Restoring functionally diverse communities enhances invasion resistance in wetlands

1. Understanding community reassembly and diversity interactions is particularly relevant when plant invaders are part of the assembly pool and can compromise restoration goals. If initial species composition has a lasting effect on community structure, manipulating the assembly pool could help increase biotic resistance to invasion.<br>2. We conducted a large plant community restoration experiment in a wetland under high propagule pressure from Phragmites australis (common reed) and investigated species and functional diversity effects of the restored communities on biotic resistance during early assembly using a diversity-interaction model framework. Functional groups differed mostly in life longevity and canopy height and flooding condition was incorporated in all models.<br>3. Experimental assemblages were invaded by 20 exotic or native species including P. australis and two woody taxa (Salix sp., Populus deltoides). Increasing species and functional group richness of the restored communities generally reduced invaders abundance. <br>4. Functional diversity effects were explained by functional group identity (selection effect) and pairwise group interactions (complementarity effect) but, interestingly, their relative contribution varied with invaders. Functional group interactions determined overall community invasibility, whereas functional identity was important against specific invaders. <br>5. The strong performance of fast-growing annuals against seed invasion of P. australis emphasized priority effect. Flooding contributed to reduce invaders abundance except for P. australis.<br>6. Synthesis and applications. Understanding the circumstances under which diversity effects of restored communities prevail can inform ecological applications. Wetlands are highly susceptible to invasion and wetland restoration is a major issue worldwide. This is one of the few tests of the diversity-invasibility relationship in wetlands and one of the first quantifying the relative contribution of complementarity-diversity effect and selection effect to invasion resistance under fluctuating environmental conditions. <br><br>