The biology and ecology of Lepidium latifolium L. in the San Francisco estuary and their implications for eradication of this invasive weed

Lepidium latifolium (perennial pepperweed) is a widespread and invasive weed that is problematic throughout the western United States and internationally. The aim of this research was to improve understanding of L. latifolium biology and ecology and the influence of each on control efficacy in San Francisco Estuary tidal wetlands, with particular regard for variation along the salinity gradients in the estuary. To address these issues, I conducted four discrete but related analyses: (1) I examined how seed germination and viability of perennial pepperweed change with increasing salinity in San Francisco Estuary tidal wetland sites; (2) I determined if salinity, flooding, and native vegetation cover, influenced recruitment of L. latifolium from early developmental stages through inflorescence production; (3) I assessed the relative importance of environmental constraints to L. latifolium distribution; then compared the role of pepperweed, relative to recognized environmental constraints, in determining other wetland species distributions; and assessed the effect of prior year L. latifolium cover on native and non-native species; and (4) I evaluated response to herbicide treatments in low and high density L. latifolium stands and examined post-treatment changes in native abundance at three tidally-influenced sites representing salinity gradients in the estuary.;My research indicated that salinity reduced establishment and growth at all life history stages, from viability and germination through inflorescence development and seed production. Mature L. latifolium plants displaced native species. Mature plants were able to persist under salinities that had limited germination and recruitment; but under elevated salinities, plants were restricted to higher elevations and more well drained soils. Greatest abundance was correlated with high foliar nitrogen levels. Herbicide control improved with increasing salinity, and particularly with the combination of moderate salinities and flooding. The combination of these edaphic conditions along with a native-dominated propagule pool and the absence of other invasive species resulted in the highest control efficacy.