EFFECTS OF HEAVY EQUIPMENT ON PHYSICAL PROPERTIES OF SOILS AND ON LONG-TERM PRODUCTIVITY: A REVIEW OF LITERATURE AND CURRENT RESEARCH

Soil disturbance caused by heavy equipment used for harvesting or site preparation can have negative effects on soil properties and long-term forest site productivity. Soil compaction, churning, rutting, mixing, displacement, and removal are types of disturbance that can reduce tree root growth through their influence on soil physical, chemical, and biological properties. Removal or displacement of surface soil can also expose deeper subsoil, which may be less suitable for root growth due to its greater bulk density and soil strength. Most information on the effects of soil disturbance on root growth is derived from studies of agricultural crops under carefully controlled laboratory and greenhouse conditions. In contrast, trees are long-lived and have great capacity to adapt to restrictive soil conditions through morphological and distributional changes in their root system. This adaptive capacity limits our ability to predict how trees will respond to soil compaction under field conditions. In most soils, compaction reduces subsequent root growth in direct proportion to the degree of compaction. The long-term significance of observed short-term reductions in growth will depend on whether reduced root growth is associated with a) an overall reduction in rootable soil volume and quality or b) simply a slower exploitation of the pre-disturbance rooting volume. The likelihood or severity of growth impairment from soil disturbance is difficult to generalize because disturbance influences or interacts with other site-specific, growth-determining factors. In most situations, short-term growth of seedlings or saplings on skid trails is less than on adjacent non-skid trail areas; this may or may not reduce ultimate stand yield. Soil disturbance can also have unanticipated secondary effects on tree growth. A good example of this is enhanced seedling growth found on some disturbed sites due to reductions in competing vegetation. Some soils are more resilient to physical impact than are other soils, and mitigative measures or corrective treatment are not always needed. Compaction of some coarse-textured soils can actually improve moisture relations for tree growth. Loamy soils are more susceptible to compaction; clayey soils are more susceptible to rutting and churning. The time required for natural recovery from compaction varies with soil physical characteristics, chemical characteristics, climate, and the severity of compaction. Recovery may be faster where soils are subjected to freezing-thawing or wetting-drying cycles. In the absence of site-specific information, the effects of compaction on forest soils may be assumed to persist for several decades. Rehabilitation of compacted soil can be attempted by mechanical manipulation, such as disk harrowing, bedding or mounding, subsoiling or ripping, and spot cultivation. If properly applied, these tillage practices can favorably alter soil properties and enhance seedling survival and growth. Tillage under non-optimum conditions (e.g., wet soil), however, can cause additional soil compaction and/or puddling, and create further risk to long-term productivity. National Council for Air and Stream Improvement This technical bulletin reviews the types of soil disturbances that can occur from common forest management operations and how these disturbances influence soil physical properties, root growth, and ultimately, stand yield. Management practices that can be used to prevent or ameliorate soil disturbance are also described.