Crested Wheatgrass (Agropyron cristatum) Rates of Spread into Native Sagebrush Steppe in Eastern Idaho

Loss of sagebrush steppe rangeland has had a large impact on sagebrush obligate wildlife. A number of factors have been associated with the decline in sagebrush steppe including conversion to cropland, urban development, invasive species and conversion to other vegetation community types. Following a 2002 fire on the INEEL, conversion of a rangeland with a sagebrush canopy to a crested wheatgrass dominated grassland was observed. Land cover change of this sort could have important impacts for management of sagebrush-obligate wildlife. This prompted questions about the ecology of crested wheatgrass in the upper Snake Rive Plain and the potential risks to remaining sagebrush steppe caused by the spread of crested wheatgrass. First among those questions were:

  • Can crested wheatgrass in range improvement and other plantings invade into nearby, good condition sagebrush steppe?
  • If so, how fast does it spread?


The objectives of this study were to assess the spread of crested wheatgrass from plantings into sagebrush steppe in the upper Snake River Plain. Specific goals included:

  • Developing a GIS layer of historic crested wheatgrass plantings on the INEEL.
  • Mapping the present extent of certain crested wheatgrass communities.
  • Estimating the rate of spread into adjacent good condition sagebrush steppe.


During 2003, two sites were selected for study. One site was along Lincoln Boulevard and one at Tractor Flats. The Lincoln Boulevard crested wheatgrass planting was conducted to revegetate roadside and ditches for this paved roadway. The road was originally built in 1952, but was gravel and not paved. The road was upgraded in the early 1970s (exact date is uncertain) and again in 1991. Aerial photographs from 1976 showed that the road had been paved and that the vegetation immediately adjacent to the roadside (primarily the ditches) was different from that further away from the road. Archival photographs of this section of road in 1981 clearly show that this different vegetation is crested wheatgrass. This suggests that crested wheatgrass was planted sometime before 1976. The roadsides were again planted with crested wheatgrass after road upgrades in 1991. The native vegetation type in this area is primarily Wyoming big sagebrush steppe. This area has not been grazed by livestock since the 1940s.

Tractor Flats was planted to crested wheatgrass in 1955 to revegetate an area infested with halogeton (Halogeton glomeratus). This crested wheatgrass community was mapped in 1965 from aerial photographs as part of a vegetation community mapping project. Native vegetation of the area is Wyoming big sagebrush steppe. This area is part of the U.S. Bureau of Land Management (BLM) Twin Buttes Allotment and is grazed by sheep in spring.

The boundaries of the crested wheatgrass invasion were mapped with global positioning system (GPS) receivers. Receivers used were Trimble ProXL and GeoIII. The GPS receivers collected data at a rate of one point every three seconds. All the data was differentially ( 1-5 m [ 3.3-16.4 ft]) corrected using Pathfinder Office. The corrected files were then exported to Arc shapefiles, converted into coverages, and edited to fix overlapping boundaries. Different approaches for mapping the extent of spread of crested wheatgrass were used at each of the two study locations, Lincoln Boulevard and Tractor Flats.

At the Lincoln Boulevard site, a GIS coverage of the extent of spread was created by walking with a GPS as described above on a path following the furthest crested wheatgrass plants from Lincoln Boulevard. This was done on both the east and west sides of the road. To calculate how far the crested wheatgrass had spread, a line was digitized over Lincoln Boulevard on an existing GIS coverage for roads. Then, the crested wheatgrass boundary coverage was converted to points using ARCPOINT and the NEAR command was used to measure the distance from each of those points to the Lincoln Boulevard line.

At the Tractor Flats site, a GIS coverage of the extent of spread of crested wheatgrass was created by using GPS to map the extent of spread in areas near existing roads and at some remote areas. In four areas, sections of the boundary were mapped. Using the GPS data and a SPOT 10 m (32.8 ft) image as a guide, ArcEdit was used to create a polygon to estimate the total area now inhabited by crested wheatgrass. NODESNAP was used at 20 m (65.6 ft) to add lines to connect the GPS measurements. GENERALIZE was then used to smooth out the GPS lines as they had small loops and a very irregular texture. The Tractor Flat polygon created for estimating the spread of crested wheatgrass is an estimate based on the actual boundary lines mapped.


At the Lincoln Boulevard site, the mean distance from the road centerline to the farthest crested wheatgrass individual was 447.0 m (1467 ft) with a maximum distance of 818.6 m (2686 ft). On average, more than 50 percent of the crested wheatgrass points have spread 300 to 500 m (984 to 1640 ft) from Lincoln Boulevard. The distribution varies from one side of the road to the other. On the west side, more than 70 percent of the points are in the 300 to 500 m (984 to 1640 ft) range. The majority of points on the east fall between 400 and 600 m (1312 to 1969 ft) from Lincoln Boulevard with approximately 13 percent in the 600 to 700 m (1969 to 2297 ft) range. On the east, more of the points have spread farther from Lincoln Boulevard. If the roadside was first planted with crested wheatgrass in 1976, based on aerial photos taken that clearly illustrated the presence of a vegetation boundary on either side of the road, the rate of spread was 16.5 m/yr (54 ft/yr ).

At Tractor Flats, crested wheatgrass increased its coverage from 692.5 ha (1710.0 acres) in 1965 to 1708.7 ha (4222.3 acres) in 2003. This translates to a spreading rate of 26.7 ha/yr
(66.0 acres/yr) or about 18.7 m/yr (61 ft/yr).

On the upper Snake River Plain, crested wheatgrass does invade beyond the area planted into otherwise good condition sagebrush steppe. Rate of spread by crested wheatgrass in an area spring-grazed by sheep was similar to that in an area not grazed by livestock.


Investigators and Affiliations

Valerie Sheedy, Graduate Student, Idaho State University Department of Geosciences, Pocatello, ID

Keith T. Weber, GIS Director, Idaho State University-GIS Training and Research Center, Pocatello, ID

Nancy F. Glenn, Assistant Research Professor, Department of Geosciences, Idaho State University, Boise, ID

Roger D. Blew, Ecologist, Environmental Surveillance, Education and Research Program, S.M. Stoller Corporation, Idaho Falls, ID

Michael Jackson, Idaho National Engineering and Environmental Laboratory, Idaho Falls, ID

Funding Sources
INEEL Student Outreach and Education in Remote Sensing from Bechtel BWXT Idaho, LLC
Idaho State University Office of Research and the GIS Center


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