New research on conifer cutting and prescribed burning helps resource managers and landowners plan sagebrush conservation projects that deliver the best results for the bird and the herd.
For the past 200 years, conifers have been taking over sagebrush country in the western United States. The expansion of woody species is a theme in arid landscapes around the world, and poses a threat to wildlife and working lands that depend on native grasslands.
As species like juniper and pinyon pine encroach onto rangelands, it changes the ecosystem. Expanding conifers reduce available forage for livestock and wildlife, alter fire regimes, increase soil erosion, decreases water availability, and diminish habitat for species like sage grouse and songbirds that need healthy, wide-open sagebrush landscapes to survive.
For instance, as these trees infill rangeland, it increases the fuel available for fires. This in turn can lead to more severe and more frequent wildfires, which then increases the risk of invasive weeds like cheatgrass taking over.
A new study released in the scientific journal of the Society for Range Management, Rangeland Ecology and Management, analyzed how to most effectively treat conifers expanding to sagebrush landscapes. This research can help resource managers and landowners plan conifer removal projects that deliver the best results for the bird and the herd.
Rachel Williams from the U.S. Fish and Wildlife Service and eight other scientists from various universities and agencies looked at 10 sites across the western United States where juniper and pinyon trees had been removed through either prescribed burning or mechanical cutting. They measured vegetation cover and density on untreated and treated plots three and six years post-treatment.
Based on their findings, the scientists recommend tree cutting or other mechanical tree reduction when conifer cover is still low—Phase I to early Phase II expansion, based on the Tree Dominance Index.
Cutting trees resulted in more tall grass and shrub cover post-treatment than prescribed burning. Once relative tree cover increased to 50% (Phase II to early Phase III expansion), the study found that cut sites regenerated in an herbaceous-dominated state, though tall bunchgrasses did still recover.
Retaining shrubs like sagebrush increases ecosystem resilience, and helps resist to weeds like cheatgrass.
The authors state that if cut treatments are delayed to Phase III expansion, the treated sites may require revegetation to achieve a grass-shrub mix. They also recommend following up at mechanically treated sites to remove any missed or new saplings.
This study also found that prescribed fire can be effective for controlling trees on cooler and wetter sites where the risk of cheatgrass is minimal. However, results showed that prescribed fire should be avoided where sagebrush is considered an important component, and on warmer sites where cheatgrass risk is high.
Learn more about the impacts of woodland expansion in this special issue of Rangeland Ecology and Management released in January 2017, or view these 15 brief presentations by scientists: