Topic 4 Systems Invaded by Annual Grasses: The Science

This section explores the science behind the invasion of annual grasses in sagebrush ecosystems. Scroll down the page to read each sub-section, or click the Science drop-down navigation to go directly to a sub-section.


In the following video, Matt Germino describes the fire/exotic grass cycle. Click the Play button to watch.


Invasive species have caused and will continue to cause numerous problems to the sagebrush ecosystem. Although 30 or more species of plants can be considered invasive in the range of the sage-grouse (see Miller et al. 2011, table 10.4), the two primary species of concern to fuels managers are the invasive grasses cheatgrass and medusahead. These grasses were introduced into the sagebrush ecosystem in the late 19th century and have spread ever since. Although more dominant in the western (Great Basin) range, these species are at least a local threat throughout Greater Sage-Grouse range. Some of the essential points to remember concerning invasive grasses, fire, and the sagebrush ecosystem are:

Since 2000, individual fires exceeding 100,000 acres in the sagebrush steppe have become a near annual occurrence in the Great Basin. These fires, occurring under conditions of long-term drought, extreme fire hazard, high winds, low humidity, and multiple starts make direct attack and control very difficult. Recent observations suggest many megafires are linked to cheatgrass-dominated areas which serve as primary ignition points and facilitate spread within large, contiguous stands of sagebrush (Maestas et al. 2016).

Wildfires in the sagebrush steppe expand quickly and can affect hundreds of thousands of acres of sage-grouse habitat in a matter of days; the Long Draw (2012) and Buzzard Complex (2014) fires in southeastern Oregon both had multiple hundred-thousand-acre runs in a single burning period with a rate of spread between 10 and 15 miles per hour. This map (from Chambers et al. 2017, Figure 7) shows the distribution of fires over a 15-year period in the Great Basin.

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Click on map for a printable PDF version.

In lower elevations of the Northern Great Basin sagebrush steppe (below 4000 ft), the fire return interval has been reduced from 50 to 100 years to less than 10 years in some places (Great Basin Factsheet Series #5).

Making the problem even worse going forward, annual grasses that typically invade lower elevation sagebrush communities are now expanding into mid elevations following wildfire (Great Basin Factsheet Series #5).

As fires gradually deplete native perennial grasses and forbs, these previously more resistant sagebrush communities have become susceptible to conversion to invasive annual plant dominance (Davies et al. 2011), as shown in the photos from Miller et al. (2014).

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A Wyoming big sagebrush/bluebunch wheatgrass-Indian ricegrass system with a severely depleted understory and cheatgrass cover near 5% (left photo, taken in 2008). Following a prescribed fire invasive annuals dominate the understory (right photo, taken in 2010). Resilience and resistance to invasive annuals and potential seeding success is low with a Resilience and Resistance score = 9.

Sagebrush ecosystems are at a tipping point when invasive annual grasses dominate the herbaceous understory. There are limited management options and few science-based solutions to these situations.

The invasive grass-wildfire feedback cycle has serious potential negative consequences for sage-grouse populations in the next several decades. In this video, Pete Coates discusses the current rate of burning and sage-grouse population forecasts.

What can be done?

Begin by watching Matt Germino describe techniques and emerging science to combat the invasive annual grass threat.

Emerging science on various aspects of invasive grasses, their control, and restoration techniques offer some hope of solving the fire/invasives problem and breaking the cycle. These include:

  • Techniques to help the establishment of perennial grasses;
  • Better weather prediction tools to help predict seed germination and survival to establishment;
  • Seed coatings to accelerate or delay germination to increase establishment;
  • Use of seed transfer zones to obtain plant materials better adapted to local sites;
  • Use of herbicides to help eradicate of exotic annuals, including where, when and why and avoiding collateral damage;
  • Development of weed-suppressive bacteria (e.g., Pseudomonas fluorescens) to reduce the competitiveness of invasive annual grasses;
  • How soon grazing should be allowed after fire and seeding;
  • Soil health restoration (e.g., restoration of cryptobiotic crusts) that can inhibit cheatgrass germination.

Given projected climate change and longer fire seasons across the western United States, fuels management represents a proactive approach for modifying large fire trends. A key toward successful application of fuels management techniques in systems invaded by invasive annual grasses is the concept of Resistance and Resilience.

Resistance and Resilience

This concept is more fully explained in the Lesson on Resistance & Resilience, but we need to understand how resistance and resilience applies specifically to systems invaded by annual grasses. The concept of Resistance and Resilience is also critically important when designing fuels treatments and considering their long-term consequences, especially in warm and dry sites with low resistance and resilience. In this video, Rick Miller discusses the concept of resistance and resilience.

Factors that impact resistance and resilience include:

Sagebrush ecosystem resilience to disturbance and resistance to annual grass invasion is closely linked to soil moisture and temperature as shown in these graphs. Warm, dry sites (at the left side of the horizontal axis) with low productivity typically occur at lower elevations and have lower resistance and resilience (and are more vulnerable).

These contrast with cold, moist sites (at the right side of the horizontal axis) with greater productivity that occur at higher elevations and have higher resistance to invasion and higher resilience to disturbance. These graphs also illustrate how disturbance and other stressors lower the given resilience and resistance at any given point on the soil moisture and temperature gradient (shown by the red lines on the graphs).

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Potential resilience and resistance to invasive annual grasses reflect the biophysical conditions of an area, and soil temperature and moisture regimes provide a useful indicator of these conditions at multiple scales. Resilience to disturbance typically increases with more favorable environmental conditions for plant growth and reproduction. Resistance to cheatgrass and other exotic annual grasses is strongly influenced by climate suitability for establishment and persistence.

