From green
to gold
The leaves show their autumn colors as the seasons change
By Callan Bentley
Flat Hat Variety Editor
Autumn is a breathtaking time in Williamsburg. Indeed, across the country, the switch in seasons casts a crisp new light on the land. Humidity becomes more bearable with the drop in temperature, and the wind tints the cheeks of college students rosy. And, of course, the leaves change color.
The color change in the leaves of deciduous trees is one of nature's most fantastic phenomena, from the aesthetic point of view. It could be seen as nature showing off just one more time before she slips into the frozen dormancy of winter: a continental clamor for attention, if you will.
It would be
anthropomorphic to assign emotions to the trees and climate, but there is good
advice here. It is my firm belief that everyone should spend at least half a
day each autumn doing nothing but staring at the trees.
There are items
of interest beneath the oranges and yellows, as well. Why leaves change color
is almost as interesting as simply being in awe of their beauty.
"One person may see pretty colors and the next person may see something
else… It's quite a phenomenon," Dr. Martin Mathes, a botanist in the
College's botany department, said.
The cells which
make up the leaves of mature trees contain small structures called chloroplasts.
Chloroplasts serve as the site of photosynthesis in plants. There, plants convert
the energy in sunlight into chemical bonds. They change simple compounds (like
carbon dioxide and water) into more complex ones (for instance, the energy-rich
molecule glucose). It is from the plants' production of sugars that the Earth's
food chain is based.
Without plants,
there would be no plant-eating caterpillars. Without caterpillars, there would
be no caterpillar-eating birds. Without birds, the world would obviously come
to an abrupt end.
Okay, it might not be as dramatic as that, and birds aren't really the lynchpin of global health, but the point remains: Directly or indirectly, everything is based on plants, and if plants disappeared, so would everything else.
Several kinds
of molecules can be found inside the chloroplast. Mainly of interest to us are
the pigment molecules, such as the green chlorophylls, orangey yellow
carotenes, red betacyanins, and pale yellow xanthophylls.
All the different colors are present year round, as each plays a part in photosynthesis,
but the chlorophyll is usually present in much larger quantities than the other
pigments.
In the fall,
however, a number of factors cause the chlorophyll to break down. The combined
stress of reduced temperatures, shorter days, and drier air causes a change
in the plants' hormones. I'll anthropomorphize again: The plant realizes that
it's getting less and less of a return on it's food synthesis business, and
decides to shut down the glucose-production operation until conditions improve.
In winter, these
shorter days and chilly temperatures and dry air are all conditions that are
going to inhibit the driving force of photosynthesis, sunlight. If the leaves
aren't going to do anything useful, then the plant had better get rid of them.
Keeping the leaves encourages loss of water through tiny holes on the bottom
of the leaves, called stomata. If the plant sheds its leaves, however, it can
limit its water loss and simply wait out the cold season. An additional benefit
is that the plant reduces its total surface area by dropping its leaves. A large
surface area is more area for snow and ice to build up on, the weight of which
can rip limbs from trees, and squash shrubs to the ground. A slimmed-down plant
has a greater chance of making it through the winter with minimal damage from
ice.
So the chlorophyll
breaks down and the plant stops producing any more, with the result that the
other, more durable pigments show through. Different concentrations of these
other molecules cause the different colors on different species of trees, or
even among leaves on the same tree. The plant draws the leaf's nutrients into
the stem where they can be used to nourish the plant in the spring.
The color change
is officially referred to as "senescence," which refers to the process
of growing old and dying. The term "abscission" is used to describe
the process associated with the leaves falling off the trees.
Abscission leaves
trees bare and the ground covered in a layer of decaying leaves. These shed
leaves will be broken down by bacteria into their component compounds and returned
to the soil. Next year's growth will be based in part on this replenished stock
of molecules.
As Mathes put
it, "The fall coloration is visible evidence of natural recycling."
Few nutrients
are in the leaves however, and not too many animals consume them. "There's
not much in the way of nitrogen, phosphorus, or potassium in those leaves,"
Mathes said. "They just serve to increase the water-holding capacity of
the soil. Overall, the process improves the soil quality."
The accumulation
of fallen leaves affects the soil's pH, a measure of its acidity. Certain species
of plants prefer either acidic or alkaline soil to grow in. Azaleas, for instance,
do best when planted surrounded by pine needles, which in their decay lower
the soil's pH.
The science
of leaf senescence and abscission isn't just for academics, though. Numerous
practical aspects can be considered. As no consumer will buy yellow broccoli
at the grocery store, there is a profit for whoever figures out a way to keep
broccoli greener, longer. This applies to all crops sold to buyers, including
such general-use plants as cotton.
"There's
a scientific interest, and if we can control that, and economic importance as
well," Mathes said, opining why people of all inclinations should pay attention
to their surroundings in general, and leaves in particular.
When you're home this Thanksgiving break, you don't need to think about economics or xanthophyll while you gaze at the fall foliage. All you have to do is marvel to yourself that the world could come up with a display as spectacular as the one you are seeing. Once that realization is accomplished, you can start raking the lawn.