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

Authors: SCHANTZ, MERILYNN - Oregon State University; Sheley, Roger; JAMES, JEREMY - University Of California Agriculture And Natural Resources (UCANR)

Submitted to: Biological Invasions
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/21/2014
Publication Date: 5/6/2014
Citation: Schantz, M.C., Sheley, R.L., James, J.J. 2014. Role of propagule pressure and priority effects on seedlings during invasion and restoration or shrub-steppe. Journal of Arid Environments. 17(2015):73-85. https://doi.org/10.1007/s10530-014-0705-2.
DOI: https://doi.org/10.1007/s10530-014-0705-2

Interpretive Summary: Invasive annual grass-dominated ecosystems pose a significant challenge for practitioners seeking to restore native perennial grasses to arid rangelands. Seeding can be an effective management strategy, but has variable success and thus, modifying seeding practices may increase perennial grass recruitment in annual grass invaded ecosystems. In this study, we evaluated how annual and perennial grass seeding time, seeding rates, and water availability affected annual and perennial grass performance through their life history (germination to adult). We found that perennial grass density was highest two years following seeding where annual grass seeding rates were lowest, perennial grass seeding rates were highest, and annual and perennial grasses were seeded in autumn.

Technical 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.