Q: I have a simple question. Why do deciduous trees change color and lose their leaves? Is it because their roots or leaves get cold?

A: My colleague, Dr. Gary Wade, says that it’s the simple questions that lead to wisdom!

Leaf color and leaf shed are complex physiological and chemical process within the plant. As leaves grow in the spring, the rate of photosynthesis increases until the time the leaf is fully expanded.

From that point on, the efficiency of photosynthesis declines and the leaf gradually deteriorates in its rate of photosynthesis and food production. As photosynthesis declines, so does the amount of green chlorophyll within the leaf. As chlorophyll declines, other pigments in the leaf are expressed, such as the red anthocyanins and the yellow carotenoids.

Shorter day lengths and cooler temperatures in fall speed up the decline of photosynthesis. When the leaf can no longer photosynthesize enough to satisfy the tree, the leaf is shed from the plant.

Winter dormancy of deciduous plants is a genetically programmed process that allows a plant to adapt to limitations in the environment, such as extreme cold, while conserving energy for the next year’s growth cycle.

In evergreen species, photosynthesis declines but does not cease in winter. The plant continues to produce enough food via photosynthesis in winter to sustain itself, but not enough to force new growth.

Fall Color in the Forest and Landscape