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Hardwood Management:

Stand Improvement Cutting in Natural Stands of Temperate Hardwood Forests

 

Douglas Frederick, North Carolina State University
Pablo Donoso, Universidad Austral de Chile

 

Introduction

 
Improvement cutting in natural stands of temperate hardwood forests is one of best long-term investments in the Americas.  Ironically, very little improvement cutting is currently being done in North or South America.  There are several reasons for this, including a lack of knowledge and experience of the opportunities gained from conducting these operations. Other explanations include limited markets for small-diameter logs and scarcity of operators or landowners willing to do improvement cutting, although these seem to be less of a limitation in South America where small logs can be marketed or used for fuel wood.

 

What is Improvement Cutting?

 
Improvement cutting is a partial cut to “improve” a stand by removing individual trees (stocking reduction) in even-aged or uneven-aged stands that are defective, have poor form, are not favored species, have poor growth or may be damaged by insects, disease, or other agents (Smith et al. 1997, Helms 1998, Nyland 2002).  Improvement cutting will improve overall stand health, growth, quality and value for the landowner.

Within even-aged communities, improvement cuttings represent one possible intermediate treatment after the stand passes out of the sapling stage and before it reaches financial maturity (trees 10-20cm (4-8 inches) in average diameter).  In uneven-aged stands, improvement cuttings may include any cutting to release good trees from competition by undesirable ones of comparable age (Nyland 2002). 

 
Improvement cuts can serve to regulate relative density and/or maintain a specific stand structure or balance among age classes (Nyland 2002).  Thinning is another term that refers to stocking reduction but this term is more frequently applied to uniform, single species stands such as pine, (Pinus spp.), Douglas fir (Pseudotsuga menziesii) or eucalyptus (Eucalyptus spp.) where quality issues are not as important as in natural, mixed hardwood species.

 
Improvement cuts are particularly applicable to mixed mesophytic hardwoods (Carvell 1973).  In these stands, improvement cuts should be applied in stands that have attained 12-15m in height, at 15-20 year intervals (Carvell 1971, Sander 1977, Hicks 1998). In mixed oak stands on above-average sites, improvement cutting can be conducted in stands greater than 80 years old (Hicks 1998).  In mixed hardwood stands in the warm-temperate rainforests of Chile, shorter intervals would be more appropriate, but there is no published data. Longer intervals are necessary in cool-temperate forests in eastern North America or Patagonian forests in Chile and Argentina, the latter usually dominated by single species (Nothofagus pumilio).

 
A landowner with multiple management objectives such as high-quality veneer or sawtimber of specific species can use improvement cutting to efficiently achieve these goals, especially with timber prices at all-time highs. Improvement cutting in Appalachian hardwoods in Eastern North America can result in a two to three fold increase in the proportion of volume in grade 1 and 2 saw-logs (Trimble 1970, Nyland 2002).

 
On the other hand, a landowner may have firewood, aesthetics or wildlife as primary or concurrent goals and improvement cutting can easily be modified to achieve these goals as well.  Improvement cutting results in fast-growing, healthier stands that are less susceptible to pests and consequently improve the forest ecosystem.  Improvement cuts can also provide a regular source of wood for a landowner or income to help defray management costs and property taxes.

 

When Should a Landowner Consider an Improvement Cut?

 
Improvement cuts are generally appropriate in young, pole-size, even-aged stands (or cohorts in uneven-aged stands) with a good mix of desirable species, adequate stocking (trees per hectare) and growing on average or better sites with reasonable access (Frederick 2004).  Stands in the 20-40 year age class (and 10-20m in total height, depending on region) are best candidates for improvement cutting (Table 1). 

 

Table 1 – Predominant tree genera in natural secondary hardwood forests in North and South America that are excellent candidates for improvement cutting

 

North America

 

          Northern Hardwood Forest – Predominate Genera:  Acer - maple, Betula-

           – birch, Fraxinus. – ash, Fagus – beech, Quercus – oak, Tilia – basswood,

          Liriodendron – yellow poplar, Picea - spruce, Tsuga - hemlock and Pinus – pine.

 

          Central Hardwood Forest – Predominate Genera: Quercus - oak, Carya - hickory 

          Juglans – walnut, Liriodendron – yellow poplar, Fraxinus- ash and Ulmus – elm.

 

          Southern Hardwood Forest – Predominate Genera: Quercus – oak, Carya -

          Hickory, Acer – maple, Fraxinus – ash, Liquidambar – sweetgum, and Nyssa –

          Tupelo / blackgum.

