3.5. Regeneration Methods: Deferment

Description

A two aged regeneration method in which a new age class develops from seeds that germinate under overwood that is not removed. The microenvironment ranges from exposed to shaded depending on the number of residual trees retained. This regeneration method modifies the seed-tree or shelterwood methods by omitting or indefinitely deferring the overwood removal or seed-tree removal cuts. The two cohorts are thus the newly established cohort, and the deferred seed trees or overwood.

Deferment of the removal cut can be prescribed to meet a range of timber and non-timber landowner objectives (Smith and Miller 1991):

• seed for new cohort,
• shelter for new cohort,
• food and habitat for wildlife,
• protection of water quality,
• increased growth of overwood,
• improved biodiversity and stand structural complexity, or
• enhanced visual quality and aesthetic appeal.

Extent

The deferment system is not as common as any of the even aged systems (Smith et al. 1989). It is more common in forests where wildlife, recreation, or aesthetic objectives are as important as timber production. Deferment is most often used by federal and state land agencies, followed by industrial forests, and is least commonly employed by small non-industrial private landowners (Miller et al. 1997a). Most of the research available on deferment silviculture comes from the central Appalachian Mountains (e.g. Miller et al. 2004) and Pacific Northwest (e.g. Zenner et al. 1998).

Cuts Involved

The most common application of the deferment method involves one cut. The single cut is the seed-tree or establishment cuts described in the seed-tree and shelterwood sections. The result is a two aged stand following this cut and establishment of a new cohort.

Common Variations

The deferment regeneration method is alternately known as the leave tree, reserve tree, or irregular shelterwood methods. In the Pacific Northwest variants of this system are sometimes called green tree retention to clarify that the retained stand structures are live, and not snags or woody debris (Smith et al. 1997).

Ecological Considerations (Silvics)

As with the seed-tree and shelterwood systems, it is important to carefully select the trees to be retained as overwood (Smith and Miller 1991). Traits to be selected for in deferred overwood include:

• desirable species for management objective
• high live crown ratio (> 40% for hardwoods, >30% for conifers)
• desirable form
• straight stem
• no form defects such as forks, sweep, or ramicorns
• no evidence of rot
• no die-back from the top
• windfirm
• young enough to maintain vigor until the end of the rotation
• large pole class or very small sawtimber
  • is low in value today
  • will increase in value over time
  • will not outgrow the mills

It is important to note the effect that deferred overwood will have on the new cohort. The greater the basal area of deferred overstory trees, the less the growth of the new cohort (Acker et al. 1998, Birch and Johnson 1992, Boyer 1993, Messina and Jenkins 2000). Some species, such as southern yellow pines, will not survive beneath competing older trees. Suppressed individuals will often die out if they are not released after they reach several feet in height. This will result in patches with poor regeneration beneath older trees. The closer newly regenerating seedlings are to the residual overwood, the slower their growth will be (Miller et al. 2006). As the older trees continue to grow, they will expand their canopies further, thus reducing the amount of light reaching the new cohort.

The retained residual trees will increase their diameter growth rates as if the stand had been heavily thinned. One of the major advantages of the deferment system is the production of valuable large diameter sawtimber and veneer logs. Diameter growth rates of overwood in many species approximately double in the five years following the regeneration harvest (Smith et al. 1989). If retained trees are prudently selected, there appears to be little risk of mortality before the next harvest (Smith and Miller 1991).

The more complex vertical forest structure found in stands managed with deferment systems has been found to be favorable to wildlife such as songbirds (Miller et al. 1995). Other species that require den trees or other similar habitat structures can be managed for by selecting suitable large-diameter trees for retention. While not desirable for timber production, trees with signs of rot or top die-back may be retained if managing for wildlife.

