Use low-impact methods to harvest vegetation: freshwater swamps

Overall effectiveness category Unknown effectiveness (limited evidence)
Number of studies: 1
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How is the evidence assessed?

Effectiveness

40%
Certainty

20%
Harms

5%

Calculating overall effectiveness category

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.}

Study locations

Key messages

One study evaluated the effects, on vegetation, of using supposedly low-impact methods to harvest vegetation in freshwater swamps. The study was in the USA.

VEGETATION COMMUNITY

Overall richness/diversity (1 study): One controlled study in a freshwater swamp in the USA reported that after seven years, a plot where logs had been extracted by helicopter only contained fewer plant species than a plot where logs had been extracted by helicopter and ground vehicles.

VEGETATION ABUNDANCE

Overall abundance (1 study): One controlled study in a freshwater swamp in the USA reported that after seven years, a plot where logs had been extracted by helicopter only contained less overall plant biomass than a plot where logs had been extracted by helicopter and ground vehicles. This was also true for the overstory and ground layers separately. However, overstory tree density did not significantly differ between helicopter-extracted and vehicle-extracted plots.
Individual species abundance (1 study): The same study found that the abundance of some individual plant species – particularly swamp ash Fraxinus caroliniana and water tupelo Nyssa aquatica – significantly differed between helicopter-extracted and vehicle-extracted plots.

VEGETATION STRUCTURE

Height (1 study): One controlled study in a freshwater swamp in the USA found that after seven years, the average height of the overstory was similar in a plot where logs had been extracted by helicopter only and a plot where logs had been extracted by helicopter and ground vehicles.
Diameter, perimeter, area (1 study): The same study found that after seven years, the average stem diameter of overstory trees was similar in helicopter-extracted and vehicle-extracted plots.

About key messages

Key messages provide a descriptive index to studies we have found that test this intervention.

Studies are not directly comparable or of equal value. When making decisions based on this evidence, you should consider factors such as study size, study design, reported metrics and relevance of the study to your situation, rather than simply counting the number of studies that support a particular interpretation.

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
Referenced paper
Aust W.M., Schoenholtz S.H., Zabest T.W. & Szabo B.A. (1997) Recovery status of a tupelo-cypress wetland seven years after disturbance: silvicultural implications. Forest Ecology and Management, 90, 161-169.

Please cite as:

Taylor N.G., Grillas P., Smith R.K. & Sutherland W.J. (2021) Marsh and Swamp Conservation: Global Evidence for the Effects of Interventions to Conserve Marsh and Swamp Vegetation. Conservation Evidence Series Synopses. University of Cambridge, Cambridge, UK.

Where has this evidence come from?

List of journals searched by synopsis

All the journals searched for all synopses

This Action forms part of the Action Synopsis:

Marsh and Swamp Conservation

Related Actions

Actions	Effectiveness	Category
Use low-impact methods to harvest vegetation: brackish/saline swamps Action Link	No evidence found (no assessment)	Synopsis Link
Reduce frequency of vegetation harvest: brackish/saline swamps Action Link	No evidence found (no assessment)	Synopsis Link
Change season/timing of vegetation harvest: freshwater swamps Action Link	No evidence found (no assessment)	Synopsis Link