 

          Appalachian Hardwood Forest – Predominate Genera: Quercus – oak, Prunus –

          Black cherry, Liriodendron – yellow poplar, Fraxinus – ash and Acer – maple.

 

South America

          

           Warm-temperate Nothofagus Forests in Chile – Predominate Genera:  

            Nothofagus, Laurelia, Eucryphia, Drimys, Laureliopsis, Persea, Aextocicon

 

            Cool-temperate Nothofagus Forests in Patagonia of Chile and Argentina –

             Predominate Genera:  Nothofagus and Austrocedrus

 

             Warm-temperate Evergreen forests of Chile - similar to Nothofagus forest but

              usually without Nothofagus and with Weinmannia in the upper canopy, conifers

              of the Podocarpaceae family and various species in the Mirtaceae family

    

 

There are millions of hectares of such sites in the Americas that have resulted from past timber harvesting or agricultural abandonment and these sites are prime candidates for improvement cutting.  Such stands often have an inferior mix of species and the stems are of low quality and are growing slowly due to intense competition.  Also, many stems will be of sprout-origin and these must be handled correctly during an improvement cut.  Current stand conditions do not always reflect the “potential” of the site.  In fact, the site may be an excellent hardwood site and it will only take a series of improvement cuts to maximize this potential for stand growth and value.  

 
There are hardwood stands for which improvement cutting is not an option.  In even-aged communities, these may be stands that are either too young or are mature or stands that have been severely high-graded to the point that there is inadequate stocking of desirable species. Even-aged stands that have suffered insect, disease, storm or extensive logging damage may not be good candidates. For these stands, a regeneration cut (final harvest) such as clearcutting or some form of two-step cutting (e.g., shelterwood or deferment) may be the best option.  Similarly, in severely high-graded or understocked uneven-aged stands, more effort should be expended to regenerate the stands rather than improving the quality of the existing trees.

 
Improvement cutting is designed to “improve” the existing stand and regeneration is not a consideration except in forest types where valuable shade-tolerant species can form an understory and the objective is to build a two-age forest or rebuild an uneven-aged stand.  Once an even-aged stand has had several improvement cuts and contains only large, high-value stems, a decision must be made on how to regenerate the stand. In unmanaged uneven-aged stands, several conditioning cuts may be necessary to bring the stand to a balanced structure and appropriate stocking (Nyland 2002). These conditioning cuts are primarily conducted through improvement cuts, i.e. cutting of defective trees within all size/age classes.

 
Improvement cuts cannot be made indefinitely in an even-aged stand, but research supports that three to five cuts or more at 10 to 20 year intervals are possible in temperate forests of North America (Frederick 2004) and South America (Donoso et al. 1993).  Even removing 25 to 35% of the basal area with each cut will not result in under stocking if the original stand is well stocked; a major consideration in making a decision for improvement cutting (Lamson and Smith 1978).  Because of increased growth and rapid recovery of basal area and crown closure, a stand quickly gets back to full stocking, but older even-aged stands may take more time to attain this condition.  Another improvement cutting must then be conducted, since growth rates of selected trees and net volume growth of the forest stand should be maintained.

 
The main change with a series of improvement cuts is fewer stems per hectare that are larger in diameter in even-aged stands, with greater representation of preferred species and with good form and vigor. For instance, even-aged Nothofagus stands in Chile have from 3 to 6 thousand trees per ha when they have an average diameter of 10 cm, and usually a final cut is expected when these trees are 35 cm in mean diameter and with no more than 400 trees per ha. The transition must be conducted through at least 3 and possibly 5 improvement cuttings.  Conditions and prescriptions in hardwood stands in North America are similar (Frederick 2004).

 

What Are the Mechanics of Doing an Improvement Cut?

 
Landowners must do a stand evaluation to determine if a cut is feasible (Hicks 1998).  A simple inventory noting the species, diameter, condition, stocking, and overall site quality using a limited number of plots will suffice.  By keeping track of the number of acceptable (preferred) and unacceptable stems, a determination of whether there are enough stems of sufficient quality to make an improvement cutting is feasible.

 
In North America, generally, the presence of 120 to 250 desirable (crop) trees per hectare (50-100 trees per acre), or at least 9 to 12 square meters per hectare of desirable trees  (there will be other trees present as well), indicates that such a cut may be  considered. On many sites in temperate South America, more than 30 square meters per hectare is usually the residual stocking after an improvement cut (Grosse and Quiroz 1999, Lara et al. 1999). If there are an insufficient number of acceptable stems present in the stand, then an improvement cut may not be feasible and a recommendation for a regeneration (final harvest) cut can be made using the same data.  Not all areas on a landowner’s forest may be suitable for an improvement cut, so the cut areas may be spread-out over the forest.

