Posted on 04.12.23 by Harney SWCD

Woody encroachment affects every state in the Great Plains, negatively impacting forage production, grassland species, soil moisture, and water availability. Another consequence of volatile woody fuels, like eastern redcedar, moving into grasslands and replacing native grasses is altered wildfire behavior. New WLFW-affiliated research details the risk of increasing wildfire in the Great Plains due to woody encroachment and how prescribed fires help reduce that risk.

Research led by Victoria Donovan, a WLFW-affiliated researcher at the University of Florida, and Dirac Twidwell, WLFW science advisor at the University of Nebraska, sheds light on critical differences in how wildfires and prescribed fires behave and the risks they pose in the Great Plains.

Increasing wildfire in the Great Plains, exacerbated by woody encroachment, is a serious management and human health concern. Wildfires in grasslands are less intense than wildfires in woody-encroached landscapes. Fine fuels, like grasses, burn with lower flame lengths and are easier for wildland firefighters to suppress than fires in volatile woody fuels, which burn more intensely.

In contrast, prescribed fires are set intentionally, under specific conditions and with coordinated response teams. In the Great Plains, prescribed fires are applied to grasslands that have been encroached on by woody species, which can halt or reverse encroachment and restore grassland biodiversity.

To better understand how prescribed fires behave differently than wildfires – and to assess their relative risk compared to wildfire and how they might reduce the risk of a wildfire – Donovan studied spot-fire behavior in the Loess Canyons, a renowned landscape in Nebraska that has restored a prescribed fire culture to the region to combat woody encroachment.

Specifically, Donovan employed commonly used fire behavior models to compare modeled-spot-fire distances in grasslands, encroached grasslands (grasslands with some conifer trees in them) and woodlands (grasslands that have completely converted to conifer woodlands) in both prescribed and wildfire scenarios.

Spot fires are fires that start outside of the original fire perimeter from wind-carried sparks or firebrands that ignite once they contact a new fuel source. Spot-fire distance is a critical metric for wildfire danger, because spot-fires start new wildfires independent of the one firefighters are fighting. This greatly complicates wildfire suppression efforts and increases danger to wildland fire personnel.

Researchers found that during wildfires, when high wind conditions are most likely, grassland wildfires produced a maximum spot-fire distance of 1.4 kilometers (0.87 miles) under the most extreme wind conditions. In encroached grasslands, that distance increased to 4.3 kilometers (2.67 miles), more than doubling the exposure of adjacent grasslands to spot fires. In woodlands, the maximum spot-fire distance increased to 7 kilometers (4.35 miles), exposing an average of 15,825 hectares (39,104 acres) of adjacent grasslands to potential fire ignitions, a 10-fold increase in exposure area compared to a grassland wildfire.

Prescribed fires, which are only conducted under low wind conditions, had a lower spot-fire risk at each stage of encroachment. The team found that “spot-fire distances under typical prescribed fire conditions were 2- to 3-fold less than those associated with high wind speeds that only occur during wildfires in the same stage of woody encroachment.”

This research highlights the critical, multi-functional role that prescribed fires can have in controlling woody species encroachment and reducing the risk of damaging wildfires in the Plains. In addition to the ecological benefits prescribed fires create, the team concluded that “using prescribed fire…has a high potential to reduce fire risk compared to waiting for wildfire to occur.”

Prescribed fire is one of the key tactics in WLFW’s Framework for Conservation Action in the Great Plains Biome. As demonstrated by this research, prescribed fire can significantly reduce the risk of severe wildfires in the Great Plains, along with improving grassland resiliency, restoring biodiversity, and producing better and more forage.

The USDA-NRCS offers technical and financial support to landowners in the Great Plains who are looking to control woody species encroachment with prescribed fire. Visit your local USDA-NRCS Service Center to learn more about these opportunities and specifically available assistance through the Great Plains Grasslands Initiative (GPGI) in Nebraska, Kansas, Oklahoma, and South Dakota.

Read The Paper

Spot-fire distance increases disproportionately for wildfires compared to prescribed fires as grasslands transition to Juniperus woodlands

Abstract: Woody encroachment is one of the greatest threats to grasslands globally, depleting a suite of ecosystem services, including forage production and grassland biodiversity. Recent evidence also suggests that woody encroachment increases wildfire danger, particularly in the Great Plains of North America, where highly volatile Juniperus spp. convert grasslands to an alternative woodland state.

Spot-fire distances are a critical component of wildfire danger, describing the distance over which embers from one fire can cause a new fire ignition, potentially far away from fire suppression personnel. We assess changes in spot-fire distances as grasslands experience Juniperus encroachment to an alternative woodland state and how spot-fire distances differ under typical prescribed fire conditions compared to conditions observed during wildfire. We use BehavePlus to calculate spot-fire distances for these scenarios within the Loess Canyons Experimental Landscape, Nebraska, U.S.A., a 73,000-ha ecoregion where private-lands fire management is used to reduce woody encroachment and prevent further expansion of Juniperus fuels.

We found prescribed fire used to control woody encroachment had lower maximum spot-fire distances compared to wildfires and, correspondingly, a lower amount of land area at risk to spot-fire occurrence. Under more extreme wildfire scenarios, spot-fire distances were 2 times higher in grasslands, and over 3 times higher in encroached grasslands and Juniperus woodlands compared to fires burned under prescribed fire conditions.

Maximum spot-fire distance was 450% greater in Juniperus woodlands compared to grasslands and exposed an additional 14,000 ha of receptive fuels, on average, to spot-fire occurrence within the Loess Canyons Experimental Landscape. This study demonstrates that woody encroachment drastically increases risks associated with wildfire, and that spot fire distances associated with woody encroachment are much lower in prescribed fires used to control woody encroachment compared to wildfires.

Acknowledgments: We thank landowners in the Loess Canyons who provided their prescribed burn information.

Citation: Donovan VM, Fogarty DT, Twidwell D (2023) Spot-fire distance increases disproportionately for wildfires compared to prescribed fires as grasslands transition to Juniperus woodlands. PLoS ONE 18(4): e0283816.

Permanent URL: https://doi.org/10.1371/journal.pone.0283816