Ground Cover

Dynamics in a

Sagebrush Steppe

Community

by Bruce P. Van Haveren,

Ph.D., Senior Scientist 

BLM, National Science and

Technology Center

Background

The Rocky Mountain Forest and

Range Experiment Station, USDA

Forest Service, operated the Stratton

Sagebrush Study Area in southcentral Wyoming from 1967 to 1991 in

cooperation with BLM. Past research

at Stratton produced abundant new

knowledge about sagebrush steppe

ecology and hydrology. Of particular interest is a set of ground cover

data collected from 1968 to 1981

(Sturges, 1986). Long-term data sets

such as this one are rare and can be

very valuable tools for land managers dealing with current resource

management issues. For example,

long-term ground cover data may

reveal important information about

the variability of different cover

parameters under relatively constant land-use situations.

The Stratton site is located about

30km west of Saratoga, Wyoming at

an elevation of 2340 to 2370m. The

area was grazed lightly to moderately

by domestic sheep, antelope, and

mule deer during its tenure as a

study area. Average annual precipitation was 52.6cm, three-quarters of

which was snow. The area is best

described as late seral sagebrush

steppe with mountain big sagebrush

on deeper soils, Wyoming big sagebrush and black sage on shallower

soils, and Idaho fescue, bluegrass,

needlegrass, and sedge as understory

species. Cushion plants are common

on exposed ridgetops and summits.

The average shrub canopy cover was

16 percent.

Discussion

Ground cover data were collected

triennially, beginning in 1968 and

ending in 1981, on the Loco Creek

Watershed at Stratton (Sturges,

1986). Loco Creek was used as a

control watershed for sagebrush

eradication experiments. The

ground cover data are portrayed in

Figures 1 and 2. The present analysis

is based on primary samples. Each

primary sample consisted of 500

individual line-point observations

read along five 100-ft transects,

100 observations per transect. The

data were collected from midAugust to late September of each

sampled year.

The amount of bare ground present on a site is a reliable predictor

of site stability (Packer, 1951).

Although bare ground appeared to

decline gradually between 1968

and 1981 on the Loco Creek

Watershed, there was no statistically significant (p=0.95) difference

between the two years. Similarly,

no statistically-significant differences were found for the remainder

of the ground cover components.

In spite of a snowmold fungus that

killed sagebrush on the Loco Creek

watershed (Sturges, 1986), there

was no significant difference in

sagebrush canopy cover between

1968 and 1981.

Figure 1 shows the correlation

coefficients among ground cover

components and sagebrush

canopy cover. The highly significant inverse correlation between

bare ground and litter (r=-0.83)

indicates that there are direct tradeoffs between litter and bare ground

on this site. The same, but to a

lesser extent, may be said about the

relationship between bare ground

and grass/sedge cover (r=-0.53).

The relationship between sagebrush

canopy cover and bare ground

appeared to change between 1968

and 1981 at Stratton. The slopes of

the linear regression relationships

between these two variables were

nearly identical but the intercept

(amount of bare ground when sagebrush canopy cover is 0) decreased

from 47 percent (1968) to 34 percent (1981).

RESOURCE NOTES

NO. 19 DATE 06/26/00

Range

Range

Figure 1. Average ground cover (%) data from Loco Creek Watershed.
Range

Conclusion

Hydrologic cover 1 provides a good

index of site stability. It is inversely

related to runoff potential, in that a

high hydrologic cover value would

indicate low runoff potential.

According to Packer (1951), reduced

cover (vegetation and litter) and

large bare openings are indicative of

low site stability and high erosion

potential. Hydrologic cover was

quite stable over the 14 years of

observations, ranging from 58 to 62

percent, and exhibiting a coefficient

of variation of 34 percent. By

comparison, bare ground ranged

from 21 to 28 percent, with a

coefficient of variation of 43 percent. A strong inverse relationship

(r = -0.82) existed between bare

ground and hydrologic cover.

Litter was the largest component of

hydrologic cover, contributing an

average of 82 percent. The contribution of litter to hydrologic cover

increased from 74 percent in 1968

to nearly 90 percent in 1978 and

1981. However, this increase was

not statistically significant. There is

a highly significant correlation

between litter and sagebrush aerial

cover. The litter derived from sagebrush canopies tends to be larger,

does not oxidize as rapidly as litter

from grasses, sedges, and forbs, and

thus persists longer in the harsh

environment found at the Stratton

site. A linear regression analysis of

the relationship between litter and

sagebrush aerial cover showed that

the relationship did not change

over the measurement period.

Ground cover at Stratton was

remarkably stable over the 14-year

measurement period, in spite of a

significant drought event during

1976-77. Hydrologic cover exhibited a low variability during the measurement period and should be

evaluated further as a potential

indicator of landscape stability and

ecological condition.

Contact

Bruce P. Van Haveren, Ph.D.,

Senior Scientist

BLM, NSTC, RS-140

Building 50, Denver Federal Center

P.O. Box 25047

Denver, CO 80225-0047

phone (303) 236-0161

fax (303) 236-6450

email bvanhave@blm.gov

Literature Cited

Packer, Paul E. 1951. An approach

to watershed protection criteria.

Journal of Forestry 49(9):639-644.

Sturges, David L. 1986. Responses

of vegetation and ground cover to

spraying a high elevation, big sagebrush watershed with 2,4-D.

Journal of Range Management

39(2):141-146.

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Range

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Figure 2. Aerial cover (%) data from Loco Creek Watershed.

1

Hydrologic cover is defined here as the sum of litter, grass/sedge, forb, cushion plant, and shrub components of ground cover