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Non-native species with growth forms that are different from the native flora may alter the physical structure of the area they invade, thereby changing the resources available to resident species. This in turn can select for species with traits suited for the new growing environment.
We used adjacent uninvaded and invaded grassland patches to evaluate whether the shift in dominance from a native perennial bunchgrass, Nassella pulchra, to the early season, non-native annual grass, Bromus diandrus, affects the physical structure, available light, plant community composition and community-weighted trait means.
Our field surveys revealed that the exotic grass B. diandrus alters both the vertical and horizontal structure creating more dense continuous vegetative growth and dead plant biomass than patches dominated by N. pulchra. These differences in physical structure are responsible for a threefold reduction in available light and likely contribute to the lower diversity, especially of native forbs in B. diandrus-dominated patches. Further, flowering time began earlier and seed size and plant height were higher in B. diandrus patches relative to N. pulchra patches.
Our results suggest that species that are better suited (earlier phenology, larger seed size and taller) for low light availability are those that coexist with B. diandrus, and this is consistent with our hypothesis that change in physical structure with B. diandrus invasion is an important driver of community and trait composition.
The traits of species able to coexist with invaders are rarely considered when assessing community change following invasion; however, this may be a powerful approach for predicting community change in environments with high anthropogenic pressures, such as disturbance and nutrient enrichment. It also provides a means for selecting species to introduce when trying to enhance native diversity in an otherwise invaded community.