Use low-impact methods to harvest vegetation: freshwater swamps

How is the evidence assessed?
  • Effectiveness
    not assessed
  • Certainty
    not assessed
  • Harms
    not assessed

Source countries

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

  1. 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
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?

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Marsh and Swamp Conservation

This Action forms part of the Action Synopsis:

Marsh and Swamp Conservation
Marsh and Swamp Conservation

Marsh and Swamp Conservation - Published 2021

Marsh and Swamp Synopsis

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