Plant-soil feedbacks (PSF) are thought to play key roles in plant community dynamics, including succession, invasion and restoration. While negative PSFs are often thought to predominate in established native plant communities maintaining species diversity, positive PSF are thought to drive exotic species invasion. However, it is largely unknown how plant-soil feedbacks might interact with management practices such as grazing or how they are influenced by indirect effects such as plant-plant competition. We address these questions within three grassland types representative for California’s winter rain-fed rangeland systems: perennial natives (e.g. Stipa pulchra), annual exotics that are acceptable forage species (e.g. Avena spp.), and the annual noxious weed Medusahead (Taeniatherum caput-medusae). We set up a greenhouse study in which different live soil inocula derived from the three distinct grassland types under non-grazed or grazed conditions were added to experimental grassland communities in a full factorial design. In addition, single plants of each species in the grassland community (total of seven species) were grown with the same soil treatments to distinguish direct PSF from indirect effects via plant competition.
Results/Conclusions
Grazing influenced the direction and strength of observed PSF: soil inocula derived from grazed vegetation types generally decreased the strength of PSF and differences among species specific PSF diminished. Using soil inocula derived from non-grazed vegetation types, we found that the native species experienced negative PSF when grown in competition. However, when natives were grown without competition they experienced positive PSF. The annual forage grass Avena exhibited the opposite pattern: when grown in competition it experienced strong positive PSF, while experiencing negative PSF when grown individually. Contrary to our expectations, the noxious weed experienced strong negative PSF. Thus, indicating that plant soil feedbacks are not the main driver of the ongoing drastic invasion of Medusahead into California grasslands.
Our study highlights that plant-soil feedbacks can change due to indirect effects such as competition as well as in response to grazing management. A better understanding of those interactions is crucial to improve our understanding of how management actions can interact with plant-soil feedbacks to influence mechanisms maintaining diversity and ecosystem function or resulting in plant invasion.