Thinnings and Oak Management

Not everybody agrees on how and when intermediate cuttings should be made in red oak stands. However, silviculturists generally concede that stands originating from seedlings should be treated differently from those originating primarily from stump sprouts. The latter require early intermediate cuttings if tree quality matters. The former may benefit from no thinning for 40 years or longer (Carvell 1971). (Isebrands and Dickson, 1994)

In central New England, red oak commonly occurs in association with red maple and birch (Betula spp.), where site index ranges from about 55 to 75 (Hibbs and Bentley 1983, Oliver 1978, 1980). Although red maple and birch typically outnumber and outgrow the oaks during the first 2 decades of stand life, red oak ultimately emerges to form the dominant canopy because of its greater survival rate and its sustained growth rate (Hibbs 1981, Oliver 1978). Only about twice the number of red oak crop trees needed at the end of the rotation are needed at stand age 25 for the stand to be considered adequately stocked with red oak (Hibbs 1981). Recommendations thus are to delay first thinnings to stand age 45 (Hibbs and Bentley 1983, 1984). During the first 45 years, rapid height growth and the development of long, branchless boles are encouraged by maintaining high stand densities. Length of clear bole largely depends on early stem development and site quality (Carmean and Boyce 1973). (Isebrands and Dickson, 1994)

Although early thinning increases diameter growth, studies in New England have shown that merchantable height is reduced (Hibbs and Bentley 1984). The resulting value lost related to reduced merchantable height is not compensated by the gain in diameter. After stand age 45, recommendations are to reduce stand density to B-level stocking every 10 years until shelterwood cuttings are made to encourage the development of oak reproduction (Hibbs and Bentley 1983, Sampson et al 1983). The recommended rotation, assuming current utilization technology, is about 95 years when diameters of individual trees reach financial maturity at 21 to 25 inches d.b.h., depending on site quality (Hibbs and Bentley 1984). (Isebrands and Dickson, 1994)

Where northern red oaks are overtopped by long-lived competitors early in the rotation, early release may be necessary if the oaks are to attain dominance. This is especially true where red oak site index is greater than 70 and where the species must share dominance with sugar maple, American beech, yellow-poplar, black cherry (Prunus serotina Ehrh.), or other long-lived species. Where red oak is not overtopped, recommendations are to defer thinning until crop trees are at least 25 feet tall (Lamson and Smith 1978). In West Virginia, removing competitors within a 5-foot radius of 9-year-old crop trees on site index 70 did not significantly increase height or diameter growth or length of clear stem, and it did not prevent crown-class regression during the 5-year study period (Lamson and Smith 1978). Similar results were observed on site index 60 for 7-year-old trees (Trimble 1974). If thinning in young stands is undertaken, recommendations are-to do so conservatively by creating canopy gaps no more than 5 feet wide on three or four sides of selected crop trees (Sampson et al 1983). (Isebrands and Dickson, 1994)

Besides controlling composition, early thinnings also increase yields. Studies in the central hardwood region show that cubic-foot yields of oak stands are more than 50 percent greater when thinning (cleaning) is begun at age 10 than when thinning is begun at age 60 (Gingrich 1971). Although economic considerations may preclude cleaning, oak stands often can be profitably thinned as early as stand age 2S where there is a market for fuelwood or pulpwood (Carvell 1971). Stands should not be thinned after they reach about 60 years (Sander 1977). (Isebrands and Dickson, 1994)

Even though maximum yields may occur at stocking levels below B level (Dale 1972, Leak 1981), the lower limit of thinning is usually set at B level to ensure bole quality (Dale and Sonderman 1984). Thinning below B level risks increasing epicormic branching on the lower bole with consequent loss of tree value. However, stocking levels after thinning do not have a significant effect on the stem form of oaks (Hilt and Dale 1979). (Isebrands and Dickson, 1994)

Thinning within clumps of northern red oak stump sprouts in Wisconsin maximizes residual stem growth and quality when clumps are thinned to one stem before they reach 3 inches d.b.h. (age 12 to 15) (Johnson and Rogers 1984). Growth simulations indicate that thinning clumps to one stem at age 5 on site index 70 can produce boles 11.6 inches d.b.h. at age 25 when coupled with periodic thinnings around crop stems to a spacing that approximates B-level stocking. Recommendations are to thin clumps only on site index 65 or greater. (Isebrands and Dickson, 1994)

Clump thinning recommendations based on a West Virginia study (Lamson 1988) are: (1) to thin clumps when they are 10 to 20 years old, and only on good sites; (2) to select crop stems that are dominant or codominant, straight, and free of forks and other defects in the lower 17 foot bole, and that originate not more than 6 inches above groundline3; (3) to retain only the best one or two crop stems per clump. If two crop stems are retained, the stems should be far enough apart so that during their expected life span they do not fuse together at a common base (Roth 1956, Stroempl 1983); (4) to remove all intermediate and codominant trees if their crowns touch the crop stem crown, or if their crowns are above or below the crop stem crown; and [5) to thin around thinned clumps in about 10 years to sustain maximum diameter growth. Because pruning 12-year-old northern red oak stump sprouts reduced 10-year diameter increment, that practice is not recommended in young stands (Lamson 1988)4. (Isebrands and Dickson, 1994)

Timing and intensity of intermediate cuttings will depend on the natural stand dynamics of the ecosystem where they are to be applied. For example, maximizing bole quality in red oak stands in New England is facilitated by maintaining maximum stand densities rather than thinning during the first 40 years. This approach to early stand management is feasible in that region because survival and growth rates of young red oaks are high relative to those of the predominant competitors, red maple and birch. In other regions, early crop tree release may be necessary for red oak to attain and maintain dominance, especially if the competition is yellow-poplar, sugar maple, or beech. But even where competition is severe, recommendations are to defer thinning until crop trees are at least 25 feet tall. Exceptions include red oak stump sprouts, whose growth and quality can be increased by thinning clumps to one stem as early as age 5 years. To ensure bole quality, the safe lower limit for residual stand density is usually "B-level" stocking percent as defined by stocking charts and equations. Stands older than about 60 years should not be thinned. Rotation lengths for sawtimber range from about 80 to 120 years depending on management objectives, site quality, and other factors. (Isebrands and Dickson, 1994)