Managing oak in the driftless area
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The "Driftless Area" is an island of unglaciated land in western Wisconsin, southeastern Minnesota, and northeastern Iowa. This region comprises 10 million acres and roughly one third of it is forested. Having escaped the scouring action of the glaciers, the Driftless Area is much hillier than the surrounding area and the ecosystems are much older.
Where the ridges and valleys are broad and flat, the level areas have been cleared for farming, so forests are confined to the steep hillsides. But where the topography is more rolling, forests may cover the entire landscape. These forests, dominated by oak, have contributed significantly to the economy of the region. The value of oak lumber and veneer has risen, benefiting local forest industries as well as landowners. Other benefits provided by these forests include the protection of the soil and watersheds, abundant acorns which provide food for wildlife, and natural beauty which attracts sightseers and recreationists.
Recently, however, this oak resource has been declining. Oak forests have suffered as a result of increased cutting and, more importantly, poor management or lack of management. High-grading - taking the best and leaving the rest - has done dual damage to many stands: the residual, poor-quality trees have little potential for developing into merchantable timber and the harvesting process has ignored the regeneration requirements of the oaks. The oaks are often replaced by other hardwoods, generally species less valuable for either commercial or wildlife use.
Thus, harvesting and lack of good management are jeopardizing the future of oak forests in the Driftless Area.
The purpose of this guide is to recommend silvicultural practices for:
- regenerating oaks, especially northern red oak,
- increasing the growth rate of oaks in established stands, and
- rehabilitating stands that have been indiscriminately cut.
Figure 1. The driftless area
This publication is intended primarily for foresters and landowners in the Driftless Area, but the practices recommended are applicable in similar ecosystems (exceptions include those containing yellow-poplar) well beyond the boundaries of this region. Recommendations emphasize the production of high-quality sawlogs, but include suggestions for enhancing wildlife and protecting watersheds.
The regeneration strategies presented in this publication are based partly on formal research, but also on the observations of local state foresters and loggers and the results of informal trials they have conducted. Although these strategies have not been extensively tested, they are "state-of-the-art" in the region. Appendix A, p. 16, summarizes the applicable research results and implications that form the basis for the recommendations. More information is listed in "Recommended Reading", p. 15.
Management techniques for a particular species are dictated by the sites it grows on as well as its silvical characteristics and those of its associates. Although there are silvical differences among the oaks themselves, the most important differences are those occurring between oaks and the 10 or so other hardwoods that occur on each site.
Sites and site quality
Most upland forest soils in the Driftless Area originated from wind-blown silt (loess). These soils are generally moist but well-drained and rich in nutrients. However, site quality is highly variable because of differences in soil moisture and depth to bedrock and differences in slope aspect, steepness, and shape. Where single-stemmed, dominant, or codominant trees occur, site quality can be determined from site index curves (Appendix B, p. 18). Where there are no suitable trees, site quality must be estimated from soil and topographic features and plant communities (including ecological classification systems where such information exists). See Appendix C, p. 20. Poor (dry) sites (site index less than 55 feet) are usually found on narrow ridgetops or south- and west-facing, steep, convex upper slopes where soil depth is less than 20 inches. These sites are mainly occupied by a mixture of oaks: bur, black, white, and red, with bur and black more prevalent on the driest sites. Associated species are chiefly black cherry and shagbark hickory. Merchantable boles are short on such sites, partly accounting for the low yields - usually less than 10,000 board feet per acre on 80- to 100-year rotations. (Yields referred to throughout the narrative are generalized; yield tables are presented in Appendix D, p. 20.)
As site quality increases, the ecosystems become more complex. On dry-mesic sites (site index 55-65 feet) red and white oaks predominate but many other species are common, e.g., basswood, slippery elm, black cherry, shagbark hickory, sugar maple, and white ash. These are the "average" sites in the Driftless Area and most of the oak resource is concentrated here. Yields from stands managed on 80- to 100-year rotations will be approximately 15,000 board feet per acre.
The best (mesic) sites (site index 65+) occur on north- and east-facing, gently sloping, concave or lower slopes where soils are at least 36 inches deep. The dominant species are sugar maple, basswood, slippery elm, and ironwood. Associated but less prevalent species include red and white oak, white ash, bitternut hickory, and American elm. However, under certain conditions, red oak may form pure stands on such sites. Yields on the best mesic sites (site index 75) may reach 24,000 board feet per acre on 80- to 100-year rotations.
On both dry-mesic and mesic sites, natural succession aided by current harvesting practices is reducing the dominance of oaks. Where sugar maple and basswood seed is abundant, these species are rapidly becoming dominant; where these species are absent, the oaks are being replaced by a mixture of black cherry, slippery and American elm, bitternut and shagbark hickory, boxelder, and aspens. On some clearcuts, oaks may still be well represented, largely because of stump sprouts. But well-developed shrub communities often discourage and even prevent tree reproduction, especially on the dry-mesic sites.
- Acorn production. Good acorn crops commonly occur every 2-5 years, but the interval can be longer. Acorns in the white oak group mature the year of flowering; red oaks the year after. Even in a good year the oaks are sparse seed producers. A fully stocked, mature stand will yield up to 250,000 acorns per acre while a stand of sugar maple, for example, may produce in excess of 5 million seeds. Moreover, dispersion of acorns is limited: most remain directly beneath the crown of the mother tree where they fall, although a few may be spread more widely by birds and small mammals.
Fallen acorns deteriorate rapidly if not protected from drying out. Leaf cover provides some protection, but burying the seeds several inches into the mineral soil, either by squirrels or logging disturbance, increases the probability of successful germination. This not only keeps the acorns moist but also minimizes later pilfering by animals and reduces losses to insects that feed on the radicles emerging from germinating acorns.
