It was a maple-leaved viburnum shrub growing along our
driveway that captured our attention a few days ago and brought
us to a halt at the start of our hike. The closest resemblance
to the gorgeous, soft, rosy-pink fall color of this native plant
I can think of is that of the various species of Euonymus
(you-ON-i-mus), better known as the burning bush.
Another welcome and fairly early color-change to the autumn
scene is produced by the staghorn sumac. In my opinion the
blazing red-orange color of their long compound leaves is
actually enhanced when, on a calm day, the leaves with their many
smaller leaflets hang limply downward.
Having experienced 72 autumn seasons in Wisconsin, it has
become very obvious that not only do the overall colors vary in
brilliance and intensity from year to year, but so does the
timing of the eagerly-awaited event. An interesting experiment
would be to single out a favorite sugar maple tree, for example,
and photograph it on a sunny day every year from the same
location at the same time, say October 10th, or as
close to that date as a sunny day occurs. Then simply compare
your photos from year to year.
One could look back into history and accumulate many
fascinating beliefs regarding the fall leaf-color changes that
are so glorious in a surprisingly small part of the world,
fortunately including the Upper Midwest. A common but invalid
belief for many years was that the best color didn’t occur until
after the first frost had hit an area.
Tree scientists for years have believed that dry summers
followed by early fall rains, these preventing the leaves from
falling off the trees so soon, cool but not freezing nights, and
sunny dry days produce the absolute best colors.
An important function of the tree’s leaves is to produce
sugars, converted from starches, which can be stored by the tree
to act as a reserve supply. The pigment, anthocyanin
(an-tho-CY-a-nin), enters the picture now. On cool nights when
the air temperature falls below 45 degrees F. but not down to
freezing, something occurs which prevents the sugars, produced
during the day, from moving from the leaves in the tree. The
following day, especially during dry sunny weather, anthocyanin,
that blazing red pigment, is produced in the leaves.
Now three University of Wisconsin-Madison scientists have a
new theory about why autumn leaves turn scarlet and why the hues
are more vibrant some years than others. It is their contention
that the red pigments, the anthocyanins, in plants such as
maples, oaks, dogwoods and viburnums act like sunscreen.
According to the researchers, Bill Hock, Eric Zeldin and Brent
McCown, the pigments shade sensitive photosynthetic tissue in
fall while trees reabsorb nutrients from their leaves.
"Trees need to store as many of those nutrients as they can
before the leaves drop." The Wisconsin scientists argue
that the pigments protect the leaves’ dwindling ability to
generate energy during this period.
The researchers’ theory agrees with the observation that
autumn colors are best when the fall features dry weather with
bright, sunny days, and cold nights. It also makes sense of
observations that the outer leaves of maple trees, for example,
are more colorful than shaded leaves inside the canopy and leaves
on the north side.
Professor Hock says their ideas also explain why most of our
native maples and oaks in the Midwest and New England turn red,
while European species, such as the Norway maple, do not.
"None of the European counterparts of these North
American trees produce high levels of anthocyanins, Hock says.
"We think it’s because the weather in that part of the world
is cloudier and warmer during fall. European species don’t need
the protection of these pigments."
While working at the Ridges Sanctuary for many years where
northern pitcher plants are quite common in some of the more open
sunny wet swales, I noticed that the leaves of those pitcher
plants growing in open sunny locations eventually turned a
brilliant shade of maroon. The leaves of many of those plants
growing in more shaded sites remained green. I strongly suspect
that, indeed, the leaves of sun-growing plants colored up to a
reddish shade to provide protection from the ultra-violet rays of
the sun, or whatever rays could damage those leaves. What a
fine example of nature’s sunscreen.
One can talk endlessly about the flamboyant extravaganza of
October’s trees and shrubs changing color, but to really enjoy
the performance at its best you have to appear on the scene in
person.
As the great naturalist-writer Henry David Thoreau put it,
"I would have my thoughts, like wild apples, to be food for
walkers, and will not warrant them to be palatable, if tasted in
the house."
Northern red oak leaves may remain on their trees –
especially the smaller, younger ones – for the duration of the winter.
This column appeared in the Door County Advocate
on 10/19/2001.
© Copyright 2001 Roy Lukes. All rights reserved.
