Twenty-year Woody Vegetation Changes in Northern Flatwoods and Mesic Forest at Ryerson Conservation Area, Lake County, Illinois
AbstractConservationists are concerned that forest fragmentation and fire suppression are causing an increase in shade-tolerant species (e.g., maples) and a decline of fire-adapted oaks and associated species richness in midwestern forests. We tested whether such changes are occurring in flatwoods and mesic forest stands that were first sampled in 1975—1976 at the Ryerson Conservation Area in Lake County. Illinois. We re-sampled tree and shrub plots in these stands in 1997 and compared their changes over time. In 1976, the northern flatwoods was dominated by swamp white oak and white oak in large size classes, and by ash in small size classes. By 1977, these species had increased in stem numbers, resulting in a 23% increase in basal area. However, a large decline took place among mid-size oaks and shrub layer species over the 20-year period. Sugar maple dominated the mesic forest stand, where it increased in importance and now dominates all but the largest size class, which is oak-dominated. However, there was little gain in larger size classes, and basal area decreased 28%. Maples also increased in smaller size classes, whereas shrub layer species and mid-size class oaks and maples declined. Shrub layer stem density and species richness were much higher in flatwoods than in mesic forest, and a native species richness index also showed the northern flatwoods groundlayer to be more than twice as rich as the mesic forest. Tree cores indicate that declining mid-size class trees arose in the late 1800s, while older age class oaks and maples predate settlement. The changes in flatwoods are apparently due to forest canopy maturation and canopy closure, a process that probably began with fire protection after European settlement. Decline of oaks in the mesic forest may be less closely linked with fire protection, and the increase in maple saplings might have been triggered by more recent loss of canopy elms. Over-browsing by eastern white-tailed deer could have enhanced the decline of shrubs in both stands. The trends of increasing ash and maples in these stands indicate that they will become less diverse unless management can restore canopy structure that will maintain shrub layer species and allow oak regeneration. Restoration goals and applied research are needed to guide recovery. Fire appears to be the principal tool, especially in flatwoods, but it may have positive and negative effects, and supplemental cutting of fire-resistant vegetation may be required.
Volume, Page Number19, 31-51