- Germination. White oaks begin germinating in the fall immediately after the acorns drop; black and red oaks germinate the following spring. Germination is completed during the first growing season. First- and second-year seedling growth is supported by food stored in the acorn. During this time, seedlings can survive in low light. Thereafter, more light is required for continued development. Given adequate light and moisture, seedlings produce several flushes of shoot growth during a growing season.
- Drought and heat resistance. The ability to sprout vigorously after drought, fire, or other damage gives the oaks a distinct advantage over less responsive hardwood species. Even when the tops of young seedlings are killed, they will generally sprout again. Rooting depth enhances drought resistance: the tap root of new seedlings reaches mineral soil within weeks of germination. In contrast, roots of competing mesic hardwood species may be confined to the surface organic soil for years. Finally, as the oaks mature, their thick bark helps insulate them from damage by fire.
Because of their capacity to sprout and to withstand drought and heat, oaks commonly form climax associa-tions on dry sites. The moisture deficiency on such sites limits the number of mesic hardwoods and reduces the ability of all species to develop full canopies, thus allowing sufficient light for oak reproduction. Shrubs can be a problem on dry sites, however. Typically, shrub density is 10 times greater on dry sites than on moist sites, creating unfavorable conditions for oak reproduction.
- Shade tolerance. Oaks are only moderately shade-tolerant, so full sunlight is necessary if young oaks are to outgrow their competitors. That is why, without management, shade-tolerant hardwoods eventually supplant oak on the best sites. Full overstory canopies of shade-tolerant trees and dense understories of trees and shrubs combine to shade out oak seedlings, even those seedlings that have developed well for the first year or two.
Most current oak forests have developed because of the long history of repeated fires before the settlement of the region. On the dry prairie edges and savannas within the Driftless Area, where light was abundant, scattered bur, white, and black oak trees and seedling-sprouts survived recurring fires that discriminated against various other hardwood species for many years. When the pattern of repeated burning was finally interrupted by people settling on and developing the land, the oaks took over the remaining forested areas almost unhindered. Similarly, when fires on good, mixed-hardwood sites destroyed the fire-sensitive species (e.g., sugar maple and ironwood), red and some white oak replaced them, often forming pure oak stands.
Figure 2. Young oaks cannot survive long beneath a dense understory or overstory.
Susceptibility to damaging agents
- Acorns. Oaks typically produce only a small, viable seed crop. This crop is diminished by acorn-eating animals and acorn-destroying insects, which eliminate large quantities of seed each year. During fair-to-poor seed years, few acorns are left to germinate and develop into seedlings.
- Seedlings. During the first month following germination, many new oak seedlings are lost to squirrels because acorn locations are revealed by the emerging seedlings. The seedlings themselves are a favored browse of rabbits and deer. Even though the oaks readily sprout after such damage, very young seedlings sprout weakly and repeated browsing gives less-palatable species an advantage, enabling them to eventually outgrow and shade out the oaks.
- Foliage. Oaks are subject to attack by several defoliating insects from spring through late summer. A potentially serious pest in the Driftless Area is the gypsy moth, whose outbreaks may last for several years. Two or more years of heavy defoliation may kill some oaks and stress others to the point where they become susceptible to disease and other insects, especially the two-lined chestnut borer (see below).
- Stems, branches, and twigs. Oak wilt is one of the most serious threats to oaks, especially red oak. The disease is spread through wounds and root grafting and is difficult to control.
The two-lined chestnut borer attacks branches and twigs. It is a secondary insect that kills many oaks weakened by drought and other insect attacks. Several wood borers also attack oaks, degrading the wood and providing entry for decay. Logging injury to pole and sawlog trees may lead not only to decay but, more importantly, to loss of quality. Here is a classic example of where "an ounce of prevention" pays off.
- Roots. Perhaps the most significant pest attacking oak roots is shoestring root rot. Infection often follows drought or defoliation.
Details on the prevention and control of these pests are outlined in Appendix E, p. 24. But, again, the best control is prevention. The best prevention silviculturally is to: (1) maintain tree vigor by regulating density, harvesting mature trees, and harvesting or killing diseased and insect-infested trees while minimizing logging damage, and (2) promote species diversity. As a last resort, it may be necessary to spray for gypsy moth.
For greatest financial returns, timber management should aim at producing high-quality sawlogs for veneer and lumber – trees that are 16+ inches diameter at breast height (DBH) with straight, defect-free boles. Such trees are best grown on medium or better sites (site index 55+) that have been well-stocked during the critical seedling/sapling/pole stages when tree quality is established. These trees are worth up to 10 times as much as smaller, poorly formed ones that yield only low-quality sawlogs, pulpwood, or firewood.
Tree-grading rules for high-quality sawlogs specify a minimum of 14 inches DBH for grade 2 trees and 16 inches for grade 1. Premature harvesting of trees with high-quality potential can be very costly: e.g., 12-inch trees will increase in value 8- to 10-fold if grown to 16 inches. The internal rate of return for such trees during this period of growth ranges from 20 to 30 percent.
Economic maturity (when declining growth reduces the rate of return to less than about 4 percent) usually ranges from about 16 inches DBH on poor sites to 20 inches on good sites. For vigorous, veneer-quality oaks on the best sites, DBH for economic maturity can be as high as 24 inches. Depending on intensity and regularity of thinning, producing trees of these sizes will take 90 to 120 years on poor sites and 60 to 90 years on good sites.
Obtaining adequate reproduction is the most difficult and critical step in oak management. The key to success is the regeneration cut; this cut determines future composition as well as the productivity of the stand. The goal is to apply harvesting techniques that will replace mature stands or those of low potential value with oak and other desirable species suited to the site.
