Abstract
Understanding the factors that drive biotic resistance is important for designing invasion resistant wetland seed mixes. Two factors that affect biotic resistance and can be altered in a seed mix are native plant seeding density and diversity. Here we evaluated the effects of native plant functional group identity (annual forb, bulrush, grass, perennial forb, or rush) and native seeding density (low or high) on the growth of native wetland species and an invader (Phragmites australis) at two sites in the Great Salt Lake Watershed, USA, over two years. We found that the effects of the seeding treatments on biotic resistance appeared to be largely overwhelmed by water depth due to two extreme weather events (severe drought in year 1 and prolonged flooding in year 2). However, these events provided an opportunity to observe the recovery potential of the different functional groups and identify three species (Bolboschoenus maritimus and Schoenoplectus acutus from the bulrush functional group and Distichlis spicata from the grass functional group) that were able to survive the extreme weather conditions during both growing seasons at one of the field sites. Furthermore, we found that different emergent wetland restoration sites can have different plant community outcomes despite geographic proximity, possibly due to distinct site environmental conditions. These findings underscore the importance of using bet-hedging strategies (e.g., seeding diverse mixes with species with a range of hydrologic tolerances) to overcome wetland restoration challenges under hydrologic extremes, conditions likely to become more common with climate change and ever-increasing upstream water diversions.