Site conditions determine a native species’ contribution to invasion resistance in California grasslands

Background/Question/Methods

In grasslands, resident species with functional traits similar to invaders’ can reduce invasion by competing for resources.  Less is known about how resident species’ abundance affects competitive interactions.  Additionally, because many studies occur in highly controlled plots or microcosms, it is sometimes unclear how heterogeneous site conditions alter competitive interactions.  Through a field-based experiment in California grasslands, we explored whether the abundance of a native competitor affected exotic species invasion, and how invasion resistance varied across a complex landscape.  We focused on interactions between two annual forbs:  Centaurea solstitalis (yellow starthistle) a noxious invader spreading across the Western U.S., and the native Hemizonia congesta.  Past microcosm studies indicate that Hemizonia and starthistle use resources similarly in time and space and that greater Hemizonia abundance correlates with reduced invasion.  To test for a similar relationship in real ecosystems, we established plots at four sites across a mixed-grassland/oak woodland landscape.  Plots contained natural populations of Hemizonia to which we added starthistle seed.  Additionally, at one site we watered a subset of plots to investigate soil moisture’s effect on competition. We measured species’ abundances, invader biomass, and site and plot conditions including: aspect, annual grass and litter cover, soil moisture, and species richness.

Results/Conclusions

In this study, we found that greater Hemizonia abundance reduced starthistle invasion, measured as invader biomass, but only at sites with high starthistle germination. Since in grasslands, many plant-plant interactions happen at relatively small spatial scales, the higher starthistle population sizes at these sites may have resulted in strong Hemizonia-starthistle interactions and thus greater competition for limiting resources.  We found weaker Hemizonia-starthistle interactions at the one high-germination site where we added water to subplots supporting the hypothesis that species were competing for limited water supplies.  Hemizonia’s competitive effects on starthistle disappeared in sites with poor starthistle germination.  At these low-germination sites, Hemizonia abundance and starthistle biomass were positively correlated within plots indicating that plot conditions, rather than competition between species, limited invader growth.  At the site-scale, north facing site aspect appeared to reduce starthistle germination, while at the plot-scale soil moisture and annual grass cover resulted in lower germination rates.  This study supports the idea that even in complex grassland ecosystems, it is possible for a native species with traits similar to those of an invader to suppress invasion, but that the strength of this interaction will depend on site- and plot-scale factors that influence population sizes and resource availability.