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1 Definition of a Silvicultural System 4 The Decision Process Appendix 1 Answer Key
2 Major Types of Systems 5 There's More to Learn Appendix 2 Advantages and Disadvantages
3 Variations of Major Types 6 Implementation Appendix 3 References

Even-aged Versus Uneven-aged Stands

Even-aged stands

Although even-aged stands often occur naturally, they rarely exist in a pure form. Natural even-aged forests occur after a major disturbance initiates the processes involved in stand regeneration. Often, these stands maintain their even-aged structure until the next major disturbance occurs. In managed even-aged stands, the regeneration period is not greater than 20% of the length of a standard managed rotation. This time period will be reflected in the age difference between the oldest and youngest trees.

Even-aged stands generally have one age class, although two age classes can be found in some two-layered natural or managed stands. These stands generally have a well-developed canopy with a regular top at a uniform height.

Pure even-aged stands generally have a nearly bell-shaped diameter distribution. This means that most trees are in the average diameter class. However, diameter distributions should be viewed cautiously because diameter can be a poor criterion for age. The smallest trees in natural even-aged stands are generally spindly, with vigour suppressed by the overstorey.

For managed even-aged stands, regeneration cuttings are concentrated near the end of the rotation. Most forests in western North America are managed this way because it conforms to the development of many of these forests, and because even-aged management is easier to plan, conduct, and regulate. Large-scale natural disturbances, such as wildfire, have naturally favoured even-aged stands of seral species with a niche closely associated with these disturbance types.

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Uneven-aged stands

An uneven-aged stand is a mosaic of tiny even-aged clumps and scattered individuals woven together through a perpetual cycle of random disturbances. In nature these disturbances contribute to scattered mortality, allowing for temporal waves of different age-classes dispersed throughout the stand. Such disturbances may include light-intensity fires, windthrow, and insect or disease attacks. Some authors also refer to these stands as "multi-cohort" or "all-aged" stands. In nature a pure, "uneven-aged" stand probably does not exist as age classes develop with a high degree of randomness in space and time.

We will use the term "uneven-aged" because it is more widely recognized. Uneven-aged management removes some of the natural randomness to allow for more predictable stand development over the long term. Using regeneration cuttings throughout the rotation facilitates uneven-aged management, in effect giving us a perpetual regeneration.

Uneven-aged stands have at least three well-represented and well-defined age classes, differing in height, age, and diameter. Often these classes can be broadly defined as: regeneration (or regeneration and sapling), pole, and mature (or small and large sawtimber).

Uneven-aged stands have an uneven and highly broken or irregular canopy (often with many gaps). This broken canopy allows for greater light penetration and encourages deeper crowns and greater vertical structure in a stand. Most stems occur in the smallest age/size class, as regeneration quickly fills the frequent canopy gaps. This number declines through normal species competition as age/size classes increase, to the point where the largest age/size classes can be quite scattered (although distribution may be highly regular). Because regeneration is initiated in small gaps, more shade-tolerant tree species are generally favoured.

Although diameter distributions are commonly used to differentiate between even-aged and uneven-aged stands, defining stand structure by diameter distributions alone should be used cautiously. It is best to also look closely at other stand characteristics such as age and vigour of each diameter class. In true uneven-aged stands, vigour is maintained in all age classes.

 

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Graph of Stems/ha vs Diameter class.

In its ideal form, where diameters approximate age, distribution of diameters in uneven-aged management will approach the classic inverted-J form. This means that the largest number of stems fall in the smallest diameter classes. As you progress through the diameter classes, the number of stems per hectare drops in an inverted geometric fashion, giving a dipping curve relationship which looks like the mirror image of a "J" without the top.

Uneven-aged management is generally considered to be more difficult than even-aged management since all age classes are mixed together and therefore can be difficult to isolate and quantify. In British Columbia, uneven-aged management has been limited, although interest is increasing. However, uneven-aged management may be an objective for many reasons such as visuals, regeneration of shade-tolerant species, health, soil, habitat, and fire protection. (See also "Advantages of Selection Systems" in Appendix 2).

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