Silvicultural systems are defined as planned programs of silviculture treatments designed to achieve specific stand structure objectives during the entire life of the stand. The STEMS experiment uses silvicultural systems and extended rotation treatments to create diversity in forest structure that results in a variety of canopy layers (vertical structure) and spatial patchiness (horizontal structure) to enhance biodiversity and wildlife.

The seven different silvicultural systems and extended rotation treatments include:

1. Modified Patch Cut
2. Extended Rotation (non-treatment control)
3. Group Selection
4. Clearcut with Reserves
5. Uniform Dispersed Retention
6. Extended Rotation with Commercial Thinning
7. Aggregate Retention

These silvicultural systems create a range of gap sizes and frequencies that emulate natural variation in forest structure. This is consistent with the goals of ecosystem management, which are to sustain diversity in the structure, composition, and function of entire ecosystesm.

Other silviculture treatments used to create further diversity in the STEMS experiment include:

• modified thinning practices to promote vertical and horizontal heterogeneity by developing mixed-species stands, multiple canopy layers, and more diverse understories;
• retention of standing dead and downed trees and coarse woody debris for wildlife habitat. Where feasible, windthrow is retained as active recruitment of downed woody debris;
• extended rotations (with and without thinning) to develop a wide range of stand ages, tree sizes, and structures;
• retention of wildlife tree patches [WTPs], individual tree reserves, and riparian reserves to maintain elements of forest structure that provide habitat, biodiversity, scenic, or other values for at least one rotation; and
• temporarily deferred areas to retain structure until the desired forest conditions develop in the existing or adjacent stands.