Bromus inermis and Elymus canadensis but not Poa pratensis demonstrate strong competitive effects and all benefit from priority

Abstract

In the Northern Great Plains, invasive grasses Bromus inermis (smooth brome) and Poa pratensis (Kentucky bluegrass) are substantial threats to remnant prairies because they form monotypic stands that decrease diversity and alter nutrient cycling. A greenhouse study was conducted to evaluate the competitive ability of these two invaders and how priority effects influence competition. We grew each invasive species alone, with a conspecific neighbor, and a native neighbor (Elymus canadensis) to assess competitive ability. Priority treatments included planting both species concurrently, planting the invader prior to the native (by 21 days), and planting the native prior to the invader (by 21 days). Final aboveground biomass and a relative interaction index (RII) were used as response variables to determine the competitive ability of each species. Both competition and priority were found to have a significant effect on native and invasive species performance. Bromus inermis demonstrated a large competitive effect that increased with priority. Poa pratensis demonstrated smaller competitive effects but also benefited from priority. Unexpectedly, our native phytometer E. canadensis demonstrated a very strong competitive effect that, when coupled with priority, nearly excluded one of the invasive species.