Uneven-aged systems promote a variety of species, ages, and sizes within a stand. In this system, light cuts at 5- to 25-year intervals encourage regular growth and prevent severe disturbances to the stand. Each cut may include thinning, harvesting, and understory treatments that create small canopy gaps where shade-tolerant tree species can regenerate. Some sapling and pole-sized trees may need to be cut to release the best stems from competition and encourage fast growth in the reduced under-story light.
The goal is to achieve an optimum distribution of size and age classes so that that each class contains enough quality trees to replace those harvested in the next larger size class. Uneven-aged systems typically produce high quality wood because there are frequent opportunities to remove poor trees, allowing the best trees to grow longer. The visual quality of such stands may be superior to even-aged systems.
Two disadvantages of uneven-aged systems are that relatively small volumes of wood are harvested during each cut and repeated entries to the stand with heavy equipment can damage residual trees and hinder regeneration.
In an uneven-aged system, trees of all sizes are removed in each cut according to these general guidelines:
- Trees at high risk from insects, disease or other hazards
- Trees that directly compete with crop trees
- Cull trees
- Low vigor trees
- Unwanted species (to remove seed sources)
- Improve spacing (including thin dense sapling patches and stump sprouts)
- Mature trees
Tree diameter often is used as a measure of tree maturity. Consider these factors when determining an optimum maximum diameter for harvesting trees in an uneven-aged system:
- Higher quality sites normally allow trees to be grown to a larger diameter before growth rates decline significantly and decay becomes a major factor in tree value.
- Uneven-aged stands normally contain a variety of tree species, each with a different growth rate and life span that affect the optimum maximum diameter for the species.
- Each type and quality of wood product desired (such as pulpwood, sawtimber, and veneer) requires trees within a range of diameters.
- As a high-quality tree gets larger, it becomes more economically valuable due to its larger volume and higher grade. Grade is a measure of tree quality that determines the types of products (including veneer, high quality lumber, and low quality lumber) for which its wood is suitable. Attaining veneer size and grade can greatly increase a tree?fs value. Deciding whether to cut a large, valuable tree or let it grow longer must be weighed against the uncertainty of it still being alive and healthy for the next harvest.
- Your goals as the landowner affect the maximum diameter classes to keep. You may choose to extend a tree?fs life to enhance non-timber resources (such as aesthetics, wildlife food and shelter, and old growth characteristics). Extending a tree?fs life may increase the volume of valuable sawtimber and veneer, but there is also an increased risk for reduced growth rates and damage.
You will need to consider additional criteria to enhance wildlife habitat, water quality, and aesthetic values.
Figure 4-10. Selection system.