Control of exotic invaders often involves an herbicide-based approach where the target species is killed and desired plants are expected to recolonize form either dispersal or the seed bank. However, this approach can be problematic because exotics rather than natives often dominate the reestablished community. Three factors that are likely to contribute to continued exotic plant dominance following initial control efforts are1) large exotic seed bank; 2) high exotic dispersal rate/low native dispersal rate; and 3) low competitive ability of natives during early stages of restoration. Topsoil removal may be particularly promising in addressing these factors in restoration of highly invaded areas by reducing the exotic seed bank, reducing nitrogen availability, and reducing exotic allelopathic and soil microbe effects. We designed a multi-factorial experiment manipulating exotic control method (topsoil removal, herbicide, or untreated), the addition of native plugs and seed, and exotic dispersal input in an area dominated by Conium maculatum (poison hemlock) in southern California. We hypothesized that topsoil removal, reducing exotic seed input, and planting native species would improve restoration success.

Results/Conclusions

Topsoil removal plots had lower nitrogen availability, fewer exotics in the seed bank, lower exotic cover, and higher light availability than herbicide and control plots. After two years, plots receiving herbicide and topsoil removal without native addition had a reduction in C. maculatum cover but remained dominated by other exotic annual species. Among plots that were planted with plugs of native shrubs and perennial grasses, topsoil removal significantly improved survival and total cover of natives compared to herbicide and untreated plots. Results suggest that native restoration was limited by dispersal and competitive interactions, and that native seed addition in combination with reduction in the exotic seed bank and exotic cover would improve restoration success. In this system, and potentially others dominated by invasive plants, topsoil removal may have additional beneficial effects on the competitive environment than herbicide alone by altering soil resource and microbial conditions that may promote continued exotic dominance.