Land Cover Trends Project

Land-Cover Change in the United States Great Plains

By Mark A. Drummond 1 and Roger Auch 1

Introduction

One of the important stories of the United States Great Plains is the geographic and temporal variability of recent land-use and land-cover change, including the underlying causes of land conversion.  Because it is an agricultural region, land change in the Great Plains depends in large part on the available natural resources and inherent suitability of the region for crop production and rangeland grazing.  Areas with favorable soils and climate have a long history of cultivation, while those unsuited for crops are often used for livestock grazing.  Given a range of conditions and land use regimes across such a large expanse, our recent analysis shows distinct regional characteristics of land change that also vary across relatively short time-scales as human activities intersect with the environmental setting.  The prevailing climate and biophysical factors (e.g. precipitation variability, soil quality, water availability, topography) interact in complex ways with the actions of people and society through public policies, regional and global economic opportunities, population and demographic change, technological advances, and local cultural histories that facilitate the rates and characteristics of land-use and land-cover change in ecoregions. 

The Great Plains as a geographic region has been variously defined (Rossum and Lavin, 2000).  In general, the region lies between the dense eastern forests and the western mountains and deserts.  The vast, flat-to-rolling plains are used primarily for cropland, rangeland, and settlements.  Weather can be extreme and drought periodically affects the region.  Precipitation is limited in the western plains, which sits in the rain shadow of the Rocky Mountains, but increases towards the east.  Temperature has a strong north-south gradient.  The northeast and extreme northern part of the region is a glaciated area with numerous prairie pothole wetlands and has more cropland than the western plains.  The western plains are primarily a semi-arid shortgrass steppe, where streamside and groundwater irrigation from the High Plains (Ogallala) Aquifer is important.  Shrubland and forest are more prevalent in the southeast where land use practices and fire suppression have encouraged woody encroachment (Engle and others, 2008).  Large population centers tend to be on the fringes of the plains, and are more prevalent in the southeast.  There are few large cities in the interior plains, where population loss is a concern in numerous communities.  Beyond these broad generalizations, many of the ecoregions within this expansive region have contrasting socioeconomic and environmental characteristics that potentially have different effects on their land cover trends. 

To understand the rates, types, causes, and to aid in assessing the consequences of land change, the USGS interpreted and analyzed trends in the 17 EPA Level III ecoregions (Omernik, 1987; EPA, 1999) of the United States Great Plains.  A statistical sampling approach stratified by relatively-homogenous ecoregions was used in part to control for the large amount of error that can occur in assessing wall-to-wall change for multiple time-steps across large regions (Stehman and others, 2005).  A set of 10-km x 10-km sample blocks was randomly selected for each ecoregion from a uniform grid.  Land cover and its changes were interpreted at a resolution of 60-m using Landsat MSS, TM, and ETM+ satellite data for five dates, nominally 1973, 1980, 1986, 1992, and 2000.  A detailed explanation of methodology can be found in Loveland and others (2002).  The approach provides a systematic basis for understanding land-use and land-cover change and can aid in managing and planning for future human-environmental interaction. 

 

Regional Synthesis

Land-use and land-cover change was analyzed for the 17 Great Plains ecoregions as part of a national assessment of land change (fig. 1).  The 17 ecoregions cover approximately 2,231,167 km2

 

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Figure 1. Land cover of the 17 Great Plains ecoregions

 

Land-Cover Composition

 

A substantial shift in the amount of agricultural land cover, which includes all cropland and related agricultural uses except rangeland, versus grassland/shrubland occurred over the 1973 to 2000 period (table 1).  Agriculture and grassland/shrubland are the predominant land covers in the Great Plains (fig. 2, fig. 3).  Land change estimates show that agriculture was the dominant land cover in 1973, 1980, and 1986.  The total percentage of agriculture increased during each of these time steps from 45.9 percent in 1973, to 46.3 percent in 1980, to 46.4 percent in 1986.  However, grassland/shrubland likely became the dominant land cover by 1992 and reached its greatest extent in 2000 (44.5 percent of the ecoregion).  Combined, grassland/shrubland and agriculture comprise more than 88 percent of all land cover in the Great Plains ecoregions.

