October 31, 2003

  John Karlik, Farm Advisor 

                                                                          Environmental Horticulture/Environmental Science

                                                                                                                                 (661) 868-6220

        One of the most colorful displays of nature is a landscape ablaze with fall color.  In the northern United States, especially in the hardwood forests from Michigan to New England, each tree may give startling impact to the scene and no two trees are exactly alike.  Some species are noted for fall color, such as the scarlet sumac covering many Midwest hillsides and, here in California, the bright liquidambars.

Why the different shades of color?  Chlorophyll is the green pigment which makes photosynthesis possible, capturing the energy of sunlight for synthesis of sugars possible from carbon dioxide and water.  During the growing season, chlorophyll is produced as long as the plant remains healthy.

In late summer and early autumn, the spectacular unveiling of color, rather coldly described under the heading “senescence,” begins as day length triggers the process.  Metabolism in the leaf, including chlorophyll production, slows.  Nitrogen, phosphorus, and potassium are pulled back into twigs while calcium and magnesium remain in leaves.  Cells begin to break down.  How much and how fast chlorophyll is destroyed differs among plant species.  For example, Norway maple leaves lose almost all their chlorophyll while those of lilac lose only 40 percent.  The average chlorophyll loss across many species is about 85 percent.

As chlorophyll breaks down, pigments which have been present all along begin to be visible.  Carotenoids are a class of pigments with over 60 members found in plants and animals.  The most familiar carotenoid is carotene, the orange pigment found in carrots.  Another closely related chemical group, even more plentiful in plants, are the xanthophylls.  Both groups of compounds are yellow-to-orange in color.  Tree genera, such as ash and willow, which produce carotenoids and xanthophylls display these characteristic colors when they are unmasked in autumn.

But what about the reds and purples?  Plants including viburnums, Boston ivy and liquidambar can synthesize new pigments in the autumn.  The mechanism to form these compounds isn’t active at other times of the year but in autumn sugars are synthesized to form pigments called anthocyanins, named from the Greek words “antho,” a flower, and “hyanos,” dark blue.  Each compound has a particular color, which may be crimson, scarlet, blue-violet, red, purple or mauve.

Light and certain weather conditions favoring accumulation of sugars also favor production of anthocyanins.  The best conditions for glorious fall color are sunny, dry weather with cool but not freezing temperatures.  These conditions are more likely to be found in the mountains than on the valley floor.  Frost, however, does not encourage development of fall color.  Rather, leaves may be killed or injured before coloring processes are complete.

Liquidambar and Chinese pistache are among the most colorful shade trees in the southern San Joaquin Valley.  Willow, birch and poplar contribute yellows.  In riparian areas such as the Kern Canyon, poison oak provides reds and maroons.

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