ALL THE AVAILABLE RESEARCH INFORMATION AGREES that soil degradation associated with conventional forest harvesting is a major problem in British Columbia and elsewhere in the Pacific Northwest.
The extent of the harvesting-caused disturbance is influenced by certain soil conditions at the time of the activity: (e.g., soil strength, moisture content, degree of frost, and snow cover) and by particular characteris- tics of the activity, (e.g., frequency that machinery and logs pass over a given site location, and the forces [pressure and vibration] exerted by the machinery and logs on the soil). Yarding and the construction and use of haulroads, skidroads and landings have been shown to cause disturbance levels ranging from 6 to 87% of the harvested area, with deep disturbance (>25 cm depth) being recorded in the range of 0-60%. Ground- skidding systems account for 60% of the total volume harvested provincially and are responsible for the highest disturbance levels recorded.
Mineral soil exposure has been shown to range from 5% for some helicopter-or skyline-yarded areas to 70% for some tractor-yarded sites. Mineral soil exposure on ground-skidded sites generally ranges from 10 to 50% but can be reduced significantly (50%) by such practices as ensuring ample snow cover (>1 m) for skidding in winter and using planned, designated skid trails. Cable-yarding systems generally produce less mineral soil exposure, but can result in high levels of disturbance and exposure (56%).
Harvesting causes dramatic microclimatic effects in forest sites, especially in those that have been clearcut. These changes can pose severe limitations to forest regeneration, particularly because of lethal air and soil surface temperatures, frost-heaving, snow press, desiccation and wind damage.
Increases in surface soil layer (0-10 cm) bulk density have been reported in the range of 15 to 60% for skidroads and 25 to 88% for landings. These density increases generally do taper off with depth, but have often been observed to exist at depths of 30 cm or more. Although compaction effects may last for only 10 years or less at the soil surface, most evidence points to compaction effects persisting at all sampled depths for 30 years or more. Most compaction is produced during the first few passes of the equipment and logs. Subsequent passes have proportionately less effect, but may cause density levels which may alter root growth significantly. The compacting forces of operating machinery may cause soil puddling, the effects of which on soil productivity are similar to those caused by bulk density increases.
It appears that a soil bulk density value in the range of 1200-1400 kg m3 is a critical threshold above which tree root growth will be reduced for the range of soil types and tree species in B. C. The evidence indicates that compaction-induced volume reductions over entire harvest areas in interior B. C. can be projected to average 10-15%.
Moving skidding machinery over the soil and pulling mechanical site preparation (MSP) equipment (passive and powered), and dramatically altering site characteristics through the use of prescribed fire can result in soil degradation and loss of site productivity if these treatments are improperly selected or poorly applied. Of the MSP treatments frequently used in the B.C. Interior, the one with the most potential for serious, detrimental effects is windrowing. Other treatments, such as blade scarification, V-blading, and bunching can have similar influences, but generally affect a much smaller portion of the treated area. Prescribed fire can result in the substantial loss of site nutrient capital during the burn or by erosion after the burn. If a burned site is subsequently treated (and re-treated) to control competing vegetation, nutrient cycling, retention and availability may well be impaired.
Almost all the research information available relates to the short-term effects of site preparation on seedling survival and growth. Very little information is available on long-term impacts of site preparation on soil properties and site productivity. There is some evidence, however, that various common MSP treatments can significantly harm soils and hinder tree growth. For example, whole treatment area reductions in productivity caused by windrowing, bunching, and blade scarification have been shown to amount to 20, 15 and 10%, respectively. While short-term prescribed-fire effects on tree growth are reasonably well-understood, long-term effects have been more difficult to identify. Evidence has shown that tree growth on burned sites may be better, poorer, or not different at all from that seen on unburned sites.
There are few literature reports of improved stocking and/or tree growth in areas of disturbed soils (e.g., skidroads), and only some undocumented, anecdotal field observations of similar situations. There is not enough information to produce general conclusions and specific guidelines as to specific site conditions which might be improved through disturbance.
The above is from the 1990 Ministry of Forests document Impacts of Forest Harvesting and Regeneration on Forest Sites.
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