Use low-impact methods to harvest vegetation: freshwater swamps
Overall effectiveness category Unknown effectiveness (limited evidence)
Number of studies: 1
Background information and definitions
The impact of harvesting vegetation could be reduced by switching to supposedly lower-impact methods or equipment. For example, vehicles used for harvesting can compress, sink into and create ruts in wet soils. Lower-impact alternatives include: using specialised tracked vehicles or hovercraft which exert less pressure on the ground (Dubowski et al. 2013); ensuring vehicles are not overloaded and heavy (Schröder et al. 2015); and extracting harvested vegetation by hand or helicopter. When logging in swamps, directional felling may reduce the amount of collateral damage when trees fall.
To be included as evidence for this action, studies must have compared low- and high-impact harvesting methods, not just reported the effects of methods claimed to be low-impact.
Dubowski A.P., Zembrowski K., Rakowicz A., Palowski T., Weymann S. & Wojnilowicz L. (2013) Developing new-generation machinery for vegetation management on protected wetlands in Poland. Mires and Peat, 13, Article 11.
Schröder C., Dahms T., Paulitz J., Wichtmann W. & Wichmann S. (2003) Towards large-scale paludiculture: addressing the challenges of biomass harvesting in wet and rewetted peatlands. Mires and Peat, 16, Article 13.
Supporting evidence from individual studies
A controlled study in 1986–1993 in a freshwater swamp in Alabama, USA (Aust et al. 1997) reported that a plot where logs were extracted by helicopter only contained fewer plant species and less plant biomass seven years later than a plot where logs were also extracted by ground vehicles, but that both treatments had a similar overstory tree density, diameter and height. Unless specified, results summarized for this study are not based on assessments of statistical significance. After seven years, helicopter-extracted plots contained 28 plant species, compared to 31 in vehicle-extracted plots. Helicopter-extracted plots contained only 46,748 kg/m2 dry above-ground plant biomass (overstory: 41,373; understory: 173; ground: 5,202 kg/m2), compared to 65,979 kg/m2 in vehicle-extracted plots (overstory: 60,222; understory: 108; ground: 5,649 kg/m2). For overstory trees, there were no significant differences between treatments in density (helicopter: 3,539; vehicle: 3,829 trees/ha), average diameter (helicopter: 6.2; vehicle: 6.9 cm) or average height (helicopter: 7.6; vehicle: 7.5 m). The study also compared all of these metrics for individual species. The main difference was that the overstory of helicopter-extracted plots contained significantly more swamp ash Fraxinus caroliniana and significantly less water tupelo Nyssa aquatica than vehicle-extracted plots (true for biomass and density; see original paper for data and full results). Methods: In summer 1993, vegetation was surveyed in two plots in a swamp. Both plots had been clear-cut (all trees felled) in autumn 1986. In one plot, some of the cut logs were removed by helicopter. In the other plot, after removing some cut logs by helicopter, other logs were dragged around the plot with a cable skidder to simulate extraction by vehicle.Study and other actions tested