To satisfy oak light requirements, each regeneration area must encompass at least 1/2 acre (160-foot diameter circles). However, larger openings are recommended.
Small openings cost more to manage and harvest and are difficult to regulate. Moreover, such openings have a larger proportion of their area in "edge" (25 percent for 1/2-acre openings versus about 12 percent for 2-acre openings). This means that oak seedling growth will be reduced by shade from border trees. In addition, more border trees will be degraded by an increase in epicormic branches. To avoid these drawbacks, we recommend a harvest area of at least 2 acres and 200 feet wide. If deer browsing is a problem, increase the opening size to at least 4 acres.
There are no silvicultural reasons to limit maximum stand size. Generally, however, stand conditions and site should be fairly uniform to simplify the planning and application of cultural treatments. On tracts managed for sustained periodic yield and stable wildlife habitat, strive for a reasonable balance of age classes scattered over the area. Maximum stand size is determined by the area of the tract, the interval between regeneration cuts, and the number of age classes. Thus, a 40-acre tract with 8 age classes established at 10-year intervals (80-year rotation) would limit stand size to 5 acres. Even small properties can be managed to provide balanced age classes, although the gap between age classes may be wide. For example, a 10-acre forest with 5 age classes established at 20-year inter-vals (100-year rotation) would limit stand size to 2 acres. Intermediate cuts could be applied at shorter intervals, e.g., every 10 years.
The regeneration methods that will satisfy the light requirements of oak are group selection, clearcutting, and shelterwood. Group selection (which consists of openings smaller than 2 acres) is not generally recommended for timber production for the reasons just discussed. How-ever, this method can be used to maintain some oak for timber and wildlife while pursuing other management goals (e.g., aesthetics or recreation). To benefit oak regeneration, openings created by group selection must be a minimum of 1/2 acre and preferably larger.
The regeneration cuts recommended for oak also favor many other tree, shrub, and herbaceous species. Consider these special requirements of oak when applying the harvest cut:
- Competition from trees, shrubs, and certain herbaceous plants must be minor before the overstory is removed. This is particularly important when acorns or small advance reproduction (less than 1 foot tall) will be used to regenerate a stand.
- A potential source of adequate oak regeneration must be present when the overstory is removed. This can consist of (a) advance seedling reproduction, (b) trees capable of producing stump sprouts, (c) viable acorns, or (d) a combination of these sources.
- Residual overstory trees must be killed before they begin to suppress oak regeneration.
- Rapidly growing regeneration of undesirable tree species (aspens, especially bigtooth, white birch, boxelder, elms, etc.) may need to be controlled within 5-15 years following overstory removal.
Clearcutting and shelterwood are the methods usually recommended for regenerating oak. The choice depends on the regeneration potential of the stand, i.e, the presence of acorns, advance reproduction and stumps capable of sprouting. Advance reproduction and stump sprouts have the highest potential for outgrowing competing vegetation in new stands. Appendix F describes advance reproduction potential by seedling height and stump sprouting potential by species, site class, age, and DBH. Use clearcut or group selection when the regeneration potential is adequate; use the shelterwood method when regeneration potential is not adequate. The shelterwood method reduces the overstory density to favor the development of advance reproduction before the remaining overstory is removed to release it. This method should also be used before a group selection cutting when the sources of regeneration are inadequate.
In general, shelterwood is the preferred method, especially when the regeneration potential is uncertain. This system minimizes the risk of regeneration failure by retaining a seed source. Unfortunately, there is no accurate way to predict acorn numbers and viability before scheduling harvest cuts, but with shelterwood it is not necessary to coordinate the harvest with the seed crop. Moreover, moderate shade favors acorn germination and seedling survival and reduces competition during the first critical years of establishment.
The misunderstanding about "clearcutting" in forest management needs to be clarified. Some landowners and the general public often equate clearcutting with removing an "entire" forest in one harvest cut. However, in a managed forest, portions of the forest are cut periodically to create a series of balanced age classes - in essence, an "all-aged" forest consisting of small, even-aged stands distributed in space and time over the forest as a whole.
Assessing and controlling understory competition
Ferns, shrubs, and overtopping understory trees (less than 1.6 inches DBH) restrict the growth of oak seedlings and will eventually eliminate them. Thus, if a layer of such vegetation exists, or promises to develop after the overstory is cut, control is essential for the establishment of oak and other desirable hardwoods.
Control is obviously necessary if oak or other desirable reproduction is absent beneath a dense layer of such vege-tation. But if there is any doubt, the understory should be systematically assessed, applying the criteria shown in Appendix F, p. 26, last paragraph. Use the same 1/735-acre plots used for determining regeneration potential (Appendix F, Table 14a, Column G; p. 30) or similar-sized plots uniformly distributed throughout the harvest area. Control understory competition if more than 30 percent of the plots are dominated by such vegetation when relying on acorns or advance reproduction less than 1 foot tall. When relying on advance reproduction more than 1 foot tall and stump sprouts, control the understory competition if more than 70 percent of the plots are dominated by undesirable vegetation. Such vegetation can be controlled by chemical or mechanical treatment, depending on the size, density, and type of vegetation present.
Foliar herbicides must be used for ferns and can be effective on small (less than 10 feet tall) shrubs and trees. Where oak reproduction is taller than 3 feet, the oaks can be "protected" by cutting them before applying the herbicide; the cut stems will then resprout. Such intensive treatment is generally impractical when managing large forests but can be used where time and expense are not critical. Basal sprays, stem injections, and cut-stump applications are best for large shrubs and saplings. These treatments have the advantage of aiming the herbicide directly at the target plants rather than broadcasting it over all the vegetation.