Suitable Species

• US South
  • Loblolly pine (Pinus taeda)
  • Longleaf pine (Pinus palustris)
  • Eastern cottonwood (Populus deltoides)
  • Water oak (Quercus nigra)
  • Willow oak (Quercus phellos)
  • Laurel oak (Quercus laurifolia)
  • Cherrybark oak (Quercus pagoda)
• US North
  
  • Black cherry (Prunus serotina)
  • Basswood (Tilia americana)
  • Sugar maple (Acer saccharum)
• US West
  • Douglas-fir (Pseudotsuga menziesii)
  • Western hemlock (Tsuga heterophylla)
  • Western redcedar (Thuja plicata)
• Interregional
  • Yellow-poplar (Liriodendron tulipifera) S, N
  • Northern red oak (Quercus rubra) S, N
  • White oak (Quercus alba) S, N
  • Chestnut oak (Quercus prinus) S, N
  • White ash (Fraxinus americana) S, N

Economic Considerations

Due primarily to the effect of the retained overwood on suppressing the new cohort, deferment systems were found to result in only 75-82% of the net harvest revenues at each entry compared to even aged systems (Miller and Baumgras 1994). However, this disadvantage is balanced by a potential increased frequency of harvests. While even age systems in the southern Appalachians may require an 80 year rotation, it is possible to enter a stand every 40 years with a deferment system, taking out the 80-year-old reserved trees and deferring harvest on some of the smaller, 40-year-old trees until the next entry (Miller and Baumgras 1994). While timber production may be lower, and less is removed at each entry versus even aged systems, the increased frequency of entries may make this system financially attractive in regions with relatively long rotations.

When compared to clearcutting, there are added costs in a deferment system attributable to tree marking and protecting the deferred trees during harvest operations. These costs are not appreciably different from those that would be incurred in either a similar seed-tree or shelterwood system.

Societal Considerations

The aesthetic benefits of deferment systems compared to even aged systems cannot be overstated. This is particularly true in regions of the country where mountainous terrain makes clearcuts visible for many miles. Even retention of relatively low densities of deferred overwood prevents harvest areas from appearing as clearcuts. Retention of more leave trees may make harvest areas all but indiscernible from any significant distance. In areas where aesthetics are an important management objective, such as federal and state lands, deferment systems maintain constant tree cover while still allowing for active management of species with a range of shade tolerances while still realizing considerable income from timber harvests.

In the Pacific Northwest the concept of managing forests for human uses while remaining cognizant of available ecosystem structures, functions, and services, or ecosystem management was made common practice on federal lands following the 1993 FEMAT report (FEMAT 1993). As part of this policy-driven management regime, green tree retention became the commonly prescribed practice on federal lands rather than clearcutting. While not all green tree retention qualifies as a deferment system (see reserve system section), deferment is one common application of green tree retention. While the original intent of green tree retention was to manage more for 'natural' or presettlement forest structure and function by mimicking natural disturbance regimes, there remain questions as to how effective the current policy is in achieving this goal (Thompson et al. 2006).

Silvicultural System Considerations

The basal area of deferred overwood should be selected to adequately reseed the stand, provide the appropriate light environment for desired regeneration, and balance growth between the two cohorts. In Appalachian mixed hardwood stands, 20 to 40 ft² per acre is a typical target for deferred overwood (Miller et al. 1997a). Residual trees are typically selected so they are spatially well distributed across the stand in deferment systems (Miller et al. 1995). For different spatial arrangements of reserved trees, see the reserves section.

When assessing regeneration potential of deferment systems, there is an approximate 50% reduction in the percentage of stumps that sprout relative to clearcuts (Atwood et al. 2008). When compared to shelterwood systems however, 20% more stumps sprout (Atwood et al. 2008). Because stump sprouts grow taller than seedlings and establish at a greater density than seedlings for high value species such as oaks, these effects should be recognized in attempts to quantify advanced regeneration prior to harvests (Atwood et al. 2011). Steps should be taken where possible to secure adequate regeneration following the establishment cut. Removing the midstory (all trees > 1 inch dbh) is critical to obtain acceptable regeneration for many species (Miller et al. 1997a).

Minimizing damage to deferred trees is important in meeting timber management objectives. Wounds tend to have little effect on growth rates (Lamson et al. 1988), but can degrade a log or increase the risk of mortality due to insects or disease. In areas of the country with colder winters, less damage is typical for winter rather than spring or summer harvests (Miller et al. 1997b).

Establishment Treatments

No additional establishment treatments are commonly prescribed with deferment systems. Correct selection of residual trees left in the establishment cut and full removal of the midstory should result in successful establishment of the new cohort. Mechanical treatments to the seed bed, if necessary, risk damage to the valuable residual trees. Such treatments should be incorporated into the establishment cut where possible by dispersing skidding throughout the stand. If competing vegetation is problematic following the establishment cut, it can be controlled with prescribed fire or forest herbicides, as appropriate.

