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Stand Development

Chad Oliver
Yale University

A stand is a contiguous, distinguishable group of trees with a relatively uniform age distribution, composition, structure, and soil condition (Helms 1998).  The change in a stand’s appearance with time has been long noticed, and historically described as “succession.”  In the past, this succession assumed, as the name implies, that one species succeeded another as each species modified the site to conditions better adapted to the following species.  Studies of the past few decades have revealed that a different set of processes causes this change in appearance with time.  To avoid confusion with the older concept of succession, the term “stand development” is commonly used to describe the changes in a stand over time following the newer understanding of processes.

 
It is now recognized that the dominant processes occurring in a forest are as follows:

1. Intra- and inter-specific competition occurs intensely among trees and other plants for a stand’s sunlight, moisture, nutrients, and other factors—referred to collectively as “growing space” (Oliver and Larson 1996). 
2. Disturbances occur quite often to forests.  They make growing space available and thus changing the competitive advantage among existing plants as well as those beginning after the disturbance as they grow to reoccupy the available growing space.
3. Different physiological and anatomical features give different species a competitive advantage following different types of disturbances and in different soils and climates.

 
A stand develops through individual trees growing, competing, and dying at different rates—punctuated by disturbances of different types and magnitudes.  A general pathway, or trajectory, has been described that is common among many forest stands in the world following a disturbance after which no other disturbance occurs for a very long time.  The trajectory will first be described following a “stand-replacing” disturbance; i.e., a disturbance that eliminates all previous trees in the stand.  The pathway creates different “stages” in which different processes predominate.  The stages have been described in several ways, usually as variations of the following (Oliver 1981):
 

• Stand initiation:  This stage immediately follows the stand-replacing disturbance.  During this stage, the growing space has been made available by the destruction of the plants previously occupying it; and many trees, herbs, grasses, and other vegetation forms regenerate by a variety of mechanisms and grow to reoccupy the growing space.  Details of the disturbance, regenerating mechanisms, and particular soil and climate can give certain species a competitive advantage in regenerating and growing.  This stage lasts until some plants occupy the growing space continuously and exclude new plants from regenerating.  The stage can last for two or three years where the growing space is rapidly occupied or for many decades.  All plants beginning after a single disturbance are referred to as a “cohort.”

• Stem exclusion:  This stage occurs as soon as all growing space is occupied and new plants are excluded from regenerating.  At first, the different trees occupy a single layer, or stratum—a condition referred to as the “brushy stage” (Gingrich 1971).  Eventually, trees in stands of a single species may differentiate into “crown classes,” as some become larger, take growing space from others, and force the others to grow more slowly or to die.  Some species in mixed species stands commonly lag behind in height grow, become overtopped and shaded, and grow very slowly in the shade.  This process creates a “stratification” pattern (a layered appearance) in which large trees of some species occupy the upper stratum and small trees of other species occupy lower strata—even though they are all of the same age.

• Understory reinitiation:  Eventually, the initial cohort is not vigorous enough to occupy all growing space and exclude vegetation from regenerating in the forest floor.  This process allows a vegetative understory to develop, creating the understory stage.  The overstory shade keeps this vegetation from growing tall, and a distinct stratum of vegetation within one or two meters of the forest floor can persist for many decades or centuries.

• Old growth:  The stage is the hypothesized consequence of no disturbances occurring to the forest within the natural life span of the oldest trees that began in the stand initiation stage.  It can take on many forms, but the process is one of trees and other plants having all begun during the understory reinitiation stage or later.  Confusion exists between this stage and the complex, a.k.a. “old growth” structure described below.

 
When a disturbance does not remove all trees, the residual trees compete for growing space with the newly regenerating trees and create a similar stand development pattern with certain differences:  With increasing numbers of residual trees, the post-disturbance cohort is more shaded.  With increasing shade, all trees grow increasingly slowly; but trees of less shade tolerant trees suffer most and may die, causing the understory trees to consist of increasingly shade tolerant species. 

