Mapping Western Shrublands Pixel by Pixel: USGS Shrub/Grass Products

From a U.S. Geological Survey fact sheet, December 2017.

Shrublands in the western United States provide vital ecological, water, wildlife, farming and recreation services. But grazing, invasive species, conversion to farmland, climate change, urban growth and energy development are all changing them.

Shrubland change in northwest New Mexico: a sagebrush-dominated landscape (A) beside a degraded one with more herbaceous cover (B), about 1 kilometre apart. USGS.

Managing these ecosystems well means knowing where shrublands are, where and how much they have changed, and where they are likely to change next. Remote sensing is the most practical source of data for monitoring disturbance across large areas, both now and in the past, and a monitoring framework built on it can analyse change efficiently and accurately at many scales of space and time.

Nine map products

The USGS has developed nine component maps that give, for every 30-metre grid cell, in 1-percent steps:

• the percent cover of shrub, sagebrush, big sagebrush, herbaceous plants, annual herbaceous plants, litter and bare ground;
• shrub height and sagebrush height.

The components are meant to be combined and customised for many uses: rangeland monitoring, wildlife habitat analysis, resource inventories, adaptive management and environmental review.

A landscape view

The products let managers and scientists see current and past vegetation cover across whole landscapes, track change among shrubs and grasses, and think beyond one parcel and one decision at a time — while keeping most of the local detail needed for day-to-day management.

The USGS Shrub/Grass Mapping Project makes them in collaboration with the Bureau of Land Management, with help from the Multi-Resolution Land Characteristics Consortium, a group of federal agencies that produce consistent national land-cover data. When complete, they will cover most shrubland regions of the West — the most comprehensive remote-sensing measure of western shrublands to date.

USGS shrubland mapping regions, with 370 high-resolution field sampling sites and 196 validation clusters of about 10 points each. USGS.

How the maps are made

• High-resolution imagery: WorldView satellites were tasked to photograph sample sites at 2-metre resolution when the plants were at the best stage for mapping, while field crews measured the same sites at about the same time. Those measurements trained regression trees that produced 2-metre maps of each component.
• Scaling up: the 2-metre maps were combined with multiple 30-metre Landsat images and other data, again with regression-tree software, to predict each component continuously across the landscape.
• Checking accuracy three ways: against independent field measurements, by cross-validating the regression-tree models, and with spatial error maps delivered with the products that show accuracy place by place.

Breaking the landscape into fractional components also makes it possible to measure both sudden change, from fire and human disturbance, and gradual change, from grazing, invasive species and climate change.

Looking back to 1984

Once a component map exists for the current year, maps for earlier years can be made from the Landsat archive, which goes back to 1984. The USGS developed a change vector analysis that searches historical images for any spectral change from the present year. Where there has been change, the unchanged areas from the present year provide training data to update the predictions — so historical images can be classified using present-day training data. The results have proved good at detecting gradual change such as that caused by climate and grazing.

The approach is now running across large regions of the West. When complete, the record of each component from 1984 through 2017 will show how much these landscapes have changed, and when and where.

The story of every pixel

The long-term goal is to tell the story of every 30-metre pixel of western shrubland through time:

• What was the pixel in 1984?
• What is it now?
• If it changed, what changed it?
• How is it likely to change in the future?

Work is under way to compare the historical changes with climate data, wildfires, grazing records and other management data, to find the causes and project where and when similar changes may happen. One analysis links shrub cover to precipitation: with projected declining precipitation, shrub cover will likely thin.

As new satellite images arrive, the landscapes can be checked for change every year, building understanding of when, where and how much change is happening, and supporting better science and management.

Pronghorns eating Wyoming big sagebrush, with the Wind River Range behind. Tom Koerner, U.S. Fish and Wildlife Service.

Sources

• U.S. Geological Survey, U.S. Geological Survey Shrub/Grass Products Provide New Approach to Shrubland Monitoring, Fact Sheet 2017–3084, December 2017, Earth Resources Observation and Science (EROS) Center. https://doi.org/10.3133/fs20173084 — three pictures recovered from the PDF.
• Rewritten in hubnx's own words.