Fire Controls Yellow Star Thistle in California Grasslands

Abstract

within test plots. y star thistle (Centaurea solstitialis) first invaded open grassland sites within the San Francisco Bay area sometime before 1869, when a specimen was collected in Oakland. Native to Eurasia, it was apparently introduced in association with livestock use during the Mexican and post-mission period (Maddox and Mayfield, 1985). A fast-growing, aggressive annual herb, yellow star thistle reaches a height of 0.3 to 2 meters (1 to 6.5 feet), and often grows in dense stands, mainly in grasslands. It rapidly crowds out less-aggressive natives. At present it is the most widely distributed weed in California, infesting over 4 million hectares (nearly 10 million acres), or approximately 10 percent of the total surface area of the state. In addition, it has infested large areas in Washington, Oregon and Idaho, and is present in smaller numbers throughout much of the United States and Canada (Maddox, 1981). Infestations of yellow star thistle can have devastating effects on both natural and agricultural ecosystems under certain conditions. It is of economic importance when it invades grainfields, orchards and vineyards, pastures, roadsides and wastelands. In pasture lands, for example, star thistle can lower forage yield and quality, interfere with grazing, cause problems in harvesting of forage and crops, and cause "chewing disease" in horses (Maddox, 1981). In natural areas, yellow star thistle reduces wildlife forage and habitat, displaces native grassland plants and decreases native plant and animal diversity (Sheley and Larson, 1994). Sugarloaf Ridge State Park is located in Sonoma County, in the northern Coast Range near Santa Rosa. It contains a complex mosaic of vegetation types that includes chaparral, mixed evergreen forest, woodland-savanna, grassland and conifer forest. Open grasslands occupy approximately 280 hectares (690 acres) of the park’s 1,080 hectares (2,667 acres). Although these areas have been heavily influenced by livestock grazing and related agricultural practices since before 1900, they have considerable value as examples of the now extremely rare native California grassland. The current grassland complex of the park includes both a native perennial component dominated by purple needle grass (Nassella pulchra), blue wild rye (Elymus glaucus), and creeping wild rye (E. triticoides), and an variety of annual European grassland species dominated by ripgut brome (Bromus diandrus), soft brome or blando brome (B. hordeaceus), silver European hairgrass (Aira caryophyllea), wild oats (Avena fatua), and little quaking grass (Briza minor). Of special concern are scattered populations of the rare California endemic, Sonoma ceanothus (Ceanothus sonomensis), found growing on serpentine outcrops along the grassland and chaparral margins at Sugarloaf Ridge. Listed as endangered by the California Native Plant Society (CNPS), this species is endangered in portions of its limited range. There are good reasons, including the dramatic changes in the forests and grasslands during the past century, to suspect that the invasion of areas like Sugarloaf Ridge State Park by yellow sta thistler is due at least in part to dramatic reductions