The STEMS long-term experiment includes following seven treatments:
- Extended Rotation (non-treatment control)
- Extended Rotation with Commercial Thinning
- Uniform Dispersed Retention
- Aggregate Retention
- Group Selection
- Modified Patch Cuts
- Clearcut with Reserves
In the case study prepared for the APBI 402 / SOIL 502 course, the focus is on the following treatments:
1. Uncut (100% retention)
2. Dispersed retention
3. Aggregate retention
4. Clearcut (0% retention)
Selection of the adequate tree retention level is a key factor to achieving successful implementation of VRH and maintenance of the soil diversity and function. Retention levels that are too low may not have the desired impacts due to an insufficient density of living trees and may be vulnerable to disturbances such as windthrow. Dispersed and aggregated retention are the most common designs to maintain the presence of living biological legacies on a harvested site. Aggregated retention has a greater potential to preserve undisturbed patches of forest floor and root activity, but its effects are not evenly disturbed throughout the harvested area. Dispersed treatments more evenly spread the impact of living trees on soils in the harvested, but the effects on ecosystem processes and biodiversity may be less than in aggregated patches (Franklin et al. 1997).
(photo by Dr. M. Krzic)
At the aggregate retention treatment individual trees and groups of trees are left within the cutblock for the purpose of maintaining or protecting environmental values and structural diversity during at least one rotation. More than half the total area of the cutblock is within one tree-height from the base of a tree or the cutblock boundary.
(photo by Dr. M. Krzic)
The dispersed retention system retains individual trees within the cutblock for the purpose of maintaining or protecting environmental values and structural diversity during at least one rotation. More than half the total area of the cutblock is within one tree-height from the base of a tree or the cutblock boundary.
(photo by Dr. S. Grayston)
The clearcut system manages successive, even-aged stands by cutting the entire stand of trees at planned intervals (the rotation). New stands then regenerate in their place. Clearcutting provides baseline data for comparing forest health and productivity in silvicultural systems that use partial cutting.
An even-aged (60-70 yr) stand of western hemlock (Tsuga heterophylla) and Douglas-fir (Pseudotsuga menziesii) adjacent to the study treatments was also sampled.