Community structure along elevation gradients in headwater regions of longleaf pine savannas

Abstract

Savanna community structure has been described mostly at the regionallevel worldwide. Quantitative descriptions of vegetation patterns andrelationships with substrate characteristics at more localized scales havereceived less attention. Our primary objective was to examine the distributionsof herbs and shrubs/trees along local topographic gradients in headwaterregions of longleaf pine savannas in the southeastern United States. We alsoexamined whether herb patterns were structurally similar to those ofshrubs/trees along the same topographic gradients and whether patterns werecorrelated with edaphic factors. Abundance data were collected within quadratsplaced along transects from upslope savannas through mid-slope seepage bogsintolower-slope shrub/tree zones within Louisiana and Florida. β-flexiblecluster analysis and nonmetric multidimensional scaling were used to delineateherbaceous species communities. Ordination was performed separately onshrub/tree abundance data. The herb-based classifications were also used todelineate shrub/tree communities, providing an indirect means of comparingherb to shrub/tree distributions. In Louisiana, three herbaceouscommunitieswere sharply delineated along elevation gradients of several meters and werestrongly correleated with soil moisture. In Florida, three similar herbaceouscommunities were less discrete along elevation gradients of <1 meter.In both regions, shrub/tree distributions were much broader and appearedless sensitive than herbs to changes in environmental gradients. Coefficientsofvariation indicated that, in general, herbaceous species were more narrowlydistributed than shrubs/trees along localized elevation gradients in bothLouisiana and Florida. Alterations of fire regimes (fire suppression,dormant-season fires) may have resulted in expanded distributions ofshrubs/trees, but not herbs.