 
The objective of an improvement cut is future stand improvement, so making a large profit, or any profit on the first cuts should not be a primary objective (Frederick 2004). Sometimes the first cut will be a pre-commercial “investment” and in fact will cost money to implement.  However, the future stand value will offset this initial investment. Local markets for firewood or chips in addition to pulpwood and sawlogs may be used to offset initial costs, as well as government cost-sharing (subsidy) programs that may be available.

 
Most individual trees to be removed in an improvement cut will be obvious. However, because of the diversity in hardwood stands across the Americas, variation in quality and value, there will always be decisions to be made.  Keep in mind that an optimal final spacing for even-aged hardwoods prior to a regeneration harvest is about 6 X 6 meters (about 240 trees per hectare (100 trees per acre)) in cool temperate forest stands, and about 5 x 5 (400 trees per ha) in warm temperate forest stands. So, marking a stand with these numbers in mind will help in the decision process.  Stands containing understory and intermediate crown class some individuals of species having value for wildlife, aesthetics or biodiversity should be retained.

 
Basal area control is a common and very effective means to accomplish an improvement cut (Frederick 2004). In this approach, the overall stand basal area (determined using a prism or fixed area plot) is reduced by marking undesirable trees, competing with crop trees, to some predetermined basal area level.  For example, in cool temperate forests, an overstocked, even-aged stand with 30 square meters of basal area per hectare can be reduced to 18 square meters per hectare, which would still be considered fully stocked.  On productive sites, stand basal area can be reduced sometimes to as low as 11 to 14 square meters per hectare, and the stand remain fully stocked and productive. 

 
In warm temperate forests, an overstocked even-aged stand with 40 square meters of basal area (a stand between 40 and 60 years old) can be reduced to 24 square meters and continue to be fully stocked. These numbers are reflective of the proposal that stands are fully stocked when they have a residual density > 60% (Nyland 2002). In this kind of a cutting, generally the cut trees are from the smaller size classes, less desirable species, and include some co-dominant trees (perhaps even of preferred species) to adequately release the best trees and to provide sufficient harvest volume to make the cut economically feasible (Frederick 2004).  On occasion, dominant trees are also removed, especially if they are of poor form, vigor and less desirable species. 

 
An alternative approach to stocking control in improvement cutting is “crop tree release”, in which “leave” trees are individually released by thinning specifically around them (Lamson and Smith 1978, Bardon 2004).  This is often a good method for the “first” cutting in a series of improvement cuts.  The idea in both methods is to reduce competition on the residual (leave) trees either by dealing directly with specific crop trees or doing it on a stand-area basis.  Stocking charts for some hardwood types are available and can be used to help a landowner determine optimal cut and residual stocking levels for individual stands (Dale 1972, Leak 1981, Sampson et al. 1983).

 

Handling Sprouts and Epicormic Branching

 
One of the greatest challenges in marking a hardwood stand for improvement cutting is handling sprouts (Lamson 1988, Frederick 2004).  Hardwoods naturally sprout from stumps and roots when cut, so in most cases the many stems in young hardwood stands will be sprouts as opposed to seed-origin stems.  Sprouts can make very good crop trees but care must be taken in choosing the best sprouts.  Low-origin stump or root sprouts make the best “leave or crop trees” (Lamson 1988). Conversely, sprouts originating high on a stump or from multiple sprout clumps are at high risk for decay and do not make good stems for crop trees (Roth 1956). Single stems should generally be favored from multiple sprout clumps if they are cut cleanly, are of low origin, and have small diameters (Lamson 1988, Hicks 1998, Frederick 2004). Sprouts should be thinned on only the best sites (Isebrands and Dickson, 1994).

 
Species is a consideration when evaluating sprouts and there is wide variation in the “natural” susceptibility of species to wood discoloration and decay among hardwood species. In North America, diffuse porous species such as Liquidambar styraciflua, sweetgum, Liriodendron tulipifera, yellow poplar and Acer rubrum, red maple are very susceptible to discoloration and decay, and cut stems do not heal as well compared to Fraxinus spp., the ashes, Quercus spp., the oaks, Prunus serotina, black cherry or Juglans nigra, black walnut (Frederick 2004).

 
In South America, only the three evergreen Nothofagus species: N. dombeyi, N. nitida and N. Betuloides, the deciduous N. Pumilo, dominant in Patagonia, and Drimys winteri are non-sprouters. All others are vigorous sprouters including all deciduous Nothofagus species.   These Nothofagus species sprout vigorously from stumps and root collars while a common associated species, Eucryphia cordifolia commonly sprouts from roots. There are no reports of differences among these species in terms of discoloration and decay susceptibility. 

