Vegetation changes underway in oak woodland and savanna-like communities in the midwestern United States include increased tree density (thicketization) and shading and loss of diversity in the ground layer. These changes are primarily a result of reduced fire frequency. In the prairie-forest transition zone centered around Illinois, some habitats, particularly on dry-mesic to xeric sites, retain considerable potential for restoration with prescribed fire and insights are needed for directing management efforts where they can be most effective. I will examine structural characteristics of flatwoods, barrens, and dry-mesic woodland habitats with a focus on tree density, a readily assessed attribute, as a factor influencing composition and diversity patterns in ground-layer vegetation.
It is proposed that an inverse pattern of tree density and ground layer species and functional group diversity from a nested sample design would signal productive opportunities for restoration because it would indicate ground layer species were present and responding, albeit negatively, to thicketization. This will be referred to as the species attrition hypothesis. Where such a pattern is lacking would suggest two null possibilities: 1) shading has yet to limit ground-layer diversity (rare), or 2) shading has progressed to the point where remaining ground-layer species are indifferent to shade levels (more typical). Interactions between overstory and ground-layer vegetation also will be examined for ordered patterns of species and functional group losses (i.e., community disassembly) with increasing tree density. Documenting ordered patterns along the structural gradient of tree density would provide insights regarding ecological condition and restoration potential.
Results/Conclusions
Support was found for a pattern of species and functional group attrition in barrens and several flatwoods remnants. However, some examples of dry-mesic oak woodlands and certain flatwoods demonstrate extensive thicketization with prolonged fire absence resulting in a pattern fitting null attrition model 2. Efforts to restore these communities with reintroduction of fire have shown some promise with evidence of improved overstory structure and ground layer diversity; however, the seed bank may be limited following prolonged stand closure. Functional groups that appear most sensitive to thicketization and first in order to decline with extended fire absence are herbaceous vines, perennial grasses, sedges, and perennial forbs. While opportunities remain to restore aspects of oak woodland and savanna habitats in the prairie-forest transition zone, the attrition model suggests a limited time frame for the restoration of ground layer diversity even in dry to xeric sites.