The Autumn Color Show
Wisconsin fall color report
Minnesota fall color report
Many people think that cold weather is solely responsible for the color
change in leaves, but not so. Leaves begin to turn before we have any
frosts. Change in coloring is the result of chemical processes which
take place in the tree as the seasons change.
During the spring and summer a food-making process takes place in the
leaves, within cells containing the pigment chlorophyll. This gives
the leaf its green color. The chlorophyll absorbs energy from sunlight
and uses it in transforming carbon dioxide and water to carbohydrates,
such as sugars and starch.
In the fall the decrease in intensity and duration of sunlight,
and the cooler temperatures cause the leaves to stop their food-
making process. The chlorophyll breaks down, the green color
disappears and the yellowish colors or other pigments already in the
leaf become visible.
Autumn weather conditions favoring the most brilliant colors are warm
sunny days and cool, but not freezing, nights. A few hard frosts can
cause the leaves to wither and fall from the tree without changing
color. The degree of color may also vary from tree to tree. Leaves
directly exposed to the sun may turn red, while those on the shady
side may be yellow. When warm, cloudy and rainy weather dominates the
fall season, leaves tend to have less coloration.
In the Coulee Region, red and scarlet leaves can be found on red
maple, red and black oak and sumac. Purple and burgundy can be seen on
the green ash and white oak. Sugar maple is responsible for the
brilliant oranges. Hickories turn a rich gold. Yellows are common for
the trees native to the river floodplain, sliver maple, elm,
Cottonwood and white ash, as well as on the birch and aspen of the
slopes and ridgetops.
Fall color in the Coulee Region usually peaks between the 10th and
15th of October, but this can vary by a week or so either way.
For more information on why the fall colors occur, follow this
link to the U.S. Forest Service internet site.
Rod Swerman - NWS La Crosse
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