Even-aged Systems - Silviculture - Lecture Slides, Slides of Forestry

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Even-aged vs. Uneven-aged Systems

Common characteristics of even-aged stands

• Crown canopy is generally limited to a single layer elevated above the ground
• Diameters vary widely only if shade-tolerant species are present
• Only old stands have sawtimber sized trees
• Small trees have short live crown length when compared to total height
• Largest trees often have 25-40 percent live crown, depending on stand density

Even-aged vs. Uneven-aged Diameter Distributions

Reverse J-shaped

Bell-shaped (normal distribution)

Reverse J-shaped does not always indicate a true uneven-aged stand (3+ age classes)

• Concurrent application of individual practices of an uneven-aged silvicultural system during a cutting cycle harvest in a balanced uneven-aged stand
  • Treatments are applied to subunits of the stand depending on conditions within each subunit
  • Each cutting cycle harvest will support similar treatments

Uneven-aged Regeneration Systems

• Uneven-aged regeneration systems often referred to as selection systems

Characteristics of Uneven-aged Systems

• Maintains a continuous high forest cover
• Typically emphasizes the production of sawtimber sized trees

Characteristics of Uneven-aged Systems

• Selection is particularly useful for putting an irregular stand under productive management without losing existing stocking
• A selection system can be designed to obtain a sustained yield at recurring short intervals
• For sustained yield in a selection system:

Characteristics of Uneven-aged Systems

• To avoid "high-grading", each cutting should include intermediate treatments among trees other than those of the target size
• For a sustained yield, the method requires frequent and accurate inventory

General Procedure in Uneven-aged Systems

• Harvest mature trees, either single trees or in small groups
• This provides openings for regeneration of a new age class (cohort)
• "Tend" the remaining cohorts to maintain approximately equal total area in each -- among these remaining sizes, "cut the worst, leave the best"

Area regulation : this is the simplest, and is fairly easy with a group selection system, but it is difficult with the single-tree approach.

Volume regulation: harvest the allowable cut each cutting cycle -- if a stand is balanced, this is equal to the growth during the cutting cycle period

Balance vs. Irregular (unbalanced) uneven-aged stands

• In balanced uneven-aged stands with an reverse-J shape distribution, a constant ratio exists between the number of trees in successive diameter classes.
• This relationship defines the curve’s shape (steepness or flatness) and is called q (or quotient)

q =

where, Ni = number of trees in the i th diameter class Ni+1 = number of trees in next largest diameter class

i 1

i N

N 

Structural regulation and reverse J-shaped curve