Nature Bulletin No. 124 September 27, 1947
Forest Preserve District of Cook County
William N. Erickson, President
Roberts Mann, Supt. of Conservation
Jack Frost is here. Late in August the sumacs began to change from
green to crimson. Some years, in October the woods are painted red,
yellow, purple and brown, each kind of tree and shrub having its
distinctive colors. Other years the foliage is dull. In Chicago itself, as
in Europe, there is never much autumn color. Why .
Temperature is one factor. All summer, new chlorophyll -- green
pigment -- is being formed in leaves. With cooler weather all the vital
processes in plants slow down. The green quickly fades and then the
other substances in leaves begin to show their own colors. Three
substances are chiefly responsible for autumn coloration: the carotenes
are pigments which vary from pale yellow to orange red: the
xantholphylls are yellow; and the anthocyanins vary from red to blue.
Sunlight is another factor. Its intensity is reduced in cities by smoke
and dust particles in the air. In the country, the brilliant sunlight of
clear fall days causes the green chlorophyll to quickly fade and the
reds and yellows to become more brilliant. Moisture is a third factor.
In very dry autumns there is little red, the yellows are subdued, and the
browns predominate. A cool moist autumn produces brighter foliage
than a warm dry season.
The fourth factor is the nature of the plant itself. Elm, poplars,
sycamores and soft maples :How little color. White oaks turn dark red
or purple; red oaks to red or reddish brown; hickories and beeches to
golden yellow; and sassafras to a brilliant saffron. Sour gum, sweet
gum, pin oak, sumac and maple-leaved virburnum are gone of the
trees and shrubs of magnificent color in fall, and the hard maples are
famous for their flaming crimsons and yellows.
England and Europe simply do not have such a wide variety of trees
that produce brilliant fall foliage, and there may be other factors such
as milder weather and less intensity of sunlight. In our cities the shade
trees are principally elms and other species which do not possess
bright pigments, and the sunlight is less intense. So take a walk in the
country, these sunny fall days.
And who was Kilroy? Jack Frost wants to know.
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Update: June 2012