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Crested
Wheatgrass (Agropyron cristatum) Rates of Spread into
Native Sagebrush Steppe in Eastern Idaho
Background
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?
Objectives
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.
Accomplishments
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.
Results
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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