Interactions of Cattle Grazing and Climate
on Semi-arid Ecosystem Function

Duration: February 1999 - September 2003

Rangelands - lands that are dominated by herbaceous plants and shrubs - are a major type of land on all continents. Thirty-four percent of the U.S. is rangeland, with 57% of that land being owned by the U.S. government. Rangelands provide forage for native and domestic animals, wildlife habitat, watershed protection, wood products, and recreational opportunities. As U.S. populations continue to grow and become more urbanized, rangelands are increasingly valued for their scenery and open space.
Most rangelands in the western U.S. are water-limited ecosystems. Even small changes in precipitation timing can affect the biological components that maintain nutrient, water and energy movement within and through these ecosystems. In the Intermountain West (roughly, between the Rocky Mountains and the Sierra Nevadas), most precipitation comes in the winter. However, many climate models predict that summer precipitation will increase under global climate change. A shift towards summer precipitation could have major consequences for these ecosystems. It is important that land managers understand this potential shift and its likely consequences.

Human-induced impacts may either add to or counteract the effects of climatic shifts on rangelands. Many western rangelands are used for livestock grazing. Along with grazing comes trampling and the redistribution of soil nutrients. In the Intermountain West, many plants and soil organisms lack adaptations to cope with livestock grazing.

Severe grazing (i.e., inappropriate grazing seasons or excessive stocking rates) removes vegetation, reduces litter, and destroys biological crusts that protect the soil from wind and water erosion. The composition of the vegetation and the physical properties of the soil may also be altered. If the impacts are severe enough, critical thresholds of ecosystem function may be crossed, altering water availability, energy, and nutrient supplies and causing ecosystem degradation.

Our project will examine the combined impacts of precipitation timing and livestock use on rangelands in the Intermountain West. We will attempt to identify where thresholds occur between functioning and non-functioning systems along gradients of soils, climate (precipitation timing) and livestock grazing.

Design:

• Study sites will be located in grazed rangelands, along natural gradients in soil types
• Precipitation timing will be examined through the use of "rainout" shelters and field observations in sites with winter or summer dominated precipitation
• Distances from point sources of water will serve as a surrogate measure of livestock use

Measurements:

We will examine a wide range of indicators of rangeland health. Our measurements will include:

• physical and chemical characteristics of soils
• vegetation quantity and composition
• abundance of grasshoppers and other arthropods
• biological soil crust cover and composition
• soil microbial composition and biomass
• soil respiration rates
  
  
  

Modeling:

We believe that livestock grazing and climate changes will interact to produce changes in soil hydrology, nutrient cycling and nutrient composition. Using a computer model (CENTURY), we will predict changes in plant productivity, soil chemistry, and other ecosystem characteristics under specific climate and grazing conditions, for a period of 50-500 years.

Results from the field experiments will be combined with the CENTURY model output to produce science-based predictions of impacts of livestock grazing in the Intermountain West under altered precipitation regimes. The use of soils as a "common denominator" in this study will allow land managers and policy makers to use the results in prioritizing areas for conservation, rehabilitation and management actions in this region.

Products:

Expected products include annual technical reports and a final report documenting the findings of the study, scientific papers in peer-reviewed journals, model-based predictions of the sustainability of grazing under various grazing and climate scenarios, and gradient plot data.

Application of Results:

Results and model predictions will be conveyed to Department of the Interior (DOI) land managers in a series of workshops. Participants will help synthesize this information and produce a consensus report on potential management outcomes.

Collaborators:

Belnap, Jayne; Graham, Tim, B.; Sanford, Robert, L.; Svejcar, Tony; Ojima, Dennis, S.

	More information: Primary Contact: David A. Pyke, USGS Forest and Rangeland Ecosystem Science Center E-mail Dr. Pyke