Recent studies indicate that the belowground bud bank, rather than the seed bank, is the primary source of new tiller recruitment in tallgrass prairie.  Therefore, we argue that an important additional component of the “resource availability theory” is to consider the factors that determine relative ability of residents versus exotics to capitalize on pulses of new resources. We suggest that, even during episodes of new resources and adequate exotic propagule supply, a community may still be resistant to invasion if the residents possess traits that allow them to pre-empt the newly available resources more rapidly than the exotics. A recurring theme of the 2005 Millennium Ecosystem Assessment was that there is an absence of theories and models to anticipate thresholds in ecosystems. Exceeding these thresholds is expected to result in rapid degradation or collapse of ecosystem health, with slow, or negligible, recovery. Therefore, we tested the following hypotheses: 1) Grassland invasibility is regulated by a minimum threshold in belowground bud bank (BGBB) population densities, and 2) Increases in bud bank size will lead to increases in grassland stability, thereby reducing invasibility by exotic grasses. Field-collected Schizachyrium scoparium meristems were transplanted into microcosms at 0% (control), 33%, 66%, and 100% natural field densities.  After tiller emergence, seeds of three exotic species, Bothriochloischaemum, Bromus japonicus, and Lespedeza cuneata were sown in each microcosm (300 seeds m^-2). Differences in exotic species invasibility was assessed by quantifying final exotic species biomass (biomass = Aboveground Net Primary Productivity or ANPP).
Results/Conclusions

Exotic species biomass was 410.40 g, 45.21 g, 19.05 g, and 12.68 g for 0%, 33%, 66%, and 100% of natural BGBB densities, respectively. Therefore, this study indicates that grassland invasibility is associated with a minimum threshold in BGBB densities. We did not observe a relationship between the various densities of BGBB (33%, 66%, or 100%) and invasibility of these grassland communities. However, our lowest meristem density substantially increased stability of our grassland community. Our findings begin a critical step in obtaining a better understanding of belowground bud banks in rangeland responses to environmental change at the population, community, and ecosystem levels.