Aims Plants of similar life forms and closely related species have been observed to create similar types of plant–soil feedbacks (PSFs). However, investigations of the consistency of PSFs within species have not yielded clear results. For example, it has been reported that species create different types of PSFs in their native and introduced ranges. The aim of this project is to examine if four species create similar PSF types from soils collected from widely distributed areas within their introduced range. The soil for this project was collected from three areas in western North America. With this design, we aim to determine species- and site-specific ability to create PSFs and if the type of PSF created is consistent in all soil from all three collection areas. The species examined are Agropyron cristatum, Centaurea solstitialis, Poa pratensis and Taeniatherum caput-medusae.
Methods We used three-field collected soils (from northern Nevada, western Montana and eastern Montana) in a two-phase greenhouse experiment to quantify the type of PSFs created by four invasive species. The first phase was a conditioning phase wherein each invasive species created species-specific changes to the soil. The second phase of the experiment was the response phase wherein both the conditioning species and a native phytometer were grown in the conditioned soil and in unconditioned (control) soil. The final aboveground biomass was used to evaluate the effect of conditioning and to determine the type of PSF created by each invasive species.
Important findings Our results suggest that three of our four study species did show consistency in relation to PSF. Two species A. cristatum and T. caput-medusae consistently created PSF types that benefit conspecifics more than heterospecifics (and thus are 'invasive' PSF types) and P. pratensis consistently exhibited no, or 'neutral', feedbacks. The fourth species (C. solstitialis) was inconsistent: in one soil, no feedback was created; in other soil, an invasive PSF was created and in the last soil, a feedback that relatively benefited the native phytometer was created. Thus, PSFs appear to uniformly contribute to the success of two species (A. cristatum and T. caput-medusae) but not C. solstitialis nor P. pratensis.