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 Silvicultural Systems, Silvicultural Operations, and Pathways

 Chad Oliver and Glenn Galloway

Yale University and CATIE

• Originally each species was commonly considered more appropriately managed by a specific silvicultural system depending on its relative “position” in the then-accepted succession paradigm.  “Pioneer,” “mid-seral,” or “later-seral/climax” species were considered appropriately managed by clearcut, shelterwood, and selective harvesting, respectively (Barrett 1980).  A change to the more dynamic paradigm of nature showed that each tree species could be managed in a wider variety of ways (Oliver and Larson 1996).

• Silvicultural systems do not differentiate between stands with similar regeneration mechanisms but with later, drastically different intermediate operations such as weed control and thinnings of different timings and intensities.

 
Recently, stand management is increasingly being described using “silvicultural pathways” to avoid the emphasis on regeneration, to allow more specificity in management, and to encompass the more dynamic perspective of nature (Oliver and O’Hara 2004).    Each silvicultural system could follow a variety of pathways, as will be described below.  Time will determine whether “silvicultural pathways” are a used as a subset of “silvicultural systems” or as a replacement for the term.  The operations-pathways approach allows greater specificity of management and easier communication between silviculture and landscape management, financial analysis, operational planning, and quality control.
 

Ecologist are recognizing that a stand changes with growth and disturbances, with the disturbances releasing growing space that allow existing trees to expand and new trees to regenerate.  Specific characteristics of the disturbances and other factors help determine the species regenerating.  The exact pattern of growth and disturbances is unique to each stand, although similar patterns can occur to different stands.
 

The pattern of a stand at any time is referred to as the “structure”— the characteristic distributions of sizes, numbers, and species of trees, shrubs, herbs, and snags and logs (Helms 1998).  Structures have been classified, such as “open,” “dense,” “understory,” “complex,” and “savanna” (Oliver and O’Hara 2004).  Each structure provides some values, such as suitable habitat for some species, varying degrees of hazard resistance or susceptibility, different commodities, and different amounts and timing of water flow.  A stand will not remain static in its structure, so the values it provides will be constantly changing.  The trajectory of a stand’s development with growth and disturbances is referred to as a “stand development pathway.”
 

To ensure all values are provided in a forest, it is necessary to ensure different stands are in different structures. The pathways of the different stands need to be coordinated to ensure that they are always out of synchrony—that there is always a diversity of structures across a landscape--since each stand is changing with time.
 

Silviculturists intentionally direct the stand development process through one or several carefully executed actions done at appropriate times to achieve an appropriate mix of values over time in a stand.  The actions can include removing trees, replanting trees, burning the understory or residual stand after harvest, pruning, fertilizing, weed control and others.  The combination of these activities and the intervening growth creates a unique, anthropogenic stand development trajectory over time.
 

The individual actions are referred to as “silvicultural operations” and the directed changes in the stand over time are referred to as a “silvicultural pathway.”

Silvicultural operations can include regeneration, site preparation, weed control, spacing, harvesting, and other activities done at a concentrated time to change the growth trajectory of the stand.  They act as “inputs” to a stand in systems terms and to various degrees are used to mimic, avoid, or recover from natural disturbances so the stand will grow in a desired way.  Although done to a stand at a single point in time, they often require years of lead time to gather appropriate seeds and grow needed seedlings, to gain access to the stand, and to coordinate the necessary equipment, labor, materials (e.g., seedlings, chemicals, or other), and weather.
 

Silvicultural operations include biological, mechanical and labor, and logistical components.  They require equipment, labor, and expertise specifically suited for the operation, terrain, and biological conditions of the species.  A given operation is done very infrequently to a given stand—perhaps once or twice every fifty years.  However, a relatively standard set of operations is commonly done to many stands in a region because of the similarities in biology and terrain and existence of given equipment, labor, and expertise.  Operations evolve over time with changing needs, expertise, laws, equipment, and labor conditions.

Silvicultural pathways are changes in a stand’s structure over time that include both carefully designated silvicultural operations and anticipated growth and mortality of the stand between these operations.  For example, a silvicultural pathway may prescribe thinning of a crowded stand ten years from now, followed twenty years later by a harvest that leaves 12 residual trees per hectare and a planting of 1,000 trees per hectare of a given species.  The results would be a dense stand for the next ten years, a more open stand in the understory structure (and obtaining some timber) for the next twenty years, and a savanna structure for the next fifteen years (and obtaining more timber), followed by a dense structure as the stand regrows.  Another pathway could prescribe no thinning, but a harvest in 15 years, followed by planting a different species.  The flow of structures and timber would be different, or course.
 

Depending on the details of its structure, most stands can potentially follow one of several pathways at a given time, but can not follow others successfully.  For example, a particular, crowded stand in the dense condition may be amenable to being thinned, clearcut harvested, or shelterwood harvested; however, if no actions were taken for many years or if the stand were selectively harvested, the unharvested trees would probably blow over, be attacked by insects, and/or burn up—and not achieve the landowners’ objectives, assuming this destruction was not desirable.
 

Considerable expertise is needed in planning the silvicultural pathway of a stand.  A stand may be capable of responding to thinning at any time for several decades, and different silvicultural pathways could be designed that included thinning.  Ultimately, the trees may become too crowded and weakened if not thinned, and the stand may be destroyed by insects, blow over, and/or burn up whether they are thinned or not thinned.  In this case, the “window of opportunity” for thinning would have been lost.
 

In addition, different species require different operations and structures for growth.  Some oaks and bigleaf mahogany require the open structure created by clearcut harvesting to become established and survive (Oliver et al. 2005, Snook 1996), while others can become established beneath a savanna structure.
 

Various tables, diagrams, and computer models that project growth, mortality rates, and other changes over time for a given stand’s condition help foresters design appropriate silvicultural pathways; however, each tool has its strengths and weaknesses, and expertise is needed to recognize these and appropriately use and interpret the tools.
 

Silvicultural pathways of the many stands need to be coordinated so that some stands are always in each structure to ensure the many values of a forest are sustained.  This coordination requires further expertise, and specific tools can also help with this coordination.   The appropriate pathway for an individual stand may be less than optimal for achieving a single value in order to optimize the provision of many values over many stands and over a long time.
 

References:
 
Barrett, J.W.  1980. Regional silviculture of the United States, 2^nd Edition, John Wiley, New York.
 
Helms, J.A., Ed.  1998.  The dictionary of forestry, Society of American Foresters, Bethesda, MD.
 
Oliver, C.D., and B.C. Larson.  1996.  Forest Stand Dynamics.  Update Edition.  John Wiley and Sons, New York.  521 pp.
 
Oliver, C.D., and K. L. O’Hara.  2004.  Effects of restoration at the stand level.  Chapter 3 In (J.A. Stanturf and P. Marsden, editors)  Restoration of Boreal and Temperate Forests.  CRC Press, New York.  31 – 59.
 
Oliver, C.D, E.C. Burkhardt, and D.A. Skojac.  2005.  The increasing scarcity of red oaks in Mississippi River floodplain forests: influence of the residual overstory.  Forest Ecology and Management 210:  393-414.
 
Smith, D.M. 1986.  The practice of silviculture.  Ninth Edition.  John Wiley & Sons.  New York.
 
Snook, L.K. 1996. Catastrophic disturbance, logging and the ecology of mahogany (Swietenia macrophylla King): grounds for listing a major tropical timber species in CITES. Botanical Journal of the Linnean Society, 122: 35-46.

Posted:  April 2007

Updated: 23 August 2007