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Mechanical treatment can readily modify surface organic and soil layers of many forest sites. Changes in these layers can influence seedling performance (survival and growth). Additionally, the rearrangement of slash loads and the preparation of planting spots can reduce hazards and improve planter access and reduce the time taken for planting spot selection, thereby reducing costs.
Surface layers can be modified by scalping, mixing, and inverting. Suitability of each method will vary according to depth and quality of organics, soil texture, and other factors limiting seedling growth.
Seedbeds or planting spots formed by these methods may be level, raised above, or depressed below the original ground level. Also, treatments may cover the whole site, be in continuous or intermittent strips, or in discontinuous patches or spots.
Removal of excessive, loose surface organic layers exposes the underlying rich decomposed organic layers and mineral soil. Exposed mineral soil warms more rapidly and reaches higher subsurface temperatures than does undisturbed soil. In wet climates where high surface temperatures threaten stems of tender germinants, exposed mineral soil conducts heat down, away from the soil surface.
On exposed mineral soil, seedlings are initially deprived of nutrients, because the fertile surface layers are removed by scalping. However, when scalping is conducted on appropriate sites, seedling roots stimulated by increased soil temperature, extend quickly beyond the scalped area to reach nutrients in surrounding mixed or undisturbed areas. The optimum area of exposed mineral soil varies with site.
Mixing incorporates surface organic layers with the underlying mineral soil, making nutrients available to newly planted seedlings. The surface of well-mixed soils is essentially mineral in character, which enhances soil temperature much as scalping treatments do.
For fine-textured soils, mixing may be more satisfactory than scalping. Mixing organic matter with the mineral soil creates planting spots which are not as compact, so the problem of restricted root growth is avoided. If sufficient amounts of organics are incorporated, seedling susceptibility to frost heaving is also reduced.
Competing vegetation can be controlled by intense mixing, which is most easily achieved on fine-textured, relatively stone free soils. Partial or coarse mixing is suitable only where there is little competing vegetation. Mixing rarely eliminates vegetation for prolonged periods and may result in a change from one competing vegetation complex to another.
Where the forest floor (duff) is deep, mixing may not incorporate sufficient mineral soil. In such cases, the risk of seedling dessication is increased.
Planting spots consisting of overturned surface organic matter covered by mineral soil may provide several potential benefits for seedlings. Seedling roots can proliferate with the warm soil temperatures provided by the exposed mineral soil cap. Also, seedlings have immediate access to nutrients in the buried surface organic layers.
If deep enough, the mineral soil capping suppresses the resprouting of competing vegetation and retains moisture. Shallow mineral soil capping, on the other hand, may not suppress competing vegetation and mounds are at a greater risk of drying out.
Mineral soil cappings are prone to desiccation during dry periods and, therefore, seedlings with roots only in the cap may experience moisture stress. Seedlings should be planted deep with roots extending well into the humus layer. While the use of long-rooted seedlings has been tried to alleviate this potential problem, the logistics of growing and planting proved to be infeasible.
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