NATURAL REGENERATION / UNEVEN AGED MANAGEMENT

Alternative Silvicultural Systems for Managing Your Forest

Forest Landowner / Sept-Oct 1999

Larry Nelson, Bob Franklin, and George Kessler Extension Forest Resources, Clemson University

The most widely used silvicultural system for harvesting and regenerating pines in the southeastern United States is clear-cutting followed by artificial regeneration—by hand or machine planting nursery-grown seedlings.  This system is widely accepted by foresters because it is highly efficient and consistently successful.  A few important advantages of clear-cutting and artificial regeneration include minimized logging costs, stocking control (number and spacing of seedlings) and use of genetically improved pine seedlings.  Ecologically, the large clearings somewhat mimic natural disturbances from sources such as fire, insect mortality and storms, and provide ideal growing conditions for southern pine seedlings, which require open sunlight for survival and growth.  Foresters regard clear-cutting as the best system for maximizing merchantable wood produc­tion per acre, but it does not always satisfy the objectives of private landowners.
 

Recent surveys show that private landowners as a group rank timber income behind such amenities as wildlife, aesthetics, personal recreation and satisfaction of ownership as a reason for owning forestland.  Timber income may be desirable, but not at the expense of the other objectives.  In this case, alternative silvicultural systems may provide for a wider range of landowner objectives.

The systems described below are well documented in forestry literature, but are not fully utilized by foresters in the Southeast.  Although alternative silvicultural systems can provide a profitable timber crop, generally less merchantable wood is produced per acre than with clear-cutting, and more time and planning are required.  One of the major obstacles to using these systems is locating a forester that has the experience and willingness to implement them.

Uneven Age Management

Uneven-age management involves managing three or more age classes of crop tree species on the same tract of land.  Some important features of uneven-age management are as follows:

* A significant stand of timber is always present on the site.  Instead of clear-cutting, timber is harvested periodically by individual tree or group selection.

* Timber quality is improved during scheduled harvests.  Selection harvesting removes trees of poor growth and form, which in turn results in accelerated growth of the best trees. 

* This process requires little capital and provides periodic income while the stand is being improved.

* Volume production is concentrated on valuable sawtimber trees.

* Regeneration costs associated with even-age management are less with uneven-age systems.  Each harvest represents a reproduction cutting.  Removal of large trees during harvest creates openings in the stand that allow pine seedlings to regenerate from seed from adjacent trees.

* Stands are not as vulnerable to complete destruction by wildfire, ice storms, hurricanes, disease and insects as are even-age stands.

* Long-term uneven-age management results in conditions somewhat similar to old-growth forests.  Stand structure and biomass become fairly stable, but other attributes of old-growth forests such as downed woody debris and low net-growth are less likely to occur.

Essentially, uneven-age management involves harvesting timber with light thinnings and creating gaps for regeneration on a periodic cutting cycle.  The trees selected for harvest  stand, its relative position, health, dominance and rate of growth, and determine how its presence or absence will affect landowner objectives. 

Management in this manner can produce a patchy forest with groves of older trees, interspersed with groups of younger saplings, middle-age clumps and areas of reproduction.  This produces a great deal of diversity on a small scale benefiting many species of wildlife.  The forest is generally more natural in appearance compared with even-age systems. 

Some disadvantages of uneven-age management include:

   * Certain area-efficient management practices, such as prescribed burning and chemical treatments, may be difficult to apply.

   * Harvesting may be more difficult.

   * Foresters must use more complex management strategies than with other methods.

Economics

Total merchantable wood production using uneven-age management is generally less than with even-­age systems.  However, in the long-term, production of high value products such as veneer and sawlogs may be greater.   A limited number of economic comparisons of even-age versus uneven-age management show that uneven-age stands rank higher in net present value than even-age stands if initial growing stock and land are not considered a cost.  Using these systems, private landowners can maximize their rate of return because of low establishment costs.  However, other variables such as interest rate, the cost differential between sawtimber and small roundwood, site productivity and initial stocking levels also effect economic comparisons.  More studies are needed to provide forest­ers and landowners with accurate economic information over the wide range of conditions encoun­tered with uneven-age management.

The bottom line is that uneven-age stands may not be as profitable as even-age stands, but will allow a reasonable income from timber while meeting many other objectives that landowners may consider just as valuable.

Other alternatives to either clear-cutting or artificial regeneration include various even-age natural regeneration systems. Regeneration with these systems is dependent upon seed production from residual pines left after harvest or from pines adjacent to the harvested area.   Five methods are described below.

The seed tree method removes most of the trees in one cut, leaving a few well-spaced, good seed producers over the area. The number of seed trees left depends upon size, species, cone-bearing characteristics and site conditions (see Table 1). The seed tree system is the most widely used natural regeneration system and is particularly suitable for coastal plain sites.  The seed tree system works well with loblolly and shortleaf pines because of the frequency of good seed crops.  Longleaf pine is more difficult to establish with the seed tree system because good seed crops are infrequent.  The shelterwood system is recommended for longleaf.

The shelterwood system involves leaving many more trees than the seed tree system above. Generally 25 to 40 square feet of basal area per acre is left after the cut. This is  equivalent to 23 to 37 fourteen-inch trees per acre. The shelterwood system can be highly suitable on Piedmont and coastal plain sites.  An advantage of the shelterwood system is that enough volume is left to get market price for removal after regeneration is established.  And quality growth can occur while regeneration is being established.

