Landscape

Stability Indicators

for Sagebrush

Steppe Ecosystems

by Bruce P. Van Haveren, Ph.D.,

Senior Scientist, National Science

and Technology Center

Background

Sagebrush steppe ecosystems are found

primarily within the Intermountain

Semidesert Province (Bailey, 1995).

It is nearly impossible to find examples of pristine sagebrush steppe.

Most of these areas are small relict

areas, topographically isolated from

human disturbance or protected within

special-use areas. West (2000) suggests that none of the original 

pre-Euroamerican sagebrush steppe

remains. Only about one percent of the

sagebrush steppe is in late seral stage,

consisting of sagebrush-grass communities dominated by sagebrush

with a healthy understory of grasses

and forbs (Laycock 1991; West

2000). Another 25 to 30 percent of

the original sagebrush steppe is in a

stagnant late seral stage, close to a

threshold of site change. This stage is

characterized by relatively high densities of sagebrush (canopy cover >25%),

large areas of bare soil in the shrub

interspaces, and decreased grass and

forb composition. West (2000) claims

that most of the land area of the

sagebrush steppe has experienced

threshold-exceeding changes. Once a

threshold is crossed, it becomes very

difficult and expensive to move a sagebrush steppe community back across

the threshold to a late seral stage.

Indicators are needed that will help

land managers identify these thresholds.

Discussion

An ecological indicator is a characteristic of the environment, both

abiotic and biotic, that may provide

quantitative information on the status

of ecological systems. An indicator may

be a single environmental variable or

an aggregation of variables expressed

as an index. The indicator concept is

related to the concept of landscape

sensitivity, the capacity of a landscape

to resist or absorb impulses of change.

Abiotic system stability, as applied to

landscapes, is an example of a parameter that can be used to help quantify

the ecological condition of wildlands. A recent National Science and

Technology Center (NSTC) project

identified abiotic indicators of upland

landscapes in the Intermountain

Semidesert Province, with an initial

focus on sagebrush steppe ecosystems.

Success in managing and restoring

upland components of landscapes

hinges on the ability to read and react

to the ecological clues, both biotic and

abiotic, that such landscapes reveal.

Ecologically responsible land managers seek early warning signals of

change on the landscape. They want

to identify land-use thresholds or endpoints of ecological condition, they

want to know where a watershed or

ecosystem lies with respect to those

endpoints, and they wish to learn

whether the ecological trend is moving

towards or away from the endpoints.

They need to know when a landscape,

such as a headwater basin, is stable or

close to a threshold of change or

whether it is moving in a downward

or upward ecological trend. An

example of such an indicator for the

sagebrush steppe is hydrologic cover.

Hydrologic cover is defined as the sum

of live vegetal cover and litter cover

(Sturges, 1986). It is very closely related, in an inverse manner, to bare

ground (Van Haveren, 2000). It is also

inversely related to runoff potential, in

that a high hydrologic cover value

would indicate low runoff potential.

Based on numerous studies on a variety

of plant communities around the western United States, we know that hydrologic cover must be at least 70 percent

to reduce runoff from rainfall and to

protect soils from erosion. It turns out

that hydrologic cover is an excellent

indicator of upland watershed stability

for sagebrush steppe and is very easy to

measure. A review of ground cover

studies in sagebrush steppe revealed

that litter consistently is the dominant

cover component (Van Haveren, in

press). This suggests that land managers

should pay particular attention to the

litter balance on sagebrush steppe

rangelands.

Steep headwater basins in the sagebrush

steppe are often close to a threshold of

geomorphic instability. The presence of

gullies in these basins is an indication

that the threshold was crossed. The

threshold of instability appears to be a

function of valley slope, drainage area,

and valley width (Patton and Schumm,

1975; Elliott, 1989).

My observations of headwater areas in

sagebrush steppe suggest there are two

principal valley-floor types that illustrate the importance of valley width

to geomorphic instability. Type I valleys are V-shaped, as shown in Figure 1

NO. 44 DATE 04/18/01

Range

Range

North Draw

Distance, feet

Elevation, feet

200 400 600 800 1000 1200 1400

7780

0

7800

7820

7860

7880

7900

7920

7940

7960

7840

Figure 1. Valley profile for North Draw, North Fork of Savery Creek, southcentral Wyoming
for a headwater basin in southcentral

Wyoming. Valley floors are very narrow and hydrologically connected to

the adjacent hillslopes. Type II valleys,

illustrated in Figure 2, are characterized

by an inflected-U shape and often have

wide, swale-like bottoms. Because of

the slope breaks, the valley floors are

hydrologically disconnected from

the hillslopes. Type I valleys tend to

be more unstable than Type II valleys. This classification is similar to

that proposed for alluvial valleys by

Bisson and Montgomery (1996),

who differentiated between confined

and unconfined valleys.

Conclusion

Landscape analysis and classification

hold considerable promise as tools for

land managers concerned with the

integrity and health of ecosystems. It

may be possible, within a landscape

classification system, to identify landform units that are susceptible to ecological change, or close to a threshold

of change, and to combine this information with indicators of ecological

condition.

Literature Cited

Bailey, Robert G. 1995. Description

of the ecoregions of the United States.

2nd edn. Miscellaneous Publication

1391. Washington DC: USDA Forest

Service. 108p.

Bisson, Peter A. and Montgomery,

David R. 1996. Valley segments,

stream reaches, and channel units. Pp.

23-51 in Methods in Stream Ecology,

Hauer, F. Richard and Lamberti, Gary

A. (eds). Chapter 2. 1996. London:

Academic Press.

Elliott, John G. 1989. A geomorphically determined valley-erosion

threshold for reclaimed surfacemined drainage basins, northwestern Colorado. Pp. 419-429 in

Proceedings of the Symposium on

Headwaters Hydrology, Woessner,

William W. and Potts, Donald F.

(eds). Bethesda, MD: American

Water Resources Association. 708p.

Laycock, W. A. 1991. Stable states

and thresholds of range condition on

North American rangelands: a viewpoint. Journal of Range Management

44(5):427-433.

Patton, Peter C. and Schumm,

Stanley A. 1975. Gully erosion,

northwestern Colorado: a threshold

phenomenon. Geology 3:88-90.

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.

Van Haveren, Bruce P. 2000.

Ground cover dynamics in a sagebrush steppe community. Resource

Notes 19:1-2.

Van Haveren, Bruce P. In press.

Characteristics of litter cover at a high

elevation sagebrush steppe site.

Submitted to the Journal of Soil and

Water Conservation.

West, Neil E. 2000. Synecology and

disturbance regimes of sagebrush

steppe ecosystems. Pp. 15-26 in

Proceedings: Sagebrush Steppe

Ecosystems Symposium, Entwistle,

P. G.; DeBolt, A. M.; Kaltenecker, J.

H. and Steenhof, K. (comp.).

Publication No. BLM/ID/PT001001+1150. Boise, Idaho: Bureau

of Land Management. Idaho State

Office. 145p.

Contact

Bruce P. Van Haveren, Ph.D.,

Science and Research Advisor, BLM

NSTC, ST-131

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

Range

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Middle Draw

Distance, feet

Elevation, feet

200 400 600 800 1000 1200 1400

7780

0

7800

7820

7860

7880

7900

7920

7940

7960

7840

Figure 2. Valley profile for Middle Draw, North Fork of Savery Creek, southcentral Wyoming