Relationships between seasonally wet soils and occurrence of wetland plants in California

Abstract

Vegetative cover, depth to groundwater soil oxygen, and redox potential were measured at sites in California within a gradient from tidal marsh to upland pasture. Plant cover was sampled using a point-intercept method, depth to groundwater was measured in piezometers, and soil oxygen and redox was measured usingin-situ instrumentation over a year-long period. The seasonal weather pattern was typical of a Mediterranean climate with a long, dry period from May to October and a wet period from November to April. Plant species were categorized according to their ranking on the US Fish and Wildlife Service’s wetland plant indicator list. Vegetative cover was greatest during the rainy season at all nontidal sites, and the cover by wetland species reflected the trend observed in the soil moisture measurements. However, the only significant correlation was that between obligate wetland species and soil oxygen and redox potential. When soil oxygen decreased below 12% and redox potential decreased below +300 mV, obligate wetland species increased proportionately. Furthermore, the presence of upland species in the non-tidal wetlands was greater during the wet season, whereas wetland species persisted during the dry season. As a result, the wetland character of the sites as measured by a weighted average ordination scale varied throughout the year.