Land change variability and human–environment dynamics in the United States Great Plains

Abstract

Land use and land cover changes have complex linkages to climate variability and change, biophysical resources, and socioeconomic driving forces. To assess these land change dynamics and their causes in the Great Plains, we compare and contrast contemporary changes across 16 ecoregions using Landsat satellite data and statistical analysis. Large-area change analysis of agricultural regions is often hampered by change detection error and the tendency for land conversions to occur at the local-scale. To facilitate a regional-scale analysis, a statistical sampling design of randomly selected 10 km × 10 km blocks is used to efficiently identify the types and rates of land conversions for four time intervals between 1973 and 2000, stratified by relatively homogenous ecoregions. Nearly 8% of the overall Great Plains region underwent land-use and land-cover change during the study period, with a substantial amount of ecoregion variability that ranged from less than 2% to greater than 13%. Agricultural land cover declined by more than 2% overall, with variability contingent on the differential characteristics of regional human–environment systems. A large part of the Great Plains is in relatively stable land cover. However, other land systems with significant biophysical and climate limitations for agriculture have high rates of land change when pushed by economic, policy, technology, or climate forcing factors. The results indicate the regionally based potential for land cover to persist or fluctuate as land uses are adapted to spatially and temporally variable forcing factors.

Introduction

The dynamics of land-use and land-cover change are increasingly recognized as operating within a linked human–environment system that is shaped by the complex interactions of social, economic, climate, and biophysical factors (Rindfuss et al., 2004, Global Land Project, 2005, Turner et al., 2007). In practice, the organization, function, and causes of land use activities are often not adequately considered in environmental change studies. As a result, the spatial and temporal complexity of human–environmental processes and feedbacks that operate at regional to global scales are not fully understood (Liu et al., 2007). Regardless, regional analyses of the extent, types, and processes of land change are critical for further assessment of the prospects for ecological and socioeconomic sustainability (Loveland et al., 2002, Turner et al., 2007), as well as for issues of climate (Pielke et al., 2007), hydrology (Scanlon et al., 2005), carbon exchange (Burke et al., 1991, Post and Kwon, 2000, Guo and Gifford, 2002), and biodiversity (DeFries et al., 2004).

Over the past two centuries, the United States Great Plains has undergone significant land surface change as it was transformed from extensive grassland to a modern mosaic of rangeland, dryland farming, and intensive irrigated and industrial agriculture. Perceptions of the Great Plains, which have ranged from desert to agricultural oasis, have also evolved over time, in part as advances in technology and agricultural practices aided adaptation to climate variability and drought (Lawson and Stockton, 1981, White, 1994). Recent scientific thought emphasizes the longevity and sustainability of agricultural pursuits, while also recognizing the risks and vulnerability of the region to socioeconomic and environmental change and the opportunities to enable resilience (Cunfer, 2005, Parton et al., 2007). Many areas of the Great Plains may remain relatively stable producers of food, fiber, and fuel well into the future. However, other areas in the region are significantly affected or may be affected in the near future by climate change, land use policies, increased demand for biofuels, globalization, national economic conditions, declining water availability, population change, and other factors. Indeed, regional land use practices have been adapting to climate and resource variability, new technology, regulatory policy, and new global economic opportunities for decades (Easterling et al., 1993). In light of the contemporary pressures and driving forces shaping the Plains, how dynamic and diverse are the land changes in the region?

Across the Great Plains, where extensive areas of land are dedicated to livestock and cropping activities, land use patterns inevitably rely on the environmental capacity for agricultural production, as well as the human capacity to utilize available resources. However, the timing and extent of land changes are modulated by numerous socioeconomic forces. Essentially, different locations have geographic advantages or limitations for intensive crop production, rangeland grazing, or other agricultural uses that are contingent on the prevailing climate and land quality (e.g., soils, topography, and water availability). Human interactions further strengthen or diminish the characteristics of local and regional-scale change through land use policies and economic opportunities (Drummond, 2007), technological advances and agricultural inputs (Parton et al., 2007), population and demographic shifts (Gutmann et al., 2005), industrialization of agriculture (Hart and Mayda, 1998), and surface and groundwater irrigation (Kromm and White, 1992). The human–environmental land system not only enables the management of the landscape for the production of food, fiber, feed grains, and fuel but also causes feedbacks and consequences that ultimately affect the vulnerability and sustainability of the system. Because of these interacting forces, the rates, causes, and implications of land change may vary substantially across the region.

To examine land change dynamics, we analyzed the geographic and temporal variability of land use and land cover for five dates between 1973 and 2000 stratified across 16 nested ecoregions that comprise the greater Great Plains ecoregion (Fig. 1) (Omernik, 1987, CEC, 1997, USEPA, 1999). The hierarchical ecoregion framework provides a set of relatively homogenous land units (EPA Level III ecoregions) to compare, contrast, and generalize the characteristics of land conversion across the diverse conditions of a large region such as the Great Plains (EPA Level I ecoregion), which has considerable potential for regional transformation. The individual ecoregions of the Great Plains may show differential characteristics of change that ultimately relate to many of the pressing issues of land use that include providing food and fuel for a growing world population, carbon sequestration, groundwater mining, strategic habitat conservation, and climate change.

