Taeniatherum caput-medusae (medusahead grass) is an exotic annual grass introduced to North America (NA) in 1887 that has since invaded ~ 4 million ha of rangelands. In contrast, in its native ranges of Eurasia (EA), medusahead is not considered to be invasive. Why is medusahead invasive in NA, but not in its native lands? We considered two prominent hypotheses: enemy release and soil resource availability. Previous research provides evidence that increased resource availability in California soils, where medusahead is invasive, offers a partial explanation of its invasiveness.
This new study was designed to test the effect of soil (three EA native soils and three NA invaded soils), seed source (three native and invasive populations from soil collection sites), and inoculum treatment (sterile, 20 mm filtrate, whole soil inoculum) on biomass and arbuscular mycorrhizal and pathogen root counts of medusahead. We hypothesized: 1) Medusahead-invaded soils of NA are more fertile than native soils in EA and thus support invasion; 2) Invaded soils of western NA have a lower load of pathogenic organisms, which in native environments reduce the growth and invasiveness of medusahead. Interactions among seed source, soil source, and treatment significantly influenced growth of medusahead.
One of the NA soils was much richer in nutrients than all other soils, while two of the EA soils were very low in nutrients. Two of the NA soils a nd one EA soil were intermediate in nutrient concentrations. Biomass in sterile soils was related to soil nutrients rather than continent of origin. All three NA populations regardless of soil origin generally had high responsiveness to both filtrate and whole soil inoculum. In contrast, the EA populations tended to have higher or equal biomass in sterile soils than inoculated or filtrate soils. Microscopic observation showed higher potential pathogen counts in EA populations. In addition, filtrate appeared to contain “fine mycorrhizal endophytes” that are known to be beneficial to host plant growth, and may explain positive response to filtrate.
Both the resource availability and enemy release hypotheses appear to be operative in explaining invasion of medusahead. Microbial communities from filtrate and whole soil treatments in NA soils enhanced plant growth, while microbial communities in EA soils had reduced or neutral growth.