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

Grasslands of central North America 225
in southern Nebraska, central Kansas and Oklahoma, and Texas (Lauenroth et al.,
1994; Lauenroth, Burke and Gutmann, 1999).
CLIMATE
Figure 6.4 shows the precipitation gradient east of the Rocky Mountains in
the central part of the Great Plains resulting from the rain-shadow effect of
the Rocky Mountains. Annual precipitation increases from about 320 mm
at Greeley, Colorado, to nearly 900 mm at Kansas City, Missouri. Seasonal
precipitation patterns also vary from south to north in the central plains
(Trewartha, 1961). In southern portions of the short-grass prairie , summer
maxima are the rule, while further east there is one peak in the late spring -early
summer and another in late summer-early autumn (type 3b in Trewartha, 1961).
In the northern Great Plains, spring peaks are common (type 3c in Trewartha,
1961) while in the upper Mississippi Valley-Great Lakes region, summer and
autumn peaks occur (Trewartha, 1961).
The major temperature gradient is from warm temperatures in southern
grassland to cooler temperatures in the north (Figure 6.5). The gradient is
steeper for January temperatures than for July temperatures. Colder winter
temperatures in the north have many implications for both plants and animals.
However, snow cover in the north may ameliorate extremely cold air temperatures
at the soil surface. Winter temperatures in southern locations allow
cool-season plant growth almost any time that there is adequate soil moisture
(Smeins, 1994; Holechek, Pieper and Herbel, 2001).
Before European settlement, the three grassland types in the central portion
of the continent were fairly comparable in area: short-grass was 615 000 km 2 ,
mixed prairie was 565 000 km 2 and tall-grass prairie was 570 000 km 2 (Van
Dyne and Dyer, 1973). Today, the tall-grass prairie is much constricted because
of conversion to intensive agriculture; originally, it extended eastward into
southern Minnesota, most of Iowa, northern Missouri and northern Illinois
and western Indiana (Lauenroth, Burke and Gutmann, 1999). Currently, the
Figure 6.4
Mean annual precipitation
gradient for various sites on
the Great Plains east of the
Rocky Mountains.