Uneven-aged Systems

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:

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:

You will need to consider additional criteria to enhance wildlife habitat, water quality, and aesthetic values.

Figure 4-10. Selection system.