The stages that emerge from stand development pathways form very characteristic distributions of sizes, numbers, and species of trees, shrubs, herbs, and snags and logs.  These distinct distributions are known as “structures” (Helms 1998).  These structures are important because the different structures provide habitats for different species; different susceptibilities to winds, fires, insects, and other disturbances; different volumes and qualities of timber; and differences in other values. 
 

• The stand initiation stage with no residual trees is referred to as the “stand initiation” or “open” structure. 

• If residual trees exist in the stand initiation stage, it is referred to as the “savanna” structure, although the savanna structure can also be in the stem exclusion stage where a few overstory trees exist above grasses or shrubs that are occupying all of the growing space. Distinguishing between the open and savanna structures is important because some animals (e.g., the red cockaded woodpecker) distinctly need the savanna structure.

• The stem exclusion stage is generally referred to as the “stem exclusion” or “dense” structure, both with and without residual overstory trees. However, sometimes a stem exclusion structure with snags, logs, and many species in a stratified condition can be referred to as the old growth structure.

• The understory reinitiation stage is generally referred to as the “understory” structure. 

• The term “old growth” is confusing because the old growth structure can encompass more stages than just the old growth stage.  The old growth structure is sometimes referred to as the “complex” structure (Oliver and O’Hara 2004).  The understory and old growth stages, and sometimes the stem exclusion stage, as described above, can have the complex or old growth structure.  The old growth structure is often described as having old, relatively large trees, several layers of tree canopies, dead trees and logs, and other distinguishing features (Franklin et al. 1984, Parker 1989).

 
Some structural classifications distinguish between the savanna or old growth structures with and without large diameter trees; and other structural classifications divide the old growth structure into several structures.

A forested landscape commonly contains a variety of structures as each stand develops from one structure to another through a combination of growth and disturbances (Oliver et al. 1998).  For example, a stand may follow a trajectory, or pathway of being in the savanna structure, but growing to the old growth structure with the exclusion of fire.  Then, if a stand-replacing disturbance impacted it, the open and then dense structure would develop, followed by the understory structure barring a disturbance.  A disturbance in the dense structure could change the stand to the open, savanna, or understory structure; and a disturbance in the understory or complex structure could create an open or savanna structure. 

Some topographic positions on the landscape more commonly have certain disturbance types and frequencies—and consequently more commonly follow some pathways.  For example, an upper slope exposed to heat or wind could commonly be subject to wind, fire, or other stand-replacing disturbances and thus commonly have a pathway that cycles between the open and dense structure.  Protected areas could be subjected to relative infrequent disturbances and thus have pathways that cycled the stand largely within the complex structure.  Consequently, some stand development pathways, and some structures, are found more commonly in some topographic positions than in others (Camp et al. 1997).
 

References
 
Carey, A.B., B.R. Lippke, and J. Sessions.  1999.  Intentional ecosystem management: managing forests for biodiversity.  Journal of Sustainable Forestry 9, 83.
 
Camp, A., C. Oliver, P. Hessburg, and R. Everett.  1997.  Predicting late-successional fire refugia pre-dating European settlement in the Wenatchee Mountains.  Forest Ecology and Management 95:  63-77.
 
Franklin, J.F.  1984.  Characteristics of old-growth Douglas-fir forests.  Proceedings of the Society of American Foresters National Convention.  10-16.
 
Helms, J.A., Ed.  1998.  The dictionary of forestry, Society of American Foresters, Bethesda, MD. 
 
Oliver, C.D. 1981. Forest development in North America following major disturbances. Journal of Forest Ecology and Management 3. (1980-81) 153-168.
 
Oliver, C.D., and B.C. Larson.  1996.  Forest Stand Dynamics.  Update Edition.  John Wiley and Sons, New York.  521 pp.
 
Oliver, C.D., A.Camp, and A.Osawa.  1998.  Forest dynamics and resulting animal and plant population changes at the stand and landscape levels.  Journal of Sustainable Forestry 6(3/4) 1998:  281-312.
 
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.
 
Parker, G.R.  1989.  Old-growth forests of the central hardwood region.  Natural Area Journal 9. 5-11.
 
 
 
Posted:  April  2007

Updated:  23 August 2007