Silvi Culture

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  Single tree selection removes individual trees of all size classes more or less uniformly throughout the stand to maintain an uneven-aged stand and achieve other stand structural objectives. While it is easier to apply such a system to a stand that is naturally close to the uneven-aged condition, single tree selection systems are prescribed for even-aged stands, although numerous preparatory cuttings must be made to create a stand structure where the system can truly be applied. Once the uneven-aged structure approximates the balanced condition, the single tree selection system generally produces a complex mixture of small, even-aged clumps which are thinned over time to theoretically produce one mature tree. In theory these clumps should yield at least one mature tree of the specified maximum diameter, although in practice these clumps are often larger. New regeneration develops in small scattered openings created theoretically in small gaps with an area equivalent to the crown spread of a single mature tree. In practice these gaps are often larger, created through the removal of several mature trees. Since regeneration is always being recruited and larger mature trees are scattered, or in very small groups, these stands appear quite open, with many gaps. The system is generally used for the most tolerant species in an area. Using the single tree selection system to encourage species mixtures requires effort, especially where some less tolerant seral species are desired. Such stands must be opened considerably for this system to work. Since these stands are a confusing jumble of age classes, regulation of these stands tends to be complex. Usually guidelines for residual stocking, maximum diameter, diameter distribution (usually expressed by a q-ratio ), and cutting cycle are used during each entry. Smith (1993) warns against attempting to create mini, sustained yield units at the stand level. He suggests that foresters should instead focus on maintaining a continuous stock of larger trees, without excessive concern about perfectly balancing age classes. Nyland (1996) also warned about relying on the pure mathematical relationship defined by a q-ratio for diameter distributions, pointing out that q-distributions never remain stable even through one cutting cycle. Diagram of single tree selection over time. Single tree selection over time However, Nyland does recommend using all of the classic selection parameters, including q-ratios, as interim guidelines. Both Smith and Nyland suggest close monitoring and periodic remeasurement to  follow selection stands through their cutting cycles such that parameters can be adjusted to fit the  biological reality with the management objectives. All authors encourage foresters to never forget the basic tenets for successful single tree selection: to provide sufficient gaps for regeneration and to maintain vigour throughout the stand. Group Selection Systems Group selection systems also promote uneven-aged stands with clumps of even-aged trees well distributed throughout the cutting unit. Unlike single tree selection, however, these even-aged groups are large enough to accommodate some shade-intolerant seral species in addition to more tolerant climax species. Small gaps or openings are created on short intervals to develop into a mosaic of at least three or more age classes throughout the stand (see the diagram). Illustration of group selection systems. Remember that the choice of group or single tree selection must consider the resource management objectives at all levels and the existing stand and site conditions. Because of stand-level advantages, group selection or any other system cannot be viewed as a panacea in areas with many conflicting management objectives. The implications of using a broad application of one silvicultural system over a large area could be serious for one or more management objectives. The strip selection system manages age classes in regular strips rather than unspecified group shapes. This system was developed to provide advantages for managing windthrow. As with other systems using strips, the strips should be oriented perpendicular to the prevailing wind. If cutting proceeds systematically against the wind, the stand develops an aerodynamic shape, with further advantages for wind protection (see diagram).  Again, this system differs from the strip shelterwood system in that the strips are removed over three or more passes to give a truly uneven-aged stand and their removal occurs much more slowly. The cutting  period between passes will be 15-30 years, depending on the number of passes, and a regular rotation is needed to complete all passes.  As in the strip shelterwood system, the strip width in the strip selection is sufficiently narrow to create an environment that differs substantially from a clearcut and produces strips that are too small to qualify as individual stand units. Because they have the advantage of one long dimension, strips tend to be  narrower than groups. Where mature timber heights are 30-35 m, strips widths of 15-50 m will be used, depending on orientation and objectives. These systems are well suited to small skyline systems. 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. 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. Exposed soil after scarification. (Eli Sagor)
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