Fine-scale spatial variation in plant species richness and its relationship to environmental conditions in coastal marshlands

Abstract

Previous studies have shown that variations in environmental conditions play a major role in explaining variations in plant species richness at community and landscape scales. In this study, we considered the degree to which fine-scale spatial variations in richness could be related to fine-scale variations in abiotic and biotic factors. To examine spatial variation in richness, grids of 1 m 2 plots were laid out at five sites within a coastal riverine wetland landscape. At each site, a 5 · 7 array of plots was established adjacent to the river's edge with plots one meter apart. In addition to the estimation of species richness, environmental measurements included sediment salinity, plot microelevation, percent of plot recently disturbed, and estimated community biomass. Our analysis strategy was to combine the use of structural equation modeling (path modeling) with an assessment of spatial association. Mantel's tests revealed significant spatial autocorrelation in species richness at four of the five sites sampled, indicating that richness in a plot correlated with the richness of nearby plots. We subsequently considered the degree to which spatial autocorrelations in richness could be explained by spatial autocorrelations in environmental conditions. Once data were corrected for environmental correlations, spatial autocorrelation in residual species richness could not be detected at any site. Based on these results, we conclude that in this coastal wetland, there appears to be a fine-scale mapping of diversity to microgradients in environmental conditions.

Direct and Indirect Control of Grassland Community Structure by Litter, Resources, and Biomass

“…The lack of biomass effects in this study contrasts with other SEM analyses that found significant relationships between standing aboveground biomass or cover and species richness (Grace and Pugesek 1997, Gough and Grace 1999, Grace and Guntenspergen 1999, Grace and Jutila 1999, Grace et al 2000, Weiher et al 2004, Mancera et al 2005, Grace and Keeley 2006, but in those studies live shoot biomass and litter were not considered separately. Finally, I found that initial species richness and evenness remained important, indicating that, similar to other studies (Grace and Guntenspergen 1999), plot history is an important determinant of current community structure.…”

“…A number of recent studies have taken a holistic approach to examine the controls on plant diversity in a range of communities including coastal wetlands, grasslands, and shrublands (Grace and Pugesek 1997, Gough and Grace 1999, Grace and Guntenspergen 1999, Grace and Jutila 1999, Grace et al 2000, Weiher et al 2004, Mancera et al 2005, Grace and Keeley 2006. These studies used structural equation modeling (SEM) to explicitly evaluate the causal relationships among multiple interacting variables that together affect diversity (Shipley 2000, Grace 2006.…”

Floral and environmental gradients on a Late Cretaceous landscape

“…Midwestern U.S. fens as small as 80 ha can have .500 species (Amon et al 2002). Freshwater coastal marshes in Louisiana average 8-12 species per m 2 (Mancera et al 2005), and transects of temperate to subtropical North American wetland vegetation that are hundreds to thousands of meters long commonly record 50-200 species (e.g., Laliberte et al 2007, Flinn et al 2008, Peirson and Evans 2008, Carr et al 2009). The high diversity of BCR relative to other fossil assemblages probably reflects unusual preservation of herbaceous as well as the few woody plants, rather than high diversity of the original flora compared with extant floras in similar habitats.…”

Structural equation modelling detects unexpected differences between bryophyte and vascular plant richness along multiple environmental gradients

“…The direct and indirect effects of abiotic and biotic variables – for example, canopy cover and mineral content of soil (Weiher et al. , 2004), fine‐scale spatial variations in plant species richness (Mancera et al. , 2005), or time since the last fire (Laughlin & Grace, 2006) – have been reported from various natural systems.…”

“…Failure to distinguish these different paths and to assign them the correct importance could lead biologists to examine spurious relationships (Shipley, 2000). The direct and indirect effects of abiotic and biotic variables -for example, canopy cover and mineral content of soil (Weiher et al, 2004), fine-scale spatial variations in plant species richness (Mancera et al, 2005), or time since the last fire (Laughlin & Grace, 2006) -have been reported from various natural systems. Ideally, the effects of these interrelated variables should be tested experimentally, but in many cases manipulations are not possible because of the high number of factors involved.…”