Caution: Be sure to contact your state extension specialist or forestry agency for local regulations and recommendations on herbicides and their application!
Figure 3. Group selection, clearcutting, and shelterwood harvesting provide sunlight needed for oak regeneration.
Mechanical uprooting during logging or in a separate operation before or after logging can be as effective as herbicide treatment in removing saplings and shrubs. The key is to expose the roots - not cut or break off the stems, which leads to sprouting. Sugar maple and ironwood saplings are especially easy to kill by merely "riding down" the stems with logging equipment, leaving root-sprung trees and little soil disturbance. Advance oak reproduction generally survives this treatment because of its deep root system and ability to sprout after top injury. An added benefit is the advantage that the resulting soil disturbance gives to acorn germination and seedling establishment. Severe soil displacement should be avoided, however.
The best time to control understory competition is during a good seed year. Apply herbicides in late summer or early fall before the acorns drop; delay mechanical treatment until after the acorns have fallen.
Determining regeneration potential
Before selecting a regeneration strategy, you will need to inventory the advance reproduction and the overstory to determine the potential ability of the stand to reproduce itself. Advance reproduction and stump sprouts are the two most reliable sources of oak regeneration. If you find that there is not enough oak advance reproduction to replace the stand, the overstory inventory can be used to determine whether there will be enough stump sprouts to make up the difference. This information determines how the strategies selected will be applied (see figure 4, p. 7). Detailed procedures for making such an inventory appear in Appendix F, p. 26.
If advance reproduction and stump sprouting potential are inadequate, you will also need to assess acorn production and seed-tree distribution. Only “good” crops of acorns will adequately regenerate a stand. There is no objective way to estimate acorn production; if in doubt, seek the advice of an experienced local forester when the seeds begin to ripen in late summer. A stand at least 50 percent stocked with mature oak (60 or more square feet of basal area in well-distributed sawtimber trees) has the potential to be a good seed producer. (Stocking percent should be used to measure stand density when determining thinning intensity and when evaluating overstory stocking in shelterwoods. See Upland Central Hardwoods Stocking Charts, Appendix G, p. 32.)
Strategy for adequate regeneration potential
Figure 4. Regeneration strategies. Decision process for selecting regeneration strategies. Not every possible situation can be presented, so select the alternative that best fits your situation. Begin at the top of the chart and make the appropriate decisions until you have reached one of the regeneration methods.
If regeneration potential is adequate and includes advance reproduction taller than 1 foot as well as stump sprouts, clearcutting or group selection is the method of choice. When the advance reproduction is less than 1 foot tall, shelterwood is the preferred method, allowing the advance reproduction to grow to a competitive size before overstory removal. Clearcutting or group selection is still an option, however, if the need for an immediate financial return outweighs biological considerations.
Clearcutting (or group selection) is also recommended when oak reproduction potential is less than minimum but other hardwood species acceptable to the forest manager combine with the oaks to meet minimum stocking requirements. This method will produce a manageable stand of other hardwoods with an oak component.
Strategy for inadequate regeneration potential
- Acorn crop and seed-tree distribution adequate. If there is a good crop of acorns well-distributed throughout the harvest area, the shelterwood method is preferred. However, if the landowner demands an immediate harvest, clearcutting or group selection will provide adequate reproduction in most areas where a good crop of acorns is present.
- Acorn crop inadequate but seed-tree distribution adequate. If acorns are sparse, the harvest should be delayed. One option is the shelterwood system for natural regeneration (from future acorn crops). Shelterwood with underplanting is another option, or, if the owner insists on an immediate return, clearcutting or group selection followed by interplanting.
- Seed-tree distribution inadequate. In this case, planting is necessary to re-introduce oak. Use shelterwood with underplanting or use clearcutting or group selection with interplanting.
Applying the systems
Clearcutting and group selection
Before or during the harvest cut, control understory vegetation if necessary; during or after the harvest cut, remove or kill all trees larger than 2 inches DBH. Cut unmerchantable oak to promote sprouting. Cut oak stumps as low as possible to facilitate sprouting from the root collar. Use herbicides on undesirable species (such as red maple) that produce vigorous stump sprouts. When relying on acorns for reproduction, delay harvesting until after the acorns ripen (mid-October) and disturb the soil during logging or intentionally scarify. But minimize soil disturbance if the regeneration consists of advance reproduction and stump sprouts.
When using group selection, first decide on a cutting cycle, usually 10-15 years. Assuming a 10-year cycle, regenerate 10 percent of the stand to be harvested by cutting several 0.5-to-2-acre openings. Select areas to be cut that contain advance reproduction and mature trees. If advance reproduction is lacking, apply the appropriate treatments previously described. Anticipate the next harvest by controlling understory competition and applying the shelterwood method to another 10 percent of the area. In this way, the entire area will have been regenerated to oak after 10 cuts. Obviously, if a larger percentage of the stand is cut each time, fewer cuts will be needed. Areas between groups can be thinned using even-aged stocking goals or managed by individual tree selection.
Apply shelterwood seed cut and control understory competition if necessary, during a good seed year, if possible. A good acorn crop within 2 years of understory removal will usually assure adequate reproduction. However, if a good acorn crop does not occur within 3 years, understory competition may have to be controlled a second time, again, preferably during a good seed year.
Remove trees from the lower canopy. If acorns or advance reproduction are lacking, leave 70 percent or more stocking – usually 80-100 square feet of basal area per acre for mature stands. However, if acorns or seedlings are present, stocking as low as 60 percent may be allowed. If you err, err on the high side: too little stocking encourages competing shrubs and trees at the expense of oak.
