Grasslands of the World.pdf - Disasters and Conflicts - UNEP

Grasslands of central North America 221
Chapter 6
Grasslands of central North America
Rex D. Pieper
SUMMARY
At the time of colonial settlement there was extensive grassland from the prairies
of Canada to the Gulf of Mexico on mainly level topography. Great Plains grassland
is in three strips running north-south: tall-grass , mixed -grass and short-grass ,
with the tall-grass in the better watered west. Precipitation increase from west to
east (320 to 900 mm) is the main factor governing primary productivity ; periodic
droughts occur. Bison, the dominant large herbivore until the mid-nineteenth
century, have largely been replaced by cattle . About half of the beef cattle in the
United States of America are in the Great Plains. Woody vegetation types are
embedded, with the trees varying according to latitude. C4 species comprise more
than 80 percent from 30° to 42°N, while C3 species increase dramatically north of
42°N. Only 1 percent of the tall-grass remains; half of the short-grass is uncultivated;
unproductive cropland is being put back to grass. Cattle predominate;
sheep are far fewer and declining. Most land is privately owned, much in small
farms . There is extensive ranching in dry areas. Grazing is seasonal, especially in
the north with supplemental feed in winter . In favourable areas, sown pasture are
used sometimes alongside range grazing . Rotational grazing is common, although
research results on its advantage are mixed. Fire is used to suppress undesirable
plants and increase fodder production. Grassland monitoring includes long-term
ecological research sites. Introduced plants can cause problems: Euphorbia esula
is an aggressive weed and Bromus japonicus often competes with native grasses.
Many small operations are no longer economically viable, so many are being
abandoned. Livestock enterprises should remain viable, although they have to
compete with systems based on forage grown under irrigation.
INTRODUCTION
When European settlers first moved into the central portion of what is now
the United States of America (USA ), they encountered an extensive , unbroken
grassland extending from the prairies of Canada to the Gulf of Mexico and
Mexico. While this grassland was generally free from woody plants, apparently
there was a dynamic ecotone between the mountains and deserts to the
west and the eastern deciduous forests in central and northern portions of
the grassland (Bazzaz and Parrish, 1982; Gleason, 1913; Transeau, 1935). The
general impression held by many observers was that conditions in this vast