Sugar maple invasion of an old-growth oak-hickory forest in southwestern Illinois

Analysis of presettlement land survey records in an area surrounding a present 7.2 ha old-growth oak-hickory forest, Weaver's Woods, in southwestern Illinois showed dominance by Quercus alba and Q. velutina. Weaver Woods developed into a Q. velutina, Q. rubra, Carya ovata forest by 1956, and into an Acer saccharum, C. ovata, Quercus spp. forest with a basal area of 32.7 m$^2\cdotha^{-1}$by 1983. Woody stem inventories were based on trees >30.5 cm dbh in 1956, and on all woody plants >6.6 cm dbh in 1967 and 1983 along with the sampling of understory and herbaceous vegetation. Stand density has fluctuated and in 1983 was 459 trees·ha-1. Size-class distribution of all species combined has not changed. Size-class distributions of individual species show a changing composition and structure of the forest. Size-class distributions of the oaks and hickories best fit a normal or Gaussian curve, with little or no recent regeneration. Acer saccharum was the dominant species in the forest in 1983 and had a negative exponential size-class structure reflecting abundant regeneration. Asimina triloba was the dominant species in the understory in 1983, heavily shading much of the forest floor. If current trends continue, Acer saccharum will dominate the canopy and oaks and hickories will decrease to remain as scattered large individuals. The size-class distributions of Weaver's Woods over the past 27 yr, plus future trends and old-growth status related to presettlement forests in the area, are discussed.