The use of phenology and plant morphological traits to reassemble invaded plant communities
The trait-based approach to community assembly focuses on determining the key traits organisms possess and the environmental factors that filter these traits. Our study experimentally examines how phenology and plant morphological traits define vegetation structure and development during restoration of exotic annual grassland. Using native species differing in phenology and morphological traits we established restoration plots to address the question does seeding with functionally similar native forbs result in greater exclusion of the invasive grass Bromus diandrus and the invasive forb Erodium cicutarium?. Four plant functional groups were determined using phenology and morphological traits, these include: 1) perennial forbs, 2) tall winter annual forbs, 3) short winter annual forbs, and 4) summer annual forbs. In November 2012, 1m2 field plots were seeded with the four functional groups with and without invasive species. Plots were sampled throughout the growing season, at peak flowering time, and at senescence for plant species richness, density, percent cover, and biomass. Traits considered and measured included: germination time, maximum height, seed set, seed mass, and mycorrhizal fungi infection. It was hypothesized that native communities that are functionally similar to B. diandrus or E. cicutarium would be more resistant to their establishment through direct competition for resources.
Native forb density, cover and biomass varied significantly with species and treatment (P<<0.001). In general native forb functional groups had greater establishment in plots where invasive species were excluded. Plots that were seeded with both native and invasive species were dominated by invasive species regardless of functional group treatment. In these plots, cover of invasive species ranged from 58-74% with no significant treatment effect. We found no evidence for limiting similarity as seeding with functionally similar native forbs did not significantly affect the establishment and growth of invasive species in our study. Invasive plant density, cover, and biomass did not significantly differ among functional group treatments (P>0.05). Native species that were similar in many measured morphological traits related to competitive ability were poor competitors with Bromus. This may be due in part to the difference in the timing of germination after the first rain event. Phenology differences in native and invasive species give an invasive species an advantage when they promote stabilizing niche differences that then make coexistence between these phenologically offset competitors more difficult.