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Citations
... Communicated by Holly Greening Blank 2004;Reynolds and Boyer 2010). Management has been particularly challenging in SFE wetlands, as mechanical control and herbicides are often impractical or prohibited (Spenst 2006;Hogle et al. 2007;Boyer and Burdick 2010). ...
Invasive species exert disproportionate impacts in wetlands and pose particular challenges for rare species persisting at small spatial scales. In the urbanized San Francisco Estuary (SFE), which contains 90% of California’s remaining coastal wetlands, invasive and rare species often co-occur. One narrow endemic taxon, the federally listed Suisun thistle ( Cirsium hydrophilum var. hydrophilum ) is restricted to two or three locations where the invasive perennial pepperweed ( Lepidium latifolium ) has an increasing presence. Perennial pepperweed has invaded salt, brackish, and freshwater wetlands around the SFE, leading to high management concern. In this study, we investigated how perennial pepperweed may contribute to further rarity of the Suisun thistle, by conducting a removal experiment and surveying soil-plant relationships. Removing pepperweed led to a doubling of native species relative cover and an increase in native species richness by an average of one species per plot, positive effects on Suisun thistle cover, number, and reproductive output, and shifts in soil properties. Combined with survey data inside and outside of pepperweed stands, we conclude that pepperweed competes with Suisun thistle via competition for space, nutrients, and light, interferes with the Suisun thistle’s reproductive success, and alters brackish marsh soil physicochemical characteristics to further favor pepperweed. We recommend local control of pepperweed to prevent further loss of Suisun thistle. Further, the wide range of mechanisms by which this invasion may proceed if unchecked should be considered in other settings where rare or uncommon species are at risk from invaders.
Perennial pepperweed (Lepidium latifolium) is an aggressively invasive species that spreads by vegetative growth and seeds. Common methods for removal such as hand-pulling and mowing are impractical in brackish marsh environments. We evaluated the effects of two herbicide treatments (imazapyr and imazapyr + glyphosate) against a non-herbicide control (flower head removal) on invasive pepperweed and native vegetation in three habitats (bay edge, channel edge, and levee) in brackish marshes. Both herbicide treatments produced significantly better control of pepperweed than the control, but imazapyr alone took two years of treatment to produce levels of control that were similar to one year of the imazpyr + glyphosate treatment. Both herbicide treatments also reduced native cover, but the effects were more severe in plots treated with imazapyr + glyphosate than in plots treated with imazapyr alone. Effects on pepperweed were similar across the three habitats, but impacts on native vegetation were less severe in bay edge environments. Managers should consider the tradeoffs when choosing a treatment plan for pepperweed: the quick reduction of pepperweed achieved by the combination of imazapyr and glyphosate may come at the expense of creating opportunities for reinvasion by causing bare ground and/or patches of litter that are slowly recolonized by native species.
