Document: DUK-3-328-178

Species composition, species richness, and soil disturbance affect grassland susceptibility and response to invasion.

DUKES, J.S.*

University of Utah, Salt Lake City, UT 84112 USA ¹

Abstract:
I constructed grassland microcosms to study how species composition, species richness, and soil disturbance affect the susceptibility of California grassland to invasion by Centaurea solstitialis (yellow starthistle), and how Centaurea invasion affects community productivity. I constructed monocultures to study competition between Centaurea and species from four different functional groups. I created multi-species communities that differed in species richness, but not funcional group richness, to determine how species diversity affects the invasibility of plant communities. I introduced Centaurea into both newly-constructed and 1-year old microcosms of each community type to study how simulated disturbance affected community invasibility. For each community type, I compared productivity between microcosms with and without Centaurea to assess how starthistle invasions affect grassland productivity. Invasibility (Centaurea biomass production) was a decreasing, asymptotic function of species richness in both the new and 1-year old communities. The age of the communities affected their invasibility. Surprisingly, the proportion of total community biomass made up by Centaurea was greater in 1-year old communities than in new communities, with the exception of the perennial grass treatments. Although several monocultures increased total biomass production in response to Centaurea invasion, multi-species communities and monocultures of late-season annual forbs did not. Competition with Centaurea significantly decreased productionof resident species in several communities. In new microcosms, Centaurea significantly decreased production of resident species in several communities. In new microcosms, Centaurea depressed the production of resident species more in simple communities than in diverse communities. This effect of diversity on the response of resident species to the invader was not evident in the 1-year old microcosms. Results from this study generally support the hypothesis that diverse communities resist invasion more successfully than simple communities. The results also suggest that, in some cases, diversity may decrease the impact of invasion on resident species.

Keywords: grasslands

This abstract is being presented at: 9:45 AM in session:
Oral Session #19: Grassland Restoration.