Californian salt-marsh vegetation: An improved model of spatial pattern

Although tidal wetland vegetation patterns are typically related to elevati on, we hypothesized that the vertical range of a species may shift where th e topography is more heterogeneous. We examined plant species occurrences i n relation to elevation, proximity to the bay and proximity to tidal creeks at a near-pristine wetland in San Quintin Bay, Baja California, Mexico. At the whole-wetland scale, most species occurred primarily within a 30-cm el evation band (the marsh plain). However, Spartina foliosa occurred only at the bayward margin, even though "suitable" elevations were present further inland. A similar pattern was found in San Diego Bay. At the microtopograph ic scale, three species on the marsh plain were strongly influenced by elev ation, whereas four species responded to both elevation and proximity to ti dal creeks. The latter species tended to "avoid" the lower 10 cm of the mar sh plain except near a tidal creek. Species richness was thus greater (by 0 .6 species at the lowest 10-cm class) at the tidal creek margin. Better dra inage near creeks is the hypothesized cause. Our results help explain why s pecies that are transplanted to constructed wetlands do not always grow at the full range of elevations they occupy in natural wetlands. We recommend that species be introduced to their modal elevation (determined from nearby reference marshes) and that salt-marsh construction designs include topogr aphic heterogeneity (complex tidal creek networks). The analysis of broad-s cale and fine-scale patterns of occurrence also suggests new habitat nomenc lature. Elevation-based terms ("low," "middle," and "high" marsh) should be replaced by a system that considers elevation, landscape position, and con spicuous species. We suggest three habitat designations: (a) the high marsh -a 30- to 70-cm elevation range with Salicornia subterminalis; (b) the mars h plain-a 30-cm elevation range with heterogeneous topography and up to nin e common species; and (c) cordgrass habitat-the bayward portion of the mars h plain and lower elevations, all occupied by Spartina foliosa. Although th ese habitats do not have discrete boundaries, separate terms are needed for wetland restoration plans and these designations will improve recognition that vegetation patterns respond to horizontal, as well as vertical, positi on.