Soil moisture regime is that actual rise and fall of available soil moisture. The major contending factors include:

Weather (Macroclimate)

Some part of the precipitation enters the soil; solar radiation, temperature, humidity (vapor pressure deficit), and wind speed determine the amount of precipitation lost to evaporation.

Site

Slope position and microtopography affect the movement of water in and out of the soil, while slope and aspect affect the amount of solar radiation received.

Vegetative cover competes for moisture through uptake and transpiration and by intercepting precipitation, some of which evaporates before reaching the soil.  Water retention and availability are affected by the soil texture (pore size distribution determines the amount of soil water available to the plant), soil gravel and stone content, which affects the available water storage capacity (water available for use by plants), hydraulic conductivity (ability of the soil to conduct water) and root zone depth.  Soil texture is one of the more useful variables for predicting water relations.  Changes in soil texture alter soil moisture, the potential for frost heaving where frost is present and surface soil erosion and compaction.

Soil bulk density affects pore space; an increase in bulk density (e.g. by compaction) reduces water storage and decreases infiltration and drainage capabilities.  The stone or coarse fragment content of the soil affects water storage, which decreases with increasing coarse fragment content.

Increasing the organic matter can improve soil structure, making drainage and air permeability easier.   A thick covering layer can increase the soil's resistance to compaction.   Loose, undercomposed organic matter (e.g. a surface litter layer) has a poor water storage capacity greatly increasing if mixed and buried in mineral soil.

Positive effects of site preparation on soil moisture

Site preparation treatments modify the soil moisture regime either by conserving available water or by removing excess water.  For planting, the goal is to influence the moisture conditions around seedling roots, primarily by providing the planting spot with relatively high and secure moisture availability without risking oxygen deficiency.

Moisture conservation

Reducing transpiration losses can be done through the removal or killing of competing vegetation.  Moisture deficits in the rooting zone of conifers are largely result of transpiration by competing vegetation rather than evaporation.

Exposing mineral soil through the removal of the surface organic layers.

The periodic and rapid drying of the organic matter layer makes this material a poor planting spot for pine compared with mineral soil.  The VH Mulcher, used in a fashion to produce a depressed planting spot, (below the original soil surface) can be particularly effective in the case of planting by conserving soil moisture in dry climates, as snow and rainfall accumulate in a depression.  Even small micro depressions in the soil surface can improve moisture supply for seedlings located therein.   In mounds, soil moisture may be enhanced by incorporating chunky organic matter with the mineral soil.  This can maintain capillary continuity will still allowing drainage and good aeration.

Incorporation of the humus layers with the mineral soil by mixing.

This was found to increase the soil's water-holding capacity in the rooting zone.  It will also improve soil moisture by reducing the density of competing vegetation, particularly in coarser-textured soils.  On fine-textured soils mixing can increase the infiltration of moisture and can avoid capillary discontinuity in raised beds.  In the southern United States, soils on moderate to steep slopes subjected to compaction and puddling were similarly improved by cultivation.  Mixing ameliorates the soil's physical conditions and improves oxygen availability for long periods of time.  Few studies have been conducted on mixing as a site preparation treatment.  A study of bare root pine and white spruce on a loamy fine sand found the least water stress and greatest stomatal "optimization" in mixed soils, while the most stress occurred in either bare mineral soil or other site preparation treatments.

Moisture Removal

Localized drainage. Any site preparation method that produces raised planting spots, such as mounding or plowing, will drain microsites and create drier and thus warmer and better aerated planting spots for seedlings.  The initial benefits of raised (mounded) microsites have been well documented for the cold, wet soils of Sweden at high latitudes.   Mounding has also been advocated as having the potential for improving the survival and performance of out planted seedlings in cold climates in parts of British Columbia.   Mounds can be varied in construction height from slightly elevated to extremely large (24 inches) all having distinct differences.  Small mixed mounds show the most promise in a variety of climates.

Negative effects of site preparation on soil moisture

Given the many factors that influence soil moisture, it can be difficult to achieve and maintain the positive effects of site preparation.  Some of the negative effects of mechanical site preparation are as follows:

(1)  Droughty conditions can sometimes be made worse.   Studies in British Columbia have indicated that raised planting spots may desiccate during dry periods, particularly in "course-textured" soils. On these soils, mounds may further accentuate desiccation because of the capillary discontinuity created by the lack of mixing of the organic layer.  Experience from Sweden with inverted mounding, which is distinctly different than mixed mounding, indicates that in fine-textured soils, capillary discontinuity can result in an even greater risk of desiccation owing to the higher hydraulic conductivity at the surface (compared with that of coarse-textured soils), which can result in faster drying of the mineral cap.  To overcome problems associated with mound desiccation on inverted mounds, deep plant to place at least part of the seedling root system deeper than the upturned humus layer. "The incorporation of organic material in mixed mounding may be significantly different and beneficial." Deep planting may NOT be advisable for mixed mounds.

(2) Depressed site-prepared microsites may be subject to seasonal moisture stresses as well as frost heaving.  The risk of frost heaving may be increased, particularly on fine-textured soils.  The potential for frost heaving after mounding is not clear.  An increase in frost-heaving potential is possible on mounds of fine-textured soil and for mounded mineral soil over mineral soil.  A decrease is predicted for mounding in general and on inverted humus mounds.  In British Columbia, frost heaving has not been a problem following site preparation by mixing, even when mineral caps are fine-textured.  The new soil structure produced by mixing appears to be a significant factor.

Completely exposure of mineral soil can lead to modification in surface structure.  Rainfall can modify the soil surface by creating a seal that can retard infiltration, promote surface flow, and lead to erosion.  Mounds of fine-textured soils created by hand were resistant to rewetting once they had dried. This would not have been the case if the soil structure had been changed permanently and enhanced by mixing of organic material.

There is ample evidence that soil disturbance and compaction caused by forest crop harvesting and extraction adversely affect tree growth and the long-term productivity of a site, but the individual effects are variable and have not yet been well quantified.  Soil pore structure can be adversely affected.  Recommendations have been provided on the choice of machines and running gear to reduce soil damage under various soil conditions, as well as on operation methods.  The adverse effects of mechanical site preparation treatments on soil bulk density have been summarized, primarily in studies from Nordic countries, and as with harvesting equipment, the results were variable.  A report on the effects of increased soil bulk density on southern pine species stated that negative effects may be masked by the initial positive response of seedlings to reduce woody competition.  In Canada, it has been pointed out that scraping a fine-textured soil surface (e.g. by blading) can cause glazing, the closure of the soil pores at the surface, and the inhibition of water penetration. GLAZING in any plantation preparation method should be explicitly avoided.  The glazing along the sides of the furrows will restrict root egress.  Soil rooting volume will be limited.  Continuously plowed furrows may restrict tree root extension across furrows.  This may limit rooting volume and lead to future windthrow problems.