INVASIBILITY OF ROADLESS GRASSLANDS: AN EXPERIMENTAL STUDY OF YELLOW STARTHISTLE

Abstract

Roadless habitats are commonly found to be less invaded than habitats near roads, but few studies have tested whether this pattern is due to propagule limitation or to greater invasion resistance of roadless sites. We examined reasons for the lower frequency and cover of yellow starthistle (Centaurea solstitialis) in grassland sites >1000 m vs. 10 m from roads in an inland California, USA, foothill landscape. During winter 2001 and 2002, we planted 100 Centaurea seeds in 64 pairs of 30 x 30 cm plots (uncleared and cleared of aboveground plant material) at sites stratified by distance from roads (10 m and >1000 m), soil type (nonserpentine and serpentine), and aspect (cool, warm, and neutral slopes). In nonserpentine grasslands, Centaurea survival was greater in uncleared (but not cleared) near (10 m) plots than in distant (>1000 m) plots. These findings suggest that the effect of distance from roads on survival corresponds with higher aboveground biotic resistance in distant sites than in near sites. Centaurea biomass was greater in near than in distant plots (both uncleared and cleared) on nonserpentine soil, suggesting that either abiotic resistance or belowground biotic resistance could limit its growth in distant sites. These distance effects were no longer significant in models that included two biotic co- variates: native grass cover (which was higher in distant sites and negatively correlated with Centaurea performance) and bare ground (which was lower in distant sites and pos- itively correlated with Centaurea performance). On serpentine soil, there was no effect of road proximity on the performance of planted Centaurea. Our results suggest that Centaurea seeds can germinate in nonserpentine and serpentine grasslands regardless of distance from roads. Beyond the seedling stage, however, biotic resistance associated with higher native grass cover and lower levels of disturbance may inhibit Centaurea invasion of nonserpentine grassland sites that are distant from roads.