 
Another consideration when implementing an improvement cutting in hardwood stands is epicormic branching which may occur following a cutting (Hilt and Dale 1979, Lamson 1988, Frederick 2004). Epicormic branching can be a quality defect which results from adventitious buds beneath the bark of some trees that are stimulated by increased light and temperature following an improvement cut.  The incidence of epicormic branching is related to species, site and the amount of light (and temperature increase) that reaches the stem following cutting that may stimulate these adventitious buds (Hicks 1998).  Residual stocking levels will influence these variables. 

 
Certain species seem to be more susceptible to epicormic branching compared to others.  When they occur, epicormic branches can develop into permanent branches which will result in knots in the wood when the tree is cut into lumber or veneer.  Otherwise, the bole would be clear, free of knots and have higher value.  Log buyers will pay a premium for clear logs which can make epicormic branches a major liability for a landowner who is selling logs.  The possibility of epicormic branching resulting from improvement cutting must be recognized, and managed.

 
Epicormic branching is more common on certain species in North America, such as Quercus alba, white oak and yellow poplar. In South America we have not observed development of epicormic branching following improvement cutting, but following shelterwood cutting, it has been observed in species like E. cordifolia and Weinmannia trichosperma (Personal Observation, P. Donoso).   It is best to carefully consider each species to be released during an improvement cutting, and alter the amount of release appropriately.  Maintenance of residual density above 50%, or avoidance of cutting more than 35-40% of the original density in one cut should minimize epicormic branching.

 
In hardwood management for high quality sawtimber and veneer, quality is most important, so it is critical to minimize epicormic branching. In some cases, older trees with thicker bark will tend to branch less, but this is difficult to predict. In some cutting operations, suppressed and intermediate crown class trees adjacent to the most valuable crop trees should be left in place to provide shade on these stems, in order to reduce the development of epicormic branches.

 

Improvement Cutting and Logging Damage

 
It is very important to prevent logging damage to residual stems during an improvement cutting operation. This type of damage to the stem and roots can lead to substantial wood discoloration and decay which will dramatically reduce future value.  Both wood discoloration and decay are serious grading defects and may lead to much lower value or complete rejection of the logs (Hicks 1998, Frederick 2004). 

 
Skilled logging operators who are aware of the plan for long-term management of the stand are absolutely critical to the success of this approach.  One method to protect the most valuable residual stems during logging operation is to leave small “guard” trees in place around crop trees.  These small trees can later be cut or killed in-place, or left to reduce epicormic branching, as described above. Another complementary method is to leave all the trees in place along the border of logging trails and cut them after the cutting operation is over.

 
Logging damage can sometimes be difficult to detect except immediately after a cut. Landowners should have a plan in place for a post-cut inspection, perhaps even before the loggers have left.  This will facilitate returning to harvest those trees that were meant to be crop trees, but sustained sufficient damage to warrant immediate removal.  It is often difficult to tell whether a stem is sound from the bark pattern.  Becoming familiar with external indicators of interior defect is important and takes some experience. Observing cut logs and logs being sawn are valuable experiences for hardwood landowners.

 

Final Considerations

 
Improvement cutting, concurrently with quality timber production can provide excellent wildlife habitat and food for wildlife species (Harper and Moorman 2004, Smith et al. 1997). Some defective trees and snags can be left for cavities and hard and soft mast-producing trees can be retained in the stand. The logging debris and slash resulting from the improvement cut can also provide some habitat for certain wildlife species.  Improvement cutting accelerates forest succession and large trees and other characteristics of old-growth forests can be attained in shorter periods of time, if that is a societal or individual objective.

The international trends for hardwood timber prices have continued to move upward especially for the furniture-grade hardwood species. Other products from hardwood stands are also increasing in value such as short-fiber pulpwood, fuel chips and firewood.

Woody biomass for use as a liquid fuel feedstock is gaining increased attention and wood harvested through improvement cutting in hardwood stands may become an important source for this feedstock.

Temperate forest hardwood stands occupy a significant area in the Americas and they provide a wealth of products and ecological services (Nahuelhaul et al. 2006). It is critical that we manage these forests to keep them healthy, improve species composition and quality and maximize production. Many hardwood sites are currently producing far below their potential for many products and ecological services. Improvement cutting is one of the best silvicultural techniques to accomplish these goals. The benefits will be returned directly to the landowners and to the citizens who use and benefit from the products and ecological services of our temperate hardwood forests.