 

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Figure 2. Dryland agriculture in the Northwestern Great Plains ecoregion.

 

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Figure 3. Livestock grazing in the Western Gulf Coastal Plain ecoregion.

 

The total area of forest cover was 5.8 percent in 2000, which indicates a possible slight decline from its 1973 total (5.9 percent).  The total area of water remained at 1.8 percent through 1992 and increased to 2.1 percent in 2000.  Developed land cover steadily increased over the entire study period, expanding from 1.1 percent in 1973 to 1.5 percent in 2000.  Wetland (1.6 percent) and Barren (0.6 percent) remained relatively consistent through time (fig. 4).  All other land cover types remained at or below 0.1 percent.

 

Table 1. The percentages of all land cover types examined for the Great Plains ecoregions. 

1973

1980

1986

1992

2000

Estimate

85 CI

Estimate

85 CI

Estimate

85 CI

Estimate

85 CI

Estimate

85 CI

Water

1.80%

0.48%

1.79%

0.48%

1.85%

0.45%

1.81%

0.45%

2.06%

0.45%

Developed

1.12%

0.19%

1.24%

0.20%

1.31%

0.21%

1.40%

0.23%

1.55%

0.23%

Mech. Dist

0.03%

0.01%

0.04%

0.01%

0.04%

0.01%

0.07%

0.04%

0.07%

0.02%

Mining

0.06%

0.01%

0.07%

0.01%

0.09%

0.01%

0.10%

0.02%

0.12%

0.02%

Barren

0.59%

0.26%

0.57%

0.26%

0.57%

0.26%

0.59%

0.26%

0.59%

0.26%

Forest

5.92%

0.43%

5.84%

0.43%

5.84%

0.43%

5.80%

0.42%

5.75%

0.42%

Grass/Shrub

42.72%

1.64%

42.15%

1.63%

42.15%

1.62%

44.25%

1.60%

44.47%

1.60%

Agriculture

45.92%

1.66%

46.38%

1.65%

46.38%

1.64%

44.16%

1.62%

43.73%

1.61%

Wetland

1.79%

0.22%

1.78%

0.23%

1.78%

0.22%

1.81%

0.23%

1.65%

0.22%

Nonmech. Dist

0.06%

0.09%

0.00%

0.00%

0.00%

0.00%

0.00%

0.00%

0.02%

0.02%

Snow/Ice

0.00%

0.00%

0.00%

0.00%

0.00%

0.00%

0.00%

0.00%

0.00%

0.00%

 

 

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Figure 4. Wetlands dot many of the Great Plains ecoregions, including the Nebraska Sand Hills, and provide vital habitat for birds and other wildlife.

 

Overall Spatial Change

Overall, an estimated 8.2 percent (182,509 km2) of Great Plains land-cover changed at least once during the 1973 to 2000 period (table 2).  However, the overall amount of change varied substantially across the 17 Great Plains ecoregions (fig. 5).  For example, the ecoregion with the highest amount of change was the Northwestern Glaciated Plains at 13.6 percent.  The ecoregion with the least amount of change was the Lake Agassiz Plain at 1.4 percent.  Both of these ecoregions are in the Glaciated region, which includes areas of Wisconsin glaciation and former glacial lakes in the northern and northeastern part of the Great Plains.  However, some areas of the Northwestern Glaciated Plains ecoregion have irregular topography, rather than the extensive level lands associated with the Lake Agassiz Plain, that hinders agriculture.

Across the Great Plains, the characteristics of ecoregion change varied depending on biophysical factors.  For example, the four ecoregions with overall spatial change of less than 5 percent have contrasting characteristics that constrain land-cover change and enable stability.  The Western Corn Belt Plains (3.0 percent) and Lake Agassiz Plain (1.4 percent) have level topography and glacial till soils that are well-suited for maintaining intensive cultivation.  Conversely, the Nebraska Sand Hills (4.2 percent) and the Flint Hills (2.2 percent) have hilly topography and soils that constrain cultivation and favor rangeland use and grassland maintenance.  However, both types of characteristics (prime cropland versus historical rangeland) resulted in relatively stable land use patterns and low rates of change. 

