This experiment explored the possible evolutionary adaptations of the native Achillea millefolium (Asteraceae) that have occurred because of coexistence with the perennial invasive grass Holcus lanatus (Poaceae). Holcus is known to cause abiotic and biotic changes to the soil with which it associates, usually making for a more inhospitable growing environment for native plant competitors. We tested whether Achillea populations that coexists with Holcus have adapted to Holcus-altered soil. We hypothesized that coexisting, ‘experienced' Achillea populations would have greater performance when grown on soil from sites invaded by Holcus than ‘naïve' Achillea populations. Achillea seed was collected from both sites in which it coexists with Holcus and in which Holcus is absent. Likewise we collected soil from the same invaded and uninvaded sites. Seed and soil collections were replicated throughout Bodega Marine Reserve (BMR) and Point Reyes National Seashore (PRNS). Achillea seeds (‘experienced' from coexistence with Holcus in an invaded site or ‘naïve' from growth in an uninvaded site) were planted into pots containing a sterilized background soil (invaded or uninvaded site origin) and soil inoculum. The soil inoculum was of invaded or uninvaded site origin and live or sterilized. Plants were grown with a non-P fertilizer for several months in the greenhouse during which time growth measurements were taken. Aboveground and belowground biomass was harvested for dry weight measurements. Seeds from each reserve were experimentally planted in soils that only came from that reserve. Thus BMR Achillea seeds were only planted into soils from BMR and likewise for seeds and soil from PRNS. Statistical analysis was also done separately for each reserve. Results/Conclusions
Results show varying direct effects of these soil treatments on Achillea growth. Soil treatment effects depended upon the reserve from which the seed and soil were collected. There were differing significant main results and interactions on total biomass based on the site origin of the soil inoculum and whether or not it was sterilized. Additionally, a three-way interaction between soil effects and Achillea populations (‘experienced'/ ‘naïve') was significant for tests from both reserves, although not in the same direction. These results show that there is an indirect effect of Holcus on its coexisting native plants via its associated soil.