Fire has long played a leading role shaping the ecology of the Great Plains. Whether lightning-caused or human-induced, research shows that much of the Great Plains experienced fire frequently, every fourteen to less than two years depending on region. Recurrent fire helped create and maintain the diverse grassland ecosystems – short grass, mixed grass and tallgrass prairies – that exist across the Plains. Fires also controlled woody species presence throughout the region, confining trees and shrubs to sites where fire was less frequent or absent due to topographical, climatic, or other natural conditions.
Over the last couple of centuries, the removal of indigenous fire ignitions and widespread fire suppression throughout the region drastically altered this historic fire regime. In a short period of ecological time, the Plains went from one of the most frequently burned regions on the planet to one where fire became relatively scarce, allowing woody species and associated fuel loads to build and expand. Today, wildfire, specifically large, damaging wildfires, have greatly increased in frequency and severity across the Plains. The 33 documented large wildfires that burned in the Plains from 1985 to 1994 have been eclipsed by 117 large wildfires that burned in the next 10 years (2005-2014).
Woody expansion is causing a shift from grasslands to woodlands in central North America, and this shift is collapsing wildlife diversity, heightening wildfire risk, and crashing grazing land profitability. In short, woody expansion is threatening the very characteristics that make the Great Plains so productive.
Fires that deter woody expansion are driven by complex interactions between climate, fuel loads, land use, and human behavior. Dr. Victoria Donovan and her team of researchers at the University of Nebraska are at the cutting edge of understanding the various roles of fire as a driver of plant community change in central North America. We sat down with Dr. Donovan to learn more from her two new papers: Land-Use Type as a Driver of Large Wildfire Occurrence in the U.S. Great Plains and Resilience to Large, ‘Catastrophic’ Wildfires in North America’s Grassland Biome.
These two papers focus on fire in the Great Plains. Scientists have concluded that fire was once a regular part of the Great Plains ecosystem, but that’s no longer the case. Why were you and the team interested in studying large wildfires in the Great Plains?
While we know that large wildfires have been increasing substantially across forested regions of the western U.S. for some time, recent increases in large wildfire are relatively new in the Great Plains. This offers us the opportunity to understand shifting wildfire patterns in this region and possibly get ahead of some of the risks large wildfire can pose to people by learning how to better manage wildfire to prevent loss of human life and property.
Let’s start with the paper called Land-Use Type as a Driver of Large Wildfire Occurrence in the U.S. Great Plains. Your team analyzed large wildfires in the Great Plains from 1993 to 2014 and overlaid the boundaries of those fires onto a map. You then analyzed the various types of land use, which you classified into five types – grassland, woody vegetation, cropland, pasture and hay fields, and developed areas – within the fire boundaries. What were you looking for?
We wanted to understand how land use might be playing a role in shifting wildfire patterns by identifying which land-use types have the highest propensity for wildfire. Around the world, different land-use types, including agricultural and developed lands, are known to affect wildfire occurrence differently. Over the last century, the Great Plains has seen drastic shifts in land-use following Euro-American settlement, including agricultural conversion and development, along with widespread woody encroachment of grassland systems. Today’s wildfires are occurring within that altered landscape. Since our previous research showed a surge in wildfire activity in the Great Plains, a clear question for us was to determine whether this surge was tied to different land uses.
You note that in 11 of the 14 ecoregions you analyzed: “areas burned by large wildfire were primarily composed of woody vegetation and grassland.” Why are these land-use types more susceptible to large fires?
There are a number of reasons why we saw more fires in these land-use types. Grasses are one of the most flammable fuel types on the planet, and they are highly adapted to and have properties that promote frequent fire. While woody vegetation communities do not typically experience fires as frequently as grasslands, they offer an abundance of fuels that promote large fires once ignited. In contrast with grasslands, vegetation in pastures, hay fields, and crops generally have different fuel properties that make them less susceptible to fire. For instance, the plants in these areas tend to hold more moisture, which can make it much more difficult for fire to ignite and spread. Irrigation could also play a role in this. This result is consistent with research that suggests that crop fields may actually act as a barrier to fire spread in some regions of the world, which is interesting since we know that agricultural lands are some of the most frequently burned land use types elsewhere in the world. There are social reasons as well. For instance, fire suppression is more likely to be successful for a wildfire burning through a grassland than one burning through forest canopy.
It makes sense that intact grasslands burned more than crops or developed areas, but you found that woody vegetation “burned proportionately more than any other type of land-use in the Great Plains.” What do you mean by that and why is it important?
We found that of all of the land-use types in the Great Plains, large wildfire is most likely to burn in woody vegetation. This means woody vegetation is associated with the highest large wildfire risk. This is extremely important because we know that woody vegetation is increasing across much of the Great Plains by invading our grassland systems. We also know that wildfires that occur in woody vegetation are generally more difficult for us to put out. Consider a campfire: You might use a few handfuls of dried grass as a way to get the fire started, but if you don’t add some logs to it, it will burn out quickly. The same is true in natural systems. Woody vegetation generally offers more fuels that can burn longer and more intensely than grasses. To add to that, trees grow much taller than grasses. Imagine trying to put out your campfire when it is a few stories above you.
