Comparison of Presettlement, Second-growth and Old-growth Forest on Six Site Types in the Illinois Shawnee Hills

Abstract

-Witness tree data from the General Land Office 1806-1807 survey records were used to reconstruct presettlement forest community data and a landscape pattern in the Illinois Shawnee Hills region. Using topographic maps, we categorized section and quarter section corners by physiographic site type: rocky S slope, S slope, ridgetop, high N slope, low N slope and terrace. For each type, species importance values (relative basal area) and community basal area, density and average diameter were determined from witness tree data. These data were compared to plot data of second-growth and old-growth forest communities on analogous sites. Presettlement communities had lowest basal area and highest mean diameter; old-growth had the highest basal area level. Second-growth stands had highest density and lowest mean diameter. Presettlement, second-growth and old-growth forest communities of rocky S slopes, S slopes and ridgetops were compositionally similar (percent similarity = 58-84%). Quercus stellata was the major dominant on xeric rocky S slopes while Q. alba was the most important species on xeric-mesic S slopes and ridgetops. Ridgetops supported a more open presettlement community of large Q. alba and Q. velutina. On these three site types, oldgrowth stands appear to be an acceptable representation of presettlement forest. On mesic high north, low north and terrace sites, presettlement forest communities were more similar to disturbed second-growth (percent similarity = 46-79%) than to old-growth (percent similarity = 22-30%). In presettlement forest, Quercus alba and Q. velutina were the dominants on high and low N slopes; Q. alba, Q. rubra and Liriodendron tulipifera were the most important species on terraces. Acer saccharum was absent or of minor importance. In second-growth forest, Q. alba and Q. alba with Q. rubra were the dominant species of high and low N slopes, respectively. On terraces, A. saccharum dominated but Q. alba, Q. rubra and L. tulipifera also were important. Old-growth forest on high N slopes was dominated by Q. rubra but A. saccharum was the major dominant of low N slope and stream terrace communities. While both presettlement and old-growth communities represent virgin (uncut) forest on mesic sites in the Shawnee Hills, they are relatively dissimilar in composition and structure. This difference exists because our present old-growth forest is an artifact of near total protection while presettlement forest developed under a fire regime.