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Positive effects of site preparation on soil temperature
Two major goals to mechanical site preparation are to facilitate soil warming in the rooting zone and to reduce surface temperature extremes, which in turn can cause seedling heat or cold stresses.
Increased soil temperature can be achieved in the following ways:
Removing the vegetation that shades the the soil or reducing the thickness of the insulating organic layer. This will cause soil warming as a result of increased solar radiation and thermal diffusion. Soil warming will be greatest where the original organic layers, litter or slash, were thickest.
Cultivate the soil(e.g. roto-tilling). This can promote soil warming by reducing soil bulk density and thus improving drainage and aeration. By deep burial of chunky organic matter, moisture can be conserved for longer while still being accessible to seedling roots.
Manipulating microtopography through site preparation to favor microsites with a Southely aspect (or Northerly if in the Southern Hemisphere). The importance of aspect is more profound at higher latitudes, where surface with a slope of 1 degree to the south is equivalent, in terms of solar radiation received, to level ground 100 km farther south.
Draining microsites (e.g. by mounding or plowing). This has proven more effective in raising the soil temperature in the rooting zone than of scalping patches or furrowing where low soil temperature is the result of high soil moisture content.
Surface temperature extremes can be decreased by:
Removing the surface organic layer. Exposed mineral soil will maintain lower surface temperatures because heat is quickly conducted away from the surface. Heat conductivity is further enhanced by soil moisture, which increases thermal conductivity and causes cooling through evaporation. The net result is to reduce temperature extremes. Additional protection from excessive insulation can be achieved with a roughened mineral soil surface. In a trial conducted in the Great Lakes states, discing increased the amount of shade, and consequently, the survival of red pine seedlings. Spot treatments where slash, logs, stumps, and some vegetation left nearby may be of higher value as "protection" during seedling establishment.
