Community-level determinants of smooth brome (Bromus inermis) growth and survival in the aspen parkland

During the initial establishment phase, both biotic and abiotic conditions of the resident community can be important in determining the persistence of invasive plant populations. Invaders may act as passengers by tracking variation in environmental conditions (passenger model), or alternatively, may drive changes in environmental conditions which facilitate their continued invasion (driver model). We distinguish between these two models by comparing variation in resource availability among multiple plant community types to invader initial growth and survival. This study took place in the aspen parkland ecoregion of Alberta, Canada, which is a savanna-type habitat consisting of multiple plant community types and invaded by nonnative smooth brome (Bromus inermis). We characterized four community types by a suite of biotic and abiotic variables (brome seed density, plant richness, plant cover, soil pH, soil moisture, and organic and inorganic N) and performed a brome seed addition experiment. Brome seedling growth and survival were greater with increased levels of soil moisture, while growth decreased with increases in dissolved organic N, but did not vary with other environmental conditions. Both survival and growth of brome seedlings were lowest in brome-dominated areas. These results show support for the passenger model of invasion, as variation in local environmental conditions were associated with variation in brome performance. Further, brome appears to have a negative effect on its own growth, a pattern uncommon among plant invaders.