The Ascent and Spread of Annual Invasive Grasses in the Great Basin

Joe Smith, talks about his research showing that annual invasive grasses are moving up in elevation and to more northerly aspects throughout the Great Basin.

Across the West, productive rangelands that support diverse wildlife populations and rural communities are under threat from annual invasive grasses. Unlike native vegetation, these grasses offer little value as wildlife habitat or livestock forage. Further, as annual invasive grasses displace native vegetation, they fuel more severe wildland fires, deplete soil nutrients and water, and create monocultures where annual grasses alone dominate the landscape.

In the Great Basin, which covers most of Nevada and parts of Utah, Idaho, Oregon, Wyoming, and California, annual invasive grasses like cheatgrass, medusahead, and ventenata, have degraded more than 40,000 square miles of productive sagebrush rangelands. For perspective, the Arctic National Wildlife Refuge in Alaska, the largest U.S. refuge, covers 30,000 square miles.

Understanding how these exotic and unwanted species are moving across the landscape is critically important to maintaining productive rangelands for wildlife and livestock producers. New findings from Working Lands for Wildlife (WLFW) researcher Joe Smith shed light on where invasive annual grasses are spreading across three eco-regions in the Great Basin.

In this Ask an Expert, Smith discusses how his research can help land managers proactively address this threat before annual invasive grasses displace native vegetation, push out sagebrush-dependent wildlife, and threaten rural economies dependent on productive, resilient rangelands.

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In the opening lines of your paper’s abstract, you note the transition from native vegetation to exotic annual grasses is “self-reinforcing.” Will you explain what that means?

What I mean by “self-reinforcing” is that exotic annual grasses have a habit of sticking around once they get established in an area. They do this by altering the fire regime of the communities they invade. These grasses are highly flammable, they fill in the bare ground gaps that usually exist between native perennial grasses and shrubs, and they dry out very early in the summer.

Once this happens, fires can more easily ignite and travel through these areas. The invasive grasses are tolerant of burning every few years, but the native vegetation—especially sagebrush—is poorly adapted to this frequency of fire. You get a positive feedback loop where more exotic grasses lead to more fire, which then leads to more exotic grasses, and so on. Once this pattern is established, it’s very difficult to recover those native plants and get back to a “normal” fire regime.

Cheatgrass, one of the most widespread invasive annual grasses, has been present in North America for more than a century; what makes it such a severe threat now?

Cheatgrass was a severe threat 100 years ago. People like Aldo Leopold, the father of modern wildlife conservation, recognized this threat and tried to raise the alarm. But the true destructive potential of cheatgrass has come to the forefront in the last 10-15 years with the emergence of “mega-fires” in western rangelands. Since 1984, eight wildfires have grown to more than a quarter of a million acres in the Great Basin. All eight of those mega-fires happened after 2007. Numerous factors are probably responsible for this increased fire activity, but the massive expansion of cheatgrass and other exotic annual grasses has played a central role.

What makes your research different from other research into annual invasive species?

The main factor making our research new and exciting is continuity and coverage over time and space. This is the magic of working with data from the Rangeland Analysis Platform (RAP) that provides spatially comprehensive, annual, rangeland monitoring data across all western US rangelands. This powerful data can be organized and delivered so that managers can understand vegetation conditions at a resolution appropriate to inform cross-scale assessments. We don’t just see a snapshot of the problem at a specific point in time, but instead, we get to watch change unfold like a movie over the last 30 years. That gives us insights into the dynamics of this ecosystem transformation process that would be unattainable with a field-based study.

Your research found the rapid expansion of annual invasive grasses in the last two decades was “associated with a broadening topographic niche…consistent with predicted effects of a warming climate.” Will you unpack that?

There’s a pretty solid foundation of experimental field research showing that cold temperature and snowpack are factors that limit where cheatgrass and other exotic annual grasses can flourish. Higher elevations and north-facing slopes have been considered fairly safe from major infestations for this reason.

But the Great Basin, like most of the western US, has been gradually getting warmer. The winters are warming, and the summers are getting hotter and drier. Climate change is expected to open up a bunch of new high-elevation habitat for these exotic annual grasses while at the same time weakening the ability of the native plants to compete and resist invasion. Sure enough, when we looked at where these grasses have become dominant over the last 30 years, we found that they’ve gradually moved into higher elevations and onto more north-facing slopes, just as predicted.

In 2019, USDA’s NRCS in Idaho launched the “Cheatgrass Challenge,” a proactive, partnership-driven, statewide effort to tackle annual invasive grasses by focusing on “core areas” where grasses haven’t yet dominated. How does your research help identify these core areas?

The first step in our research was identifying areas where cheatgrass or other annual grasses have, essentially, already outcompeted native vegetation. That, alone, is useful information as managers strategically identify where to work—excluding those areas already too far gone. But the more important insight is that the situation is very dynamic, both because the invasion process is ongoing for many of these species and because the warming climate is steadily changing what areas are vulnerable and what areas are not. Just because areas above 5,000 feet in elevation have been relatively safe in the past doesn’t mean they’ll continue to be safe in the future. When managers identify core areas, they need to think about what those places will look like another 30 years down the road and what that means for vulnerability to invasion.

If you could leave readers with one key message from your research, what would it be?

The most important takeaway is that invasive annual grasses are on the rise (literally!…they are moving up in elevation), and they are expanding fast, which means we need to get serious and strategic about dealing with them if we don’t want the whole Great Basin to convert to an annual grassland. It’s already happened to grasslands in California, and it can happen to grasslands across the entire West if we don’t do something to stop it.

Meet the Expert

What drives your interest in rangelands and, particularly, sagebrush rangelands?

I believe there are two kinds of people in this world: people who get uncomfortable deep in the woods and people who get uncomfortable out in the open. I’m the former type. I grew up right around the crest of the Cascades in Oregon, and I always found I naturally rolled eastward toward the open sagebrush country.

How do you decide what to research and what questions or topics you want to explore?

I’m lucky to have supervisors and funders that give me the latitude to coproduce science that informs management. They give me broad questions and general goals, and I can be pretty creative as I color roughly between those lines.  I like asking questions that better inform strategic implementation of conservation. This analysis was one of those.

What was the last book you read?

Thinking, Fast and Slow by Daniel Kahneman. Winner of the Nobel Prize in Economics, Kahneman constantly questions how we know what we think we know.

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