Ecoregions with a high amount of spatial change tend to have more lands that are marginal for agriculture that, for example, may not support viable cropland under adverse climate or economic conditions, or that have significant pressure from other land change processes.  Fluctuations between grassland/shrubland and cropland in the Northwestern Glaciated Plains (13.6 percent) and the Western High Plains (11.6 percent) contributed to the high rates of overall change as marginal lands were brought in and out of production depending on commodity prices, advances in farm practices, or farm program incentives.  Cyclical clearance of mesquite, juniper, and scrub oak shrublands for rangeland improvement and to enhance water availability contributed to the high rates of change in four of the Southern ecoregions, particularly the Southern Texas Plains (11.9 percent) where climax and invasive woody vegetation are periodically removed, including through state-funded brush control programs.

Table 2.  Overall amount of change by ecoregion and regional group, between 1973 and 2000

Table 2

Land change processes other than the overall dominant fluctuations between cropland and grassland/shrubland were important in individual ecoregions.  For example, urbanization of former agricultural land played a large role in the Texas Blackland Prairies (11.1 percent).  Lake expansion caused by an increase in regional precipitation contributed to change in the Northern Glaciated Plains (7.5 percent).  The Western High Plains (11.6 percent) had the most land enrolled in the Conservation Reserve Program (CRP), which provides an economic incentive to convert environmentally-sensitive cropland to perennial cover since enacted in 1985, but also has access to the dwindling High Plains Aquifer for irrigation and confined livestock feeding operations in other parts of the ecoregion.  Because the biophysical underpinnings and the socioeconomic factors of an ecoregion vary, change in the Great Plains can be further understood by examining individual ecoregions.

 

 

see caption

Figure 5. Overall spatial change from 1973 to 2000 for all Great Plains ecoregions

 

 

Summary of Great Plains Land Change in Ecoregions

Most of the land change in the Great Plains involved the expansion and contraction of agriculture and grassland/shrubland cover types that occur across the region.  However, other changes were important in individual ecoregions where natural resources, climate, or human population caused land-cover changes that were associated with reservoir expansion, wetland transition to ponds and lakes, urban growth, and brush and forest management.  Water availability from the region’s streams and aquifers, which include the immense High Plains (Ogallala) Aquifer and the Missouri and Platte Rivers, enabled the expansion of intensive irrigation (fig. 6) and contributed to the conversion of grassland/shrubland to agriculture (Dennehy and others 2002).  However, water scarcity in areas of the Western High Plains that is caused in some areas by high rates of historical water use has also constrained expansion (fig. 7).  Climate variability also plays a role, including changes in precipitation regimes that caused wetland conversion in the Northern Glaciated Plains.  Population changes affected the conversion of agriculture and grassland in larger urban areas such as in the Southern Plains and smaller cities across the region, even though the regional effect is small.  

 

See caption

Figure 6. Cropland irrigation in the Central Great Plains ecoregion.

 

See caption

Figure 7. Open rangeland in the Western High Plains ecoregion.

 

Changes over the entire 1973 to 2000 period resulted in net increases in grassland/shrubland, developed, water, mining and mechanically disturbed cover types.  Net change in land cover (table 3 and fig. 8) is a measure of the end result of land conversions associated with each land cover type.  Gross changes indicate the total area of change (table 4), which can be much larger than the resulting net change because of cyclic changes between agriculture and grassland, and because of expansions in some locations while others have declines.  The unidirectional, and relatively permanent, changes to developed land are a notable exception to agricultural fluctuations.

 

Table 3. Net changes in land cover for the 17 Great Plains ecoregions

Table 3

 

 

Table 4. Gross and net changes in land cover classes since 1973

Table 4

 

 

See caption

Figure 8.  Area of net land-cover change during the four time intervals, for all 17 Great Plains ecoregions

 

