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Bowles, Marlin L.
McBride, Jenny
Apfelbaum, Stephen I.
Haney, Alan
Packard, Stephen

Vegetation Composition, Structure, and Temporal Change in a Midwestern Silt-Loam Mesic Savanna

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Midwest North American silt loam savannas are among the rarest of natural ecosystems due to post settlement fire protections, overgrazing and development, and their restoration management is of high priority. We resampled ten-year-old woody and herbaceous vegetation transects in the 11.7 ha Middlefork Savanna, one of two high quality mesic silt-loam savanna remnants in northeastern Illinois. In 1996, woody vegetation structure was highly skewed, with about 50 canopy trees/ha, but with 330 subcanopy trees/ha and over 14,000 stems/ha in the shrub layer. Quercus ellipsoidalis was the most important subcanopy tree, while the alien shrub Rhamnus cathartica and the dogwood Cornus racemosa dominated the shrub layer. Ground-layer vegetation at Middlefork savanna was highly structured by a canopy light gradient, causing groups of shade-, intermediate-, and light-adapted species under light mean light levels ranging from 50 to 1250 PAR. Native species richness and alien abundance were inversely related along the light gradient, with alien abundance increasing with increasing shade. Despite five management fires in the last ten years, Quercus ellipsoidalis stem densities have increased in lower mid size classes and Rhamnus cathartica has invaded the shrub layer. In relation, native species richness has increased in all but the lowest light level, but the increases have been from intermediate- and shade-adapted species. These results indicate that even under fire regimes that increase species richness, silt-loam savannas are vulnerable to alien invasion at low light levels, which is exacerbated by increasing shade from woody vegetation at higher light levels. To maintain open savanna structure at Middlefork, management fires should be conducted under burning conditions that are most likely to impact woody vegetation. We suggest experimental growing season fire treatments, especially under drier conditions that might have promoted naturally occurring fires. Supplemental cutting and use of appropriate herbicides should be used to help reduce woody vegetation, especially the alien Rhamnus cathartica.
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