Impacts of Exotic Annual Grass Invasion, Wildfire, and Restoration on Carbon Storage in the Sagebrush Steppe
USGS is investigating the impact of the annual grass-fire cycle-- and restoration land treatments aimed at slowing that cycle-- on carbon storage in dryland soils.
Threats to Sagebrush Ecosystems
Sagebrush steppe landscapes cover a vast portion of the Great Basin. In their healthy, undisturbed state these landscapes consist of sagebrush shrubs and diverse communities of perennial plants. Sagebrush is evergreen, and many other perennial plants like bunchgrasses have long growing seasons, which means plant communities are photosynthesizing and removing carbon from the atmosphere nearly year-round.
Exotic annual grass invasions threaten the function of drylands in the western U.S. and around the world. Invasive grasses like cheatgrass are quick to colonize post-wildfire, resulting in landscapes consisting of a single species rather than diverse communities. Cheatgrass has a short growing season and leaves behind mats of highly flammable dead grass—which has led to a cycle of more frequent and larger fires.
Carbon is lost from sagebrush steppe ecosystems when deep rooted, evergreen perennial plants are replaced by shallow rooted annuals that don’t capture or store as much carbon. Wildfires are also expected to degrade soil carbon stocks by releasing carbon from the soil into the atmosphere and by promoting erosion.
Restoration efforts are underway across the West, aimed at protecting habitat for wildlife, conserving carbon storage, and keeping greenhouse gases out of the atmosphere. These projects include post-fire restoration and seeding of native plants, treatment with herbicides to prevent or control exotic annual grass invasion, and fuels management to reduce fire risk. More information is needed on the relationship between soil carbon, exotic annual grasses, and wildfire to inform restoration planning.
How do Disturbances Impact Carbon Storage in Rangelands?
Our team is collecting the data needed to confirm the impacts of exotic annual grasses and wildfire on soil carbon. We’re also investigating when, where, and how restoration actions such as herbicides or perennial plant seedings affect soil carbon and the drought resilience of soils. This work addresses a key concern of land managers, the public, and national policy makers by helping identify restoration treatments that are effective in combating exotic annual grasses and the wildfires and other problems they create.
Better Methods, More Reliable Results
We designed and implemented a sampling strategy that accurately measures the amount of carbon in soil and can detect changes in soil carbon across variable landscapes.
Phase one of this project is focused on understanding the relationship of exotic annual grasses and wildfire on soil carbon at a limited number of sites.
Our sites were carefully selected. They include areas that are:
Unburned and Not Invaded
Burned and Not Invaded
Unburned and Invaded
Burned and Invaded
Sites were chosen using a combination of satellite imagery, conversations with land managers, and in-person site visits. We verified that the sites were similar in terms of all other variables, such as elevation, topography, and land use. This attention to detail ensures our results are comparable across sites and that soil carbon can be directly related to fire and/or invasion.
Within each sampling site, soil samples were collected from multiple locations to account for variability between microsites. Microsites had different plant communities, they include:
- Bare soil
- Sagebrush
- Perennial bunchgrasses
- Exotic annual grasses
We hypothesized that landscapes with diverse, native plant communities would have the largest carbon stocks. Highly disturbed areas with bare soil are predicted to have the smallest carbon stocks.
This study is unique, in that it accounts for carbon in shallow soil as well as up to one meter deep. Past studies of soil carbon relied on commonly sampled shallow soils without adequate accounting for the microsite variation that strongly affects carbon cycling in this system.
We also measure soil density and soil water infiltration, two soil characteristics closely tied to carbon content. Accurate soil density measurements are required for scaling soil carbon from the plot to the landscape. Soil water infiltration is directly related to soil carbon. Soil is a reservoir for water, and if water can't infiltrate it won't be available for plants growing in the hot summer season. Rather, it will run-off into streams or pool in low-points on the landscape.
Extracting a Bulk Density Soil Sample
Measuring Soil Water Infiltration
Phase One Results
The results from phase one of this study found that invasion and wildfire reduced soil carbon by approximately 50%. This is the first time that invasion has been definitively linked to soil carbon loss. Across the entire Great Basin, the losses we observed could amount to 17-20 trillion grams of carbon loss annually.
We found that carbon losses happen much deeper than where annual grasses are rooted. Disturbances affected deep soils just as strongly as shallow soils.
These results suggest that previous studies that only measured carbon in shallow soils likely underestimated both the carbon storage capacity of drylands and carbon losses that result from invasion and fire.
Phase one of this study revealed that carbon stored in dryland soils is less secure than previously thought. Both invasive grasses and wildfire can deplete soil carbon that was likely built up over thousands of years relatively quickly.
Implications for Land Management
Exotic grass invasions are a problem worldwide. Invasions have led to increased wildfire risk in Australia, Brazil, and Hawaii as well as in the Great Basin. This study suggests that maintaining intact sagebrush-steppe by protecting against the annual grass-fire cycle could be an effective “nature-based” carbon storage strategy. Soil carbon is central to all ecosystem functions. Preserving intact landscapes composed of diverse, native, perennial plants will also improve water retention, soil fertility, grazing opportunities, and wildlife habitat.
Future Directions
In phase two of this study, we will add more sites across a broader range of landscapes that vary more widely in their climate and soil types. We will also look at the impacts of common restoration practices—such as herbicides and seeding—on soil carbon.
Visit the web pages below to learn more about our research.
