Bromus inermis invasion of a native grassland: diversity and resource reduction

Invasion-driven diversity reduction is often attributed to decreased resource availability, but this has rarely been examined. We tested whether the invasion of native grassland by the introduced grass, Bromus inermis, was associated with reduced diversity, increased standing crop (including roots), and decreased resource availability. Diversity and evenness were significantly lower in invaded stands, but richness was not significantly different. Both shoot and root mass were significantly greater in B. inermis stands, suggesting that resource demand should be higher. Light penetration and soil moisture were significantly lower beneath B. inermis. In contrast, most nutrients (including available N) did not vary between vegetation types. Some nutrients (P, K, Ca, and Mn) were significantly more abundant beneath B. inermis, possibly reflecting the invader's tendency to invade lower landscape positions. Overall, the results are consistent with invasion-driven diversity reduction being caused by increased resource demand and decreased availability of light and water.