Intermediate Treatments

Thinning can be incorporated into deferment systems, although it is not as effective compared to even aged stands if establishment cuts are being implemented at 1/2 the rotation length. Trees in two aged stands show up to 80% less of a growth response to thinning compared to similar sized individuals in an even aged stand (Hannah 1978). This is likely attributable to them still being influenced by the deferred overwood cohort even when released from competition from their own cohort. If thinning is prescribed, it should seek to improve the growth and vigor of crop trees in both cohorts. Crop tree management would be a logical technique to integrate into a deferment system, as long as the reduced benefits to growth compared to even aged systems are fully considered.

Deferment Pros

• Improved aesthetics versus even aged systems.
• Improved wildlife habitat due to structural retention.
• Lower logging costs than seed-tree or shelterwood due to a single entry.
• Can regenerate many species by adjusting leave-tree density to match shade tolerance.
• Can produce large diameter sawtimber more rapidly with released leave trees.
• Can harvest large diameter sawtimber at 1/2 rotation length.
• Can produce a range of products from a single stand at once.
• Higher initial revenue if stand has been historically managed with this system.
• Can maintain species in a stand that would otherwise not reproduce with a clearcut.

Deferment Cons

• Less timber production and lower profits compared to clearcutting.
• New cohort will be suppressed, reducing growth.
• New cohort may not survive if too heavily shaded.
• Stump sprouting is reduced.
• May be difficult to favor very intolerant species.
• May be difficult to apply this system if long-lived trees are not present.
• Leave-trees may be degraded over time.
  • Damage from initial harvest.
  • Damage from subsequent operations such as thinnings.
  • Epicormic branching may be problematic in some species.
  • Leave trees may die and are more subject to windthrow.
  • Leave trees may grow too large for mills to handle them.

Examples

References

Acker, S. A., E. K. Zenner, and W. H. Emmingham. 1998. Structure and yield of two-aged stands on the Willamette National Forest, Oregon: implications for green tree retention. Canadian Journal of Forest Research 28:749-758. http://dx.doi.org/10.1139/x98-039

Atwood, C. J., T. R. Fox, and D. L. Loftis. 2008. Stump sprouting of oak species in three silvicultural treatments in the Southern Appalachians. Proceedings of the 16th Central Hardwoods Forest Conference; Newtown Square, PA: U.S. Department of Agriculture, Forest Service, Northern Research Station. 595 p. http://www.treesearch.fs.fed.us/pubs/13907

Atwood, C. J., T. R. Fox, and D. L. Loftis. 2011. Effects of Various Silvicultural Systems on Regeneration in Mixed Hardwood Stands of the Southern Appalachians. Journal of Sustainable Forestry 30:419-440. http://dx.doi.org/10.1080/10549811.2011.541020

Birch, K. R. and K. N. Johnson. 1992. Technical commentary: Stand-level wood-production costs of leaving live, mature trees at regeneration harvest in coastal Douglas-fir stands. Western Journal of Applied Forestry 7:65-68. https://doi.org/10.1093/wjaf/7.3.65

Boyer, W. D. 1993. Long-term development of regeneration under longleaf pine seedtree and shelterwood stands. Southern Journal of Applied Forestry 17:10-15. https://doi.org/10.1093/sjaf/17.1.10

FEMAT. 1993. Forest ecosystem management: An ecological, economic, and social assessment. USDA Forest Service, US Department of Commerce, National Oceanic and Atmospheric Administration, National Marine Fisheries Service; USDI Bureau of Land Management, Fish and Wildlife Service, National Park Service, Environmental Protection Agency, Washington D.C., USA. http://www.blm.gov/or/plans/nwfpnepa/#1993%20FEMAT%20Report

Hannah, P. R. 1978. Growth of large yellow birch saplings following crop tree thinning. Journal of Forestry 76:222-223. https://academic.oup.com/jof/article-abstract/76/4/222/4643872

Lamson, N. I. and H. C. Smith. 1988. Effect of logging wounds on diameter growth of sawlog-size Appalachian hardwood crop trees. USDA Forest Service, Northeastern Forest Experiment Station, Research Paper NE-616. http://treesearch.fs.fed.us/pubs/21801

