Long-term effects of prairie restoration : community structure and native plant population dynamics 6 years after experimental management

Restoring degraded habitats with the goal of achieving long-term ecological complexity and stability is an essential component in combatting global declines in biodiversity. The main objectives of prairie restoration are to reduce the abundance of exotic species while enhancing native species richness and abundance, but it is often difficult to extend monitoring to evaluate these as long-term goals. Understanding how initial outcomes persist or change over time is essential for evaluating treatment efficacy. Additionally, observing how specific native populations persist and spread following restoration treatments can inform future decisions regarding seeding practices and management timelines. To assess the degree to which initial treatment effects continue after project completion, I revisited remnant patches of Pacific Northwest Garry oak savanna/prairie habitat 6 years after experimental restoration treatments were applied. I evaluated the composition and structure of the plant community at each site to determine if, and how, the effects of disturbance treatments and supplemental native seeding changed in the years following experimental management. I tracked the persistence of seeded species and measured spread of their populations as a metric to evaluate longer-term success, suitability of native species for restoration, and the ability of the habitat to support native plant populations. I found that plots that received supplemental seeding continued to exhibit higher native species richness than those left unseeded, and that both seeding and disturbance treatments could positively influence the long-term pattern of native species abundance. The initially-observed effects of disturbance treatments on reducing exotic grass abundance diminished after 6 years, but nevertheless these treatments significantly influenced the population trajectories of 4 out of 8 seeded species. There was spatial advance of most seeded species’ populations, as evidenced by occurrences in previously unoccupied plots. A case study of the seeded species, Plectritis congesta, allowed for estimation of the average rate of spread per generation and quantification of the long-term spatial influence of seeding efforts. The results from my extended monitoring confirm that seed limitation of native species and difficulties maintaining the reduction of exotic grasses continue to be major barriers to success in restoration of invaded prairies.Arts, Faculty ofGeography, Department ofGraduat