Role of propagule pressure and priority effects on seedlings during invasion and restoration of shrub-steppe

Abstract

Plant invasion and restoration outcomes are largely driven by the timing and magnitude of seed dispersal, and by the performance of dispersed species in an environment. Because seed dispersal controls recruitment of newly arriving species and facilitates safe site occupation, assembly will differ depending on seed dispersal processes and variable environmental conditions. The objective of this study was to identify how annual and perennial grasses assembled when dispersal times, propagule pressure, and water availability were modified. To assess these effects, we conducted a field experiment in an annual grass invaded shrub-steppe ecosystem in eastern Oregon. We tested the effects of seeding annual and perennial grasses in autumn or delaying annual grass seeding until spring, adding water, and varying annual and perennial grass seeding rate by 150, 1,500, 2,500, or 3,500 seeds m⁻² on perennial and annual grass seedling emergence through time and final density and biomass. Providing perennial grasses a priority effect by delaying annual grass seeding until spring initially facilitated perennial grass establishment, but this effect did not persist into the second growing season. We found that if annual grass propagule pressure exceeded 150 seeds m⁻², perennial grass recruitment was limited. In addition, higher water availability increased perennial grass establishment, but was dependent upon annual grass propagule pressure. These findings suggest that seeding perennial grasses into annual grass dominated systems is more dependent upon the existing propagule pressure of annual grasses than the priority effects of perennial grasses, the propagule pressure of perennial grasses, or water availability.