 |
Autumn
by Joe McSharry
As we quickly spin through space on our annual trek around the sun, we
find our tilted hemisphere pushing us farther away from our heat source.
The nights slowly begin to engulf the days, as the cool weather sends
us searching our closets for light sweaters. The squirrels, fat and content
from a good summer, scurry about collecting acorns for the winter, reminding
us to pick the remaining summer vegetables. Canada geese fill the skies
with the sounds of autumn while accipiters silently take to wing. Snakes
move to their den sites, soaking up the rays of the sun on warm days,
moving farther into the dens on cool ones. But with all the preparation
going on in the animal world, none can compare to that of the plants.
Throughout the summer the trees are dressed in various hues of green. This is because they produce a chemical substance called chlorophyll, a green pigment essential to photosynthesis. Some of the chlorophyll is broken down each day, but the trees replace it as long as the days are long and warm. But as the days shorten, less chlorophyll is produced exposing other pigments found in the leaves. Eventually chlorophyll production is ended, allowing the brilliant yellows, golds, reds, and crimsons of autumn to show.
There are essentially three pigments found in our autumn leaves. Two of them, carotene and xanthophyll, are found in the leaves year-round. The green pigment (chlorophyll) overpowers them during the spring and summer, but these substances outlast the chlorophyll as the cool weather approaches. The xanthophylls give the leaves their bright yellows and the carotenes produce the oranges and near-reds. Varying ratios of these substances allow a wide variety of colors.
The third pigment is anthocyanin. It's what gives us the brilliant reds of autumn. Unlike the carotenes and xanthophylls, anthocyanin is produced only in small quantities throughout the summer. It isn't until the fall when conditions are right that it is produced in large quantities. The leaves require high concentrations of sugar for the formation of anthocyanin. And in order for the plant to produce large amounts of sugar, high levels of light are required. Low temperatures at night also slow the transfer of sugars from the leaves to the rest of the plant. Thus sunny days and cool nights are what give us the most spectacular fall foliage.
After several weeks it is time for the show to end. The trees have finally moved most of the minerals in their leaves to other tissues. A layer of cork-like cells called the abscission layer forms at the base of each leaf stem. This blocks the transfer of any substance to or from the leaf and weakens the bond between the leaf and the plant. Now the blowing wind, falling rain, or a busy squirrel can shake the leaves from their tenuous hold on the plant that created them.
In the city and suburbs it is common to see droves of home owners with
rake in hand, bagging the "leaf-litter" from a yard-full of
trees. But any gardener worth his or her weight in manure knows the valuable
compost that can be had from a season's worth of work. The trees have
known this all along. They drop their leaves to the forest floor, allowing
bacteria, fungi, earthworms, and scores of other tiny creatures to break
them down, sending nutrients back into the soil. Many of those same nutrients
are taken up by the tree's roots to produce new buds for another season
of energy production. And with any luck, sunny days and cool nights will
prevail, and the leaves will once again go down in a blaze of glory.