Logging Damage to Residual Trees Following Partial Cutting in a Green Ash-Sugarberry Stand in the Mississippi Delta

Partial cutting in bottomland hardwoods to control stand density and species composition sometimes results in logging damage to the lower bole and/or roots of residual trees. If severe, logging damage may lead to a decline in tree vigor, which may subsequently stimulate the production of epicormic branches, causing a decrease in bole quality and an eventual loss in value of the stand. To investigate this hypothesis, a partial cutting to remove pulpwood-sized trees (i.e., low thinning/improvement cuttlngl was performed in the summer of 1990 in a 45-year-old green ash-sugarberry stand located adjacent to the Mississippi River in west-central Mississippi, The goal of the thinning operation was to remove small trees of low-value species, particularly sugarberry, and to promote the growth of high-value residual trees, such as green ash. Thinning removed about 40 percent of the trees and 25 percent of the basal area, while increasing average stand diameter by about 12 percent. The partial cutting was also successful in increasing the relative proportion of green ash and decreasing the relative proportions of sugarberry and other species. The logging operation caused widespread damage to the residual stand, with about 62 percent of the residual trees being damaged at least to some extent. Damage to the lower bole and to exposed lateral roots of residual trees were the two most common types of damage. Damage to most trees was minor, but 35 percent of the residual trees experienced at least moderate logging damage. Sugarberry was more commonly damaged than was green ash. Most of the logging damage occurred during the skidding operation. Diameter growth of the residual dominant and codominant green ash averaged 0.39 tn. during the first year after thinning. The partial cutting resulted in only a modest increase in the number of epicormic branches per tree, with most of the increases occurring in lower-crown-class trees, already under stress from suppression prior to harvest. Residual green ash and sugar-berry trees of the upper crown classes produced very few new epicormic branches during the ffrst year after harvest. These latter data must be considered preliminary in that the possible stimulation of the production of epicormic branches in damaged trees is thought to be a more long-term response to logging damage. Partial cutting has long been an accepted practice used to control stand density and species composition. However, many managers of hardwood stands have failed to utilize this tool for fear that partial cutting will promote the development of epicormic branches on the boles of residual trees, and thus reduce their value as sawtimber. Bole quality, as manifested by log grade, is an extremely important deterrninant of the value of hardwood sawtimber. In general, USDA Forest Service log grades 1, 2, and 3 have value ratios of appnximately 13:7: 1, with butt logs much more Wviculturist at the Southern Hardwoods Laboratory, Maintained at Stoneville. Mississippi, by the Southern Forest Experiment Station. Forest Servtc+-USDA, in cooperation with the Mississippi Agricultural and Forestry Experiment Statton and the Southern Hardwood Forest Research. 9th Central Hardwood Forest Conference 248 likely to be grade 1 than upper logs (Stubbs 1986). Consequently. any siMcultura1 practice that may potentially result in log grade reduction can lead to a significant loss in value of the residual stand. Epicorrnic branches are frequent contributors to log grade reduction in partially cut hardwood stands. In the past, the popular notion among foresters was that epicormic branches developed on residual trees following partial cutting solely in response to increased sunlight on the bole. However, many researchers have demonstrated that the phenomenon of epicormic branching is not as simple as a direct response to sudden exposure to sunlight. Brown and Kormanik (1970). in an excellent review article, concluded that epicormic branching is a function of several complex factors: (1) stand density, (2) tree vigor, (3) aspect, (4) position on the bole, and (5) genetics. They maintained that, although direct sunlight has a stimulator-y effect on the release of suppressed buds, the sudden exposure to direct sunlight serves as a triggering mechanism for the production of epicormic branches’ rather than as the direct cause of the phenomenon. The concept that tree vigor plays a major role in determining the propensity of an individual to produce epicormic branches following partial cutting has been advanced by several researchers (Erdmann et al. 1985. Skill.ing 1957, Wahlenberg 1950). In each of these studies, trees of the lower crown classes (i.e.. low-vigor trees) exhibited a much greater propensity for epicormic branching than did trees of the upper crown classes (i.e., high-vigor trees). These results imply that individual-tree vigor, in combination with genetic differences in resistance to epicormic sprouting among species, acts as the primary controIling mechanism for the production of epicormic branches, while sudden exposure to direct sunlight or some other type of disturbance serves as the primary triggering mechanism for the release of those suppressed buds that eventually develop into epicormic branches. These two factors, individual-tree vigor and sudden exposure to sunlight, interact with each other to control and trigger the phenomenon of epicormic branch-