References

 

Bardon, R. E. 2004.   Hardwood crop tree release: Intermediate stand treatments on an individual tree basis. Forest Landowner: 63: 13-15.

Carvell, K. 1971.  Silvicultural aspects of intermediate cuttings. Proc. Oak Symp., USDA,  For Serv. NEFES, p 60-64.

Carvell, K. 1973.  Effects of improvement cutting s and thinnings on the development of cove and mixed oak stands. W VA Ag. For. Expt. Sta., Tech Bull, 20 pp.

Dale, M.E. 1972.  Growth and yield predictions for upland oak stands 10 years after initial thinning. USDA, For Serv., NEFES, Res. Pap. NE-241, 21 pp.

Donoso, P.J., T. Monfil, L. Otero and L. Barrales. 1993. Estudio de crecimiento de plantaciones y renovales de especies nativas en el area andina de las provincias de Valdivia y Cautin. Ciencia e Investigacion 7(2): 24-42.

Frederick, D.J. 2004.  Hardwood thinning and stand improvement: Intermediate stand treatments on an area-wide basis. Forest Landowner: 63:6-10.

Grosse, H. and I.Quiroz. 1999. Silvicultura de los bosques de segundo crecimiento de roble, rauli y coihue en la region centro-sur de Chile. In: C. Donoso and A. Lara (Eds.), Silvicultura de los Bosques Nativos de Chile. Ed. Universitaria, Santiago, Chile, pp. 95-128.

Harper, C.A. and C. Moorman. 2004. Wildlife habitat enhancement in hardwoods. Forest Landowner: 63:16-20.

Helms, J. 1998. The dictionary of forestry. Society of American Foresters, Tech. Rpt., 210 pp.

Hicks, R.R. Jr. 1998. Ecology and management of central hardwood forests. John Wiley, New York, 412 pp.

Hilt, D. E. and M.E. Dale. 1979. Stem form changes in upland oaks after thinning. USDA, For. Serv., NEFES, Res Pap. 7 pp.

Isebrands, J.G. and R.E. Dickson. 1994. Biology and silviculture of Northern red oak in the central region: A synopsis. USDA, Forest Service, NCFES, Tech Rpt 68, St. Paul, MN.

Lamson, N.I. and H.C. Smith. 1978.  Response to crop tree release: sugar maple, red maple, red oak, black cherry and yellow poplar saplings in a 9-year-old stand.  USDA, For. Serv., NEFES, Res. Pap., 9 pp.

Lamson, N.I. 1988.  Precommercial thinning and pruning in Appalachian stump sprouts – 10-year results. SJAF: 12:23-27.

Lara, A., C. Donoso, P. Donoso, P. Nunez, y A. Cavieres. 1998. Normas de manejo para renovales del tipo forestall roble-rauli coihue. In: C. Donoso and A. Lara (Eds.) silvicultura de los Bosques Nativos de Chile. Ed. Universitaria, Santiago, Chile, pp. 129-144.

 Leak, W.B. 1981.  Do stocking guides in the eastern United States relate to stand growth? J. For. 79:661-664.

Nahuelhual, L., P.J. Donoso, A. Lara, D. Nunez, C. Oyarzun, y E. Neira et al. 2006. Valuing ecosystem services of Chilean temperate rainforests: environment, development and sustainability. Published online, June 3, 2006, 10.1007/s10668-006-9033-8.

Nyland, R.D. 2002.  Silviculture: Concepts and applications. McGraw Hill, 682 pp.

Roth, E.R. 1956. Decay following thinning of sprout oak clumps. J. For. 54:26-30.

Sampson, T.L., J.P. Barrett and W.B. Leak. 1983.  A stocking chart for northern red oak in New England, Univ of New Hampshire, Ag. Exp. Sta. Bull 14, 15pp.

Sander, I.L. 1977.  Managers handbook for oaks in the North Central States. USDA, For. Serv., NCFES, Gen Tech Rpt., NC-37, 35 pp.

Smith, D. M., B.C. Larson, M.J. Kelty and P. Mark S. Ashton.  1997. The practice of silviculture: Applied forest ecology.  John Wiley, New York, 537 pp.

Trimble G.R., 1970.  Twenty years of intensive unevenaged management: effect on growth, yield and species composition in two hardwood stands in West Virginia. USDA, For. Serv., Res. Pap., 12 pp.

 

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Douglas Frederick is Professor of Forestry, Department of Forestry and Environmental Resources, North Carolina State University, Raleigh, North Carolina, USA.  Pablo Donoso is Professor of Silviculture, Instituto de Silvicultura, Universidad Austral de Chile, Valdivia, Chile

Posted 28 February 2008