Oak establishment and canopy accession strategies in five old-growth stands in the central hardwood forest region

Abstract

Using a radial growth averaging technique, decadal-scale changes in growth rates of overstory oaks were used to identify canopy accession events at five old-growth sites. A review of tree-ring chronologies yielded three growth strategies: (1) one-half of the oaks originated in a large opening and achieved overstory status before canopy closure; (2) 38% originated in a smaller opening and required a second gap event to attain overstory status; and (3) 13% achieved overstory status after an extended period of very low growth in understory shade and one or two subsequent gap releases. For trees that required a major canopy release, understory residence times averaged 89, 54, 50 and 38 years for white oak, northern red oak, black oak, and chestnut oak, respectively. Average diameters (inside bark) at canopy accession were 13.3, 11.7, 10.0, and 6.2 cm for the four oak species, respectively. Although there is some historical precedence for these values, few contemporary second-growth forests contain understory oaks of this age, particularly red oak. These long understory residence times suggest that the level of understory shade, and by inference, the abundance of shade tolerant understory species, was considerably less before 1900.