Tipping the Native-Exotic Balance: Succession in Restored Upland Prairies in Oregon’s Willamette Valley

Abstract

Restoration projects are usually assessed using short-term measures, leaving a dearth of detailed long-term successional data from restoration sites. This study examines the effects of microsite variation and restoration treatment on successional trajectories and three-to-eight-year restoration outcomes in upland prairies in Oregon’s Willamette Valley. I sought first to determine whether different upland prairie restoration units show similar successional trajectories, second, to identify which exotic species have persistent effects on restoration outcomes, and finally, to determine how variation in restoration treatment, topography, and aspect affect restoration outcomes. Permanent plots in two replicate units showed very similar trends over five years, and similar outcomes to restoration units at a second upland site. Exotic annuals were short-lived and did not prevent establishment of the primary planted species, the native grass Festuca roemeri (Roemer’s fescue). Festuca roemeri established well mid-slope and in valleys but not on ridges. Where native forbs were seeded, forb abundance was higher in plots with less F. roemeri cover, suggesting that it is not cost-effective to seed forbs along with high rates of native grasses. However, seeding F. roemeri at high rates may be an effective strategy when forb seed is not available. A suite of exotic perennial forbs, including Hypochaeris radicata (false dandelion), contributed to diversity and provided nectar resources for pollinators without becoming dominant, while other exotic forbs such as Cirsium spp. (thistle) and Dipsacus fullonum (Fuller’s teasel) required active management.