Restoring an invaded landscape through management and community assembly using multiple functional groups

Background/Question/Methods

Restoring areas with a long history of invasion to a native community can be difficult due to persistent effects of disturbance, dramatic depletion of the native seed bank, and lack of information of the original community present. Thus, supplementing the seed bank is a key component of ecological restoration and the combination of species used can affect the establishment and survival of the native community. The limiting similarity hypothesis suggests that seeding native species with similar functional traits as persistent invaders can limit non-native growth. Additionally, niche theory suggests seeding with a diverse mix of species will limit the available niche space for non-native species. We seeded a large-scale restoration experiment with a mix of ruderal herbaceous natives (Forb), perennial natives (Shrub), or a mixture of the two (Forb+Shrub) using either hand- or drill-seeding methods. The non-natives at the site were either left undisturbed, or were removed by (1) hand weeding/mowing or (2) treated with herbicide in a factorial design. Weeding treatments were repeated each spring for 3 years. Germination was measured 2 weeks after the first significant rain event each season prior to the hand removal treatment. Cover of all species was then measured in late spring after flowering.

Results/Conclusions

After three years, Forb seeded plot native species germination was 37% greater than control plots, while Forb+Shrub treatments was 33% greater, driven primarily by the forb species. There was minimal germination from native species not directly seeded to a plot suggesting limited seed dispersal among plots. Invasive forb germination was 46% lower in managed plots suggesting that the seed bank was beginning to decline. There was no difference in germination between drill and hand seeded plots. In spring, native cover was 44%, 80%, and 50% for Forb, Shrub, and Forb+Shrub seeded plots respectively. Shrub and Shrub+Forb had higher diversity and evenness than Forb plots. The decrease in shrub cover between Shrub and Forb+Shrub plots suggests that ruderal native annuals are acting similar to the non-native species and influencing perennial establishment. Since shrubs were successfully established during the first year of restoration, it may beneficial to first establish the perennial community before following up with a secondary seeding of native annuals. These results highlight the importance of considering limiting similarity in the context of plant ontogeny and the dynamics of establishment, such that traits that influence the establishment niche of native species are carefully considered in the sequencing of ecological restoration.