Timber harvesting and site preparation effects on soil quality for loblolly pine growing on the lower coastal plain of South Carolina

The Lower Coastal Plain of the southeastern United States is a major wood producing region. Harvesting timber under wet site conditions often results in extensive soil compaction, rutting, soil displacement, and waterlogging. The objectives of this experiment were to determine: (i) if soil disturbance changes key soil properties and processes; (ii) if soil disturbance reduces loblolly pine productivity; and, (iii) if disturbance can be mitigated with site preparation practices? Three 20-ha, 20-yr-old loblolly pine ( Pinus taeda L.) plantations were harvested under wet and dry conditions to create a broad gradient in soil disturbance. Key soil properties were integrated into a Soil Quality Index (SQI) and compared to aboveground productivity of 2-yr-old loblolly pine trees growing on closely-spaced (30 by 30 cm) bioassay plots planted across the gradient of soil disturbance. The soil physical properties were used to determine the least limiting water range (LLWR), the range in soil water content within which root growth is not limited. Soil compaction and deep rutting reduced the LLWR. Retention of logging slash improved the LLWR for compacted and rutted soils. Site preparation improved the quality of the soil physical environment across all levels of soil disturbance. Soil disturbance had no effect on soil chemical or biological properties as evidenced by no change in soil pH, ECEC, base saturation, available P, or net N mineralization with disturbance. The base saturation exceeded 80% on all sites, with Ca saturation controlling soil pH. The high base saturation buffered any redox-induced changes in soil chemistry that would have resulted from disturbance. The results showed that high fertility is an important mechanism for buffering the potentially-negative effects of soil disturbance on the soil nutritional environment. Site preparation changed soil chemical properties, but the changes were probably associated with tillage effects on organic matter and clay content, not redox processes. The SQI showed that surface soil compaction and deep rutting reduced soil quality, mainly by decreasing the LLWR and aeration depth. Site preparation mitigated the effects of most disturbances on soil quality, evidenced by similar aboveground biomass production among soil disturbance classes after bedding. A regression model was developed for predicting aboveground biomass production as a function of SQI. SQI explained 73% of the variation in aboveground biomass production. The regression model showed that compression tracks and rutting decreased aboveground biomass production compared to undisturbed soils. (Abstract shortened by UMI.).