Wapato Valley Mitigation and Conservation Bank’s primary invasive species is reed canarygrass (Phalaris arundinaceae). Wapato Valley Bank is located in Clark County, Washington on 876 acres of diverse floodplain habitats in the Columbia River Estuary that have been altered over the last 170 years to meet human needs. Invasive plants are difficult to curb in wetland restoration as they have few competitors and can grow in a variety of habitats. There are few ways to control reed canarygrass, including flooding the area by altering topography. The Bank will excavate a minimum of 45 cm of soil to get below the root system and create a wetland surface low enough to reduce reed canarygrass’s competitive advantage. Recent restoration projects in the Lower Columbia Estuary used a scrapedown method to remove non-native species and prepare restoration areas for tidal inundation. There have been speculations that spontaneous native plant growth in these areas were from a seed bank response rather than native seed recruitment. We wanted to learn if the Wapato Valley Bank site has a viable native seed bank below 45 cm. We extracted 300 soil cores to 140 cm below the soil surface. Samples were exposed to flood treatment to mimic the tidal hydroperiod of the proposed restoration condition. A subset of samples was sent to the Oregon State University (OSU) seed lab to identify seeds. This provided verification that our growing conditions were not limiting germination, and a comparison of the total seedbank with what germinated in the greenhouse.
Results/Conclusions
We found that there is no effective seed bank beneath the ground surface. Seeds that did germinate likely resulted from sample contamination during extraction. The samples we sent to OSU also came back with no viable seeds below the surface layer. The absence of sprouts is likely due to the wet conditions below the soil surface, and the duration of burial. Additionally, microtopography in the field can cause spotty seed dispersal. It is possible that we missed some pockets of seeds. However, the lack of seed germination, and similar results from the seed lab indicate we cannot rely on a seedbank to provide a meaningful component to revegetation after restoration construction. We do expect to see germination from native seed recruitment coming in from flood water and adjacent wetland areas. We intend to replant with native seed and plant material to ensure a native plant population after restoration.