Clear-cutting in strips enables you to make periodic harvest cuts while managing even-age units.  The clearcut strips may be of any length, but preferably about 200 feet wide.  Strips should be perpendicu­lar to the direction of prevailing winds to ensure good seed dispersal.

Seed-in-place involves clear-cutting the stand after the peak of seed fall, but before the start of germination.  This method is best applied during a four to five-month winter logging period.  The system can be used only when an ample crop of seed is available.   Seed-in-place is not recommended for longleaf pine.

Seedlings-in-place involves clear-cutting a stand during the summer following a good seed year.   Newly germinated seedlings spend a portion of the first growing season in the shade of the current stand.  This system allows more certainty of having a satisfactory number of seedlings following germination than the seed-in-place system.

Advantages of natural regeneration are:

* Lower establishment cost

* Lower establishment cost

* Less labor and heavy equipment are required

* Less soil movement

* No problem with the geographical origin of the seed

* Reduced tip moth damage to new regeneration

* New seedlings have a better root system than with planted seedlings

* Less immediate visual impact

Disadvantages include:

* Less control over spacing and initial stocking

* Rotations are potentially longer

* Risk of seed tree loss

* Yields are generally slightly lower than with artificial regeneration

* No use of genetically improved seedling stock

* Pre-commercial thinning is often necessary because of overstocked regeneration

Natural regeneration can be highly favorable in situations where the landowner desires a low invest­ment in regeneration, is not necessarily managing for maximum wood production and values a more natural looking forest with minimal physical impacts resulting from silvicultural practices.

Stand Regeneration

Pine stands that have relatively low stocking as a result of storm damage, various harvesting prac­tices or for other reasons can often be rehabilitated at low cost.   USDA Forest Service studies have shown that stands with 25 percent stocking or slightly lower can achieve an acceptable stocking level of 60 percent within 15 years or less if the stand has at least 5 square feet of initial basal area.

Stands that are candidates for rehabilitation may have as little as 50 to 80 trees per acre in various size classes.  If the trees are of good form and vigor and are uniformly distributed, stocking levels will increase rapidly if competing hardwoods are effectively controlled.  In one study, a stand with 30 percent initial stocking (2 cords of pulpwood and 257 board feet of timber) produced 6.2 cords of pulpwood and 2,772 board feet of timber after only ten years.  Rehabilitation costs ranged from $45 to $50 per acre for controlling hardwoods.

Landowners should be aware that effective stand rehabilitation is a management option that can provide a reasonable rate of return if conducted properly.  However, each stand must be evaluated on its own merit and should be done so with the assistance of an experienced forester.

Mixed Pine-Hardwood Management

Most professional foresters reject the idea of managing pine and hardwood on the same site or in the same stand for economic and logistical reasons.  Pine species generally bring better merchantable value on the majority of sites across the Southeast, and thus are preferred over hardwoods.  Hardwood components of mixed stands exist at the expense of the more valuable pine.  Furthermore, most foresters have little or no training on how to manage for more than one species at a time.  Nevertheless, for aesthetics, wildlife or other objectives, some landowners may prefer to retain hardwoods on a portion of their land.

Recent Forest Service studies provide guidelines for establishing stands of mixed pine and hard­wood.  The argument for this type of management is that a large percentage of our upland forests are comprised of mixed stands or low grade hardwoods, and many private landowners simply do not have the resources ($150 to $250 per acre) required to convert to pure pine stands.  Why not manage for both?  Guidelines specify felling and burning techniques for establishing pine among  pines per acre after six growing seasons.  At this time pine growth was competitive with the hardwoods.

Options are also available for retaining hardwoods in otherwise uneven-age pine stands.  Where hardwoods are uniformly distributed throughout the stand, their presence will result in less pine production.  As a rule of thumb, pine basal area will be reduced by two square feet for each one square foot of hardwood.

An alternative to retaining hardwoods on each acre is to leave hardwoods in certain parts of the stand while maintaining pure pine in the rest of the stand.  This option can allow hardwood retention on certain terrain features such as drainages or north-facing slopes with the highest hardwood site quality.   Concentrating hardwoods in certain areas can provide cover and corridors for wildlife and can be convenient for cutting firewood.

Summary

Studies have shown that much of the nonindustrial private forestland in the Southeast is only producing at one-half or less of its wood-growing potential.  Reasons given for this lack of management by many landowners have been high investment costs, long payback periods and a concern that traditional forestry as practiced in the South could degrade aesthetics and wildlife habitat.   The above-mentioned silvicultural alternatives have the potential to increase wood production off many nonindustrial private lands while providing for and enhancing many of the non-timber concerns.  These alternatives are low-cost and can improve wood growing possibilities on hundreds of thousands of acres that might not be managed otherwise.   Consider them and explore how they may fit into your woodland management plans.

Portions of this article were excerpted or adapted from: 

Baker, J. B., M. D. Cain, J. M. Guldin, P. A. Murphy and M. G. Shelton. 1996.   Uneven-Aged Silviculture for the Loblolly and Shortleaf Pine Forest Cover Types.  USDA For.  Serv.  Gen. Tech. Rep. 50-118. 65 pp.

 Williston, H. L., W. E. Balmer and D. H. Sims. 1993.  Managing the Family Forest in the South.  USDA For.  Serv.  Manag.  Bull.  R8-MB1