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 is 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 three discrete but related analyses: 1) examined how seed germination and viability of perennial pepperweed change with increasing salinity in San Francisco Estuary tidal wetland sites. 2) determined if salinity, flooding, and native vegetation cover, influenced recruitment of L. latifolium from early developmental stages through inflorescence production. 3) 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. 4) 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 indicates that salinity influences every early life history stage, from viability and germination through inflorescence development and seed production. Adult plants respond to salinity at a regional scale, with declining abundance corresponding to increasing salinity. Within sites salinity continues to be a significant determinant of abundance, but plants are more closely tied to foliar nitrogen and flooding surrogates (mean soil particle size, and elevation). Patterns of herbicide efficacy resembled patterns of recruitment such that plants weakened by salinity, and more so by the combination of salinity and flooding were most susceptible to treatments. Recommended control programs will evaluate site attributes as well as control probabilities in the development of a comprehensive management strategy.