When applying the seed cut, you can "mark to leave," designating unmarked trees for cutting. But be sure the logger clearly understands this arrangement!
Leave the best dominant and codominant trees, including desirable non-oak species if needed to meet stocking goals. Also leave all unmerchantable oaks capable of producing stump sprouts until the final removal cut. Remove all other trees larger than 2 inches DBH, including seed-producing trees of undesirable species (especially boxelder) and, if necessary, control stump sprouts with herbicide. The final stand should appear parklike, with no subordinate crown layers and no major gaps in the main canopy.
Remove the overstory when the advance reproduction meets the minimum stocking standard described in Appendix F, p. 26. Residual tree control should not be necessary; these trees should have been controlled during the seed cut.
All harvest cuts
When relying on acorns, disturb the soil. Otherwise, minimize soil disturbance by scheduling the removal cut for winter when there is snow cover and the soil is frozen. When logging in other seasons, restrict skidding to select-ed areas. This will discourage establishment of intolerant, pioneer tree species. Also, it should not be necessary to control the ground vegetation that follows the removal cut. The oak reproduction may seem to be buried under the dense, vigorous, herbaceous vegetation, but it will emerge from this layer after 4-6 years.
Artificial regeneration offers optional methods for establishing oak, but should be used only as a last resort. Planting is expensive: seedlings must be grown and transported as well as planted. Direct seeding is also uncertain; dependable methods have yet to be developed. However, planting (or seeding) is sometimes necessary, either to reforest open fields or to supplement clearcutting, group selection, or shelterwood when seed trees are inadequate. Guides presented here were developed mainly for red oak, but the biological principles should apply as well to white, black, and bur oak.
General guides to planting
Oaks grown for timber production should be planted only on good sites - site index 65+. If dry sites, i.e., less than site index 50, are planted for other than timber, plant white, black, or bur oak.
Plant large seedlings (at least 3/8-inch stem diameter) with fibrous root systems (at least six permanent, first-order lateral roots). For ease of handling, clip tops and roots to about 8 inches in length. Tops should be clipped not more than 10 days before planting; roots can be clipped earlier, e.g., in the nursery before storage and transport. Take good care of stock during transport and planting and use proper planting techniques, e.g., plant stock with spreading root systems in holes, using shovels or augers if necessary. Control competition for about 3 years after planting.
On forest sites, spacing can be wide (20-25 feet) because the intervening areas will be stocked with natural reproduction (even though it may not be oak) that will create the density necessary for developing quality trees. In open fields, trees should be planted 5-8 feet apart in rows that are 10-12 feet apart. This closer spacing will promote earlier crown closure, which will in turn shade out understory weeds while producing high-quality trees. Such spacing will allow access by mechanical equipment while still providing an adequate selection of crop trees.
Figure 5. Large diameter oak seedlings with a fibrous root system survive best.
Consider planting with other desirable species suitable to the site, such as walnut and white ash, but which may be missing because of indiscriminate harvesting. If there is a market for pine in your area, alternate rows of pine with oak on field sites. This will provide early weed control and income to help offset the cost of establishing oak.
Protection from browsing and girdling by animals may be necessary. Discourage rodents by keeping the area immediately surrounding the seedlings as weed free as possible during the early years. To discourage browsing by larger animals, you may have to fence entire stands or individual trees. Minor to moderate browsing can usually be tolerated, however. Tree shelters can protect and promote the growth of individual seedlings, but should be used sparingly because of their cost. The shelters can be used to assure the survival and growth of a minimum number of seedlings (50 perhaps, but no more than 100 per acre).
Underplanting with shelterwood
Planting under shelterwood gives the seedlings time to overcome planting stress before the overstory is removed. Having this "head start" allows the largest seedlings to outgrow the competing vegetation that invades following overstory removal.
Cut from below to leave 50-60 percent stocking in dominant and codominant trees. Control understory vegetation and stump sprouts of undesirable species, if necessary. Plant in early spring of the first year following the shelterwood cut. Irregular spacing is acceptable. Plant in spaces lacking oak seed trees or where potential competition is minimal. If planting stock is smaller than recommended (see figure 5), plant three times as many seedlings as crop trees desired; if the stock meets the recommended specifications, plant two seedlings for each tree wanted. Planting these "extra" seedlings assures the establishment of the desired number of trees without additional control of understory competition following overstory removal.
Remove the overstory during the dormant season 3-6 years after planting; seedlings should be at least 2 feet tall. Any seedlings damaged during harvesting will resprout. Stump sprouts of undesirable species left during the seed cut should be controlled. Competing trees that invade after overstory removal can be removed during the sapling stage, if necessary.
Interplanting with clearcutting
Control understory vegetation before or immediately after overstory removal. Harvest the merchantable trees and remove or kill all residual trees. Control stump sprouts of undesirable species.
Plant seedlings in early spring of the first year following the harvest cut. Irregular spacing is acceptable. Con-trol invading competition around planted seedlings during the establishment period, usually 3 years; this precludes the need for "extra" seedlings.
Planting open fields
During the fall before planting, prepare the site by applying herbicide followed when feasible by plowing, disking, or rototilling. Treat either the entire site or strips at least 6 feet wide. Plant seedlings at desired spacing and control competing vegetation as needed, usually for 3 years.
Direct seeding is rarely successful, chiefly because of acorn pilfering by rodents and lack of competition control. However, the following procedure should increase your chance of getting good results.
Collect mature acorns as soon as they drop. Keep them cool and moist during handling and storage. Place them in a tub of water and discard the floaters. Sow white oak in the fall, as soon as possible after collecting; sow red oak immediately in the fall or, if necessary, delay until the following spring.
