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Regional Trends of Biological Resources — Grasslands

Prairie Grasses

Tall-grass Prairie

USGS photo by J.T. Lokemoen: Tall-grass prairie.  
Fig. 3.   Tall-grass prairie.

Tall-grass prairie (Fig. 3) is the wettest of the grassland provinces and is predominantly composed of sod-forming bunch grasses. Like other grasslands, the tall-grass prairie has species originally from different geographical sources (Simms 1988). Grassland groupings of the tall-grass prairie are the bluestem prairie from southern Manitoba through eastern North Dakota and western Minnesota south to eastern Oklahoma, and the wheatgrass, bluestem, and needlegrass area from south-central Canada through east-central North Dakota and South Dakota to southern Nebraska.

Three additional areas are associated with tall-grass prairie: the Crosstimbers, a band of grassland and oak savanna at the southern edge of the bluestem prairie in Kansas to the Trinity River in Texas (KŘehler 1964), the Blackland Prairie south of the Crosstimbers (Gould 1962), and the rice prairies. The rice prairies are former coastal prairies that have been converted to rice production (Hobaugh et al. 1989). The original vegetation in rice prairies was mainly tall grass and extended across 9,000 square kilometers, largely along the Texas coast and inland as much as 125 kilometers, and into Louisiana. Little coastal prairie remains; Attwater's Prairie Chicken National Wildlife Refuge in Texas is the single major remnant.

Since 1830 declines in the area of tall-grass prairie within specific states and provinces are estimated to be 82.6 to 99.9% (Table 1) and exceed those reported for any other major ecological community in North America (Samson and Knopf 1994). Iowa, for example, has barely 12,140 hectares remaining of its original 12 million hectares of tall-grass prairie. Less than 1% of the presettlement tall-grass prairie remains in Manitoba, Illinois, Indiana, and North Dakota. Minnesota and Missouri, states active in prairie conservation, work with less than 9% of the presettlement tall-grass prairie. Tall-grass prairie remains important to ranching in the Osage and Flint Hills of Kansas and in tracts in South Dakota, Oklahoma, and Texas.

Mixed-grass Prairie

USGS photo by F.L. Knopf: Mixed-grass Prairie  

One can envision the short-grass and tall-grass prairies intergrading just east of an irregular line that runs from northern Texas through Oklahoma, Kansas, and Nebraska, northwestward into west-central North Dakota and South Dakota (Figs. 1 and 2). The perimeter is not well defined because of the array of short-stature, intermediate, and tall-grass species that make up an ecotone between the short-grass and tall-grass prairies (Bragg and Steuter 1996). In general, the mixed-grass prairie is characterized by the warm-season grasses of the short-grass prairie to the west and the cool- and warm-season grasses, which grow much taller, to the east (Fig. 4). Because of this ecotonal mixing, the number of plant species found in mixed-grass prairies exceeds that in other prairie types. Estimated declines in area of native mixed-grass prairie, although less than those of the tall-grass, range from 30.5% in Texas to over 99.9% in Manitoba (Table 1).

    Fig. 4.   Mixed-grass prairie in
    Nebraska Sandhills.

Short-grass Prairie

USGS photo by F.L. Knopf: Short-grass Prairie  
Fig. 5.   Short-grass prairie in Laramie Plain, Wyoming.

The short-grass prairie extends east from the Rocky Mountains and south from Montana through the Nebraska panhandle and southeastern Wyoming into the high plains of Oklahoma, New Mexico, and Texas (Figs. 1 and 2.). The short-grass prairie landscape (Fig. 5) was one of relatively treeless stream bottoms and uplands dominated by blue grama and buffalo grass, two warm-season grasses that flourish under intensive grazing (Weaver et al. 1996). Buffalo grass reproduces both sexually and by tillering sprouts from the base of grass clumps. Unlike the more eastern species, short-grass prairie species remain digestible and retain their protein content when dormant.

Declines in short-grass prairie have generally been much less than those of tall-grass and mixed-grass prairies (Table 1). However, perhaps in no other system than short-grass prairie are historical and evolutionary impacts of grazing so apparent (Knopf 1996). Clearly, birds endemic to the short-grass prairie express life-history characteristics and habitat use in response to grazing (Fig. 6). The mountain plover responds to highly disturbed sites, the chestnut-collared longspur to moderately grazed areas, and the Baird's sparrow to sites with taller grasses. In the mid-1800's the numbers of individuals of native mammal species—bison, prairie dogs, pronghorn, elk, grizzly bears, and gray wolves—rivaled or exceeded those now in the African Serengeti (Howe 1994). Major antigrazing structures evolved in plants: thorns and spikes; thick or hard tissues difficult to bite, chew, or digest; and secondary compounds difficult to digest. These structures have arisen through the long coevolutionary association between plants and animals with grazing on grasslands.

Figure 6
Fig. 6.   Importance of coevolution between grazing and native prairie bird distributions and abundances (after Knopf 1996).

At present, extensive areas of short-grass prairie are dominated by invasive perennial and annual species, whose presence is attributed to overgrazing by domestic livestock and dryland farming (Weaver et al. 1996). To the south, specifically the Texas high plains, much of the short-grass prairie is now farmland or shrubland invaded by prickly pear cacti and oaks. Only the short-grass prairie and, to a lesser extent, mixed-grass prairie remain in public ownership. These areas are largely on the national grasslands managed by the U.S. Forest Service.

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