Third-year Growth and Bole Quality Responses T0 Thinning in a Red Oak-sweetgum Stand on a Minor Streambottom Site in West-central Alabama’

Four thinning treatments were applied to a red oak-sweetgum (Quercus spp.-Liquidambar styraciha L.) stand on a minor streambottom site in west-central Alabama in 1994: (1) unthinned control; (2) light thinning to 70-75 percent residual stocking; (3) heavy thinning to 50-55 percent residual stocking; and (4) B-line thinning to desirable residual stocking for bottomland hardwoods, as recommended by Putnam and others (1960). The thinning operation was a combination of low thinning and improvement cutting to remove most of the pulpwood-sized trees as well as sawtimbersized trees that were damaged, diseased, of poor bole quality, or of an undesirable species. Prior to treatment, the stand averaged 196 trees per acre with a basal area of 121 f? per acre. Quadratic mean diameter was 10.7 in., while stocking averaged 107 percent across the 24-acre study area. Light thinning reduced stand density to 63 trees and 62 ft2 of basal area per acre, increased quadratic mean diameter to 13.5 in., and reduced stocking to 69 percent. Heavy thinning reduced density to 49 trees and 64 ff of basal area per acre, increased quadratic mean diameter to 15.5 in., and reduced stocking to 52 percent. B-line thinning produced stand characteristics intermediate between those resulting from light and heavy thinning. Thinning increased 3year diameter growth of residual trees, across all species, but there were no significant differences among the three levels of thinning. Thinning also increased diameter growth of codominant trees, but not dominant trees, when averaged across all species. All levels of thinning, except heavy thinning, increased the production of new epicormic branches on the butt log, across all species, but all levels of thinning resulted in fewer than four new branches after 3 years. All levels of thinning increased epicormic branching on sweetgum, but only B-line thinning Increased epicormic branching on red oak and only light thinning increased epicormic branching on hickory (Carya spp.). In general, the production of new epicormic branches on the butt log was greatest on low-vigor, lower-crown-class trees. lNTRODUCTlON Thlnning regulates stand density and dramatically increases dlemeter growth of residual trees. Growth of individual trees has been improved in several hardwood forest types such La central upland oaks (Hilt 1979, Sonderman 1984b), Allegheny cherry-maple (Prunus spp.-Acer spp.) (Lamson 1885, Lamson and Smith 1988). and mixed Appalachian hardwoods (Lamson and others 1990). In general, the heavier the thinning, the greater the diameter growth response of individual trees. However, very heavy thinning may reduce residual stand density to the point where standlevel basal area growth and volume growth are greatly diminished. Site occupancy is less than optimum because the stand does not fully realize the potential productivity of the site. Recommended minimum residual stocking levels necessary to maintain satisfactory stand-level growth and to ensure full occupancy of the site are 46 to’65 percent in central upland oaks (Hilt 1979) and 45 to 60 percent in cherry-maple stands (Lamson and Smith 1988). Residual stand density equivalent to 52 percent stocking in a young water oak (Quercus nigra L.) plantation appeared to be sufficient to promote adequate stand-level basal area growth following thinning, whereas a residual stocking level of 33 percent created a severely understocked stand that will be unable to fully occupy the site for many years to come (Meadows and Goelz, in press). Degradation of bole quality of residual trees is also sometimes associated with increased thinning intensity. In upland oaks, the number and size of live and dead limbs on the boles of residual trees increased significantly as residual stocking decreased (Sonderman 1984a). On the other hand, Sonderman and Rast (1988) found that the production of epicormic branches on residual oak stems decreased with increasing thinning intensity. The proportion of dominant and codominant trees in the residual stand increases as the intensity of thinning increases. These vigorous, uppercrown-class trees are less likely to produce epicormic branches than are less vigorous, lower-crown-class trees (Meadows 1995). Consequently, a properly designed thinning should improve average bole quality throughout the residual stand. In many stands, however, there may be a trade-off between improved diameter growth and the potential for adverse effects on bole quality of residual trees as thinning intensity increases and residual density d e c r e a s e s . In most mixed-species hardwood forests, a combination of thinning and improvement cutting is also used to improve species composition of the residual stand (Meadows 1996). In general, the goal is to decrease the proportion of lowvalue trees and thus increase the proportion of high-value trees, Although most important at the first thinning, improvement of species composition and residual bole quality should also be a major consideration at all subsequent thinnings in mixed-species stands. These four components of thinning-increased diameter and volume growth of individual trees, increased stand-level basal area and volume growth, maintenance or . enhancement of bole quality, and improved species composition-are critically important for the profitable management of hardwood stands for high-quality sawtimber production. Thinning regimes should ideally be designed to optimize value of the stand, thereby synthesizing these four components. However, because maximization of all four components is not likely, some compromises or trade-offs in expected benefits must be accepted. ’ Paper presented at the Tenth Biennial Southern Silvicultural Research Conference, Shreveport, LA, February 16-18. 1999. ’ Principal Silviculturist and Principal Forest Biometrician, USDA Forest, Service, Southern Research Station, Stoneville, MS 38776, respectively.