Agricultural Change

Agriculture, which is the main land use in the Great Plains, had the greatest amount of net change with a decline of 2.2 percent (nearly 48,800 km2) of the total area and the second highest amount of gross change (8.4 percent; approximately 186,750 km2) between 1973 and 2000 (table 4).  Only three of the seventeen ecoregions had a net gain in agricultural land; the Nebraska Sand Hills (1.4 percent of ecoregion), the Southern Texas Plains (0.9 percent of ecoregion), and the Edwards Plateau (0.6 percent of ecoregion).  Most ecoregions had an early expansion of agriculture that occurred between 1973 and 1980 when economic opportunities for overseas grain exports increased and public policies encouraged farmers to expand.  Technological changes, particularly the advent of center pivot irrigation, also caused agriculture to expand where water was available.  Some expansion continued through 1986 in several ecoregions, although at a slower rate.  The earlier gains in agriculture were more than offset between 1986 and 2000 by conversions from agriculture to grassland.  The conversion to grassland was important throughout most of the Great Plains, except for the Edwards Plateau where forest clearance and agricultural expansion were the leading changes between 1973 and 2000.

Between 1973 and 1986, much of the increase in agriculture occurred in ecoregions that had substantial amounts of land that was marginal for cultivation, particularly in the group of Western Plains ecoregions.  Conversely, ecoregions with a climate advantage, level topography, and suitable soils, such as the Western Corn Belt and the Lake Agassiz Plain, tended to be relatively stable or have small declines in agriculture.  The largest decline between 1973 and 1986 occurred in the Texas Blackland Prairies (-2.4 percent of ecoregion), where agriculture transitioned to grassland/shrubland.  The ecoregion also lost substantial land area to development, which continued throughout the entire study period. 

Between 1986 and 1992, when the Conservation Reserve Program took effect, all ecoregions lost agriculture and most continued to decline to 2000.  The CRP, which is a federal program to convert erodible croplands and other environmentally sensitive areas to perennial cover including native grasses, began enrolling land in 1986.  The total amount of CRP land in the U.S. reached a plateau in the early 1990s.  The extent of CRP varies across the Great Plains, with the Western High Plains ecoregion having the most (9.5 percent of ecoregion) and the Edwards Plateau having the least (0.0 percent of ecoregion) (USDA). 

The most common conversion over the 1973 to 2000 study period was from agriculture to grassland (106,560 km2).  However, agricultural declines were also caused by other factors including urbanization, land abandonment, reservoir construction, wetland expansion, and mining.  The trends of change over the four time intervals indicate that land cover in the Great Plains is capable of significant fluctuation enabled by the underlying biophysical conditions and the pressures of land use and socioeconomic change (fig. 9 and fig. 10).

 

 

 

See caption

Figure 9. Net gains and losses associated with agricultural land-cover change for the four time periods

 

 

See caption

Figure 10. Maps of the net change in the 17 Great Plains ecoregions for agriculture, grassland, developed and forest cover types.

 

Grassland/Shrubland Change

Grassland/shrubland had the greatest amount of gross change (8.7 percent of Great Plains; 193,375 km2) and the second highest net change (1.7 percent of Great Plains; 38,975 km2) in the Great Plains between 1973 and 2000 (table 4).  In many regards, the characteristics of change are inverse to those of agriculture (fig. 11).  The total area of grassland/shrubland declined between 1973 and 1986 as agriculture expanded.  Whereas between 1986 and 2000, grassland/shrubland had large gains that came primarily from the decline in agricultural land cover caused by land use abandonment for social, economic or biophysical reasons and by the incentives of the CRP that paid farmers to convert highly erodible cropland to grassland cover.  For example, agricultural gained from grassland/shrubland as irrigation expanded between 1973 and 1980, affecting nearly 1.5 percent of the semi-arid Western High Plains, which was followed by the expansion of grassland/shrubland between 1986 and 2000 that affected approximately 7.5 percent of the ecoregion.  Overall, the extent of grassland/shrubland change was highest in the Great Plains between 1986 and 1992, supporting the premise that the CRP and other drivers of agricultural abandonment were important forces of grassland/shrubland expansion, including the need for a smaller agricultural land base that stems from the ongoing intensification, industrialization, and consolidation of agriculture.

Across the Great Plains, grassland/shrubland was converted to developed land (3,131 km2) and also transitioned to forest (2,973 km2), although the net result in ecoregions was often an increase in grassland/shrubland because gains offset the losses to other land cover types.  For example, the Texas Blackland Prairies had a net gain in grassland/shrubland during all time intervals as agriculture was abandoned or de-intensified, despite a relatively large amount of urban expansion.  Additional gross land-cover changes occurred as cyclic changes due to mechanically disturbed land that was alternately cleared of shrubland and then regrew (4,490 km2), which occurred primarily in ecoregions of the Southern Plains (fig. 12).

