The spread and rise to dominance of exotic species into the semi-arid grasslands of the western United States represents one of the most dramatic biological invasions to date. Currently, 90% of grasslands within California are dominated by exotic species, most notably exotic annual grasses from Europe. The research conducted here evaluates the effects of the widespread exotic annual grass, Bromus diandrus, and its thatch (i.e. dead persistent litter) on the physical and community structure in a central California grassland. The controls over Bromus diandrus persistence and spread in grasslands were also evaluated.

Observational comparisons between native and exotic-dominated grassland patches were used to determine that exotic-dominated patches had more thatch and 3x less light at the soil surface than neighboring native patches. There were fewer species associated with exotic-dominated patches and species traits, such as timing of growth, seed size and plant height, were important in determining the distribution of species in the two patch types.

Thatch and Bromus diandrus density were manipulated to evaluate the underlying mechanisms leading to native forb decline in grasslands invaded by Bromus diandrus. Thatch inhibited the performance of native forbs, while competition from living Bromus diandrus had little effect. There was also a positive Bromus diandrus-thatch feedback, whereby Bromus diandrus performed best with moderate amounts of thatch. The positive Bromus diandrus-thatch feedback may be one important factor contributing to the rise to dominance and persistence of this species in many grasslands.

Given the negative effects of Bromus diandrus and its thatch on native species performance, I sought to understand the controls over its spread under predicted global change conditions. Bromus diandrus spread was facilitated by N enrichment and constrained by prolonged drought periods achieved through repatterned rainfall treatments.

The invasion of the exotic annual grass, Bromus diandrus, into the grasslands of California has altered important plant resources, like light, and lead to the decline of many native forb species. Largely these biological impacts are mediated through the production of thatch, rather than living Bromus diandrus, indicating that management actions focusing on thatch removal will be the most promising for reducing the impact of this species.