Bud bank density of warm-season grasses as regulators of grassland invasibility

Background/Question/Methods

Exotic species invasions have serious consequences for native species populations, biodiversity, and ecosystem processes. Thus, identifying factors that influence invasions by exotic plant species is of critical importance. Fluctuations in resource availability may be a key factor controlling invasibility, with successful invasions occurring only intermittently, when resource enrichment or release coincide with the availability of propagules of exotic species.  Thus, invasions will occur during “windows of opportunity” when propagules can capitalize on newly released or available limiting resources. In perennial grasslands, the belowground bud bank (BGBB) plays a fundamental role in local plant population persistence, structure and dynamics; all important factors determining invasibility. We suggest that, even during episodes of new resources and adequate exotic propagule supply, a community will still be resistant to invasion if the residents possess traits that allow them to pre-empt available resources more rapidly than the exotics. Using greenhouse and field studies (Oklahoma and Kansas) we tested the following hypotheses: 1) Grassland invasibility is regulated by a minimum threshold in BGBB population densities, and 2) Increases in bud bank size will lead to increases in grassland stability, thereby reducing invasibility. 

Results/Conclusions

At peak biomass, our greenhouse microcosms had mean exotic species biomass (g) of 410.40, 45.21, 19.05, and 12.68 for 0%, 33%, 66%, and 100% of native BGBB densities, respectively.  Similar results were observed in a complementary field study where native BGBB densities were reduced. The 2-year average of exotic species biomass (g) at the Oklahoma site was 166.29, 29.32, 6.37, and 2.99 for 0%, 33%, 66%, and 100% of native BGBB densities, respectively. The 2-year average of the Kansas site was 17.45, 0.17, 0, and 0 for 0%, 33%, 66%, and 100% of native BGBB densities, respectively. Our study indicates that grassland invasibility is closely associated with a minimum threshold in BGBB densities. In both our greenhouse and field studies 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 (0%) substantially decreased stability and increased invasibility of our grassland community. Our findings begin a critical step in obtaining a better understanding of BGBB in rangeland responses to environmental change.