Grassland/shrubland increased or was relatively flat in most ecoregions with the exception of small declines in the Central Great Plains (-0.2 percent of ecoregion), Southern Texas Plains (-0.9 percent of ecoregion) and the Nebraska Sand Hills (-1.4 percent of ecoregion).  Declines in the Southern Texas Plains were caused by multiple factors including brush removal for livestock grazing and invasive control, agricultural expansion, and increases in developed land and mining.  Grassland/shrubland in the sparsely populated Nebraska Sand Hills declined primarily as cropland expanded along the margins of the ecoregion.

See caption

Figure 11. Gains and losses associated with grassland/shrubland-cover change

 

See caption

Figure 12. Brush clearance in the Southern Texas Plains ecoregion.

 

Developed Land Change

New developed land (commercial, industrial, residential, transportation, water treatment, and other similar land uses) increased by 0.4 percent (9,545 km2) of the total ecoregion area between 1973 and 2000 (fig. 13).  As a percentage of change, developed land increased 38.3 percent from its estimated 1973 extent.  New developed land was found primarily around regional and sub-regional service, retail, and manufacturing centers located in micropolitan (populations greater than 10,000 but less than 50,000) and metropolitan (populations greater than 50,000) areas. There are at least 106 micropolitan and 46 metropolitan areas within or partially included in our Great Plains study area (U.S. Census, 2003). 

Other developed land included recreational development and in some cases additional industrial infrastructure. Sections of the original Interstate Highway system were completed during the first two time intervals (1973-1986) and subsequent additions of major multi-lane roads to the federal and individual state highway systems occurred throughout the study period. Some increased amenity based development was found in localized areas, usually around existing or newly built reservoirs. Several isolated industrial complexes, such as power plants, were built between 1973 and 2000.

New developed land cover had three primary sources that accounted for 97% of the increase over the 1973 to 2000 period (fig. 14).  Developed land cover gained an estimated 5,173 km2 from agricultural land cover and an estimated 3,117 km2 from grassland/shrubland land cover. Forest land cover, the third leading source, accounted for an estimated 995 km2 of new developed land. The ratios of conversion to new developed land cover remained relatively stable across the study period, although forest land cover conversion to development increased from 6.6 percent of the new developed land during the first time interval to 12.5 percent of the new development between 1980 to 1986 and remained near that level through 2000.  Small amounts of wetland, compared with the other land cover types, were developed.

At the ecoregion scale, the Texas Blackland Prairies had the greatest increase in developed land cover (3.8 percent of the ecoregion), gaining 1,932 km2, which is approximately 20 percent of the net developed land increase in the larger Great Plains. This ecoregion includes an axis of large urban areas, starting with Dallas in the north, south through Austin, and terminating in San Antonio.  Other leading ecoregions in terms of the extent of the ecoregion affected by an increase in developed land cover include the East Central Texas Plains (1.3 percent) and the Western Gulf Coastal Plain (1.2 percent).  Growth in the Western Gulf Coastal Plain included coastal industrial and amenity-based development.  However, many sparsely populated ecoregions experienced only small amounts (>0.5 percent) of increased developed land cover especially those with extensive rangeland.  This group included the Northwestern Great Plains, the Northwestern Glaciated Plains, and the Southwestern Tablelands.

See caption

Figure 13. Urban expansion in the Texas Blackland Prairies ecoregion.

 

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Figure 14. The average annual contribution of land cover sources to new developed land in the 17 Great Plains ecoregions, 1973 to 2000

 

Water Change

Climate in the Great Plains ranges from semi-arid to humid and can be highly variable both spatially and temporally, with long periods of drought in some ecoregions and cycles of above average precipitation in others.  Because of climate variability, many ecoregions rely on water storage in reservoirs to irrigate crops, to provide for livestock, for confined-feeding operations, or for household consumption.  All of these factors relate to land change. 

