GRIDLEY, CA (MPG) - The streets of Gridley are glowing with autumn color. Winter is on its way.  Deciduous trees are dropping their leaves. What makes the colors of autumn leaves vary in depth of color from year to year?

During the long days of summer sunlight, trees cast shade created by leaves that use sunlight to convert water and carbon dioxide into sugar.  This is the process of photosynthesis, which feeds the tree and its leaves. Chlorophyll, responsible for the green color we see in leaves. It is an important pigment that absorbs light energy used in photosynthesis.  As days grow shorter and temperatures begin to cool, trees respond by producing less chlorophyll, and photosynthesis slows and eventually comes to a stop. When that happens, yellow, orange and brown pigments in the leaves (called carotenoids) begin to show through. Carotenoid is always present under the chlorophyll and does not change in response to weather. Some trees produce pigments called anthocyanins in their leaves. Creating red leaves. Scientists offer several reasons for why some trees produce the anthocyanin that leads to bright reds. When a series of warm autumn days and cool non-freezing nights occurs, it’s a good year for red fall foliage. Primarily, anthocyanins protect leaves from excess sunlight and enable trees to recover remaining nutrients. This helps trees prepare for the next growing season. 

Deciduous trees begin to release their leaves when cool temperatures trigger a hormone that sends a chemical message to the leaves to “let go.” Located where the leaf stem meets the branch is a thin bumpy line of microscopic cells called Abscission cells.  These cells “cut” the leaf away from the stem when they receive that message.  When the air moves them, these dangling leaves fall away. If the abscission cells do not develop, wind will not blow the leaves off. This is why some trees keep their brightly-colored fall foliage long after other trees drop their leaves.

A severe drought can delay the arrival of fall color by a few weeks. A dry fall with plenty of warm sunny days and cool nights like has happened recently contributes to vibrant late fall foliage.