Quantifying large-scale impacts of cattle grazing on annual burn probability in Napa and Sonoma Counties, California

Abstract

Wildfire in California is an increasing threat to life and property. The expansion of urban and suburban development into wildlands limits risk-reduction options like prescribed burning, whereas large-scale mechanical and herbicide treatments can be cost prohibitive and unpalatable to the public. Cattle grazing is a low risk, affordable treatment not frequently considered for use explicitly for fuels reduction in California. To examine the impact of cattle grazing on fire in Napa and Sonoma Counties, California, we quantified its effects as change in average annual burn probability. Probabilities were calculated for 2001–2017 using mixed-effect regression models in combination with a range of grazing intensities and extents. These grazing scenarios were designed to represent current grazing conditions, ungrazed conditions, adding grazing to high priority landscapes, and grazing the full study area. We estimated that under current grazing conditions, cattle grazing reduces average annual burn probability 45% (from 9.9% to 5.4%) compared to ungrazed conditions. Adding grazing to high priority landscapes as identified by the California Department of Forestry and Fire Protection (CAL FIRE) decreased their average annual burn probability by 82% (from 7.6% to 1.4%) compared to under current grazing conditions. Of the scenarios assessed, grazing high priority landscapes heavily while maintaining the current extent and intensity of grazing on other rangelands provided the best return in terms of decreased burn probability per additional area grazed. Finally, we demonstrated how our methodologies can be utilized by fuel managers and planners to identify key areas for treatment with cattle grazing. Our findings suggest cattle grazing provides benefits to the study area by reducing overall burn probability, and that extending its use to treat fuels in priority areas in and around the wildland urban interface could provide further fire-risk reduction on community-adjacent lands. Land managers may find cattle grazing a valuable long term fuel-management tool at the landscape scale.

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