add two-- mine tailings
"Basically," she says, "the sites are such harsh environments
for the plants that there are few species that can tolerate the
conditions. These conditions do not change for a long time."
Also, during early stages, there is natural selection of
strains of plants--known as ecotypes--that can survive the harsh
conditions. Once there are tolerant survivors on the tailings,
offspring of these plants can begin to spread, she theorizes, and
there is then a surprising jump in the rate of colonization. Her
data show that there is, indeed, a marked increase in plant cover
on sites 20-35 years old.
The data also show another plateau in the rate of vegetation
spread from sites 35-65 years old.
"This may be caused by a nutrient limitation," she states.
"Lack of organic matter in the soil may halt the further development
of the vegetation."
A second increase in vegetation cover after 65 years includes
tree species as an important component--cottonwood, slippery elm,
box elder, and black cherry. These trees have been present from
earlier phases of colonization but during the later years their
size is sufficient to moderate the environment. An increase in
species diversity--the different kinds present--also takes place
during this next period, with the addition of native prairie
forbs such as bergamot, columbine, verbena, and little bluestream
grass.
These species tend not to be very abundant but are scattered
over the sites.

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