For spring sowing of red oak, store acorns over winter in 4-mil polyethylene bags at 35-37 degrees F. or bury the bags several feet deep in well-drained soil. In spring, remove the acorns from storage and pre-germinate them by raising the temperature to about 40 degrees F. for 10-14 days. Continue to keep them moist. Germination will probably begin in storage and accelerate when the temperature is raised. Sow only those acorns with developing radicles; broken radicles will resprout. An advantage of spring planting is that you know you have sown viable acorns.
After controlling understory vegetation, seed openings at least 1/2 acre in size (the larger the better) that are fairly free of litter and logging debris. Sow acorns 1-2 inches deep in spots several feet wide, 3 or 4 acorns to a spot. Sow at least twice as many spots as trees wanted. Control competing vegetation as needed. As added insurance, you may want to cover some of the sowed acorns with tree shelters. Bury the bottoms of the shelters several inches deep to discourage digging by squirrels.
Intermediate (improvement) cuttings are intended to (1) insure survival and increase growth of the desired species, (2) reduce the time to harvest, (3) improve quality of the trees, (4) increase tree vigor, and (5) enhance wildlife habitat. In managed forests, these practices determine how much of the potential established by the regeneration cut will ultimately be realized; in previously mismanaged forests, such cuttings can increase productivity.
Intermediate cuttings may provide salable products, but often the trees removed are too small or low in quality to be marketable. Nevertheless, if there is a local market for high-quality sawtimber, investment in one or more noncommercial thinnings may more than offset the cost (Appendix H, p. 32). In fact, thinning to increase the growth rate of high-quality sawtimber trees generally produces a 10-20 percent rate of return because of the short investment period and the low cost of releasing large trees. For best results, confine noncommercial cuttings to good sites (site index 55+) that support high-quality trees. But if there is a local market for pulpwood or fuelwood, poorer sites may qualify for such treatment. In any case, select for release a limited number of trees that will produce grade 2 or better sawlogs. The key is to minimize costs without sacrificing effectiveness.
Identifying crop trees
Except for liberation cuts that remove the overstory to release established reproduction, intermediate cuts should aim at providing adequate growing space for pre-selected crop trees - those with the potential to produce high- quality sawlogs or veneer. Specifically, a crop tree should have:
- A crown in the dominant or codominant class.
- A diameter larger than the surrounding trees.
- A vigorous crown - dense, dark green foliage.
- Few epicormic branches below the main crown or lower 20-25 feet of bole of pole-size and larger trees. (Poor crown vigor and the presence of epicormic branches portend more epicormic branching after thinning. Well-developed crowns usually inhibit development of epicormic branches.)
- A straight, well-formed bole nearly free of live branches and large dead branches for the first 20 feet, if pole size or larger, or half the total height, if a sapling. Three or four scattered branches less than 2 inches in diameter are acceptable.
- No V-shaped forks. (U-shaped forks above 25 feet are acceptable.
- No large cracks, seams, or wounds.
- Stump sprouts are acceptable if they meet the above criteria and originated at or below ground level.
Releasing crop trees
Release no more than 100 crop trees per acre - 90 trees would result in a fully stocked stand of 16-inch DBH trees that could be grade 1. With continued management, such a stand could provide harvests of 30 16-inch, 30 19-inch, and 30 22-inch trees. In stands with more than 100 potential crop trees per acre, some good trees may have to be cut to make more room for others. This will be rare, however; most stands will have fewer than 100 stems meeting crop tree specifications. Ideally, crop trees should be 20-25 feet apart. But if they are scarce or unevenly distributed, two trees as close as 10 feet can be selected as long as they are treated as one tree when thinning. Figure 6, on the next page, illustrates crown release.
Figure 6. Crown release will stimulate crop tree growth.
Release each crop tree by removing the adjacent dominant, codominant, and intermediate trees whose crowns touch or encroach on that of the crop tree. Free all sides of saplings and small poles and at least three sides of larger trees.
However, in sawtimber stands, if any trees competing with large crop trees have the potential to develop into grade 2 or better within 10 years, leave them until they attain the higher value.
In noncommercial cuttings, cut or kill only those trees preventing crown expansion of the crop trees. Do not ap-ply noncommercial release in areas devoid of crop trees.
In commercial cuttings, after the crop trees have been released, remove enough other marketable trees in the main canopy to reach the stocking goal. Trees below the main canopy can be cut if they are salable; if not, leaving them will have little detrimental effect on the growth of the crop trees.
CT = Crop tree. Source: How to release crop trees in precommercial hardwood stands. USDA Northeastern Forest Experiment Station pamphlet NE-INF-80-88.
Pruning is rarely necessary in well-stocked, natural, hardwood stands. Maintaining good stocking until the trees are 40-50 feet tall will discourage development of large branches and encourage natural pruning of the lower 20 to 25 feet of the bole. Open-grown trees in natural or planted stands may need pruning, however. Most large branches (and poor bole form) result from damage to the apical leader. In such cases, clip one of the new leaders within several years of formation. Prune live branches with a saw close to the bole without wounding the stem; prune dead branches through the dead wood outside the live branch collar. Confine pruning to the first 16-foot log or half the total height, whichever is less. Thus, a 20-foot tree may have to be pruned a second or third time as it grows in height.
Applying intermediate cuttings
Seedling and sapling stands
The goal of intermediate cuttings at this stage is primarily to remove hindrances to height growth. Any or all of three kinds of treatments may be appropriate:
- Liberation cuttings - to prevent or relieve suppression from overtopping residual trees.
- Cleaning - to prevent or relieve competition from fast-growing, less desirable trees in the same age class.
- Stump-sprout thinning - to assure the survival of stump sprouts with little risk of decay and with high-quality potential.
