Uneven-aged Management - A "Natural" Approach to Timber
With timber prices down for many products, less interest in investing in costly regeneration after harvest, and greater interest in a more natural approach to forest management, many landowners are looking for alternatives to the traditional plant, thin and harvest approach of timber management. Uneven-age management is an alternative that sustains the forest cover, provides income at more frequent intervals, minimizes regeneration costs and provides many wildlife habitat and recreational benefits.
An uneven-aged stand is a group of trees that differ significantly in ages; by convention, the spread of ages exceeds 25% of the planned life span for an age class.
In uneven-aged management, we remove mature trees, or groups of them, leaving gaps and young trees to grow, allocating a portion of the growing space to regeneration.
To contrast, we remove all of the trees when harvesting an even-aged stand in order to allocate all of the growing space to regeneration. When growing southern pines for timber, an even-aged management scheme is commonly used. To see the advantages and disadvantages associated with using these reproduction methods, visit our Even- vs. Uneven-Aged Reproduction Methods section.
In both cases, we are opening up a temporary void in the ecosystem so that the space can be occupied by a new age class to replace the one removed in the harvest.
For more information on converting an even-aged stand to uneven-age, read the University of Florida Extension Publication, Opportunities for Uneven-age mangement in second growth longleaf pine stands in Florida.
Structure of an Uneven-Aged Stand
A balanced uneven-aged stand contains age classes which occupy roughly equal amount of ground space per acre.
Each cutting in a selection system must:
- regenerate a new age class
- harvest mature trees and excess numbers in each age class
- balance the age class distribution
Balance among age classes in uneven-aged stands can be achieved by influencing the distribution of diameter classes through tending operations.
During each cut, remove:
- the mature age class and replace it with regeneration of a new age class
- the excess trees per diameter class so that the growing space is allocated equally among residuals of each class
The immature trees to leave standing:
- those of best quality, soundness, and vigor
- those having the best chance for survival and growth
- the species you wish to include in your mix
To improve the quality of the stand over time, cut among the immature age classes according to the following priority:
- remove the defective and diseased trees
- remove high risk trees that might not survive another cutting cycle
- remove low value trees of any species
- thin the least desirable species (see our Thinning page for more information).
- remove excess numbers of desired species
If we were to make a graph showing the diameter (in inches) class distribution of a balanced uneven-aged stand, with diameter classes on the x-axis and the number of trees per acre on the y-axis, the curve would resemble an inverse "J". This structure is called an Arbogast structure.
Sample Arbogast distribution from Davis and Johnson, 1987
The Selection System
The selection system is a silvicultural program which creates and maintains an uneven-aged stand.
The selection system includes:
- a selection method to harvest mature trees and regenerate a new age class to replace them
- tending (thinning) of the immature age classes
In the selection system, regeneration, tending, and harvesting all take place concurrently.
Individual Tree Selection
Individual or single tree selection is a selection system in which openings are created to regenerate a new age class in the space previously occupied by individual mature trees.
Individual tree selection will result in gaps with relatively low light levels. Therefore, this method is best suited to regenerate shade-tolerant species.
Group selection is a selection system in which openings are created to regenerate a new age class in the space previously occupied by groups of 2 or more mature trees.
The gaps created by a group selection cut will usually receive sufficient light levels to regenerate shade-intolerant species, given an adequate seed source.
When using the group selection system,
- identify family groups of mature and immature trees
- harvest groups of mature trees from cutting area
- thin the immature trees to maintain a balanced stand structure (see the Structure of an Uneven-Aged Stand)
All selection systems share the following features:
- mature trees are removed from a roughly fixed proportion of the stand area
- a new age class is regenerated in the space previously occupied by mature trees
- regeneration, tending, and harvest occur concurrently
Important note about terminology: Selective cutting is not the same as the selection system!
You may hear the word selective cutting used but selective cutting refers to:
- culling or high-grading
- it is an exploitive cutting that removes only certain selected species, size classes, or high value trees without regard to the future stocking or health of the stand
Even vs. Uneven-aged Reproduction Methods
Even- and uneven-aged reproduction methods have several advantages and disadvantages associated with them. When choosing a reproduction method it is first necessary to determine:
- what you seek
- what species and stand conditions will satisfy your objectives
- the type of stand you want, given the species sought or uses intended
- what physical or economic constraints you face in achieving the desired results
EVEN-AGED UNEVEN-AGED Advantages Disadvantages Advantages Disadvantages Allows almost complete control over stand establishment Supply of seed or seedlings must come from outside of the stand to be established, unless shelterwood or seed tree reproduction methods are employed Seed supply comes from the residual stand, ease and certainty of natural regeneration with some species (especially loblolly) Depending on species, control over stand establishment may be more difficult to achieve Less root competition in the regenerated stand Higher potential for damaging erosion to occur Lower potential for damaging erosion to occur Dependign on size of cuts and species, may be more root competition in the residual stand Suitable for regenerating shade-intolerant tree species. Shade-tolerant species may be overly exposed Depending on size of cuts and species, may be suitable for regenerating shade-tolerant tree species Depending on size of cuts and species, may not be suitable for regenerating shade-intolerant tree species Enables use of genetic improvements, if necessary, when planted Usually less biologically diverse, particularly young stands Usually more biologically diverse May be difficult to use genetic improvements, if necessary Relatively simple to achieve a balanced, sustained yield of forest products Modest income at thinnings, significant income only at final harvest Growth capacity of site can be converted to revenue at short intervals More challenging to to achieve a balanced, sustained yield of forest products Costly site preparation, including use of pesticides and heavy equipment, is usually necessary Site preparation is usually unnecessary May be higher risk of disease, insect problems, and wildfire Depending on density and other variables, may be lower risk of disease, insect problems, and wildfire