Canopy Architecture of Natural and Planted Cordgrass Marshes: Selecting Habitat Evaluation Criteria

Abstract

In order to set standards for restoration or mitigation "success" of salt marsh habitat designed for an endangered southern California bird, I explored the bird's nesting requirements, compared constructed habitats that lack the bird with natural habitats that support it, and selected habitat assessment criteria that best distinguish suitable and unsuitable nesting habitats. Selected attributes of canopy architecture are recommended for assessing the suitability of intertidal cordgrass (Spartina foliosa) marshes for the endangered Light-footed Clapper Rail (Rallus longirostris levipes) of southern California. Nests are built at °145 cm above Mean Lower Low Water (MLLW = 0 tidal datum), with nest rims at °165-170 cm MLLW; however, extreme high water is 232 cm MLLW during the March-July nesting season, and tidal inundation is a major cause of nest failure. Cordgrass that is >60 cm tall appears necessary for birds to weave a canopy that allows the nest to float upward, but not away, as the tide rises. Cordgrass height distributions and density data describe attributes of canopy architecture that assess Clapper Rail habitat value better than previously used measures (i.e., cover, biomass, mean height, maximum height). Height histograms differ for planted marshes (which do not support Clapper Rails) and natural marshes. The constructed marshes have few plants over 60 cm, while most stems in natural marshes exceed 60 cm. In natural marshes, cordgrass heights increase with freshwater flooding and nitrogen enrichment. Reference data from natural marshes that are used by Clapper Rails indicate that the standard for "suitable habitat" should be a density of at least 100 stems/m2 with at least 90 stems/m2 >60 cm, of which at least 30 stems/m2 are >90 cm in height. High interannual and spatial variability indicates the need for several (e.g., 20) years of data for assessment purposes and a large data base for reference wetlands.