Cheatgrass germination, growth and reproduction appear to be optimal under relatively warm and dry to moist regimes, limited by low and sporadic precipitation under dry regimes, and generally constrained by colder regimes. Therefore:

  • Areas with warm soil temperature and dry soil moisture regimes typically have low potential resilience.
  • Areas with cool to moderately cold soil temperature and relatively moist soil moisture regimes have high potential resilience.

Low elevation sites with low resistance and resilience are difficult, but we cannot ignore them. Watch the video of Jeff Rose discussing his thoughts on management of low Resistance and Resilience sites.

General Management

The sage-grouse habitat resistance and resilience matrix (Chambers et al. 2017, Table 8) can be used to develop general management strategies for invasive plant species (Chambers et al. 2017).

The sage-grouse habitat resilience and resistance matrix is based on resilience and resistance and Greater Sage-Grouse breeding habitat probabilities. The rows show the ecosystem’s relative resilience to disturbance and resistance to invasive annual grasses (1 = high resilience and resistance; 2 = moderate resilience and resistance; 3 = low resilience and resistance). The columns show the landscape-scale sage-grouse breeding habitat probability (A = 0.25 to < 0.5 probability; B = 0.5 to < 0.75 probability; C = ≥ 0.75 probability).

Click the image to view a PDF the matrix and associated instructions and information.

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Understanding the Feedback Loop: Wildfire, Invasive Annual Grasses, and Key Habitats

The feedback cycle between recurring wildfires and annual grass expansion has been repeatedly cited as one of the dominant threats to sage-grouse persistence range-wide (USFWS 2013, Knick et al. 2011). Recent research (Coates et al. 2015) provides context on current wildfire and sage-grouse trends across the Great Basin. Specifically, this research examined the relationship between the locations of fires, the location of highly valued habitats (lek clusters), and patterns of Resistance and Resilience. This work also projected likely future trends for sage-grouse populations based upon different rates of burning and precipitation scenarios.

Some of the important findings from this research are summarized here:

The total area burned in the Great Basin increased annually, though the absolute amount and rate of increase in burning depended on the Resistance and Resilience (R&R) index class (Coates et al. 2015, Figure 5).

The total burned area at sites with low R&R increased at a faster rate and showed more total area burned through time (red dashed line) than sites with moderate R&R (orange dashed line), which in turn increased faster with more area than sites with high R&R (yellow solid line).

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These trends are spatially illustrated in this map (Coates et al. 2015, Figure 6) showing the cumulative burned area of fires across the Great Basin from 1984 to 2013, by Resistance and Resilience class. During this time period, the area of low R&R burned (13,173 km2) was over twice the area of moderate R&R burned (6,390 km2) and over three times the area of high R&R burned (4,349 km2).

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Click on map for a printable PDF version.

Total area burned in the Great Basin increased by an average of 153 km2/yr. The area burned was higher in low precipitation years (red dashed line) and lower in high precipitation years (blue solid line), but the rate of increase per year remained about the same (Coates et al. 2015, Figure 4).

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Coates et al. (2015) took their analysis one step further by modeling the potential effects of differing levels of reduction in the rate of annual area burned on sage-grouse population trajectories. To do this, they first developed a map of sage-grouse “concentration” or “core” areas, which are clusters of leks in high quality habitat areas (Coates et al. 2015, Figure 3). The core area concept represents polygons where 75% of breeding males are concentrated within 10% of total area in the Great Basin. These areas correspond to critically important habitats where sage-grouse exhibit high site fidelity.

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Primary Management Implications

The primary management implications related to “core” areas are illustrated in Coates et al. 2015, Figure 12, which illustrates projected sage-grouse population change from 2015 to 2045.

The projections were modeled under three different scenarios of decreased rates of burning in core areas (columns; 25%, 75%, or 99% reduction in annual cumulative burned area), under three different levels of annual precipitation (rows; low, medium, and high precipitation). The black horizontal line in each graph represents stable population trends in that scenario. The blue bands represent populations in core areas, while the red bands represent populations outside of core areas.

The key take home messages are:

  • Reducing the rate of annual cumulative burned area by only 25% in defined ‘core areas’ (left column) did little to prevent population declines under any of the modeled precipitation conditions.
  • Reducing the rate of annual cumulative burned area by 75% (middle column) in core areas slowed the rate of population decline under below-average precipitation conditions (top), stabilized population growth under normal precipitation conditions (middle), and resulted in population growth under above-average precipitation conditions (bottom).
  • A near complete cessation of fire in core areas (right column) resulted in increased population growth under all precipitiation conditions, especially for normal (middle) and high precipitation (bottom).


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Wildfire, Invasive Annual Grass, and Sage-Grouse Summary

The research related to invasive annual grasses clearly shows:

  • The frequency and size of wildfires has increased across the Great Basin, driven strongly by post-fire invasive annual grass (primarily cheatgrass) dominance.
  • The distribution of sage-grouse and their breeding habitats are highly clustered. Protecting core areas where birds are concentrated is an important conservation strategy to improve long-term sage-grouse persistence.
  • At current rates of burning, continued declines in Great Basin sage-grouse populations are expected over the next three decades under most climate scenarios.
  • Effective fire suppression, which constrains cumulative area burned in core areas, is an essential management tool over the next three decades.
  • During this period, failure to constrain cumulative area burned by 75% or more in core areas is projected to result in large-scale declines in sage-grouse populations across the Great Basin.
  • Successful post-fire Emergency Stabilization and Rehabilitation following fire can offset losses resulting from wildfire and is not incorporated into the projections.

Click Play to listen to Pete Coates' conclusions from his research.


Next explore the Land Management Tools section to learn about tools for managing invasive annual grasses and associated wildfire.