Effects of eradication and restoration treatments on Italian thistle ( Carduus pycnocephalus )

Abstract

Low elevation grasslands in California long have been dominated by Mediterranean grasses, but many areas still have large native forb populations. Alien forbs invade these grasslands, displacing both native and other alien species. Italian thistle is a noxious alien herb that has recently invaded these grasslands, including ungrazed blue oak ( Quercus douglassii ) and interior live oak ( Quercus wislizenii ) stands in Sequoia National Park. Here, Italian thistle tends to dominate under oaks and has the potential to substantially alter the foothill ecosystem by displacing native plants and acting as a ladder fuel that can carry fires into the oak canopy. We tested the effects of selectively reducing Italian thistle populations alone and in combination with restoration of native species. Two thistle eradication techniques (clipping and the application of clopyralid herbicide) and two restoration techniques (addition of native forb seeds or planting native grass plugs) were used. After two consecutive years of treatment we found: a) clipping was not effective at reducing Italian thistle populations (clipping reduced Italian thistle density in some areas, but not vegetative cover), b) herbicide reduced both Italian thistle density and vegetative cover for the first two growing seasons after application, but cover rebounded in the third growing season, c) native forb cover and species richness were not significantly affected by clipping or spot-treating with herbicide, d) the grass and forb addition treatments by themselves were not effective at reducing Italian thistle during the course of this study and e) sowing annual forb seeds after clipping resulted in greater forb cover and moderately reduced Italian thistle vegetative cover in the short term.