Are vegetation-environment relationships different between herbaceous and woody groundcover plants in barrens with shallow soils?

The extent to which woody vegetation exhibits more expansive community structures and different relationships with environmental variables than herbaceous plants is poorly understood in savannas and barrens worldwide, especially those with shallow soils. We explored this question in oak barrens, which are savanna habitats characterized by shallow soils, in southern Ohio, USA. Groundcover plant aerial cover and environmental data were collected using 75 randomly located 1-m(2) quadrats in 3 barrens. A combination of non-metric multidimensional scaling (NMDS) and beta-flexible cluster analysis revealed 3 distinct herbaceous plant assemblages that varied in abundances of C-4 and C-3 graminoid species and several forbs. Herbaceous vegetation patterns were correlated with soil acidity, soil depth, plant available water, and especially soil organic matter and quadrat slope. By contrast, woody plants were more widely distributed than herbs, and woody vegetation patterns were strongly correlated only with soil organic matter, although they were weakly associated with quadrat slope and an index of tree influence. The expansion of woody plants into barrens is likely not restricted by most edaphic factors; thus, barrens are highly susceptible to woody plant encroachment that has been fostered by anthropogenic alterations to natural fire regimes (suppression, dormant season fires). We hypothesize that fires that arc intended to mimic natural lightning fires during the growing season will be most effective in deterring woody plant encroachment.