Soil legacy effects of an invasive grass and implications for restoration

Background/Question/Methods

A history of ranching and agriculture in Southern California has transformed the native coastal sage scrub (CSS) in this region to nonnative grasslands. At Satwiwa cultural center, in the Santa Monica Mountains, Phalaris aquatica is an invasive bunchgrass that was removed in patches from approximately 2006-2013. Native species recruitment into the bare ground where invasive P. aquatica had previously grown was minimal, despite intact CSS bordering the treatment areas. This suggests soil legacy effects of P. aquatica. Our objective was to determine which native plants will survive best in the post-P. aquatica soils and if additional mitigation is necessary to improve native plant establishment. Our primary question was: do certain CSS species and P. aquatica germinate, grow, or survive better in CSS or post-P. aquatica soils? Using a fully factorial design we sowed 100 seeds of three natives (Artemisia californica, Salvia leucophylla, Baccharis pilularis) and the invasive grass into separate 4” pots filled with either intact CSS soil or post-P. aquatica soil, collected in and around the post-P. aquatica site. N = 10 pots per treatment. After seven months of growth in a nursery the seedlings were removed from the soil, washed, dried, and measured.          

Results/Conclusions

In the 2-Way ANOVA, the species were significantly different in most of their seedling growth responses. All of the factors in the model (Species, Soil, and Soil X Species) were statistically significant. Invasive P. aquatica consistently had the greatest growth in CSS soil but had 50% more germination in post-P. aquatica soil. In the 1-Way ANOVA,  A. californica growth was over 50% higher when grown in native CSS soil in almost every measurement. It may be possible that soil restoration methods will improve the restoration success of this species and others. S. leucophylla and B. pilularis may be good nurse species for restoration because their growth was not affected by soil type. The data indicates there are soil legacy effects of the invasive grass, and that effects of the soil on native plant growth and the conditions that support P. aquatica, deserve further study. Future research will focus on compositional analysis of the microbial communities in both soil types and improved native plant establishment through inoculation of native seedlings with CSS soil before planting into the post-P. aquatica soil. Initial findings of the inoculation experiment reveal a lower A. californica mortality when unsterilized vs. sterilized native soil inoculum is used.