Grazing intensity effects on fire ignition risk and spread in sagebrush steppe

Authors: Orr, Devyn; Bates, Jonathan - Jon; Davies, Kirk

Submitted to: Rangeland Ecology and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/26/2022
Publication Date: 10/1/2022
Citation: Orr, D.A., Bates, J.D., Davies, K.W. 2022. Grazing intensity effects on fire ignition risk and spread in sagebrush steppe. Rangeland Ecology and Management. 89:51-60. https://doi.org/10.1016/j.rama.2022.08.004.
DOI: https://doi.org/10.1016/j.rama.2022.08.004

Interpretive Summary: The size, frequency, and severity of wildfires is increasing on many rangelands, particularly in the western United States. This poses a significant threat to plant and wildlife habitat and human health and livelihoods, and so there is a strong need for land management practices that reduce wildfire risk. However, preventing wildfires across vast and rugged landscapes can be challenging and expensive. Livestock grazing is one potential solution for reducing wildfires in these landscapes, but may vary depending on the intensity of grazing. On sagebrush-dominated rangelands, grazing cattle eat grass and other non-woody vegetation. In this way, cattle can remove plant material that would otherwise build up and dry out in the summer once the spring growing season is over. Because of this, we expected that more intensive grazing would result in cattle consuming more of the total grass and non-woody vegetation, reducing overall dry plant material and also reducing the potential for that dry material to catch on fire or spread an existing fire. To evaluate this prediction, we examined the effects of grazing intensity (light, moderate, high) on 1) grass and other non-woody vegetation, and 2) fire ignition and initial spread in a sagebrush-dominated rangeland in eastern Oregon, USA. As expected, we found that the effects of grazing on grass and other vegetation varied depending on grazing intensity. More intensive grazing resulted in less vegetation and more bare ground. This in turn affected the probability of fire ignition and initial spread to surrounding vegetation. Furthermore, we found that the relationship between grazing intensity and fire ignition was only important in dry years when there was less rain during the growing season. These results suggest that land managers may be able to use livestock grazing for wildfire management in sagebrush-dominated rangelands. Grazing at moderate and high intensities appears to have a stronger effect on reducing wildfire risk than grazing at low intensities, and grazing is especially effective under drier conditions (when wildfires are most likely to occur).

Technical Abstract: Wildfire activity is accelerating on many rangelands worldwide, yet the potential for grazing to be used as a fire management tool remains largely unknown. Particularly, little is known about the influence of grazing on ignition and initial spread of fire, and how these vary by differences in grazing management. We investigated effects of grazing intensity (light, moderate, high) on fuel characteristics, fire ignition and initial spread during the wildfire season in a native-dominated shrub steppe in eastern Oregon, USA. We found that differences in grazing intensity have differential effects on fuel profiles (cover, height, moisture, biomass) with resulting impacts on fire behavior, but these relationships varied across study years. In particular, grazing had a stronger effect on ignition probability in drier years. Fire behavior in lightly grazed plots were similar to ungrazed plots, while moderate grazing was similar to high intensity grazing. Results of this study highlight that grazing can be useful as a tool for wildfire management, and grazing at moderate and high intensities can reduce the probability of fire propagation in native-dominated sagebrush ecosystems. Further, the effects of grazing are context-dependent and therefore may depend on specific objectives and environmental conditions.