We know many grasslands are transitioning to woody vegetation in the Great Plains because we have removed the frequent fires that used to burn through grasslands. We also know that tree planting is a common practice across much of the Great Plains and these have served as seed sources for invasion into more intact grassland systems. What our results suggest is that if we continue to promote woody vegetation across the Great Plains, we are also going to be increasing our risk for large wildfires that are more difficult for us to control.
Let’s shift gears to the second paper called Resilience to Large, ‘Catastrophic’ Wildfires in North America’s Grassland Biome. Will you explain what you mean by “resilience”?
Resilience is the amount of change that an ecosystem like a grassland can experience before shifting into something else, like a desert. When we were considering resilience in this study, we were tracking whether vegetation cover in an ecosystem was able to recover following wildfire or transition to an alternative type of ecosystem. For example, we wanted to know whether grass cover recovers to the same level as before the wildfire or whether a different vegetation type takes over after wildfire.
So, your team examined how the landscape, and specifically how the vegetation community on that landscape responded to large, severe wildfires. First off, how did you find the information and data you analyzed? Secondly, what did you find?
We were lucky enough to gain access to an amazing new data set from the Rangeland Analysis Platform, a project led by researchers from the University of Montana in collaboration with the Bureau of Land Management and the Natural Resources Conservation Service’s Working Lands for Wildlife team. It provided us with information on vegetation cover across much of the Great Plains that hadn’t been available over such large areas before. What this meant was that we could ask questions about the outcomes of large wildfires on vegetation across vast extents of the Plains to gain a fuller picture of wildfire in this region.
While we know that fire was historically an important part of the Great Plains, there is also concern in this region that fire can cause irreversible changes to vegetation. For instance, there are a number of programs that promote re-seeding following wildfire to prevent desertification, that is, the conversion of a vegetated area to bare ground. What we found was that the Great Plains is highly resilient to wildfire. At the biome level, all vegetation returned rapidly to pre-fire levels. In every ecoregion, we saw rapid recovery of perennial and annual forbs and grasses. Our findings echo over a century of research demonstrating grasslands in the Great Plains are highly resilient to wildfire.
Was there one vegetation community or one location within the Plains where fire did cause a long-lasting change in the type of vegetation cover?
Yes, in the northwestern Great Plains ecoregion we saw persistent decreases in tree cover following wildfire over our study period. This helps confirm findings from more localized studies that indicate that fire has the ability to control and remove woody vegetation in some rangeland systems. The northwestern Great Plains is dominated by ponderosa pine trees, which can be rather fire sensitive compared to some re-sprouting species in the southern Great Plains, like mesquite.
Does drought play a role in how vegetation responds to wildfire?
We found that drought can amplify the immediate response of vegetation to wildfire. For instance, if a wildfire occurred under more severe drought conditions, there was generally a greater initial loss in perennial forbs and grass cover. However, we didn’t find that drought had any impact on long-term losses of vegetation cover. In other words, wildfires burning under more extreme drought conditions did not impact the probability of vegetation recovery, though recovery may take longer than if wildfire had burned under moist conditions.
Your team found that woody vegetation burns disproportionately more often than other land-use types, and that in some places it is the one vegetation community that is least likely to return. Given that encroaching woody species have a negative impact to water, wildlife, and other resources on the Great Plains, that seems like it could be a good thing for rangelands. Is that the case?
Yes and no. For wildlife and vegetation that depend on a prairie environment, wildfires can provide a pathway for re-gaining rangeland lost to woody vegetation. In fact, some landowner groups in the Great Plains and elsewhere have viewed wildfire as a restoration mechanism that can be used as a launching point to prevent the re-establishment of problematic woody species like Eastern redcedar. On the other hand, we know that large wildfires can pose a risk to human communities, particularly, when they occur in more volatile woody fuels. How do we balance that? We utilize prescribed fire. The Great Plains was one of the most frequently burned regions in the world historically (with some locations burning every two years or less). Vegetation in the Great Plains has a long history of thriving with fire, which our research helps to emphasize. Frequent applications of prescribed fire is an effective way to help reduce wildfire risk while re-integrating an important ecological process into grassland ecosystems.
What are the key takeaways that you want people to understand from your papers?
- In the Great Plains, our grasslands are able to recover rapidly following fire, even under extreme drought conditions.
- Removing fire from these systems may have inadvertently increased the risk of large wildfires by allowing woody encroachment.
- Using fire as a management tool could help reduce large wildfire risk in some regions by reversing and preventing further woody encroachment.
Meet the Expert
Will you tell us a bit about your research interests and what you’ll be focusing on next?
I am really interested in understanding how processes like fire structure and shape ecosystems. In this rapidly changing global environment, I think understanding this will be key to building ecosystem resilience and managing ecosystems that we depend on. I have a background in wildlife research, so I am hoping to take the findings that I have learned here and integrate them with wildlife data to get a better picture of how animals, plants, and processes like fire shape one another and how we can use these interactions to plan for and adapt to future change.
What keeps you busy when you’re not working on a research project?
I am big into travel. I love to explore the wilderness of different countries, learn about new cultures, and try new and delicious foods. While I am saving up for my next trip, I am camping, hiking, kayaking, and volunteering with my local animal shelter or youth organization.
Cake or pie?
Definitely cake and preferably chocolate.