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Silvicultural treatments

Tree and stand conditions change over time as trees grow in size and as they interact with other plants, animals, and ecological processes. Silvicultural treatments are applied to change, accelerate change, or maintain the condition of trees and stands. For example, by applying selective herbicides after planting, a desired tree species can be given a head start in growth that allows it to out compete other vegetation. By thinning a stand, the remaining trees can develop into larger stems at a rate faster than if the stand was not thinned.

Many potential silvicultural treatments might be used to change, accelerate change, or maintain tree and stand conditions. Those that are typically used to foster improved tree growing conditions and/or improved growth and yield include:

  • Choice of species and site
  • Site preparation
  • Planting
  • Spacing
  • Weeding & Cleaning
  • Thinning
  • Pruning
  • Fertilization
  • Logging slash distribution

The combination of treatments used in a silvicultural system can have large impact on growth and future yields. For example, a stand managed with all of the above treatments may produce as much as four times the yields of the region wide average of such stands without treatment.

Species and Site Selection

The choice of tree species to plant on a site is an early and very important step in starting or regenerating a forest. Tree species can have very different requirements in terms of soil nutrient and moisture resources, and sites can vary widely in the extent or character of resource regimes. By careful matching of species and sites, the chances of achieving a healthy and productive forest are greatly improved. A useful approach for determining appropriate forest composition (including choice of commercial timber species) is to determine the native forest ecosystem, habitat type, or plant community of your site. There are several detailed regional references that provide the tools to make this determination including the Minnesota Department of Natural Resources web page on Minnesota's Native Plant Communities and the Wisconsin Forest Habitat Type Classification System (PDF, 146K).

Site preparation

Given a particular site, there may still be important steps available to ensure that the planted trees have the best possible start. By controlling (either removing or slowing) the growth of competing vegetation, the desired trees face less competition for site resources and can thereby establish dominance in competition for resources. Additionally, site preparation can prepare very specific microsite conditions favorable to tree growth, e.g., soil loosening, moisture, insect, and disease control, competing vegetation management, etc. These effects can be accomplished manually, mechanically, and/or with herbicides. In terms of the remaining overstory vegetation, tree species also differ widely in shade tolerance. Depending on the tree species, leaving overstory trees that provide shade can increase the chance of successful establishment by reducing heat and moisture stress. However, for shade intolerant species (typically pioneer or light demanding species), full sunlight will prove most effective. For shade intolerant species, excessive shade will decrease initial growth. Once the seedlings grow higher than neighboring competing vegetation, they are typically described as "free to grow". Competition is then managed by thinning rather than herbicides or other control techniques. Recommendations for site preparation are given for both tree species and existing site conditions. Details of these recommendations for typical sites and tree species are available in the tree specific guides sections.

 
[photo] Exposed soil after scarification. (Eli Sagor)
Exposed soil after scarification. (Eli Sagor)
 
   

The most commonly used site preparation techniques in the Lake States are:

  • TTS disk trenching
  • Leno scarification
  • Combined mechanical/chemical application with a boom sprayer at the back of TTS

 

Planting

Proper planting is crucial for tree survival and growth. If a tree is planted too deep or too high with respect to the soil surface, the seedling may be unable to access site resources effectively and will die or develop slowly with greater susceptibility to diseases and insects and thereby face increased risk of mortality or lessened tree quality. It is particularly important that the root system makes full contact with the soil and be positioned to favor normal development and growth. Poor planting can also increase susceptibility to windthrow later in life.

[graphic] Illustration of how to plant a seedling

Figure 1. How to plant a seedling.

[graphic] Illustration of proper planting depth
Figure 2. Proper planting depth.

Spacing

  [photo] Red pine planting. Southwest Wisconsin. (A. Ek)
  Red pine planting. Southwest Wisconsin. (A. Ek)

Spacing of seedlings is an important consideration. Spacing, or planting density will vary with objectives, forest type and condition, and species. For more details on spacing, see the information about planting in the tree species specific guide sections. Also, download a DNR handout about seedling stock types (PDF, 253K).