Thinning to stimulate lateral crown development and diameter growth at this stage is not recommended. It may jeopardize quality development and is of questionable economic value.
- Liberation cuttings. Overhead shade can either kill reproduction of intolerant species (and the oaks are among the first to go) or cause the young trees to develop poor form as they bend toward canopy openings. So it is essential to remove trees left after a regeneration cutting that are shading desirable reproduction. Remove tolerant understory, mid-canopy, and large vigorous trees immediately after the harvest cut. Scattered main canopy trees occupying up to 15 square feet of basal area per acre can be left as long as 15 years. The key always is to cut or kill the overstory trees before they begin to develop deep shade over a large amount of reproduction. Generally, all overstory trees should be removed at once, but, at the owner's discretion, some can be left for longer periods to benefit wildlife. Killing rather than cutting the trees can also provide valuable wildlife habitat (see Protecting and Enhancing Nontimber Resources, p. 12).
- Cleanings. Remove fast-growing saplings of other species that overtop or will overtop the oak saplings, except in stands more than 50 percent stocked with dominant and codominant oaks (based on 1/735-acre plots, see Appendix F). The best time to do this is when stand height averages at least 25 feet (10-20 years old, depending on site). Such stands are more accessible both physically and visually, fewer trees will need to be cut, and new stump sprouts will not compete with the overstory trees. Stands that contain extremely fast-growing species, notably red maple (especially stump sprouts) and boxelder, may need to be treated earlier. At any rate, apply the cleaning cut before the red oaks fall below the codominant class. Remove the trees directly affecting the oak crowns as well as the dominant trees just beyond the crown periphery that are capable of rapid crown expansion. Stump-sprout growth will not be a problem in the shaded understory, so chainsaw felling is as effective as, and less expensive than, applying herbicides. Do not cut oaks to release oaks except to remove poorly formed dominant oaks that are crowding better ones.
- Stump Sprout Thinning. Thin stump sprouts during the cleaning operation or separately in stands that are predominately oak. Do this at about age 10 when the sprouts are 2-3 inches in diameter. Generally, thin to one sprout per stump or, in sparsely stocked stands, two widely spaced sprouts. All sprouts must be connected to the stump below or near the groundline. Favor dominant or codominant stems with good form. On steep slopes, retain a sprout on the uphill side. When cutting the competing sprouts, be especially careful not to damage the one being left.
Pole and sawtimber stands
Oak stands managed for high-quality sawtimber should be kept fairly dense until the lower 20-25 feet of the boles are essentially free of live branches. This will generally be when the trees are 40-50 feet tall or between age 30 (for site index 70) and age 45 (for site index 50). When this stage has been reached, thin to stimulate diameter growth and harvest merchantable trees, if pulpwood or fuelwood markets are available.
The first thinning should be heavy, reducing stocking to 50 percent. For pole stands, this will be about 50 square feet of basal area per acre. First thinnings applied later should be to 60 percent stocking for pole stands and 70 percent for sawtimber. Allow thinned stands to grow to 90-100 percent stocking before thinning again. All subsequent thinnings should be to 60 percent stocking; intervals will range from 15 to 20 years. Concentrate on releasing the best trees in the main canopy while cutting merchantable trees of all sizes. These thinning regimes should produce fast-growing, good quality trees and rapid stand-volume growth.
A final harvest cut of mostly large (16+ inches DBH) trees would require rotations of 80-90 years on site index 70, 90-100 years on site index 60, longer on lesser sites, and at least two commercial thinnings. When no thinning is done, these rotations would have to be extended 20-30 years.
The thinning systems described here are not suitable for high-graded stands. Invariably, undesirable growing stock dominates such stands, although scattered good growing stock usually provides the basis for a productive future harvest.
You can generally rehabilitate these stands by applying several improvement cuts. Release the good crop trees and thin heavily if there is a market for pulpwood or fuelwood. If there is no market, apply a noncommercial, crop-tree thinning. When the few good trees reach harvest size, regenerate the stand.
One critical principle in successful intermediate cuttings is: avoid or minimize damage to the crop trees. Careful planning and supervision of the cuttings are the key to carrying out this principle. Most damage to stems and roots occurs during skidding. Careful operation of the equipment is more important than its size. Make sure your machine operators understand that injury-free trees are essential to your long-range management goals. If necessary, restrict some equipment use and require shortwood logging.
Protecting and enhancing nontimber resources
Silvicultural treatment of oak forests, especially harvesting, inevitably affects the value of non-timber assets: wildlife, soil and water, recreation, and aesthetics. Careless or indiscriminate cutting will usually detract from these non-timber assets, but well-planned cuttings can protect and even enhance them with little or no loss of timber production.
Oak forests managed for timber production consist of a series of stands of different ages dispersed throughout the entire oak forest. This mixture of regenerating, sapling, pole, and sawtimber stands results in both vertical and horizontal diversity of vegetation: each age class and its associated "edge" provides habitat for specific kinds of wildlife. This varied landscape pattern promotes productive and generally stable wildlife populations. In addition, there are several ways to further increase wildlife production and diversity.
- Promote vertical diversity. When regenerating stands, leave snags and scattered live trees that do not have deep, vigorous crowns. Kill the live trees before they begin to suppress the reproduction. As mentioned previously in the discussion of liberation cuttings, scattered trees up to 15 square feet of basal area per acre can be left up to 15 years. Thin sapling and pole stands heavier than necessary for timber production alone - to 40-50 percent stocking. Cleaning and release of scattered crop trees do not open the stand enough to stimulate understory vegetation.