Messina, M. G. and J. Jenkins. 2000. Loblolly pine stand early development under reserve-tree silvicultural systems in East Texas. Southern Journal of Applied Forestry 24:11-16. https://doi.org/10.1093/sjaf/24.1.11

Miller, G. W. 1996. Epicormic branching on central Appalachian hardwoods 10 years after deferment cutting. Res. Pap. NE-702. Radnor, PA: U.S. Department of Agriculture, Forest Service, Northeastern Forest Experiment Station. http://www.treesearch.fs.fed.us/pubs/9397

Miller, G. W. and J. E. Baumgras. 1994. Efficient silvicultural practices for eastern hardwood management. Pages 23-35 in Opportunities for the hardwood industry to address environmental challenges; Proceedings of the 22nd annual hardwood symposium of the Hardwood Research Council; 1994 May 12-15; Cashiers, NC. Hardwood Research Council, Memphis, TN. http://nrs.fs.fed.us/pubs/4298

Miller, G. W., J. E. Johnson, and J. E. Baumgras. 1997a. Deferment cutting in central Appalachian hardwoods: An update. Forest Landowner 56:28-31, 68. http://www.treesearch.fs.fed.us/pubs/14206

Miller, G. W., J. E. Johnson, J. E. Baumgras, and R. G. Bustamente. 1997b. Two-age silviculture on the Monongahela National Forest - Managers and scientists assess 17 years of communication. Pages 123-133 in Communicating the role of silviculture in managing the national forests: Proceedings of the National Silviculture Workshop. 1997 May 19-22; Warren, PA. Gen. Tech. Rep. NE-238. Radnor, PA: U.S. Department of Agriculture, Forest Service, Northeastern Forest Experiment Station. http://treesearch.fs.fed.us/pubs/15167

Miller, G. W., J. N. Kochenderfer, and D. Fekedulegn. 2004. Composition and development of reproduction in two-age Appalachian hardwood stands: 20-year results. Pages 171-181 in Silviculture in special places: Proceedings of the National Silviculture Workshop; 2003 September 8-11; Granby, CO. Proceedings RMRS-P-34. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station. http://www.treesearch.fs.fed.us/pubs/7224

Miller, G. W., J. N. Kochenderfer, and D. B. Fekedulegn. 2006. Influence of individual reserve trees on nearby reproduction in two-aged Appalachian hardwood stands. Forest Ecology and Management 224:241-251. http://dx.doi.org/10.1016/j.foreco.2005.12.035

Miller, G. W., P. B. Wood, and J. V. Nichols. 1995. Two-age silviculture: An innovative tool for enhancing species diversity and vertical structure in Appalachian hardwoods. Pages 175-182 in Forest health through silviculture: Proceedings of the 1995 National Silviculture Workshop, Mescalero, New Mexico, May 8-11, 1995. Gen. Tech. Rep. RM-GTR-267. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station. http://www.treesearch.fs.fed.us/pubs/23499

Smith, D. M., B. C. Larson, M. J. Kelty, and P. M. S. Ashton. 1997. The Practice of Silviculture: Applied Forest Ecology. Ninth edition. John Wiley & Sons, Inc., New York, New York. ISBN: 047110941X

Smith, H. C., N. I. Lamson, and G. W. Miller. 1989. An esthetic alternative to clearcutting? Journal of Forestry 87:14-18. https://doi.org/10.1093/jof/87.3.14

Smith, H. C. and G. W. Miller. 1991. Deferment cutting in Appalachian hardwoods: the what, whys, and hows. Pages 33-37 in Uneven aged silviculture of upland hardwood stands workshop notes; 1991 February 25-27. Virginia Cooperative Extension Service and Virginia Polytechnic Institute and State University, Blacksburg, VA. http://www.treesearch.fs.fed.us/pubs/14184

Thompson, J. R., K. N. Johnson, M. Lennette, T. A. Spies, and P. Bettinger. 2006. Historical disturbance regimes as a reference for forest policy in a multiowner province: a simulation experiment. Canadian Journal of Forest Research 36:401-417. http://dx.doi.org/10.1139/x05-247

Zenner, E. K., S. A. Acker, and W. H. Emmingham. 1998. Growth reduction in harvest-age, coniferous forests with residual trees in the western central Cascade Range of Oregon. Forest Ecology and Management 102:75-88. http://dx.doi.org/10.1016/s0378-1127(97)00108-4