Several contemporary research issues help to frame the regional-scale land-cover changes affecting the Great Plains, including a significant historical redistribution of human population and demographics. Population has declined and aged in many rural areas since the 1930s, although there may not be a close relationship between modern rural population loss and land-cover change across most of the Great Plains (Gutmann et al., 2005). Population has stabilized or increased in a few locations of expanding agricultural industry, including Finney County, Kansas where confined feeding operations and meat packing plants provide employment opportunities (Broadway, 1990, Harrington and Lu, 2002). Large cities and their surrounding areas have gained population, which can have a detrimental effect on the local extent of agricultural land as urban areas, exurban settlements, and industry gain water rights and expand onto cropland and pasture (Parton et al., 2003). Total population in the region increased by about 50% between 1970 and 2000; however many rural counties had net population loss, while there were substantial gains in urban and peri-urban areas (Wilson, 2009). This is linked to decreases in farm numbers, larger farm sizes, and decreased labor needs of modern agricultural production (Hart, 2003).

Public policies and subsidies that incentivize or delimit access to natural resources have a variable impact over time and space. This includes policies that promote or mitigate the use of energy sources, water resources, and environmentally sensitive land. The Conservation Reserve Program (CRP) established by the Food Security Act of 1985, which has encouraged landowners to retire millions of hectares of highly erodible and environmentally sensitive cropland from production using 10–15 year contracts, has had a substantial effect on land use patterns while also improving wildlife habitat, water quality, and soil carbon and nitrogen storage (Riebsame, 1990, Gebhart et al., 1994). Retired land is planted to native and cultivated grasses, windbreaks, and other cover types allowed by the initial program and subsequent Farm Bills. Although some rangeland and native grasslands may be newly tilled even as potentially less-diverse CRP grassland is established, the more than 7 million hectares of Great Plains CRP land benefits numerous birds and other wildlife species (Higgins et al., 2002). If the economic and social incentives to keep farmland in CRP weaken, then a significant amount of land could be put back into production and perhaps have a detrimental effect on local ecosystem services.

Efforts to establish biofuels as a substitute energy source could influence a trend away from land retirement (Searchinger et al., 2008). For example, the expanded use of various cultivated grasses in the drier western plains that are useful for biofuel production could cause large areas of land to be dedicated to biomass crops, although questions remain about the ramifications of such changes (Rosenberg and Smith, 2009). The amount of corn used for ethanol production in the United States tripled between 2003 and 2008, while the worldwide demand for food and livestock feed accounted for a much higher (greater than 90%) amount of the global increase in wheat, corn and other grains (Trostle, 2008). This suggests that global demand for food and feed as population and demographic factors evolve will continue to be a significant factor for future land change, and suggests a need to explore biomass sources that do not impact food production.

Climate variability and change pose risks to farmers, biota, and human well-being. Future variability of summer temperature, evaporation, and precipitation may stress the wetland and riparian ecosystems and other habitat, as well as put additional pressure on land use and a limited water supply (USGCRP, 2009). Access to water, including the High Plains Aquifer, has enabled agricultural intensification and expansion, although declining water availability and drought takes a toll on land use. Water-levels of the aquifer declined by a geographically weighted average of more than 11 ft. (200 million acre-ft.) between predevelopment and 2001 and had a greater than 50% loss of saturated thickness in the southwestern part of the Texas Panhandle due to land use (McGuire, 2003). Saturated thickness is highly variable across the aquifer, and recharge rates are generally low compared to pumping rates (Dennehy et al., 2002). Limits to the water supply have reportedly caused farm abandonment in areas of the semi-arid High Plains (Walsh, 1980, Nellis et al., 1996, Wu et al., 1999, Kettle et al., 2007). This has occurred even as industrial agriculture, crop irrigation, and confined feeding operations expanded and integrated around readily available, but declining, water supplies (Kromm and White, 1992, Harrington and Lu, 2002).

Woody plant encroachment onto grasslands and savannas, such as in the southern plains, may significantly alter carbon sequestration dynamics and contribute to a carbon sink (Hibbard et al., 2003, Wessman et al., 2004), as well as affect soil moisture and other biota. Climate and land use factors contribute to the expansion. Encroaching brush and trees are sometimes cleared by landowners as part of rangeland management and habitat enhancement. The regional extent of woody encroachment in the southern plains may be extensive (Mitchell, 2000); however, the amount of subsequent clearance is unclear. The dynamics of woody growth and clearance affects land use patterns, biodiversity, soil carbon and other environmental factors. Global carbon management depends, in part, on land and soil conditions in the grassland and agricultural regions. The storage of soil carbon differs spatially and temporally across the Plains depending on environmental characteristics such as drought but also on land-use change and the intensity, type, and time-span of cultivation (Parton et al., 2005).

Regional agricultural land use changes occur within a global context of an increasing human population and changing demographics that affects the demand and preferences for agricultural products. A projected 34% increase in global population and a more affluent and urban society may necessitate a 70% increase in food production by 2050 (Food and Agriculture Organization of the United Nations [FAO], 2009). Part of the equation for meeting that demand is to ensure that food production has the capacity to adapt to changes in climate and to other pressures such as increased biofuel production (FAO, 2009). Global population growth coupled with the effects of regional climate variability and drought on food supplies could increase the demand for agricultural land in the Great Plains. Given these pressures, regions must balance land-use change and the provision of ecosystem goods with the unintended consequences to climate, carbon, water, biodiversity and other ecosystem services (DeFries et al., 2004, Ramankutty et al., 2008).

In many regards, the theoretical underpinnings of land use and land-cover change are still being developed beyond the broad interpretations of the von Thünen model of declining bid-rent as the distance to market increases (Walker and Solecki, 2004), although Lambin et al. (2000) discuss several theoretical concepts useful in agricultural land use models. Central to the von Thünen model is the assumption that land, for a given location and its environmental attributes, will be allocated to the use that earns the highest profit or surplus with variability of agricultural rent dependent on climate, land quality, and socioeconomic factors (Polsky and Easterling, 2001). The land rent concept provides a basic framework to help characterize successive land changes and their relationship to potential economic forces and proximate causes.