Why Leaves Change Colors
Every autumn, cottonwood, quaking aspens and willow are transformed into
colorful hues of gold, orange and rust. Before long, their leaves
will fall and again become part of another cycle that feeds the
soil. What cause this yearly cycle, and what determines which color
the leaves turn?
During the spring and summer, leaves actively produce foods necessary
for plant growth. This food making process takes place in the numerous
cells within the leaf. Within these cells are Chloroplasts, which
contain the chlorophyll pigments that are responsible for the green
color of plants. The leaves also contain lesser amounts of other
pigments, primarily xanthophylss (yellows) and carotenoids (yellows,
oranges and reds).
Most of the year, these other pigments are masked by the greater
amounts of chlorophyll present in the leaves. But in fall, when
changes in temperature and the period of daylight occur, the leaves
stop their food producing activity. Soon the chlorophyll begins
to break down, the green color disappears, and the yellows, oranges
and reds slowly begin to emerge, giving the leaves their fall splendor.
The intensity of color is determined by the plant's response to
complex gradients of temperature and moisture. Fall weather conditions
favoring formation of brilliant autumn color are warm, sunny days
followed by cool nights with temperatures below 45º. Sugar
production increases during the daytime, but cool nights prevent
movement of sugar from the leaves. From the sugars trapped in leaves,
the red pigment called anthocyanin is formed. When fall weather
is consistently cloudy or rainy, and the nights warm, the leaves
usually have less intense coloration. The smaller amounts of sugar
made during periods of less sunlight moves out of the leaves during
the warm nights, reducing the conversion of excess sugars into pigments.
Before the leaves gracefully spin from their leafstalk, a special
layer of cells develops and gradually severs the tissues that support
the leaf. A small leaf scar is the only evidence that leaves once
adorned these deciduous plants.
This decscription is by Anne Halford, a botanist with the Bureau of Land Management Office in Bishop California