Seed Mix Performance with Environmental Stressors and Invasion: Implications for Wetland Restoration

Seed-based revegetation is increasingly pursued in wetland restoration, but seed and seedling mortality is high and can be further limited by environmental stressors, like hydrology and salinity. The interactive effects of hydrology and salinity on native plants, especially in the presence of invasive plants, are less clear. In greenhouse and mesocosm experiments, we tested different hydrologic and salinity conditions on the growth of native seed mixes and Phragmites australis, a widespread wetland invader in North America. The greenhouse experiment tested two mixes (forb mix, graminoid mix), while mixes in the mesocosm experiment combined forbs and graminoids chosen for broad and diverging environmental tolerances. In both experiments, reduced water negatively affected native plants and P. australis, while increased salinities reduced native plant growth but had minimal impacts on P. australis. In the greenhouse experiment, forbs greatly reduced P. australis growth only under benign conditions but maintained growth under increasing abiotic stress despite the presence of P. australis. In the mesocosm experiment, native plants reduced P. australis growth across all conditions, indicating that mixes chosen for broad environmental tolerances can sufficiently reduce invasive growth even when negatively affected by abiotic stressors. Our results demonstrate the varying drivers of invasion resistance: sometimes environmental conditions and species' tolerances limit invasives and other times, the composition and competitive potential of the native community is more important. These results also inform wetland restoration practices, i.e., choosing species for revegetation mixes with broad environmental tolerances and, when possible, manipulating wetland environmental conditions to reduce invader performance.