- Retain live trees with cavities. During intermediate cuttings, select at least 4 or 5 trees that are not interfering with crop trees. If necessary, leave a weak-crowned tree adjacent to a crop tree. When the cavity trees begin to crowd a crop tree, kill them by girdling or applying herbicides and leave standing.
- Increase acorn production. Thin early and heavily. When regenerating a stand, leave groups of trees on poor sites, such as narrow ridges and steep, south-facing slopes. Retain and favor other mast-producing trees and shrubs, e.g., black cherry, hickories, dogwood, and hazelnut, that are not competing with crop trees.
- Create and maintain herbaceous openings. Keep logging landings free of trees and shrubs. Widen scattered areas along access roads or where several skidding trails intersect. Prevent the establishment of woody vegetation in these areas except for scattered, fruit-producing shrubs.
- Manage forest/field borders. Create a 20 to 30-foot "edge" of mast-bearing trees and shrubs (where high-quality timber trees cannot grow anyway). For added diversity and winter cover, plant scattered groups of slow-growing conifers, such as spruce and redcedar. Encourage or plant fruit-bearing trees and shrubs, including those that produce “showy” flowers. And, of course, retain trees with cavities and those of scarce species.
Soil and water
Most soil and water problems associated with forest operations stem from soil disturbance during regeneration and harvest cuttings and from road construction and maintenance. Careful planning, design, location, and construction of access roads are essential to the prevention of soil erosion and stream sedimentation. But the mechanics of road construction is outside the scope of this publication; contact your local Soil and Water Conservation District for information on assistance and regulations.
To protect soils and water quality during silvicultural operations:
Minimize soil displacement during site preparation
- Select methods that mix the soil in place only 3-4 inches deep.
- Use disks, rock rakes, anchor chains, or patch and row scarification machines.
- Do not "blade" the soil.
Minimize soil displacement during skidding
- Stay uphill and away from stream channels and avoid long, straight, steep trails.
- Log when soil is dry.
- Construct broad-based dips and lead-off ditches to prevent water from flowing down the trails.
- On erosion-prone sites, log when the ground is frozen or snow-covered or winch uphill to a more level area.
Figure 7. Irregular or feathered woodland edges are more appealing than straight edges around clearcuts.
Minimize accumulation of water on landings
- Locate landings on level or nearly level sites, away from streams and poorly drained areas.
- Control the flow of water to, across, and from the landings.
- Direct the water into the surrounding forest rather than to the skid trails and roads.
Protect water bodies from sediment
- Leave strips of undisturbed vegetation to serve as infiltration zones for surface water. Trees could be harvested but ground disturbance should be minimized.
- Promptly remove tree tops from water bodies.
- Leave buffer strips between the harvest area and water: 25 feet on level sites and at least 100 feet on 40-percent or greater slopes.
Protect stream beds and banks
- Minimize stream crossings.
- Build appropriate structures to protect stream beds and banks and permit free flow of water.
- Ford streams only when beds and banks are stable. Protect landings, skid trails, and temporary roads by regenerating immediately after logging.
Apply herbicides according to federal and state standards to prevent contamination of surface and ground water.
Visitors to forests like to see natural-looking landscapes, undisturbed by human activities. Inevitably, oak management (or any forest management) falls short of that ideal; the removal of most large trees in regeneration cuts and the subsequent exposure of stumps, slash, and brush are considered unsightly by many recreational users.
However, these objectionable features can be minimized by the careful planning and execution of management practices.
For visually sensitive areas, group selection may be the method of choice because the openings created are smaller than those under other systems and thus less apparent. Moreover, a continuous forest cover and a variety of tree sizes are always visible. Shelterwood may be better aesthetically than clearcutting because the overstory is removed in two or more stages instead of one and the tree seedlings grow out of the brush sooner. But these differences may be too subtle to justify recommending shelterwood over clearcutting. Regardless of the method selected, certain things can be done to reduce the visual impact of the regeneration cuttings:
- Minimize the evidence of logging by using as much of the felled material as possible. Cut the remaining slash close to the ground and scatter large accumulations. Remove the residue from landings as well as all debris associated with human activity.
- Screen clearcuts along roads and streams with uncut or partially cut buffer strips.
- Avoid straight borders between cut and uncut areas; the borders can be shaped to harmonize with natural occurring shapes (streams, lakes, hillsides, etc.) "Feather" forest edges by partial cuts or leave individual or groups of trees in the regeneration edge to "soften" the break line (see figures 7 and 8).
- Leave snags and scattered trees during overstory removal for the benefit of wildlife, but be sure forest users understand their purpose.
- Deliberately create vistas that provide panoramic views of natural attractions.
Figure 8. In rolling topography, design clearcuts to blend into the landscape.
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This publication was made possible by a Focus Funding grant from the U.S. Forest Service, Northeastern Area State and Private Forestry. The Minnesota, Iowa, and Wisconsin Departments of Natural Resources originated the idea and outlined the content for the publication.
About the authors
Rodney Jacobs retired from the U.S. Forest Service after 33 years of service, first with the North Central Forest Experiment Station as a research forester in the silviculture of northern hardwoods and later with State and Private Forestry where he provided technical information and assistance on hardwood silviculture and management(with emphasis on oak). He holds a bachelor's degree in forestry from Pennsylvania State University and a master's in forestry from the University of Michigan.
Robert Wray retired from the North Central Forest Experiment Station after more than 40 years with the Forest Service, mostly as a research writer/editor at three different experiment stations. He holds a bachelor's and master's degrees in forestry from the University of Michigan.
Project Manager: Melvin J. Baughman, extension specialist and associate professor, Department of Forest Resources.
Editor: Richard Sherman, Educational Development System
Layout and design: Jim Kiehne, Kiehne Graphics
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