COS 136-2
Fine-scale predictions of perennial pepperweed (Lepidium latifolium) invasion and herbicide success in a brackish tidal marsh
Perennial pepperweed (Lepidium latifolium, LELA) is an aggressively invasive crucifer that is a major concern for coastal management. This plant invades a wide variety of habitats including saline wetlands, riparian areas, and upland rangelands. Traditional management actions such as hand removal and mowing are impractical for controlling pepperweed in wetland habitats. Understanding the physical factors that influence invasion and the efficacy of herbicides to control LELA in saline environments will improve management plans. We mapped LELA occupancy in a saline marsh at San Pablo Bay National Wildlife Refuge (San Francisco Bay, California) before and after spraying with imazapyr. We also collected environmental data including elevations, locations of channels, and vegetation types from freely-available sources. Pixels for each dataset were 2 m by 2 m. We combined LELA locations and environmental data into two generalized additive models (GAMs): one describing invasion risk and a second describing the probability that the herbicide spray would reduce LELA cover.
Results/Conclusions
LELA is most likely to invade marshes along drier, elevated areas such as levees, rather than wetter interiors of marshes. The type of vegetation present also has a significant effect on the probability of a pixel being invaded by LELA. Maps that we created using these results successfully reduced the area managers need to search for LELA by identifying areas where LELA is unlikely to establish. The herbicide imazapyr was generally very effective for reducing the size, number, and cover of LELA patches; however imazapyr is most effective for controlling LELA in dry areas such as the sides of levees. Our results suggest that management plans for controlling LELA should focus on detecting and spraying plants as they establish along levees, before patches spread into wetter, less accessible areas of marshes.