October 6, 2020
Autumn leaf color is a phenomenon that affects the normal green
leaves of many deciduous trees and shrubs, in which they take on
various shades of yellow, orange, red, purple, and brown. Leaves
change color during the autumn because the amounts of their various
pigments change as the leaves prepare to fall from the trees.
All leaves gradually lose chlorophyll throughout the growing
season, and this loss speeds up before leaf fall. Chlorophyll
captures solar rays and uses the resulting energy in the manufacture of
the plant's food – simple sugars which are produced from water and
carbon dioxide. These sugars are the basis of the plant's
nourishment but in their food-manufacturing process, the chlorophylls
break down and are being continually "used up"; still, during the
growing season the plant replenishes the chlorophyll so that the supply
remains high and the leaves stay green.
Chlorophyll is the most abundant membrane protein on earth and as it
degrades, hidden pigments of yellow (xanthophylls) and orange
(beta-carotene) are revealed. These pigments are present throughout the
year, but the red pigments, the (anthocyanins) do not show until
roughly half of chlorophyll has been degraded. The amino acids released
from the process are stored all winter in the tree's roots, branches,
stems, and trunk until the next spring, when they are recycled to grow
new leaves for the tree.
As autumn approaches, daylight hours shorten, and temperatures drop,
the veins that carry fluids into and out of the leaf are gradually
closed off by a layer of special cork cells at its base. Water
and mineral intake into the leaf is reduced, slowly at first, and then
more rapidly, and during this time, the amount of chlorophyll in the
leaf begins to decrease. Often, the veins are still green after the
tissues between them have almost completely changed color.
Sugar maple is one of our most spectacularly colorful trees during the
autumn. Its abundance in some areas is a result of extensive
planting along roadsides during the eighteenth and nineteenth centuries
to provide a source of sap for maple sugar. It is likely that its
natural distribution will move towards the north over the next century,
with the anticipated increase in temperatures due to the accumulation
of greenhouse gases. Over time, the autumn colors of our forests may
diminish as conditions become less favorable for this tree.
Although always impressive, autumn leaf colors vary from year to year,
and seem to be more intense in some regions where it is economically
interesting because the colors are a huge tourist draw, worth hundreds
of millions of dollars in revenues every year..
The right weather during the autumn can promote more intense color
production. The reds (anthocyanins), which require sunlight for
production, are enhanced by cold and sunny days. Rainy and windy
weather during the autumn can knock leaves down prematurely thereby
shortening the color display at its peak. Summer drought
conditions stress trees and they may thus lose their leaves earlier or
start color production prematurely. The result is a reduction of color
during the peak of the season. Adequate summer rains promote good tree
health, leaf retention and, therefore, color production during the
Thus, it has been determined that three factors seem to influence the
autumn leaf color-show. The timing of color change and leaf fall
are primarily regulated by the calendar; that is, the increasing length
of night. None of the other environmental influences--temperature,
rainfall, food supply, and so on--are as unvarying as the steadily
increasing length of night during autumn. As days grow shorter, and
nights grow longer and cooler, biochemical processes in the leaf begin.
A succession of warm, sunny days and cool, crisp but not freezing
nights seems to bring about the most spectacular color displays.
Lots of sugars are produced in the leaf but the cool nights and the
gradual closing of veins going into the leaf prevent these sugars from
moving out. These conditions--lots of sugar and lots of light--spur
production of the brilliant anthocyanin pigments, which tint reds,
purples, and crimson. Because carotenoids are always present in
leaves, the yellow and gold colors remain fairly constant from year to
The amount of moisture in the soil also affects autumn colors and as
soil moisture varies greatly from year to year so does the
color. A late spring, or a severe summer drought, can also
delay the onset of fall color by a few weeks. A warm wet spring,
good summer weather, and warm sunny fall days with cool nights should
produce the most brilliant autumn colors.
We are told that autumn colors were different in the past, and they
will likely continue to change during this century due to land-use
changes, introduced pests and diseases, and climate change from fossil
fuel emissions. We should enjoy them while we have them.