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Bowles, Marlin L.
Jones, Michael D.

Chronological Change in Old-Growth Forests of the Chicago Region

Date created
Report to the Illinois Department of Natural Resources and the Chicago Wilderness.
Despite strong evidence for canopy closure and successional replacement of oak by sugar maple in Chicago region forests, no studies have documented such changes using chronological data from tree rings, nor have they compared rates of change among different stand types. Central questions addressed in this study are whether forest canopies were more open in the 1800s, whether canopy processes have changed over time, and whether accession into canopy gaps has shifted from oaks in the 1800s to non-oak species, primarily maples, in the 1900s. To provide this information, we analyzed change in tree growth from tree rings cored in 1997 and 1998 from 24 forest stands thought to have either undisturbed or lightly disturbed canopies representing maple, red oak, and white oak stand types in the Chicago region. We also compared growth among these stand types with four savanna stands, testing the hypothesis that oak stands were more open and had growth characteristics similar to savannas in the 1800s. There were 27 occurrences of pre-1810 trees among 11 sites. White oak had the greatest number, 17, and the oldest recorded tree, which originated in 1687. Sugar maple was the second most abundant pre-settlement tree, with four individuals. There were two pre-1810 bur oaks and shagbark hickories, and a single swamp white oak. The greatest concentrations of these trees occurred at Lloyd Woods, which had 4 maples and 2 white oaks, and at St. Francis Woods, which had five white oaks. Stands developed under canopy gap processes appear to represent either old growth or lightly disturbed conditions included 89 % of the maple stands, 71 % of the white oak stands, and 50 % of the red oak stands. The remaining stands (about 30 %) appear to have completely regenerated following disturbance in the early to mid 1800s. Growth in 87.5 % of the stands was significantly correlated with the Palmer Drought Severity Index during the 1895-1949 period and growth among many stands was also highly correlated, indicating a landscape-scale climatic effect on tree growth. However < 30 % of the variation was accounted for by drought severity, indicating that other factors had strong effects on tree growth. Variability in growth among stand types, as indicated by the Coefficient of Variation, was lowest in savannas, intermediate in white oak stands, unstable in red oak stands, and high and more stable in maple forests. This suggests more uniform growth under open canopies occurred in white oak stands in the 1800s. Mean growth rates were also highest in savannas, but tended to be lower and more similar in forest stands. These results suggests that canopies of the oak stands tended to be more open in the 1800s, but not as open as savanna stands. Responses to canopy gap process differed among white oak, red oak, and maple species groups, with oaks having greater direct accession into canopy gaps, and maples having greater releases into gaps from suppressed understory conditions. Among all stand types, the greatest number of canopy accessions occurred between 1850-1899, with more responses by oaks than maples. However, maple dominated stands also had substantial maple regeneration during that period. Over time, there was a shift from direct establishment of trees into canopy gaps toward greater numbers of gap releases of trees that had been suppressed in forest understories. These results support a hypothesis that oak forest canopies have gradually become more closed. Nevertheless, canopy gap processes were operating in oak forests in the 1800s, and also allowed oaks to access canopies of maple stands during that period. An important implication for management and restoration drawn from this study is that although oak and maple stands had more open canopies in the 1800s, presumably due to landscape fire, they also operated under canopy gap processes. However, oak regeneration into canopy gaps was far more common than that of maple. Forests of today appear to have greater canopy cover, with gap access almost restricted to non-oak species. Research is needed to understand whether restored fire processes that reduce non-oak species, primarily maple, will allow oak regeneration into canopy gaps in these forests.