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

Grasslands of central North America 245
influential in the development of grassland vegetation and animal communities .
Since grasslands operate as ecosystems , it may be futile to try to isolate causal
factors, since some internal components may interact with others to change
the nature of the system. For example, Larson (1940) considered that heavy
bison grazing prior to settlement helped maintain the short-grass prairie .
However, bison evolved under grassland conditions – vegetation, climate,
other herbivores and predators – and attaching causality to one component
may involve circular reasoning.
Sauer (1950), in contrast, argued that grasslands were maintained by periodic
fires. Without fires, grasslands would progress to woodlands or forest .
Indeed, grasslands of central North America have developed under frequent
fire regimes, caused both naturally and by man (Flores, 1999; Sauer, 1950).
Flores (1999) suggested that natural lightning fires had results that differed
from those set by native Americans. He argued that maintenance of southern
Great Plains grasslands populated by large grazing animals depended on fire
management by native Americans.
One prominent feature of grasslands is a variable climate, with periodic
droughts a common feature (Dix, 1964). The drought of the 1930s that resulted
in the dust bowl in the Great Plains has been well documented (Albertson and
Weaver, 1942; Robertson, 1939; Savage, 1937; Weaver and Albertson, 1936,
1939, 1940; Whitman, Hanson and Peterson, 1943). During the drought of
the 1930s, blue grama abundance was reduced dramatically (by as much as
70 to 80 percent) in Kansas (Weaver and Albertson, 1956). In North Dakota
mixed prairie, blue grama was reduced to about 40 percent of pre-drought
(1933) levels in 1936–37 (Whitman, Hanson and Peterson, 1943). Other species
negatively affected by the drought were western wheatgrass , needle-and-thread
and prairie junegrass (Koeleria macrantha ). The one species not affected
by the drought was threadleaf sedge (Carex filifolia ) (Whitman, Hanson and
Peterson, 1943).
In western Nebraska, tree ring analyses indicated that over the last 400 years
nearly 160 were “drought ” years and 237 “wet” years (Weakly, 1943). Borchert
(1950) reported that drought years tended to be clumped, with an average
duration of nearly 13 years, compounding the effects of the drought.
Geologically, the Great Plains consists of a valley between the relatively
young Rocky Mountains to the west and the older Appalachian chain to the
east. The valley is drained by the Missouri and Mississippi River systems ,
which have carried sediment from both the Rocky and Appalachian Mountains
and sorted and deposited these sediments (Dix, 1964).
In geological time, the central North America n grasslands have undergone
many transformations since the late Cretaceous period (Axelrod, 1958; Dix, 1964;
Donart, 1984). At this time the deciduous Pan Tropical Forest covered most of the
present USA (Axelrod, 1979). From these forests, two distinct forests developed
during Eocene times: the Arcto-tertiary and Neotropical-tertiary forests (Dix,