Overall, the changes affecting water cover accounted for approximately 4.5 percent (20,766 km2) of the total gross land-cover changes detected in the seventeen ecoregions during the 1973 to 2000 time period. Total net change was much less, at an estimated 5,905 km2 or a 14.7 percent increase from the 1973 base amount.

The top two changes affecting water cover were both associated with wetland land cover. Wetland to water conversion was the leading change (8,456 km2) and water to wetland conversion was the second leading change (4,485 km2), occurring primarily in the Glaciated Plains ecoregions. Much of this change was driven by variable climatic conditions of dry years that caused lakes and ponds to become seasonal wetlands, and wet years that caused open water to persist in former wetland areas.  The wetter than normal conditions during part of the 1990s in the Northern Glaciated Plains and the Northwestern Glaciated Plains ecoregions (the prairie pothole region) caused most of the wetland to water expansion (Garbrecht and Rossel, 2002).

The Northwestern Great Plains, the ecoregion containing four of the six main stem Missouri River reservoirs, experienced similar water and wetland changes but for slightly different reasons. Changes in upstream water availability (decreased mountain snow pack, regional droughts) and reservoir management in response to these conditions likely contributed to decreased reservoir storage that caused wetland vegetation to grow in certain areas, especially embayments formed by tributary streams. The reservoirs experienced these changes in late 1980s and early 1990s, but mostly recovered by 2000 (U.S. Army Corps of Engineers, 2004).

Conversions of grassland/shrubland to water and water to grassland/shrubland were the third and fourth leading changes affecting water land cover in the greater Great Plains. Changes from grassland/shrubland to water resulted primarily from the construction of reservoirs of various sizes across the region and by flooding from expanding lake beds. A large number of the new reservoirs were used for livestock water supplies in the more arid ecoregions such as the Northwestern Great Plains, Central Great Plains, Southern Texas Plains, and Southwestern Tablelands. Many of these “stock dams” or “stock tanks” as they are commonly called cover only a few hectares in size. Other, larger reservoirs with multiple uses were also created.  The natural flooding of grassland/shrubland occurred primarily in the Glaciated Plains, where wetter than normal years caused some glacial lake beds to increase in size. These water bodies may persist at larger surface areas for years before cyclic climatic conditions reduce their size during periods of drought and more typical precipitation patterns (Shapely and others, 2005; Todhunter and Rundquist, 2004). Water to grassland/shrubland resulted when reservoirs, especially small stock dams, dried up during below normal precipitation conditions.  Overall, water cover had a small net gain of an estimated 268 km2 from grassland/shrubland land cover between 1973 and 2000.

 

Wetland Change

Changes in wetland cover, the fourth leading gross change in the Great Plains, are closely associated with water cover.  As a percentage of the seventeen ecoregions land area, wetland land cover decreased 0.1 percent. As a percentage of the 1973 wetland area, wetland land cover decreased an estimated 7.7 percent (3,085 km2) by 2000.  The net decline was caused primarily by wetter climatic conditions that allowed wetland areas to be replaced by persistent open water.

Much of the expansion of wetlands occurred in the Northern Glaciated Plains ecoregion, part of the prairie pothole region, when the wetter than normal climatic conditions in the mid-1980s and mid- to late 1990s caused many temporary and seasonal wetlands that were previously farmed to stay persistently wet, keeping them out of crop production.  The Western Cornbelt Plains, traditionally included in the prairie pothole region before its wetlands were mostly drained or modified in the late 19th and early 20th centuries to enable cultivation, also experienced similar wetland expansion on agricultural land, but only a small fraction when compared to the neighboring Northern Glaciated Plains (an estimated 136 km2 versus 1,356 km2).

Similar changes from agriculture to wetland found in the Western Gulf Coastal Plain had different factors influencing this type of change. Substantial ground subsidence in coastal areas may have created wetland conditions on former agricultural land (Davidson and Mace, 2006). Several National Wildlife Refuges were also created in the ecoregion, including the Cameron Prairie National Wildlife Refuge in southwest Louisiana, where former agricultural land was converted back to wetlands (U.S. Fish & Wildlife Service, 2009).

