Temporal instability in Chicago's upland old growth forests
Appeared InDraft located at TMA Records Box 109, Folder 49
AbstractWe tested the assumption that sugar maple is replacing oaks in Chicago region forests by re-sampling 28 old growth upland forests that were first sampled in 1976. These stands were dominated by sugar maple, red oak, or white oak. Maple basal area increased over time, and its stem densities increased in smaller size classes in maple and red oak stands. Oaks decreased in basal area and in stem densities in all stands, with the greatest loss in lower to mid size-classes. In the shrub layer, these changes were accompanied by declines in dominant shrubs and a shift toward dominance by tree saplings in maple and red oak stands. Ages of declining oak cohorts indicate they originated in the mid-1800s, probably soon after European settlement reduced fire frequencies; however old-growth canopy oaks and maples are of pre-settlement origin. Causes of oak mortality and shrub decline appear to be increasing shade from canopy closure associated with forest succession and stand maturation. Maple has played a role in these processes in maple and red oak stands, but not yet in white oak stands. We attribute these changes to loss of landscape-scale fires that maintained more open forests in the early 1800's, although over-browsing by white-tailed deer has recently contributed to loss of shrub layer vegetation on some sites. As these stands shift to canopy gap replacement processes, oaks are likely to decline further, as maples that now dominate the sapling layer in most stands will most likely replace canopy oaks. A high priority for management and research should be to understand how to restore natural fire processes that will maintain oak dominance and shrub layer diversity in these stands.
Volume, Issue, Page Number3, 2, 5-16