Forest Dynamics Across Three Century-Length Disturbance Regimes in the Illinois Ozark Hills

Abstract

Forest community dynamics in the Illinois Ozark Hills was studied over a 300 y period that included three sequential disturbance regimes. Presettlement forest community patterns were reconstructed using witness tree data taken from the 1806–7 General Land Office survey records. Species, stem diameter and distance from the corner were obtained for a total sample of 958 witness trees on 479 section and quarter-section corners. Data were separated by site type based on 13 aspect and slope positions, but later were combined into six Ecological Land Type Phases (ELTPs) using cluster analysis and percent similarity. At the end of the presettlement and Disturbance Phase 1 (ca. 1810), Quercus and Carya species dominated forest communities (combined Importance Value, IV100 = 68–76) on the Southwest Slope, South Slope, Ridgetop and North Slope ELTPs. The combined IVs for all early successional species ranged from 74–81 while IVs of late successional fire-intolerant mesophytic species ranged from 19–26. Using mortality/damage data on mesophytic stems after prescribed fire and growth data on healthy stems, we estimated that the presettlement forest of mid to upper slopes and ridgetops developed under a fire return cycle of 30–45 y, a range that permitted a modest proportion of mesophytes to attain tree size. Forest communities on the cool moist Low Slope and Terrace ELTPs were dominated by mesophytic species, mostly Fagus grandifolia and Acer saccharum (combined IV100 = 50 and 63, respectively). We believe forest composition at the time of the land survey in 1806–7 primarily reflected the long-term effect of Early American burning during the 1600s and 1700s. The start of the Disturbance Regime 2, distinguished by the severe 1810–11 earthquakes that damaged thousands of hectares of forest, was followed by a period of settlement (ca. 1820–1930) characterized by a high level of fire, grazing and timber cutting. Based on early records, this level of disturbance was judged to be greater than that of presettlement time. The present forest was established in the mid to late 1800s and early 1900s of Regime 2. To provide continuity and reliability, tree, sapling and seedling data on the present forest were collected in all ELTPs from nested quadrats located at randomly selected, but continuously forested section and quarter-section corners from which witness tree data were previously recorded. In the presently mature forest overstory, Quercus and Carya had higher combined importance values on the Southwest Slope, South Slope and Ridgetop ELTPs (combined IV100 = 73–88) than presettlement communities on these ELTPs. On the North Slope ELTP, a sharp decrease in Quercus importance was partially offset by increases in Carya and Liriodendron tulipifera importance. On the Low Slope ELTP, there was an increase in the Quercus, Carya and L. tulipifera component from the presettlement forest to the present forest. The Terrace ELTP community was composed primarily of early successional lowland species in contrast to the forest of presettlement time. Disturbance Regime 3 of the last 75 y (ca. 1930–2005) was marked by near complete protection with the cessation of fire that accompanied the development of national and state forests, wilderness areas and wildlife refuges. This third period served as a reference to demonstrate the effect of absence of disturbance on the Quercus–Carya community. The absence of fire permitted major increases in mesophyte importance in the present understory (combined IV100 = 50–77) and stem growth into small tree stratum of nearly all Quercus–Carya dominated community types of all ELTPs. Based on a structure-for-time projection, related research analyses and an absence of disturbance that will permit continued growth of mesophytic species, the Illinois Ozark Hills may be the first contiguous forest region in central North America to completely convert from a Quercus–Carya to an Acer–Fagus dominated ecosystem.