunlight, it would take too much energy for some trees to keep their leaves healthy. So deciduous trees lose their leaves for the winter. Evergreen trees have a different strategy for dealing with winter's challenges (which is a topic for another time!).

Temperature, precipitation, and soil moisture are all factors which contribute to plants dropping their leaves in autumn, but the primary agent is light, or rather, the lack of it. After the autumn equinox, the shorter days with longer nights triggers chemical changes, causing the tree to seal off vessels that supply the leaves with nutrients and water. The combination of reduced light, lack of nutrients, and less water starts the process of the green chlorophyll to break down. As the green fades, the other pigments in the leaves, which were present all along, but were masked by the green chlorophyll, begin to reveal other bright pigments. In time the leaf is blown off from the sealed cut, or falls from its own weight. 

Leaves containing primarily anthocyanin pigments will appear red. Carotenoids can be orange, yellow or red. Xanthophylls are yellow-brown. Some leaves, like oaks, contain tannins which will produce brownish colors. A chemical process called paper chromatography of a plant extract will separate the various pigments present which can reveal some of the pigments found in leaves. This photo illustrates pigments found in mint leaf extract.

Elevation, latitude and weather all affect the timing and intensity of fall colors. Higher elevations and northern latitudes produce earlier autumn colors in trees. In general, autumn weather with lots of sunny days, dry weather, and cold, frostless nights will produce the most vibrant palette of fall colors. Some trees that can produce vivid colors include maples, gingkos, aspen, birches, Japanese maples, liquidamber, cherry, redbud, Chinese pistache, and dogwood. 

In the Central Valley we usually don't get the glorious colors like the Sierra Mountains or the east coast, but we do get some color which usually starts in early November. So, enjoy the autumn jewels since it occurs only for a brief period each fall!

Denise Godbout-Avant has been a UCCE Stanislaus County Master Gardener since July 2020.