Vascular plants structure wetland ecosystems. To examine mechanisms behind their influence, plants were studied under different scenarios of change: experimental manipulation of cover, invasion, and response to flushing regimes. I tested the hypothesis that wetland plants alter benthic communities through modification of abiotic factors, with cascading effects on microalgae and invertebrate communities. Major plant effects were observed in all systems studied, but the magnitude of, mechanisms behind, and exact consequences of plant alterations depended on the particular combination of physical and biological stresses within the habitat along the marine to terrestrial continuum. Manipulation of plant cover and light regime, combined with natural abundance isotope studies in a mid-elevation salt marsh of Mission Bay, CA revealed how two dominant plants, Spartina foliosa and Sarcocornia pacifica (formally Salicornia virginica ), regulate light, temperature, and moisture, thereby influencing the abundance of benthic diatoms and the relative importance of microalgal-feeding invertebrates. Tamarisk ( Tamarix spp.), normally a freshwater invader that has recently colonized the salt marsh in Tijuana Estuary, was studied in 3 marsh zones with mensurative benthic assessment techniques and stable isotope enrichment experiments. Results demonstrate that this plant has (1) impacted the mid-marsh environment most, (2) accelerated salt marsh succession towards a more terrestrial environment by creating drier, less organic-rich sediments and an altered macroinvertebrate community (decreased densities of gastropods and marine oligochaetes, more insects) and (3) entered the food web through a broad range of invertebrate consumers. Using similar approaches, the ephemeral seagrass, Ruppia maritima , abundant in lagoons during periods of inlet closure, was also shown to play a key trophic role in structuring wetlands in southern California. Results of faunal characterization and isotope enrichment studies within San Dieguito Lagoon suggest that food webs in these environments are driven by detrital and epiphytic production. Increased representation of detritivores in R. maritima habitats relative to unvegetated mudflat appears linked to animal feeding preferences and the ability of consumers to utilize R. maritima . In summary, this research developed several experimental methods by which to isolate structuring mechanisms of vascular plants in wetlands and allowed us to make generalizations across abiotic gradients in salt marsh ecosystems.

Perennial pepperweed (Lepidium latifolium) is an aggressive, non-native herbaceous weed displacing native vegetation in marshes, floodplains, prairies and rangeland throughout California. You can even find it growing along roadsides and highways. But does the plant’s ubiquity really mean it is an ecological problem warranting costly eradication efforts?

We hypothesized there would be differences in the microbial and infaunal communitiesprimarily responsible for degrading macrophyte vs. microalgal carbon. Specifically we predicted that:

The invertebrate community degrading macrophyte-derived carbon will be significantly different from those degrading algal-derived carbon.

The microbial communities (bacteria and fungi) degrading macrophyte-derived carbon will be significantly different from those not using this carbon source.

Plant cover is a fundamental feature of many coastal marine and terrestrial systems and controls the structure of associated animal communities. Both natural and human-mediated changes in plant cover influence abiotic sediment properties and thus have cascading impacts on the biotic community. Using clipping ( structural) and light ( shading) manipulations in two salt marsh vegetation zones ( one dominated by Spartina foliosa and one by Salicornia virginica), we tested whether these plant species exert influence on abiotic environmental factors and examined the mechanisms by which these changes regulate the biotic community. In an unshaded ( plant and shade removal) treatment, marsh soils exhibited harsher physical properties, a microalgal community composition shift toward increased diatom dominance, and altered macrofaunal community composition with lower species richness, a larger proportion of insect larvae, and a smaller proportion of annelids, crustaceans, and oligochaetes compared to shaded ( plant removal, shade mimic) and control treatment plots. Overall, the shaded treatment plots were similar to the controls. Plant cover removal also resulted in parallel shifts in microalgal and macrofaunal isotopic signatures of the most dynamic species. This suggests that animal responses are seen mainly among microalgae grazers and may be mediated by plant modi. cation of microalgae. Results of these experiments demonstrate how light reduction by the vascular plant canopy can control salt marsh sediment communities in an arid climate. This research facilitates understanding of sequential consequences of changing salt marsh plant cover associated with climate or sea level change, habitat degradation, marsh restoration, or plant invasion.

Within the Huntington Beach Wetlands (HBW), we hypothesized that Brookhurst and Magnolia Marshes (marsh areas isolated from tidal influence for approximately 100 years) would have significantly different physical characteristics (salinity, temperature, moisture, sediment parameters), more terrestrial plant, algal, invertebrate and microbial communities and altered food web structure from neighboring Talbert Marsh (a marsh area whose tidal connect was restored 20 years ago). In addition, we hypothesized that once Brookhurst Marsh was restored (Summer 2009), these differences would decrease and with time, resemble those of Talbert Marsh. This restoration of structure and function will occur on differing time scales; we predicted that physical parameters would quickly become similar while algal, invertebrate and microbial communities would begin to change on a month-year time scale. Changes to the plant community and the food web structure are anticipated to occur on a year-decadal time scale (beyond the funding cycle for this project).

The Rush Ranch Open Space Preserve (Rush Ranch) is located at the northwestern edge of the Potrero Hills and includes the largest remaining undiked tidal wetland within the Suisun Marsh region of the San Francisco Estuary. The brackish tidal wetlands grade into the transitional vegetation and undeveloped grasslands of the Potrero Hills, and we describe diverse vegetation that reflects the estuarine position, land use history, and hydrogeomorphic complexity of the site.

We present a useful framework for future study of vegetation at this San Francisco Bay National Estuarine Research Reserve site. Rush Ranch includes four major estuarine geomorphic units that are widely distributed in the region and support vegetation: subtidal channel beds, fringing tidal marsh, tidal marsh plain and tidal marsh–terrestrial ecotone. These are distinguished by small variations in hydrology and elevation, as noted and described through field observations and historic vegetation-mapping data. We discuss vegetation within each of these landforms, considering each vegetation community as a function of changing physical environment and biological interactions. Past land use and exotic plant species invasions have substantially altered Rush Ranch's tidal marsh vegetation patterns. Our results indicate 27% of the current estuarine wetland-associated flora at Rush Ranch are exotic species, and several are highly invasive. Despite these influences, Rush Ranch’s position in the landscape provides important and increasingly rare habitat linkages between the tidal marsh and upland grasslands, which allows great potential for restoration and enhancement. We present a detailed flora and vegetation analysis by hydrogeomorphic setting to provide an ecological framework for future monitoring, research, and adaptive conservation management at Rush Ranch.