Enemy #1 is actually two distinct species of a small, annual grass with Euro-Asian ancestry that have taken over many arid lands in the west. In desert ecosystems characterized by well spaced shrubs and perennials, these invasive grasses can spread into a carpet. This new source of fuel that can carry a wildfire across a landscape where fire is historically rare and enormous fires unheard of. Starting in 2005, enormous wildfires burnt through southern Nevada, fueled in a large part, by these invaders. Allow me to introduce these Bromus Brothers:
Bromus rubens, also known as Red Brome, is the grass more commonly found in southern deserts and lower elevations. Standing from 1 to 10 inches tall, it frequently grows in single stems, but occasionally, in bunches. The seed heads are upright.
Bromus tectorum, commonly known as Cheatgrass, is commonly found in the middle-elevations of the southwest, and throughout the mountain west. Again, usually under 10 inches tall, single stems or in clumps if they are really healthy and happy. Here, the seed heads droop.
I know, they don’t look that evil. How can an annual grass, less than a foot tall, be so bad? Well, first off, I am still finding their seeds in my wool socks, even though it has been 8 months since I last hiked between them. Secondly, and perhaps more importantly, huge swaths of Nevada now look like this:
That’s me, carrying a white sampling frame through a post-fire survey plot. Working on a project to monitor the success of a post fire seeding treatment, my job was to put that frame on the ground 10 times at every site, and count every individual plant found within it. At sites like this, the data would read: Bromus tectorum, 500, native perennial grass, 1, and maybe a handful of other tiny natives. Across thousands of recently burned acres, we documented the desert covered with a carpet of these grasses. I used to have nightmares in which I could not count all of the brome….it would just go on and on and on. But, the problem with these grasses is more that just my personal torment.
I talked with Cayenne Engel, a researcher at the University of Nevada, Las Vegas, about the sorrid relationship of the Bromus grasses with wildfires. Cayenne studies the fire history of southern Nevada, among other interesting things, so she’s spent plenty of time with the Bromes. (Disclaimer, she’s also a friend of mine)
Kate: So, can you explain the relationship of the Bromus grasses to wildfires?
Cayenne: The theory of the grass fire cycle is that once they [the bromes] spread,the make an area more susceptible to fire by providing a continuous fuel load. Then, once a fire comes through, it eliminates the native species. The Bromus comes back in force after the disturbance, making the area even more susceptible to fire, while the natives take a long time to reestablish.
Kate: So how much data backs this up?
Cayenne: There have been a couple of papers, including 1 by Brooks in 2006, showing that areas after fire were coming back with increased brome. However, in my surveys, not in every site does the brome come back like gangbusters. Brome is subject to the same limitations that the natives are, like the local weather and climate. If there is no rain, the brome doesn’t come back either. So at some sites, like higher elevation, it will come back in a carpet, but at other sites, it won’t. So the grass fire cycle does happen, isn’t necessarily a rule or a law. It’s a possibility, not a rule.
Kate: What gives the brome the ability to out-compete the natives?
Cayenne: Here, as much as anything their phenology gives them a competitive edge. They emerge early in the growing season, establish early in the growing season, and the theory is, they use resources early. They draw down resources and prevent the natives from establishing. The bromes have very prolific reproduction, so they can increase their numbers very quickly.
Kate: How does the brome spread? Do cattle carry the seeds?
Cayenne: It was first observed in the early 1900s. As much as anything, due to insects and rodents, herbivores will carry them, there’s just so many [seeds] that once you get a patch it’s easy to spread from there. Cows can spread it, but I wouldn’t assume that cattle are the primary vector, but cattle do keep down the natives. That selective pressure against the natives gives the brome an advantage. In the Tule desert, which is heavily grazed, our research plots are almost nothing but brome. The brome is so prolific [in producing seeds] that spreads very quickly.
Kate: How long can the brome seeds survive in the seed bank?
Cayenne: We know, we’ve done some studies brome is pretty sensitive to heat. 100 C will kill brome seeds, which is a low soil surface temperature for a wildfire. If you have a really tough fire, the seeds are dead, which is an impediment to the grass fire cycle. The general theory is that brome seeds can only last about 2 years in the seedbank, but can last to 3 or 4. Alex’s data [a collegue at UNLV] showed that about 90% of the seeds are knocked out by 18 months, they are no longer viable. The natives are more durable through time. The native annuals have evolved to deal with the variable conditions here. That’s why we have the famous desert bloom, the seeds wait for a good year. Some annuals can last in the seedbank well over a decade.The brome is just so more prolific, so even if the seedbank is wiped out, in the case of an intense fire, there is so much propagule pressure from all around, that it re-invades pretty quickly, it creeps in from the outside.
Kate: So is there any hope? What can we do?
Cayenne: There’s the idea that you just need to apply the pressure to kill the plants for a couple years, because the seedbank runs out fairly quickly. Get rid of a majority of the mature seeding individuals [with a pre-emergent pesticide] and the seedbank will become inviable pretty quickly.
Thanks Cayenne! Hopefully, the research that she, and others, are working on will find a way to beat back the brome and allow the natives to re-establish. However, as she explained, not all sites become carpeted with brome after a fire. Understanding the subtle differences between sites will help researchers understand how to to encourage some areas to break out of the grass fire cycle.
Although counting brome can get depressing, I owe a thank you to everyone I used to work in Nevada’s burned wilderness with, because they made a tough job fun. I owe James and Carl for lending me some of the photos included here as well. Thanks! If anyone is interested, the link to the giant BLM report on post fire seeding treatments that our work contributed to is here.