Read the published paper (in PDF format):
Rapp, J., T. H. Shear, and D. Robison. 2001.
Forest Ecology and Management 149:241-252
Abstract
My objective was to compare the impacts of helicopter and rubber-tired skidder harvesting on the structure and function of a blackwater forested wetland eight years after harvest. This study was initiated in 1991 in a cypress-tupelo forest adjacent to the South Fork Edisto River in South Carolina. At that time, a complete block design comprised of three treatments with five replications was established; the treatments being clearcut with helicopter extraction, clearcut with skidder extraction, and an unharvested reference site. In 1992 and 1993, comparisons between treatments for soil, water quality (Perison 1997), and vegetation (Pavel 1993) were made using the unharvested reference site that was immediately adjacent and upstream from the harvest treatments. In both 1992 and 1993, there were differences in soil nitrogen, soil temperature, cellulose decomposition, and ammonium concentrations in soil water 20 cm below the soil surface (Perison 1997).
On this site during the summer of 1998, eight growing seasons after harvest, I again compared soil and water quality parameters, as well as the structure and composition of the plant communities of the three treatments (helicopter, skidder, and control). Cellulose decomposition at 5 and 10 cm below the soil surface was significantly greater in the harvest treatments than the unharvested treatment. Decomposition was dependent upon depth to the water table, and decreased along the elevational gradient from north to south in all treatments. Sedimentation rate was significantly greater in the harvested treatments than the unharvested treatment. Eight years after clearcut, the ability to trap sediment was nearly double in the harvest as compared to the unharvested treatment. The increase in sediment trapping ability is attributed to surface coarseness caused by a dense understory. Ordination of the overstory vegetation plots using their importance values (relative basal area and relative frequency) show no separation of vegetative communities by treatment, but rather, by block along an elevational and hydrologic gradient. Ordination of understory vegetation plots using their average percent cover show a separation of communities between the harvested and unharvested treatments along a light availability gradient.
The treatment differences found two and three growing seasons after harvest were no longer present eight growing seasons after harvest (except cellulose decomposition rate). This indicates that the impacts to soil and water caused by harvesting and extraction in this swamp were short-lived. Harvesting increased sediment trapping in the forest - a function often considered an important wetland environmental service. Additionally, the vegetative structure was changed, though the composition was not adversely affected by harvesting with either extraction method.
In the long-term, neither skidder or helicopter extraction cause greater damage to the physical and biological properties of the swamp. This study suggests that except for extreme circumstances, such as prolonged ponding, skidder extraction can be used, and helicopter extraction (which is much more expensive) saved for the most inaccessible sites.
Literature Cited
Pavel, C.M. 1993. An assessment of timber harvesting on the biomass, species diversity, and stand structure of the vegetation in a South Carolina bottomland hardwood forest. Masters Thesis, North Carolina State University, Raleigh, 78 pp.
Perison, D.M. 1997. The effects of timber harvest and soil disturbance on soil processes and water quality in a South Carolina blackwater swamp. Ph.D., North Carolina State University, Raleigh.