Vegetation, Composition, Structure, and Chronological Change in a Decadent Midwestern North American Savanna Remnant
AbstractFire-maintained oak savannas on silt-loam soils essentially disappeared from midwestern North America soon after European settlement because of fire suppression and agriculture. As a result, there are no precise models for restoring this vegetation, and its management and recovery needs are uncertain. The Wolf Road Prairie Nature Preserve, Cook County, Illinois, contains a 2-ha savanna remnant that persisted on mesic silt-loam soils because grazing was excluded and because it was frequently burned until about 1965. We used tree increment cores to document temporal change in this savanna's tree density and canopy structure. The savanna has an open overstory canopy of mature Quercus macrocarpa (bur oak) and a dense 30-year-old subcanopy of Q. macrocarpa and Q. ellipsoidalis (northern pin oak) that matured from oak grub sprouts after cessation of annual wildfires. Herb-layer sample plots were classified into four species groups corresponding to a measured light intensity gradient that correlated with canopy tree density. The ground layer was dominated by shade-tolerant species that presumably have spread into former savanna openings, while shade-intolerant species, such as prairie plants, were restricted to the few remaining light gaps. Plot species richness and number of prairie species were highest at highest light levels, while alien species richness was highest under canopy shade. Forbs were the most important species group (78.6 Importance Value); woody species were much less important (18.8 IV), and graminoid vegetation was only a minor component that was restricted to light gaps. Camassia scilloides was the most important forb and was distributed bimodally across the light gradient. Rubus allegheniensis was the most important woody species, and Carex pensylvanica was the most important graminoid species. The site retains important components of savanna vegetation and could be restored to former savanna composition and structure, helping provide elements of a savanna restoration model. Restoration would require a return to high fire frequencies and reduction of the subcanopy to its former grub layer, which should increase abundance of light- and fire-adapted species and reduce alien species. However, restoring former structure may be difficult, partly because of the fire resistance of maturing subcanopy oaks and because the absence of graminoid fuel species will reduce the effectiveness of fire. In addition, landscape-scale fire processes, which might be most effective, will be difficult to implement in this small fragmented natural area. As a result, experimental cutting or girdling of oaks should be used to supplement fire in opening canopy gaps.
Volume, Page Number18, 14-27