Plant-Soil Feedbacks May Underlie Competitive Dynamics Between Co-Occurring Invasive Annual Grasses Bromus tectorum...

Plant communities are shaped by interactions among plants and soil legacies. Plant-soil feedbacks (PSF) may alter invasion dynamics and change plant community species composition. There have been field observations of the invasive annual grass, Ventenata dubia, replacing another invasive annual grass, Bromus tectorum, though previous work has not found evidence of competition that would support these shifts. The goal of this study was to determine if PSF via soil legacies and competition are present between co-occurring invasive annual grasses B. tectorum and V. dubia, and if so, quantify their effects on interactions with these species. We measured aboveground per capita biomass of these species grown in biculture with three field soils (neutral, B. tectorum, V. dubia) over two experimental Phases in a greenhouse. We evaluated biomass in Phase 1 with field-conditioned soil, and Phase 2 in soil conditioned from Phase 1. We used generalized linear mixed models to determine the effect of soil origin and plant density on biomass, and non-linear analysis to quantify competition regression coefficients. The linear analysis showed intraspecific competition negatively impacted both species in both Phases, but by Phase 2 interspecific competition was weakened in conspecific soil, suggesting positive PSF. Intraspecific competition coefficients were always greater than interspecific coefficients from the non-linear analysis across soil origins. These results are suggestive of a positive PSF which may alter the strength of competition between these species in a biculture. Understanding how PSF influences interactions between these species provides insight into how their invasion dynamics may change over time.