| Bioindicators are biotic
species, (plants, animals, etc.), that display noticeable changes if
surrounding environmental conditions change.
Scientists have found that certain organisms can be correlated
with certain pollutants. The
changes displayed by the organism are measurable through various
methods, therefore can offer quantitative information on ecological
resources.
Common Milkweed is considered to be a
bioindicator of ozone. Milkweed
shows signs of damage from ozone by showing stiples, or dark polka dots,
all over the leaves, losing leaves, turning strange colors, or not
growing very big. Photographs
of injured plants can be viewed on the Forest
Health Monitoring Program's web site.
Common milkweed (Asclepias syriaca)
usually has a solitary, simple stem (1.5-6 feet tall) though clumps of
multiple stems can be found. Leaves are opposite, oval, and 2-10 inches
long. The surface of the leaf is hairy beneath and smooth above. The
leaf stem is short and thick. Milkweed exudes a thick white sap from any
cut or broken surface. Milkweed is common in fields, meadows, and along
roadsides from New Brunswick to Saskatchewan, south to Georgia and
Tennessee and west to Iowa and Kansas.
Milkweed flowers from June to August. The
flower actually consists of many flowers in a large cluster on a stalk.
A flower stalk may originate from the tip of a branch or stem or at the
junction of a leaf with the stem. The flowers are usually pollinated by
insects because the pollen is lumped in waxy masses not conducive to
wind fertilization. Very few of the flowers produce large, healthy pods.
The distinctive seeds of the milkweed are dispersed by the wind catching
the long silky hairs. Studies of seed dispersal indicate seeds typically
drift 25 to 100 feet before settling to earth.
There are many other plants that serve as
bioindicators for ozone if milkweed is not found in your area.
A brief list includes, Blackberry, Huckelberry, Spreading
Dogbane, Black Cherry, Choke Cherry, and Sassafras.
Please consult the Forest Health Monitoring Program’s web site
for more bioindicators and information.
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