Structural and compositional differences between native and non-native dominated grasslands

Background/Question/Methods: The impacts of non-native plant species on vertical and horizontal physical structure are rarely quantified, yet are likely to be important in determining what species can coexist with the invader. Impacts on physical structure and diversity may be greater when the life history of the non-native species differs from that of the native dominant species. Non-native annual grasses are dominant features in many of California’s grasslands and these invaded grasslands often contain or abut small patches of grassland dominated by native perennial bunchgrasses. We used paired neighboring non-native and native-dominated grassland patches to evaluate how the shift in dominance from the native perennial bunchgrass, Nassella pulchra, to the early season, non-native annual grass, Bromus diandrus, affects habitat structure and plant community composition and how species traits correlate with which forb species will occur within each grassland type. We hypothesized that grasslands dominated by B. diandrus would have more continuous vegetation and leaf litter cover, resulting in lower light availability and that this would have an impact on community composition and select for certain traits, such as offset flowering phenology, seed size and height.

Results/Conclusions: Our findings supported the hypothesis that B. diandrus grasslands were structurally unique from N. pulchra grasslands by having more homogenous vegetation and leaf litter cover and lower light availability. Surveys in two separate rain-years revealed that grasslands dominated by B. diandrus had lower total and native richness and diversity than those dominated by N. pulchra. Across both rainfall years, species height was an important trait in determining where a species was found with short statured species more commonly found in N. pulchra grasslands and tall species being more prevalent in B. diandrus patches. Flowering phenology was an important trait in a wet year with early season species being more abundant in B. diandrus grassland and later season species being more abundant in N. pulchra patches. In a dry year, small seed size was important in the N. pulchra grasslands and large seeded species were more commonly associated with B. diandrus dominated grasslands. These findings provide evidence that a non-native species can alter physical structure thereby reducing light availability in a habitat that historically has not been light limited. This change in resource availability has a negative impact on native grassland species that are unable to cope with low light levels.