Propagule Pressure and Priority Seeding Effects on the Demography of Invasive Annual and Native Perennial Grass Species

Background: Annual grass invasion and dominance creates a self-sustaining cycle that promotes wildfires and reduces forage abundance. Restoring native plant species to annual grass-invaded ecosystems is fundamental to fostering self-sustaining native plant communities. Aims: We sought to clarify varying restoration strategies on invasive annual and native perennial grass assembly, including strategically modified seeding times, seeding rates, and added water. Methods: We tested the effects of seeding perennial grasses in autumn, spring, or seeding half in autumn and the remaining half in spring, adding water, and varying annual and perennial grass seeding rates on annual and perennial grass life history. Results: While varying perennial grass seeding times did not affect perennial grass germination rates, annual grass germination rates were highest when perennial grasses were seeded in autumn. Seeding perennial grasses in spring produced the highest adult perennial grass density in the first-growing season, but adult perennial grass density in the second-growing season was greatest when seeding occurred in autumn. Second-growing season perennial grass density was highest where annual grass seeding rates were lowest and perennial grass seeding rates were highest. Adding water in the first-growing season produced almost two-times more second-year adult perennial grasses compared to where water was not added. Conclusions: High water availability during the seeding year appears to be the most important factor for retaining perennial grasses in annual grass-invaded ecosystems.