The effects of landscape position on plant species density: Evidence of past environmental effects in a coastal wetland

Here we propose that an important cause of variation in species density may be prior environmental conditions that continue to influence current patte rns. In this paper we investigated the degree to which species density vari es with location within the landscape, independent of contemporaneous envir onmental conditions. The area studied was a coastal marsh landscape subject to periodic storm events. To evaluate the impact of historical effects, it was assumed that the landscape position of a plot relative to the river's mouth ("distance from sea") and to the edge of a stream channel ("distance from shore") would correlate with the impact of prior storm events, an assu mption supported by previous studies. To evaluate the importance of spatial location on species density, data were collected from five sites located a t increasing distances from the river's mouth along the Middle Pearl River in Louisiana. At each site, plots were established systematically along tra nsects perpendicular to the shoreline. For each of the 175 plots, we measur ed elevation, soil salinity, percent of plot recently disturbed, percent of sunlight captured by the plant canopy (as a measure of plant abundance), a nd plant species density. Structural equation analysis ascertained the degr ee to which landscape position variables explained variation in species den sity that could not be explained by current environmental indicators. Witho ut considering landscape variables, 54% of the variation in species density could be explained by the effects of salinity, flooding, and plant abundan ce. When landscape variables were included, distance from shore was unimpor tant but distance from sea explained an additional 12% of the variance in s pecies density (R-2 of final model = 66%). Based on these results it appear s that at least some of the otherwise unexplained variation in species dens ity can be attributed to landscape position, and presumably previous storm events. We suggest that future studies may gain additional insight into the factors controlling current patterns of species density by examining the e ffects of position within the landscape.