Community structure along elevation gradients in headwater regions of longleaf pine savannas
Abstract
Savanna community structure has been described mostly at the regional level worldwide. Quantitative descriptions of vegetation patterns and relationships with substrate characteristics at more localized scales have received less attention. Our primary objective was to examine the distributions of herbs and shrubs/trees along local topographic gradients in headwater regions of longleaf pine savannas in the southeastern United States. We also examined whether herb patterns were structurally similar to those of shrubs/trees along the same topographic gradients and whether patterns were correlated with edaphic factors. Abundance data were collected within quadrats placed along transects from upslope savannas through mid-slope seepage bogs into lower-slope shrub/tree zones within Louisiana and Florida. β-flexible cluster analysis and nonmetric multidimensional scaling were used to delineate herbaceous species communities. Ordination was performed separately on shrub/tree abundance data. The herb-based classifications were also used to delineate shrub/tree communities, providing an indirect means of comparing herb to shrub/tree distributions. In Louisiana, three herbaceous communities were sharply delineated along elevation gradients of several meters and were strongly correleated with soil moisture. In Florida, three similar herbaceous communities were less discrete along elevation gradients of < 1 meter. In both regions, shrub/tree distributions were much broader and appeared less sensitive than herbs to changes in environmental gradients. Coefficients of variation indicated that, in general, herbaceous species were more narrowly distributed than shrubs/trees along localized elevation gradients in both Louisiana and Florida. Alterations of fire regimes (fire suppression, dormant-season fires) may have resulted in expanded distributions of shrubs/trees, but not herbs.