Allergy
Epidemic
by
Thomas Leo Ogren
Before 1950 most of the
billions of trees that made up the US urban
forest were trees grown from seedlings. A great
number of these trees, ash, box elders, many
maples, ginkgo, aspens, poplars, cottonwoods,
mulberries, pepper trees, junipers, willows, and
other species, were dioecious, or
separate-sexed, trees. Since these were mostly
seedling grown trees about half of them would
have been male and half female.
Starting in 1949 with the
USDA Yearbook, TREES, an emphasis on planting
male street trees was promoted, pushed because
the males did not produce ?litter.? This
?litter-free? or ?seedless? trend became
more and more common and today there are a good
number of tree species where it is now almost
impossible to find any grafted varieties for
sale that are NOT male.
Around 1950 these
separate-sexed trees were represented in urban
areas with a ratio of approximately 50% female
trees. These large female trees produced no
pollen of their own and did not contribute at
all to pollen allergy. What is often overlooked
though, is that these same large urban female
trees were also wonderful natural ?pollen
traps.? In separate-sexed species the
female flowers often have large clusters of
pistils with broad, sticky stigmas that are
positioned in the branches in such a way as to
trap windborne pollen.
For catching and stopping
airborne pollen of, for example, Red Cedar
pollen, there is no organism in nature as
perfectly designed for this job as a large
female Red Cedar tree. For trapping and stopping
airborne pollen of ANY species, the most
efficient creation is the female of that
species.
Not only did these billions
of female trees produce no pollen themselves,
they were also highly effective natural
?air-scrubbers,? or pollen removers. (
Stigmas of unisexual female flowers are
electrically positive, +, and the airborne
pollen grains are actually negative,-, thus the
two are mutually attracted.)
Today?s urban forests have
very few of these large female trees left. As
the old female trees died off naturally, or from
harsh urban conditions, or as they were cut down
because they produced ?litter,? they were
usually replaced with male clones or with
monoecious species that also produce large
amounts of airborne pollens.
In a 1982 USDA booklet titled
?Genetic Improvement of Urban Trees,? a
method was described whereby male-only trees
could also be propagated from the monoecious
species; thus we now not only have an over
abundance of males from naturally separate-sexed
species, but we additionally have many male
trees from species that in nature never were
unisexual.
In the year 1950, American
Elm trees were the predominant street trees in
thousands of neighborhoods across the United
States and also in many other countries around
the world. DED, or Dutch Elm Disease, swept
across the land killing off literally billions
of elms. Ulmus americana, the American Elm, is a
tall stately vase-shaped deciduous tree that is
perfect flowered. Elm flowers have both the male
and female parts in the same flowers and are
largely pollinated by insects, especially
honeybees and butterflies.
As DED moved westward,
killing off almost all the elms in its path, the
dead elms were cut down and were replaced
usually with unisexual flowered trees, the
majority of which are wind-pollinated. Elm trees
themselves do shed some limited amounts of
airborne pollen, and allergy from elm pollen was
not in the least bit rare. However, in most
instances the replacement trees for the elms
produced far greater amounts of airborne pollen.
In many areas tree pollen makes up more than 70%
of the total urban pollen load.
These wind-pollinated urban
trees usually lack nectar sources in their
flowers and thus, with the loss of the elms, not
only did we get huge increases in ambient
pollen, but at the same time countless numbers
of urban bees and butterflies lost their major
early spring food source.
Our urban forests are now
heavily dominated by asexually propagated,
wind-pollinated trees. Fifty years ago less than
5 percent of our population suffered from
allergies. Today it is estimated that some 38%
of the US population now has allergies. As the
number of people with pollen-allergies grows,
attitudes toward trees themselves are already
changing. The honeybees and the butterflies,
once so common, are themselves disappearing in
many areas.
So what are we to do? How can
we clean up this bio-pollution and bring back
the bees and the butterflies? The answer is
actually quite simple. First, we need more
diversity in our urban plantings. Never again
should we over-rely on just a few species.
Second, we need to start
planting as many non-polluting female-only,
pollen-trapping trees and shrubs as possible.
Thirdly, we should also increase the planting of
those perfect-flowered trees that are known to
have especially low allergy potentials.
The bombardment of urban
pollen, the resulting epidemics of allergy, and
the terrible loss of biodiversity, all of these
were avoidable, man-made problems. Now is the
time to start getting back to the benevolent
urban forests of yesteryear.
This
article was written by the author of
Allergy-Free Gardening
by Thomas Ogren
Info/Order
this book.
About The
Author
Thomas Ogren is the author
of "Allergy-Free Gardening", published by Ten Speed Press. His
two previous books are used by adult literacy programs throughout the
United States, and are published by Sundown Press, and New Readers
Press, Syracuse, New York. Tom?s trademarked Ogren Plant
Allergy Scale, the first plant-allergy scale in existence, is
being used by the USDA to develop allergy rankings for all major US
urban areas. He can be reached at
This e-mail address is being protected from spam bots, you need JavaScript enabled to view it
. To learn more about Thomas and his work,
visit www.tenspeed.com
or www.allergyfree-gardening.com
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