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Broad infestations of invasive, non-native vegetation have transformed wetlands around the world. Ludwigia hexapetala is a widespread, amphibious invasive plant with a creeping growth habit in open water and an erect growth habit in terrestrial habitats. In the upper San Francisco Estuary of California, L. hexapetala is increasingly terrestrializing into marshes and this expansion may be facilitated by allelopathy. We conducted the first field-based study on L. hexapetala allelopathy to determine whether (1) three allelochemicals known to be exuded by L. hexapetala are expressed in situ, (2) the allelochemicals are detectable in leaves, soil, and water, and (3) allelopathic expression varies by season, salinity, and growth habit (open water "patch" vs. terrestrial marsh "interface" locations). Water, soil, and L. hexapetala leaves were collected in two freshwater sites and two oligohaline sites in the upper San Francisco Estuary in summer 2021, fall 2021, and spring 2022. Myricitrin and quercitrin, known allelochemicals, and salipurposid, a newly identified polyphenol, were detected in water, soil, and leaves. There were significant differences in allelochemical concentrations under fresh versus oligohaline conditions in water and soil, but not leaves. All three allelochemicals generally had higher concentrations in patch versus interface locations, suggesting that L. hexapetala allelopathy plays a greater competitive role in open water than terrestrial habitats. Leaf concentrations of each allelochemical varied seasonally; however, both myricitrin and salipurposid had heightened concentrations in spring. These results suggest that herbicide application in early spring may be most effective in controlling L. hexapetala terrestrialization from open water to marshes. [ABSTRACT FROM AUTHOR]

Spartina foliosa is native to the San Francisco, California, Bay and Estuary. Spartina alterniflora was introduced to the Bay in the 1970s and subsequently hybridized with S. foliosa. Backcrossing created an invasive hybrid swarm able to outcompete S. foliosa within its tidal range and spread into higher and lower intertidal zones, drastically altering ecological communities. The San Francisco Estuary Invasive Spartina Project (ISP) has implemented a treatment program that has reduced the net area of invasive Spartina by 96 % from its peak of 323 hectares. An endangered bird, the California Ridgway's Rail, takes advantage of habitat characteristics supplied by hybrid Spartina, prompting treatment restrictions in some marshes to preserve hybrid Spartina for the benefit of rails. We investigated changes in the population genetics of the invasion after 10 years of eradication efforts and after three subsequent years without eradication in some locations. Our investigation covered three areas: sites surrounding and including the restricted treatment marshes; sites in the southern Bay similar to proposed salt pond restoration projects; and samples collected Baywide. Our results support previous descriptions of a system of self-fertile hybrid plants that supply seed for mostly localized recruitment into available habitat. Compared to published work from 2003 to 2004, before large-scale eradication efforts began, genetic diversity has generally declined and inbreeding within hybrid populations has increased during and after the period of treatment by the ISP. These observations are perhaps due to pollen limitation as a result of the elimination of most hybrid plants by the ISP. We did not detect strong population genetic boundaries Baywide, but a genetically distinguishable localized introduction was detected in 10 of 11 Bay regions, suggesting leptokurtic dispersal. The restricted treatment areas may allow for continued introgression with S. foliosa, recolonization, and longer persistence of these invasive plants in the San Francisco Bay. Proximity to hybrid Spartina threatens the success of major restoration projects. [ABSTRACT FROM AUTHOR]

Regional, ecosystem-level conservation projects with significant vegetation management components require planning, coordination, and responsive management strategies to minimize negative impacts and maximize ecological benefits over time. The California State Coastal Conservancy's Invasive Spartina Project (ISP) offers an example of a complex, ecosystem-scale weed eradication effort guided by regional conservation goals. We review the management framework developed by the ISP, describe decision thresholds used for site-specific management transitions over the project's 15 years, and present strategies being used to address major challenges to project completion. These strategies include developing genetics and weed mapping approaches to aid with identification of hybrids between the introduced Spartina alterniflora and the native Spartina foliosa. The ISP also developed a tidal marsh restoration project to enhance habitat for an endangered bird, the California Ridgway's rail ( Rallus obsoletus obsoletus), that uses tall, dense forms of hybrid Spartina as high tide refugia and nesting substrate. By 2014, the ISP had installed over 300,000 native plants and recorded a greater than 96 % estuary-wide reduction in hybrid Spartina (from 323 ha to 12 net ha) despite treatment restrictions imposed at 11 sites since 2011 to protect the rail. Approximately 80 % of the remaining hybrid Spartina occurs in areas currently restricted from treatment, delaying project completion. The successes and setbacks of the ISP illustrate the complexities of achieving ecosystem-level conservation goals dependent on large-scale vegetation management. [ABSTRACT FROM AUTHOR]