Using Remote Sensing to Monitor Changes in Vegetation Cover
Photo of sagebrush and distant buttes.

Vegetation monitoring site northeast of Cedar Mountain, Sweetwater County, Wyoming. 

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Monitoring Vegetation Change from Space

Disturbances such as livestock grazing, exotic species invasion, conversion to agriculture, urban expansion, energy development, and other development have altered and reduced sagebrush ecosystems by half since European settlement. It is crucial to understand the distribution of and variability in sagebrush habitat in order to understand current and future conditions in sagebrush systems. Habitat components, such as vegetation cover and shrub height, can provide a baseline for monitoring long-term changes in habitat conditions. Understanding the relations between habitat conditions and drivers of change, such as development and climate, is critical for understanding current and future distribution and characteristics of sagebrush habitats, as well as the locations and rates of change. However, measuring change on the ground can be expensive, time-consuming and labor-intensive.

USGS scientists have developed an affordable, repeatable set of protocols which serve as the cornerstone of sagebrush habitat monitoring in the WLCI region. Our approach entails combining field sampling data and remote sensing (satellite data) for estimating the percent cover of sagebrush, plant litter, and bare ground across the entire WLCI landscape. This work extends beyond traditional category-based cover-type mapping, focusing instead on making continuous cover predictions for sagebrush habitat characteristics at multiple spatial scales across the WLCI landscape. With repeated predictions over time, the USGS can evaluate and quantify the amount and distribution of change in sagebrush habitat features. This information is critical for understanding current and future distribution. Additionally, it provides input to a broad spectrum of WLCI research and applications, including projects associated with quantifying and monitoring sage grouse, pronghorn, and pygmy rabbit habitats.

 

Key Findings

  • We demonstrated the ability of high-spatial-resolution satellite imagery to serve as a potential surrogate for repeated ground measurement.
  • Bare ground was the most accurate component mapped, Wyoming sagebrush the least.
  • Bare ground decreased by total area of almost 53 km² from 2006 to 2010, herbaceous cover decreased by 63 km², big sagebrush decreased by approximately 11 km², and Wyoming big sagebrush decreased by approximately 19 km².
  • Litter increased by almost 64 km²,  shrubland (including many species other than sagebrush) increased by approximately 52 km² and all other species of sagebrush increased by approximately 9 km² from 2006 to 2010.
  • From 2006–2010 approximately 6% of change in sagebrush habitat was due to fire, 4% was due to human disturbance, and 90% was due to other effects such as climate.
  • Total shrub and sagebrush canopy taken between 2008-2016 showed an increasing trend with 53% of the shrub transects and 72% of the sagebrush transects showing a significant change. Conversely, bare ground was trending downward over this same period, with 28% of transects showing a significant change.
  • We demonstrated the ability of component predictions to potentially monitor vegetation change related to precipitation variation over time.

Study Objectives

The focus of this work is to use remote-sensing tools and protocols for monitoring long-term changes in vegetation cover across the WLCI region.

  • Evaluate and estimate sagebrush habitat change between 1988 and 2006.
  • Develop multivariate, statistical models to predict sagebrush cover and height across the sagebrush steppe in Wyoming.
  • Analyze long-term trends in components of sagebrush habitat in Southwest Wyoming
Schematic of scaling from field vegetation sampling to satellite imagery.

We developed field collection and training protocols for vegetation sampling to extrapolate landscape models from 1 m quadrat field measurements to satellite imagery at multiple scales. Click to enlarge.

What Is Remotely Sensed Data and What Can Be Learned From It?

Remote sensing is the acquisition of information about an object from a distance. Often the term is used to describe the gathering of data using satellite- or aircraft-based sensors to detect, classify, and characterize objects on Earth. Remote sensing is often identified as a key information source to facilitate ecosystem-wide characterization, monitoring, and analysis; however, there is a lack of approaches that characterize sagebrush with sufficient and accurate local detail across large enough areas to use this approach to its fullest potential. We developed a new remote sensing sagebrush characterization approach using three scales to characterize four primary continuous field components (percent bare ground, percent herbaceous cover, percent litter, and percent shrub), and four secondary components (percent sagebrush [Artemisia spp.], percent big sagebrush [Artemisia tridentata], percent Wyoming sagebrush [Artemisia tridentata Wyomingensis], and shrub height.

To understand patterns of change within sagebrush habitats across the entire WLCI region, including historical changes we need to monitor long-term changes in vegetation cover with satellites. Satellite imagery allows evaluation and quantification of the amount and distribution of changes in vegetation cover including shrubs, sagebrush, herbaceous vegetation, litter, and bare ground over large areas. The entire WLCI area was analyzed for vegetative change using satellite remote sensing between two time periods, 2006 and 2010. In addition, we monitor and analyze WLCI vegetation change with ground measurement. Long-term vegetation-monitoring plots across 260 marked transects in two intensive study sites on the ground are measured annually, along with 2-m and 30-m satellite data. These transects, measured every year since 2008, offering both valuable monitoring insight and a means to understand and validate patterns of annual ground level change compared to the remotely-sensed protocols.

Bar graph showing total cover change for five sagebrush habitat components for 3 time periods from 1988 to 2006.
Map of spatial distribution of the change in bare ground from 2006 to 2010 in the WLCI region.
Photo showing sagebrush habitat in the snow.

Total coverage changes for each of five sagebrush habitat components from the periods 1988–1996, 1996–2006 and 1988–2006. Click to enlarge.

The spatial distribution of change in bare ground between 2006 and 2010 in the WLCI region. Red represents areas where bare ground has increased, and blue represents areas where bare ground has decreased. Click to enlarge.