contains an excel format and shapefile format of the vegetation survey data collected within Humboldt Bay during the summer of 2012.
We recorded vegetation data within a 0.25 m2 quadrat concurrently with elevation surveys. Data were taken at every fourth (25%) elevation point (n=740 quadrats; Fig. 4). We measured height (mean and maximum, measured within 0.05 m) and visually estimated percent cover for each species within each quadrat. This allowed us to develop a relationship between plant species, elevation and tidal datum across all sites. We also characterized the most common species, which were defined as those found at>10 % of the plots. Plant species frequency was plotted relative to MHW. This comprehensive vegetation dataset allows the baseline characterization of the salt marshes and allows for modeling of SLR in relation to plant communities. Taxonomic nomenclature for plants follows USDA Plants (http://plants.usda.gov).
Vegetation was sampled at 740 points concurrently with elevation surveys (Table 2). Vegetation plots collected at the Manila marsh south of State Highway 255 (n=157) were not analyzed, because the survey was conducted during January which was a period of vegetation senescence and may bias results. This part of the survey was conducted opportunistically to connect marsh survey to bathymetric surveys completed on the shoals adjacent to this marsh, as well as having a low elevation site that had not been subjected to Spartina treatment.
A total of 28 species were recorded across all marsh sites (Table 4). All of the surveyed marshes had extensive cover of S. pacifica which was the characteristic marsh species of our study sites since it occurred across a wide elevation range. However, species richness and cover varied widely by sites. Plant species were organized along an elevation gradient related to their inundation and salinity tolerance (Table 6, Fig. 10-12). Vegetation results were presented by site to illustrate differences in species composition and occurrence relative to mean high water across the different sites (Fig. 13-19). Nine species were the most common and occurred at a minimum of 10% of the vegetation plots (Table 5). Sarcocornia pacifica was the most common species surveyed across sites, occurring at 463 of the 583 (79.4%) vegetation plots. Distichlis spicata was the second most common species (55.8%), followed by Jaumea carnosa (50.8%), Spartina densiflora (37.9%), Castilleja ambigua ssp. humboldtiensis (25.7%), Limonium californicum (20.9%), Triglochin maritima (17.7%), Plantago maritima (15.6%) and Triglochin concinna (15.4%).
Spartina densiflora, a non-native invasive plant species, was found at all sites. Removal efforts have been underway at Humboldt Bay NWR since 2004, with the earliest restoration carried out in the Mad River Slough site (Pickart 2012, available on website) which was evident from the vegetation survey results (Fig. 9). Areas in which dense stands of S. densiflora had recently been removed were dominated by bare ground and contained early successional species (e.g. Sarcocornia pacifica). We observed that if S. densiflora had been removed in the recent past, there seemed to be higher diversity of plant species, although this could be due to areas being in the early stages of succession. Vegetation results were framed within the context of ongoing S. densiflora removal efforts at individual study sites.