Other changes affecting wetland land cover were grassland/shrubland to wetland (an estimated gross 448 km2) and wetland to grass/shrubland (an estimated gross 285 km2) transitions.  These types of changes took place primarily in the Glaciated Plains ecoregions and several of the Southern Plains ecoregions as changes in precipitation contributed to fluctuations in wetland extent.  Forest to wetland changes were also found in the Southern Plains ecoregions where reservoirs for metropolitan drinking sources were constructed. During the filling of these reservoirs, upland forest was killed off and replaced by wetland conditions. The Western Gulf Coastal Plain has a wide variety of wetlands within it, including the western range of bald cypress, a commercially valuable wetland tree. This ecoregion experienced the most conversion from wetland to mechanically disturbed land. The Western Gulf Coastal Plain was also the leading ecoregion for conversion from wetland to developed land cover, primarily because of increased urbanization and industrial growth.

 

Forest Change

Net forest cover declined by -0.2% of the total area of the Great Plains (approximately -3,750 km2) over the study period.  However, most forest change was in the Southern Plains.  The largest net decline occurred as agriculture expanded over the 1973 to 2000 period (approximately -1,350 km2).  Loss of forest to developed land, clearcutting (mechanically disturbed land), and reservoir construction contributed to the rates of forest cover change (fig.15). 

 

see caption

Figure 15.  Gross changes in forest cover between 1973 and 2000, shown as declines and increases in forest as related to other land cover types. 

 

Mining Change

Net gains of less than 0.1 percent of the total area of the Great Plains (approximately 1,410 km2) occurred in mining land cover over the 1973 to 2000 period.  Although a small amount of change, this may have resulted in more than doubling the amount of mining land cover.  Most of the increase came from the expansion of mining on agriculture and grassland/shrubland land covers.  Mining changes were primarily due to revegetation of reclaimed mine sites, primarily as grassland/shrubland; the expansion of quarries, mineral excavations, oil and gas extraction; and a small amount of mining that was abandoned or purposely converted to ponds for recreation or wildlife. 

 

Transitional Lands

Most changes in transitional land cover types (i.e., mechanically disturbed and non-mechanically disturbed) occurred as three main processes that resulted in a net expansion of mechanically disturbed land and a net decline of non-mechanically disturbed land.  Increases in mechanically disturbed land occurred as forest cutting expanded in the Southern Plains ecoregions (874 km2), and as large amounts of gross change, associated with cycles of woody shrub clearance and regrowth, resulted in a small net gain of mechanically disturbed land.  Changes in non-mechanically disturbed land (approximately 1,802 km2 of gross change and -1,050 km2 of net change) occurred as grassland/shrubland was burned and then reestablished following fire disturbance.

 

Barren Land Change

A gross change of 3,413 km2 in the barren land cover was mostly associated with natural fluctuations in the extent of natural water bodies and grassland/shrubland cover, including transitions to and from barren caused by changes in the precipitation regime, which resulted in a small net change of -30 km2 of barren land extent. 

 

Summary

The highest amount of change in the Great Plains was due to fluctuations in agriculture, much of it on marginal land, as economic opportunities, socio-cultural dynamics, agricultural technologies, and government farm policies changed.  Most ecoregions in the Great Plains saw an overall decline in agricultural land cover facilitated by the Conservation Reserve Program. The decline followed an earlier expansion between 1973 and 1980 at the expense of grassland/shrubland.  However, the trajectories and causes of change vary substantially by ecoregion, giving some indication of the complexity of land change processes operating across the Great Plains. 

The drier Western Plains ecoregions and the brushy Southern Plains ecoregions had relatively high rates of change because of substantial fluctuations between agriculture and grassland/shrubland.  Ecoregions with favorable biophysical conditions tended to have lower rates of change.  Variability in precipitation, temperature, and drought had effects on land change that ranged from veiled to obvious.  The expansion of surface water was an important story that primarily occurred in the Glaciated Plains ecoregions and was tied to climate and human activity.  Water cover also increased as reservoirs were constructed across most ecoregions for crop irrigation, livestock, industry, and residential use. 

All ecoregions in the Great Plains saw small but steady increases in developed land cover. 

