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Prescribed Fire, Grazing Impact Sierran Forests

Cheatgrass, native to the Mediterranean region, is one of the most widespread weeds in the American West. It forms dense patches and takes early season moisture from native plants. Cheatgrass also increases fire fuels, is unpalatable, and reduces range quality.

Two important disturbance factors in the western United States-grazing and fire-have also been linked to plant invasions. Disturbances that create an imbalance in distribution of resources, such as sunlight, soil nutrients, and water, may alter species composition through shifts in resource availability. These, in turn, may create conditions favoring invasion of non-native species and elimination of native species.

Recent USGS research from the Western Ecological Research Center in the southern Sierra Nevada Mountains shows that grazing by different types of livestock in foothill woodlands may alter the composition of plant species and the distribution of vegetation types.

Because these ecosystems have been heavily invaded by non-native grasses and forbs, low-to-moderate livestock grazing is no longer a major factor in shifts of native/non-native plant dominance. However, nonnative plant dominance is affected by the grazing patters of different species: cattle grazing favors a different type of invasive species than horse grazing.

This research program focused attention on the role of fire in both ponderosa pine and mixed coniferous forests. Across these sites, species diversity is not immediately altered by fire; however, within the first 3 years of a prescribed fire program, high-fire intensity patches exhibit significant increases in species diversity. These patches are also the most susceptible to invasion by non-native plant species.

In these Sierran ecosystems, the threat of invasive species is concentrated in lower elevations and decreases markedly at higher elevations. This pattern occurs in part because most invasives in this region are annual plants, which tend to decline in both species number and dominance with elevation. The lower elevation ponderosa pine forests are potentially most susceptible to new invasions. Particularly troublesome is the apparently recent expansion of non-native cheatgrass in these forests in Kings Canyon National Park.

As is the case with species diversity in general, the expansion of cheatgrass is strongly correlated with localized patch-level fire intensity. Because of this apparent relationship between fire and cheatgrass, prescribed burning has been temporarily halted in these forests. Early control of this apparent invasion is of concern to resource managers in Sierra Nevada parks, and a more detailed study of fire and other concerns regarding cheatgrass invasion are currently being studied by USGS.