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Ecological Basis of Silviculture

Silviculture is the ecological art and science of managing forest stands to meet landowner objectives. It is also the applied ecology portion of forest management. Forest management considers the entire forest, which is made up of numerous stands; while silviculture deals with individual stands. Landowner objectives may include timber management, wildlife management, aesthetics, and recreational opportunities.

Silviculture is based on two basic ecological patterns. The first is succession, or the way forest communities develop over time. The second is disturbance, or an event that destroys all or part of an existing forest community. These patterns are natural phenomena in all forest types and take place on many different scales. Succession and disturbance are related because succession cannot be altered without disturbance. Plant communities develop through succession and are altered through disturbance. In a natural situation, succession and disturbance are chaotic. Disturbance events are unpredictable, both spatially and temporally.

Even though silviculture is based on natural processes, it does not precisely mimic them. Through the use of silvicultural techniques, natural processes are allowed to take place to produce desired conditions. An understanding of the underlying ecological principles is essential in comprehending silviculture and forest management.

Succession

Succession may follow two basic patterns, primary succession or secondary succession. These two basic types of succession are addressed in more detail later in this chapter. Silviculture most often mimics secondary succession, since some plant community generally occupied the site before it was subjected to disturbance. In order for succession to begin, some sort of disturbance has to take place. After the disturbance, new plants invade the site and begin to grow. Succession is accurately described as occurring along a time continuum, starting with year zero and continuing until another major disturbance. Left to their own devices, forest stands go through four distinct stages of development: stand initiation, stem exclusion, understory reinitiation, and steady state (Oliver and Larson 1990).

Succession: stand initiation

The first successionary stage is stand initiation. During this stage, water, nutrients, and sunlight are plentiful due to the lack of existing vegetation. In the South, plants quickly occupy the site and begin to compete for available resources. Herbaceous plants seed in and existing rootstocks sprout. Plant diversity is high relative to midsuccessional stages, since species with varying levels of shade tolerance all occupy the site simultaneously. Plants that reproduce from rootstocks and plants that are shade intolerant have a competitive advantage during stand initiation.

Succession: stem exclusion

As a stand matures, resource limitations occur. On upland sites, either water or nutrients may be in short supply. On bottomland sites, sunlight is usually the limiting factor. When available resources begin to limit the growth and establishment of new plants, the stand is in the stem exclusion stage. At this point on the successionary continuum, shade-intolerant understory species begin to disappear; and the plant community becomes dominated by trees. Fast-growing, shade-intolerant tree species generally overtop competing vegetation, and competition for available resources is extreme. Shade-tolerant species usually have slower growth rates and tend to lag behind. As this stage progresses, stratification occurs, usually resulting in a well defined midstory and overstory.

Succession: understory reinitiation

Shade-intolerant tree species are usually replaced in the overstory by midtolerant species during the understory reinitiation stage. As shade-intolerant species reach full height, other species begin to out compete them for available resources. Gap-phase dynamics begins to occur during this stage. Trees, or groups of trees, die for many reasons and are replaced either by trees that are presently in the midstory or by new reproduction. The forest canopy begins to become more heterogeneous, allowing sunlight to penetrate from above and from the sides. As trees die, resources are allocated to remaining individuals, many of which respond with increased canopy growth and diameter growth. With increased sunlight reaching the forest floor, herbaceous plants become established and flourish. Depending on forest type, species composition may shift, with shade-intolerant species giving way to more shade-tolerant ones.

Succession: steady state

The steady-state stage of succession is anything but steady, but it does tend to perpetuate itself to some extent. In many southern forest types, this stage exists only in varying degrees, with fire (historically) being the major contributing factor in arresting or setting back succession. This stage is a continuation of the understory reinitiation stage and is marked by small-scale disturbances that contribute to gap-phase dynamics. As gaps continue to form and develop over time, structure and species composition become quite complex. The presence of many gaps in various stages of development creates stand conditions where trees of many ages, sizes, and species exist simultaneously. In many systems, mature trees on the edge of gaps are more susceptible to mortality due to increased exposure, creating an expanding gap pattern of development over time.

Disturbance

Disturbances vary in severity, frequency of occurrence, and predictability. Generally, certain types of disturbance are more common in particular forest types. Low-intensity ground fires were common in southern pinelands and were characterized by high frequency and low severity. Windthrow during storms is a common disturbance in bottomland hardwood forests where trees have shallow root systems in moist soils.

An inverse relationship also usually exists between severity and frequency of disturbance. Frequent, low-intensity disturbances usually affect only part of the plant community. Low-intensity groundfires in pine stands detrimentally impact hardwood midstory and understory species but do not harm the pines in the overstory. In bottomland hardwoods, however, fires are infrequent and may potentially set entire stands back to the stand initiation stage.