Most of the increases in developed land occurred as urbanization in the Southern Plains ecoregions on agriculture and grassland/shrubland land cover.  Cycles of shrubland clearance in the Southern Plains contributed to high rates of gross land-cover change.  To understand land-use and land-cover changes in greater detail, this report is followed by summaries of change for the seventeen ecoregions of the Great Plains.

1 Mark A. Drummond and Roger Auch – U.S. Geological Survey

 

 

REFERENCES

Davidson, S.C. and R.E. Mace, 2006, Aquifers of the Gulf Coast of Texas: An overview.  In, eds. R. E. Mace, S.C. Davidson, E. S. Angle, and W.F. Mullican III. Aquifers of the Gulf Coast of Texas, Feb 2006, Report 365, Texas Water Development Board, Austin.

Dennehy, K.F., D.W. Litke, and P.B. McMahon, 2002, The High Plains Aquifer, USA: groundwater development and sustainability, Geological Society, London, Special Publications, v. 193:99-119.

Engle, D.M., B.R. Coppedge, and S.D. Fuhlendorf, 2008, From the Dust Bowl to the Green Glacier: Human activity and environmental change in Great Plains Grasslands, In, ed. Q.W. Van Auken, Western North American Juniperus Communities: A Dynamic Vegetation Type, 313p, Series: Ecological Studies, Springer, New York, Vol. 196, p. 253-271.

Garbrecht, J.D., and Rossel, F.E., 2002, Decade-scale precipitation increase in the Great Plains at the end of the 20th century, Journal of Hydrologic Engineering, vol. 7, no. 1.

Loveland, T.R., Sohl, T.L., Stehman, S.V., Gallant, A.L., Sayler, K.L., and Napton, D.E., 2002, A strategy for estimating the rates of recent United States land-cover changes: Photogrammetric Engineering and Remote Sensing, v. 68, no. 10, p. 1091–1099.

Omernik, J.M., 1987, Ecoregions of the conterminous United States: Annals of the Association of American Geographers, v. 77, no. 1, p. 118–125.

Rossum, S. and S. Lavin, 2000, Where are the Great Plains? A cartographic analysis, Professional Geographer, 52(3):543-552.

Shapley, M.D., Johnson, W.C., Engstrom, D.R., and Osterkamp, W.R., 2005, late-Holocene flooding and drought in the northern great plains, USA, reconstructed from tree rings, lake sediments, and ancient shorelines, The Holocene, v. 15, no. 1, p. 29-41.

Stehman, S.V., Sohl, T.L., and Loveland, T.R., 2005, an evaluation of sampling strategies to improve precision of estimates of gross change in land use and land cover, International Journal of Remote Sensing, v.26, no. 22, p. 4941-4957.

Tarrant Regional Water District, 2007, TRWD history timeline, accessed at http://www.trwd.com/prod/AboutUs_History.asp, February 13, 2009.

Todhunter, P. E.  and Rundquist, B.C., 2004, terminal lake flooding and wetland expansion in Nelson County, North Dakota, Physical Geography v. 25, p. 68-85.

U.S. Army Corps of Engineers, 2004, Summary Missouri River final environmental impact statement master water control manual review and update, U.S. Army Corps of Engineers Northwestern Division, accessed at http://www.nwd-mr.usace.army.mil/mmanual/Summary.pdf, February, 19, 2009.

USDA (U.S. Department of Agriculture).  1920-2002.  Census of Agriculture, Geographic Area Series.  USDA National Agricultural Statistics Service, U.S. Government Printing Office, Washington, DC. 

U.S. Bureau of the Census.  2001.  2000 Census of Population and Housing: Summary File 1 United States.  U.S. Bureau of the Census, Washington, D.C.

U.S. Census Bureau, 2003, State-based Metropolitan and Micropolitan Statistical Areas Maps, accessed at http://www.census.gov/geo/www/maps/stcbsa_pg/stBased_200306_orig.htm, Feb. 10, 2009.

U.S. Environmental Protection Agency, 1999, Level III ecoregions of the continental United States, Revised March 1999, map.

U.S. Fish & Wildlife Service, 2009, Cameron Prairie National Wildlife Refuge facts, accessed February 26, 2009 at http://www.fws.gov/southeast/pubs/facts/camcon.pdf .

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