WLFW Research Details How Wildfire Follows Cheatgrass In the Great Basin

New study clarifies the relationship between annual grass expansion and wildfire

In the Great Basin, it’s not a question of chickens and eggs, but which leads to more of what – wildfires or invasive annual grasses. New research from Joe Smith, a WLFW-affiliated researcher at the University of Montana, and the USDA’s Agricultural Research Service flips the assumption that wildfires are the primary driver of annual grasses becoming dominant by showing that most invasive annual grass state transitions in the Great Basin have occurred in areas that haven’t experienced recent fire. This information helps inform a more comprehensive conservation strategy to better protect sagebrush country rather than relying solely on fire management.

There is no doubt that invasive annual grasses like cheatgrass, ventenata and medusahead are degrading sagebrush habitat in the western U.S. The Great Basin has been particularly affected by expanding invasive annual grasses, which reduce native plant diversity, diminish habitat for sagebrush-dependent wildlife, and disrupt nutrient cycles. On lands in the Great Basin managed by the Bureau of Land Management (BLM), recent research estimates that annual grass production has tripled since the early 1990s.

Historically, the sagebrush ecosystem had sparse understories and experienced infrequent wildfires. Invasive annual grasses increase the amount and continuity of fine fuels in sagebrush habitat, aiding both ignition and spread of fire and, ultimately, increasing the frequency and severity of wildfires. These more frequent and severe fires result in high mortality for sagebrush shrubs, which then take decades to re-establish.

Because invasive annual grasses readily colonize disturbed areas, their dominance is reinforced following a fire. The result is the “grass-fire cycle” where areas with abundant invasive annual grasses facilitate fire, which then allows more invasive annual grasses to colonize, which promote more fire, and so on. Because these grass-promoted fires are then colonized by more annual grasses, it has long been assumed that fire is a primary driver of invasive annual grass state transitions.

But that assumption hadn’t been studied at a broad scale until now.

Using remotely sensed data from the Rangeland Analysis Platform, on-the-ground vegetation surveys conducted by the BLM, and information from the interagency Monitoring Trends in Burn Severity dataset, Smith and his team compared areas that had transitioned to invasive annual grass dominance with the occurrence of fire.

Their analysis unveiled four major findings:

1. Most transitions to annual grass dominance since the early 1990s have occurred unaided by recent (within the last decade) fire;
2. Areas that transition following fire are typically highly invaded before burning;
3. The outcome, in terms of functional composition, is similar regardless of whether the transition was preceded by a fire; and
4. Annual grass dominance is highly persistent even in the absence of fire.

From 1994 to 2020, the team estimated that more than 25,000 square miles (66,000 km²) of the Great Basin, an area nearly 12 times the size of Yellowstone National Park, had transitioned to annual grass dominance. However, more than three-quarters, or 77%, of that area transitioned without burning in the previous 10 years. They found that fire acts as a catalyst, but is not a necessary condition, for areas to transition to annual grass dominance.

These findings suggest that annual invasive grasses, as opposed to fire, drive most vegetation transition in the Great Basin. As such, management efforts intended to reduce annual grass expansion that focus solely on wildfire suppression and post-fire rehabilitation have little chance of meaningfully slowing annual grass expansion into sagebrush habitat. Fire management plays an essential role in protecting people, infrastructure, and lands impacted by altered fire regimes, but fire management alone is not an effective strategy for preventing ever-increasing annual grass invasions.

The authors suggest that adding proactive management strategies focused on invasive annual grass management will better protect sagebrush habitat from invasive annual grasses. For example, the Defend the Core approach emphasizes protecting intact sagebrush cores from invasion by annual grasses through practices like enhancing resistance and managing annual grass seed pressure. Once cores are protected, managers can work to expand them into areas where annual grasses may be present but aren’t dominant. These strategies are enabled by spatially explicit conservation designs and are the foundation of WLFW’s Framework for Conservation Action in the Sagebrush Biome, which directly addresses invasive annual grasses as a primary threat in sagebrush country.

This analysis makes clear that managing fire is not the same as managing invasive annual grasses, and that effectively slowing the spread of invasive annual grasses in the Great Basin requires both. More than 97% of fires in the Great Basin are controlled before they grow large, yet annual invasive grasses continue to expand. Failing to adopt proactive management strategies will jeopardize wildlife, grazing economies, and exacerbate already difficult fire-management challenges.

FIRE NEEDS ANNUAL GRASSES MORE THAN ANNUAL GRASSES NEED FIRE

Abstract: Sagebrush ecosystems of western North America are experiencing widespread loss and degradation by invasive annual grasses. Positive feedbacks between fire and annual grasses are often invoked to explain the rapid pace of these changes, yet annual grasses also appear capable of achieving dominance among vegetation communities that have not burned for many decades.

Using a dynamic, remotely sensed vegetation dataset in tandem with remotely sensed fire perimeter and burn severity datasets, we examine the role of fire in transitions to and persistence of annual grass dominance in the U.S. Great Basin over the past 3 decades.

Although annual grasses and wildfire are so tightly associated that one is rarely mentioned without the other, our findings reveal surprisingly widespread transformation of sagebrush ecosystems by invasive annual grasses in the absence of fire.

These findings are discussed in the context of strategic management; we argue a pivot from predominantly reactive management (e.g., aggressive fire suppression and post-fire restoration in heavily-infested areas) to more proactive management (e.g., enhancing resistance and managing propagule pressure in minimally-invaded areas) is urgently needed to halt the loss of Great Basin sagebrush ecosystems.

Citation: Joseph T. Smith, Brady W. Allred, Chad S. Boyd, Kirk W. Davies, Andrew R. Kleinhesselink, Scott L. Morford, David E. Naugle, “Fire needs annual grasses more than annual grasses need fire,” Biological Conservation, Volume 286, 2023.

Acknowledgements: Funding was provided by the U.S. Department of Agriculture’s Agricultural Research Service. The authors thank R. O’Connor and three anonymous reviewers for providing comments that helped improve the manuscript.

Permanent URL: https://doi.org/10.1016/j.biocon.2023.110299