Note that the data published in the 2002 State of the Nation’s Ecosystems Report as well as the 2003 and 2005 Web-Only Updates have been superseded by the 2008 Report and thus should be used with caution. For the most recent data, purchase the 2008 Report from Island Press.

The Indicator

This indicator reports the acreage of grasslands and shrublands using land cover data based on satellite measurements from the National Land Cover Dataset (NLCD). For this indicator, pastures and hay-lands were included; however, they were included within farmlands for that system’s extent indicator and the national extent indicator. Pastures and hay-land are included in this indicator because many fall within the description of grasslands and shrublands given in the introduction of this chapter, and because it is not clear how well the satellite data distinguish them from less-managed grasslands. (Note that in the NLCD the classification “pasture/hay” is defined as areas of grasses, legumes, or grass-legume mixtures planted for livestock grazing or the production of seed or hay crops.)

The U.S. Department of Agriculture Economic Research Service (ERS) has carefully tracked changes in different land uses over the past 50 years in its “Major Uses of Land” series (see http://www.ers.usda.gov/data/majorlanduses/). Its accounting for cropland and forest land is consistent with the approach taken in this report, and its trend in urban area was adopted for comparison purposes (see national extent indicator). However, the ERS category that is closest to the definition of grasslands and shrublands used in this report is “grassland, pasture and range.” This category, which included 578 million acres for the lower 48 states in 1997, is inconsistent with the definition used in this report because land is included based on grazing activity rather than on the land cover classification; there was no obvious way to reconcile the differences in definition adequately so that the ERS data could be used to track trends in grasslands and shrublands.

The extent of grasslands and shrublands (shown in this indicator) is a key aspect in understanding this ecosystem. Additional indicators in this chapter provide information on other key parameters. In addition, there have been attempts to provide overall ratings of the “ecological condition” or “health” of these lands. One potential measure of ecological condition is seral stage. The concept of rangeland health has been addressed by the National Academy of Sciences (Committee on Rangeland Classification 1994). However, ecological condition is expressed differently at multiple scales, including sites and landscapes, and presently does not lend itself to synthesis at a national scale. Aggregation of site-level rangeland condition data to a national assessment is particularly problematic (Mitchell 2000).

The Data

Data Source: The data for the lower 48 states are from the NLCD, which has a resolution of approximately 30 meters on a side. The NLCD is a product of the interagency Multi-Resolution Land Characterization (MRLC) initiative (see the technical note for the national extent indicator.

Data for Alaska are from a vegetation map of Alaska by Flemming (1996), based on Advanced Very High Resolution Radiometer (AVHRR) remote-sensing images with an approximate resolution of 1 kilometer on a side. The following groupings of classes were used (see http://agdc.usgs.gov/data/projects/fhm/#G [Statewide Vegetation/Land Cover] and the technical note for the national extent indicator. Briefly, the following are Flemming’s (1996) classes that were included within grasslands and shrublands: alpine tundra & barrens (#3); dwarf shrub tundra (#4); tussock sedge/dwarf shrub tundra (#5); moist herbaceous/ shrub tundra (#6); wet sedge tundra (#7); low shrub/lichen tundra (#8); low & dwarf shrub (#9); tall shrub (#10); and tall & low shrub (#23).

Presettlement estimates of grass/shrub land cover were derived from data provided by Richard J. Olson, Oak Ridge National Laboratory (personal communication). These data were first published in Klopatek et al. (1979). This dataset provided potential area of Kuchler vegetation types. A set of Kuchler vegetation types provided by the Vegetation/Ecosystem Modeling and Analyis Project (VEMAP) program (http://www.cgd.ucar.edu/vemap/lists/kuchlerTypes.html) was used to select a set of grassland and shrubland vegetation types from Klopatek et al. (1979). While there are minor differences between the Kuchler naming conventions in the Klopatek et al. (1979) and VEMAP lists, the overall suite of vegetation classes matches quite well, and the resulting estimate is considered reasonable.

Data for recent changes in “non-federal grasslands and shrublands” are from the U.S. Department of Agriculture A Natural Resources Conservation Service National Resources Inventory (NRI) program. NRI uses the term “rangelands,” which is consistent with our definition of grasslands and shrublands, except that the NRI data used here do not include pasture or lands enrolled in the Conservation Reserve Program. Data from 1982, 1992, and 1997 are derived from the NRI Summary Report (revised December 2000), tables 5 and 8. See http://www.nrcs.usda.gov/technical/NRI/1997/national_results.html.

Data Limitations/Caveats: In the discussion section of the indicator text, an attempt was made to place bounds on the loss of grasslands and shrublands since the time of European settlement. This was done to give the reader a sense of the change; however, this estimate should be interpreted with caution. There are two caveats in particular. The satellite data used to estimate the acreage of pasture do not indicate whether or not the land is heavily managed (i.e., plowed and seeded). Depending upon the division of pastures between relatively heavily managed and relatively lightly managed (i.e., more natural in character), grasslands converted to pasture could represent a significant addition to the estimates of area converted. Also, a considerable amount of the land that is now classified as pasture is located in the East and was probably forest. Hence, to say that grasslands and shrublands declined 40 to 140 million acres since European settlement ignores the fact that more of the original grasslands and shrublands may have been lost but these losses were offset by gains in eastern pastures.

Data Access: Please see the information contained within the technical note for the national extent indicator.

References

Committee on Rangeland Classification. 1994. Rangeland health: New methods to classify, inventory and monitor rangelands. Washington, DC: National Academy Press.

Flemming, M.D. 1996. A statewide vegetation map of Alaska using a phenological classification of AVHRR data. 1996 Alaska Surveying and Mapping Conference, Anchorage, Alaska.

Klopatek, J.M., R.J. Olson, C.J. Emerson, and J.L. Joness. 1979. Land-use conflicts with natural vegetation in the United States. Environmental Conservation 6:191–199.

Mitchell, J.E. 2000. Rangeland resource trends in the United States: A technical document supporting the 2000 USDA Forest Service RPA Assessment. Gen. Tech. Rep. RMRS-GTR- 68. Rocky Mountain Research Station, Fort Collins, CO.