Assessing the Impacts of Rangeland Restoration on Carbon Sequestration and Co-Benefits for Drought Resilience in the Sagebrush Steppe and Mixed Grass Prairie
Cheatgrass and Medusahead
Plant-Soil-Environment Laboratory (FRESC)
USGS is investigating the impact of the annual grass-fire cycle-- and restoration land treatments aimed at slowing that cycle-- on carbon storage in dryland soils.
Threats to Sagebrush Ecosystems
Sagebrush steppe landscapes cover a vast portion of the Great Basin. In their healthy, undisturbed state these landscapes consist of sagebrush shrubs and diverse communities of perennial plants. Sagebrush is evergreen, and many other perennial plants like bunchgrasses have long growing seasons, which means plant communities are photosynthesizing and removing carbon from the atmosphere nearly year-round.
Exotic annual grass invasions threaten the function of drylands in the western U.S. and around the world. Invasive grasses like cheatgrass are quick to colonize post-wildfire, resulting in landscapes consisting of a single species rather than diverse communities. Cheatgrass has a short growing season and leaves behind mats of highly flammable dead grass—which has led to a cycle of more frequent and larger fires.
Carbon is lost from sagebrush steppe ecosystems when deep rooted, evergreen perennial plants are replaced by shallow rooted annuals that don’t capture or store as much carbon. Wildfires are also expected to degrade soil carbon stocks by releasing carbon from the soil into the atmosphere and by promoting erosion.
Restoration efforts are underway across the West, aimed at protecting habitat for wildlife, conserving carbon storage, and keeping greenhouse gases out of the atmosphere. These projects include post-fire restoration and seeding of native plants, treatment with herbicides to prevent or control exotic annual grass invasion, and fuels management to reduce fire risk. More information is needed on the relationship between soil carbon, exotic annual grasses, and wildfire to inform restoration planning.
How do Disturbances Impact Carbon Storage in Rangelands?
Our team is collecting the data needed to confirm the impacts of exotic annual grasses and wildfire on soil carbon. We’re also investigating when, where, and how restoration actions such as herbicides or perennial plant seedings affect soil carbon and the drought resilience of soils. This work addresses a key concern of land managers, the public, and national policy makers by helping identify restoration treatments that are effective in combating exotic annual grasses and the wildfires and other problems they create.
Better Methods, More Reliable Results
We designed and implemented a sampling strategy that accurately measures the amount of carbon in soil and can detect changes in soil carbon across variable landscapes.
Phase one of this project is focused on understanding the relationship of exotic annual grasses and wildfire on soil carbon at a limited number of sites.
Our sites were carefully selected. They include areas that are:
Unburned and Not Invaded
Burned and Not Invaded
Unburned and Invaded
Burned and Invaded
Sites were chosen using a combination of satellite imagery, conversations with land managers, and in-person site visits. We verified that the sites were similar in terms of all other variables, such as elevation, topography, and land use. This attention to detail ensures our results are comparable across sites and that soil carbon can be directly related to fire and/or invasion.
Within each sampling site, soil samples were collected from multiple locations to account for variability between microsites. Microsites had different plant communities, they include:
- Bare soil
- Sagebrush
- Perennial bunchgrasses
- Exotic annual grasses
We hypothesized that landscapes with diverse, native plant communities would have the largest carbon stocks. Highly disturbed areas with bare soil are predicted to have the smallest carbon stocks.
This study is unique, in that it accounts for carbon in shallow soil as well as up to one meter deep. Past studies of soil carbon relied on commonly sampled shallow soils without adequate accounting for the microsite variation that strongly affects carbon cycling in this system.
We also measure soil density and soil water infiltration, two soil characteristics closely tied to carbon content. Accurate soil density measurements are required for scaling soil carbon from the plot to the landscape. Soil water infiltration is directly related to soil carbon. Soil is a reservoir for water, and if water can't infiltrate it won't be available for plants growing in the hot summer season. Rather, it will run-off into streams or pool in low-points on the landscape.
Extracting a Bulk Density Soil Sample
Measuring Soil Water Infiltration
Phase One Results
The results from phase one of this study found that invasion and wildfire reduced soil carbon by approximately 50%. This is the first time that invasion has been definitively linked to soil carbon loss. Across the entire Great Basin, the losses we observed could amount to 17-20 trillion grams of carbon loss annually.
We found that carbon losses happen much deeper than where annual grasses are rooted. Disturbances affected deep soils just as strongly as shallow soils.
These results suggest that previous studies that only measured carbon in shallow soils likely underestimated both the carbon storage capacity of drylands and carbon losses that result from invasion and fire.
Phase one of this study revealed that carbon stored in dryland soils is less secure than previously thought. Both invasive grasses and wildfire can deplete soil carbon that was likely built up over thousands of years relatively quickly.
Implications for Land Management
Exotic grass invasions are a problem worldwide. Invasions have led to increased wildfire risk in Australia, Brazil, and Hawaii as well as in the Great Basin. This study suggests that maintaining intact sagebrush-steppe by protecting against the annual grass-fire cycle could be an effective “nature-based” carbon storage strategy. Soil carbon is central to all ecosystem functions. Preserving intact landscapes composed of diverse, native, perennial plants will also improve water retention, soil fertility, grazing opportunities, and wildlife habitat.
Future Directions
In phase two of this study, we will add more sites across a broader range of landscapes that vary more widely in their climate and soil types. We will also look at the impacts of common restoration practices—such as herbicides and seeding—on soil carbon.
Visit the web pages below to learn more about our research.