Tidal Wetland Vegetation in the San Francisco Bay-Delta Estuary

摘要

We analyzed tidal wetland vegetation patterns in the San Francisco Bay–Delta estuary (Bay–Delta) and discuss their significance for future conservation. Our objective was to conduct a balanced, random sample of six historic “remnant” tidal wetlands along a salinity gradient that ranged from southern San Pablo Bay to the lower Delta. We also wanted to compare diversity among these sites at five different scales, ranging from small subplots to total species per site. We randomly established twenty 0.1-ha plots at each site, and sampled ten 7-m2 circular subplots (subplot scale) in each plot for species presence and cover. We calculated total species per plot (plot scale) and total species per site (site scale) based on these subplot data. We calculated importance values for each species found in subplots based on frequency of occurrence and proportion of total cover at each site. In addition, we recorded species found within the 0.1-ha plot but not sampled in subplots (plot_all scale), and calculated the total species for each site based on these plot_all data (site_all scale). Thus, richness at each site was evaluated at five different scales that ranged from 7 m2 to 20,000 m2. We conducted a one-way ANOVA that compared mean richness among three scales with continuous data: subplot, plot, and plot_all. At lower estuary (San Pablo Bay) sites, richness values at these three scales were significantly lower than upper estuary (Suisun–Delta) sites. In Suisun–Delta sites, significant differences in richness varied, depending on sampling scale. Rush Ranch, a brackish wetland, had the highest average number of species per plot, whereas Sand Mound Slough, a freshwater wetland, had the highest average number of species at the subplot scale and the most total species at the site_all scale. Sand Mound Slough also had the highest number of unique species (22) compared to the other sites. We found a strong negative correlation (r2 = 0.99) between total species number and salinity (the most in the Delta at the lowest salinity, and the least at China Camp with the greatest salinity). Our analysis suggests the following conservation recommendations: (1) restore habitat for freshwater plant species diversity in the Delta; (2) restore transitional brackish wetland habitat in salt marshes of the San Pablo Bay and San Francisco Bay; and (3) focus on control of pepperweed (Lepidium latifolium) in brackish marshes.