Study

Effects of managed impoundments and herbivory on wetland plant production and stand structure

  • Published source details Johnson R.L.A. & Foote A.L. (2005) Effects of managed impoundments and herbivory on wetland plant production and stand structure. Wetlands, 25, 38-50.

Actions

This study is summarised as evidence for the following.

Action Category

Actively manage water level: brackish/salt marshes

Action Link
Marsh and Swamp Conservation

Exclude wild vertebrates: brackish/salt marshes

Action Link
Marsh and Swamp Conservation
  1. Actively manage water level: brackish/salt marshes

    A replicated, paired, before-and-after, site comparison study in 1991–1994 of four brackish marshes in Louisiana, USA (Johnson Randall & Foote 2005) found that active water level management typically had no significant effect on the height of the two dominant plant species, but had mixed effects on density. Before intervention, height and density of both saltmeadow cordgrass Spartina patens and American bulrush Schoenoplectus americanus were similar in all marshes. Over the three following years, managed and unmanaged marshes contained cordgrass stems of a similar height in eight of eight comparisons, and bulrush stems of a similar height in five of eight comparisons (data reported as height categories). In the other three comparisons, bulrush stems were shorter in managed marshes. Managed and unmanaged marshes contained a similar density of cordgrass in eight of eight comparisons (managed: 280–1,420 stems/m2; unmanaged: 350–1,520 stems/m2) but a lower density of bulrush in four of eight comparisons (for which managed: 20–60 stems/m2; unmanaged: 80–210 stems/m2). In three of the other four comparisons, bulrush density was similar in managed and unmanaged marshes. Methods: From 1992, water levels were controlled (drained in spring, rewetted in summer and flooded in autumn and winter) within two impounded marshes. In two adjacent marshes, the water level was not managed (no drawdown). Plant stems were counted and measured in each marsh five times before water management began (1991–1992) and eight times after (1992–1994). Some monitoring plots, both managed and unmanaged, were also fenced to exclude herbivores.

    (Summarised by: Nigel Taylor)

  2. Exclude wild vertebrates: brackish/salt marshes

    A replicated, paired, controlled study in 1991–1994 in four brackish marshes in Louisiana, USA (Johnson Randall & Foote 2005) found that fencing to exclude nutria Myocastor coypus increased the height and density of American bulrush Schoenoplectus americanus but not saltmeadow cordgrass Spartina patens. Bulrush was taller in fenced plots than open plots in 14 of 14 comparisons over 42 months (data reported as height categories). There were more bulrush stems in fenced plots in 12 of 14 comparisons (for which fenced: 70–240 stems/m2; open: 10–60 stems/m2). Cordgrass was taller in fenced than open plots in only 4 of 14 comparisons, with no significant difference in the others (data reported as height categories). There were more cordgrass stems in fenced plots in only 1 of 14 comparisons, with no significant difference in the others (for which fenced: 360–1,350 stems/m2; open: 270–1,750 stems/m2). Methods: In April 1991, forty 9-m2 plots were established across four brackish marshes (10 plots/marsh). Twenty of the plots (five plots/marsh) were fenced (4 x 6 cm plastic-coated mesh) to exclude nutria (and other large mammals). The other 20 plots were left open. Every 3–6 months for three and a half years, stems of the dominant plant species were counted and measured in two 25 x 25 cm quadrats/plot.

    (Summarised by: Nigel Taylor)

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