 

Weeding and Cleaning

  [photo] Manual brush control (B. Palik)
  Manual brush control (B. Palik)
   

Until the planted trees have grown beyond the height of competing vegetation (sometimes termed weeds), there remains a high probability of mortality. Controlling (reducing) competition can greatly reduce mortality of desired tree species. Some typical types and applications of weeding and cleaning are:

  1. Mechanical weed control (removal or destruction) by hand or with machines. This is typically expensive and the least effective method as certain plants can grow back quickly.
  2. Herbicide control with chemicals applied to the competing vegetation by injection of stems or ground or aerial spraying of foliage. Such chemicals can be selective in terms of differential effects on various plants. For example, certain chemicals will control broadleaved plants but not harm pines if applied in the right season and at the appropriate rate. Also see Best Management Practices for silvicultural Chemicals from USDA Forest Service, (PDF, 141K).
  3. Biological control can use shade of overstory trees to slow growth of competing herb and woody vegetation until the desired trees are beyond the influence of this competition.
  4. Prescribed burning can be used to kill competition. However, this requires that the desired trees are fire resistant. While competing plants may resprout, retarded growth (particularly after several burns) may provide sufficient time for the desired trees to capture needed site resources.

The most commonly used weeding and cleaning techniques in the Lake States are:

  1. Chemical weed control
  2. Wickless applicator (brush saw with chemical dowsing)
  3. Aerial application

Pruning

Pruning is the removal of the lower branches of a tree. You should never prune more than half of the tree height. For economic reasons, only trees which will be in the final mature forest (crop trees) should be pruned. You should prune (in either one or two steps) pole size trees up to 9', for one log of knot free sawtimber or veneer, or 17', for two logs of knot free sawtimber.

Pruning is usually done during the dormant season (fall and winter) because trees can be damaged more easily during the summer. Dead and live branches should be cut close to the stem to reduce the time of healing. It is important not to damage the branch collar to ensure optimal healing.

[graphic] The key to proper pruning is to protect the tree stem and trunk. To avoid tearing the bark and stem wood and to facilitate healing, make a small cut just (known as the wedge or notch) beyond the branch collar. Then make your second cut just beyond the notch from top to bottom. Once the branch has broken free at the notch, make a third cut parallel to and just beyond the branch collar to reduce the length of the stub.

Figure 3. The key to proper pruning is to protect the tree stem and trunk. To avoid tearing the bark and stem wood and to facilitate healing, make a small cut just (known as the wedge or notch) beyond the branch collar. Then make your second cut just beyond the notch from top to bottom. Once the branch has broken free at the notch, make a third cut parallel to and just beyond the branch collar to reduce the length of the stub.

 

[graphic] Pole size trees (hardwoods 5 to 11 inches and conifers 5 to 9 inches dbh) can be pruned one or two lifts (9 to 17 feet) which results in one or two logs of knot free sawtimber or veneer.

Figure 4. Pole size trees (hardwoods 5 to 11 inches and conifers 5 to 9 inches dbh) can be pruned one or two lifts (9 to 17 feet) which results in one or two logs of knot free sawtimber or veneer.

Thinning

Thinning reduces mortality (or salvages it before it occurs) by reducing the number of trees per acre. The remaining trees then have more site resources to draw from and typically grow faster and healthier. By thinning at regular intervals, one can be assured that stress due to overcrowding is avoided. Thinned trees can then develop stronger root systems and be less prone to windthrow. The species composition of a stand can also be influenced by thinning, e.g., depending on which tree species are cut and which are retained. If sawlogs or veneer logs are sought, thinnings would focus on developing large and high quality stems. Thus thinning can improve growing conditions, species composition, tree quality, and the economic value of the stand. Importantly, poor thinning choices can reduce quality and economic values (e.g., highgrading or always taking the best trees and leaving the worst). However, well planned thinning can provide increases in timber values and economic returns. Specific recommendations for thinning are provided in the tree specific guide sections.

[graphic] Forest stand before thinning (top) and after row thinning (bottom). In this thinning example, every third row is removed.

Figure 5. Forest stand before thinning (top) and after row thinning (bottom). In this thinning example, every third row is removed.

Regeneration Harvesting

As described above, thinning can remove mature trees and, consequently, is an application of harvesting in your stand. However, foresters often make a distinction between removal of trees to improve growth of remaining trees (thinning) and removal of trees to facilitate establishment of new regeneration or age cohort. The latter removal is termed a regeneration harvest. In reality, there are no clear boundaries between thinning and regeneration harvesting; thinning may allow establishment of a new cohort of trees, or regeneration harvesting may lead to improved growth of residual trees, if some are left in the stand. Rather these terms are used to communicate the primary purposes of the treatment, i.e., improved growth of residual trees or new regeneration. Regeneration harvests can be applied in many different ways which result in distinctly different stand age structures. Recall that silvicultural systems are often named for these age structures, regeneration and harvesting approaches (clearcut, seedtree, shelterwood, selection). Details on regeneration harvesting considerations are provided in species specific guides.

 

 
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North Central Region Forest Management Guide: A cooperative project of the USDA Forest Service and University of Minnesota.
USDA Forest Service - Northern Research Station
Last Modified:  05/25/2006