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Many environmental factors affect seedling performance in planting spots, or in seedbeds prepared for natural or artificial seeding. Of prime importance are temperature, moisture, soil aeration, soil physical characteristics (texture and structure), soil nutrients, light, vegetation, frost, insects and diseases, and small damaging mammals. Changes in one of these factors may affect others, causing a chain reaction. It is, therefore, important to recognize the interaction of cause and effect relationships. The following is an overview of some of the important relationships.
Low soil temperature limits the survival and growth of tree seedlings in the cool climates of northern latitudes and upper elevations. The forest floor commonly builds to a considerable thickness because low temperatures reduce the rate of decomposition. The soil beneath a thick forest floor is insulated from the sun's rays and therefore remains cool. Removal of the forest floor to expose mineral soil allows the soil to warm up. Roots grow faster in warm soil than in cold soil and also absorb nutrients more readily. In cold climates, the lower roots of planted seedlings may die 10 to 15 cm below the soil surface.
Various methods of site treatment can remove the surface duff layer, however, excessive scalping should be avoided (see "5, Site Degradation," this section). Although the effects of exposing mineral soil are generally beneficial, exposure of fine-textured mineral soil can result in frost heaving and reduced availability of nutrients.
In warm, dry climates, such as the Interior Douglas-fir Zone, vegetation depletes soil water so that newly planted seedlings are exposed to drought stress. By controlling competing vegetation, more soil water is available to the tree seedlings.
Scarce soil water may be conserved in dry climates by creating sunken planting spots. Snow and rain will accumulate in these depressions.
In wet climates, or in water accumulating habitats in drier climates, excess soil water occupies space in the soil which should be occupied by soil air. Raised planting spots, subsoiling and drainage ditches can increase the amount of soil air around seedling roots. Roots require adequate oxygen in the soil to grow and to absorb water and nutrients, therefore, early performance can be enhanced by providing a well-drained planting spot.
Excess soil water can also exacerbate problems of low soil temperature, as wet soil requires more energy to warm than well-drained soil.
To grow and absorb nutrients, roots require adequate oxygen in the soil. Pore space must be sufficient for roots to grow and penetrate the soil, and for water to filter through. Compacted soils are more poorly aerated and also have a greater risk for runoff and surface erosion. Site preparation treatments, such as mixing and subsoiling, tend to loosen the soil and increase soil air.
Both very fine-textured and very coarse-textured soils can have adverse effects on seedling performance. Seedling roots grow through compact, fine-textured soils with difficulty. When the planting spot is exposed, fine-textured soil, access to soil water and nutrients may be restricted and the risk for frost heaving is increased. Conversely, very coarse-textured soils drain rapidly so that seedlings may suffer moisture stress. Very coarse-textured soils also generally lack adequate amounts of soil nutrients.
A compact layer close to the soil surface restricts the volume of soil seedlings can use. Consequently, care should be taken not to reduce further the effective rooting depth of shallow soils by deep scalping. To allow deeper root penetration, the compact layer can be broken up by subsoiling. This will also improve drainage, and may relieve the problem of a high water table restricting root penetration.
In most soils, the surface organic matter and the uppermost mineral soil horizon contain most of the nitrogen used by seedlings. Removal of these horizons by mechanical site preparation treatment leaves the nutrient-poor subsurface soil as the growing medium. In medium to moderately coarse-textured soils, where root growth is not restricted, the increase in soil temperature resulting from exposure of mineral soil can more than make up for initial nutrient deprivation. With increased soil temperature further contributing to growth, roots can readily extend to the nutrients of surface layers at the edge of scalped patches.
However, in fine-textured soils roots cannot quickly reach the nutrients in surrounding undisturbed soil and overall seedling growth is restricted. In this case, or when scarce nutrients overlay a coarse-textured soil, a treatment such as mixing, which retains soil nutrients in close proximity to the seedling roots, is preferred.
In soils where organic matter and nutrients extend deeper into the soil profile, removal of the uppermost horizons by site treatment may have little adverse effect on nutrient supplies.
In wet climates and habitats, tree seedlings may be smothered when surrounding shrubs, herbs, grasses and ferns are flattened by snow. Competing vegetation may also reduce the heat and light available to seedlings to suboptimal levels. In drier climates, competing vegetation deprives seedlings of soil water. When attempting to control competing vegetation by scalping surface soils to remove plant roots, consideration should be given to soil texture.
In some cases, a competing vegetation problem may be introduced to a site where treatment stimulates rhizomes or creates seedbed at an inappropriate time of the year.
Tree seedlings can be deprived of adequate light by surrounding competing vegetation. Research has shown that seedling growth is retarded at levels below 50% shading. Removal of shading vegetation is therefore an important objective of site treatment, but should be carried out using methods appropriate to the site.
Frost during the growing season can be harmful to seedling performance. Buds may be killed and growth distorted or set back. Frost later in the season can damage succulent growing shoots causing appreciable growth reduction. Less damage by frost may occur where seedlings are growing in exposed mineral soil or on raised planting spots (mounds). Seedlings growing on raised planting spots may escape frost damage if the pool of cold air in a frost pocket is shallow. Sufficient heat may be re-radiated by exposed mineral soil during the night to keep seedlings warm enough to avoid the effects of frost. Small patches of exposed mineral soil will not re-radiate sufficient heat to affect frost damage. Planting spots with partial or no disturbance to the duff can be very susceptible to growing season frosts, especially in low lying frost pocket locations. Frost pocket locations treated with herbicide that removes surrounding vegetation without exposing mineral soil can be especially prone to growing season frost damage. Seedlings planted on mounds may be more susceptible to chinooks and to damage from early spring frosts.
High temperature at the soil surface may damage the stems of the tender germinants of natural regeneration in hot climatic zones. Some species are more susceptible to damage than others. The succulent stems of recent germinants are most susceptible to damage by high soil surface temperatures.
High temperatures combined with low relative humidity can desiccate seedlings even if moisture is available in the soil. When the rate of transpiration is faster than the rate at which the seedling can replenish the moisture, the seedling will suffer drought stress. Non-living shade, such as cull logs, stumps or shade cards, unlike shade from other plants, cools the seedling without depleting soil moisture.
The spruce weevil (Pissodes strobi) can have a devastating effect on established spruce plantations. This weevil prefers vigorous, open-grown saplings and causes terminal dieback of at least two years' growth. Since some protection may be afforded by a canopy of hardwoods, site preparation methods that completely exclude a hardwood canopy can make spruce susceptible to weevil damage. Contact the regional entomologist to identify where a hazard of spruce weevil exists.
Phellinus and armillaria root rots spread when the roots of the newly established forest contact infected stumps of the previous stand. Removal of infected stumps has been used to control re-infection of the new stand. However, stump removal is expensive and caution must be taken to avoid unacceptable site disturbance.
Mistletoe on residual Douglas-fir or lodgepole pine may be controlled by a site preparation method that damages the infected residual seedlings sufficiently for them to desiccate and die and provides sufficient soil disturbance to allow subsequent regeneration from pine cones on the ground. Drum choppers are suitable for this treatment.
On sites with sufficient vegetation cover to protect the snowshoe hare from predators, browsing damage may delay indefinitely regeneration of species such as interior spruce. This is also true for mice and voles that girdle seedling stems. Vegetation cover reduction through site preparation gives the seedlings time to grow to a height where small mammal damage is less inhibiting. Caution must be used when prescribing row-type treatments, such as disc trenching, which can actually increase browsing by ungulates and trampling by domestic cattle. Seedling damage may also increase in the vicinity of slash piles, which provide cover for small mammals.
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