Study

Restoration response of relict broadleaf marshes to increased water depths

Actions

This study is summarised as evidence for the following.

Action Category

Raise water level to restore degraded freshwater marshes

Action Link
Marsh and Swamp Conservation

Actively manage water level: freshwater marshes

Action Link
Marsh and Swamp Conservation
  1. Raise water level to restore degraded freshwater marshes

    A replicated, before-and-after study in 1998–2008 of two marshes on a floodplain in Florida, USA (Toth 2010) reported that raising water levels by filling drainage/flood control channels had mixed effects on cover of plant groups, but consistently reduced overall plant species richness and vegetation cover. Unless specified, results summarized for this study are not based on assessments of statistical significance. Before intervention, both marshes were dominated by wetland-characteristic grasses (24–52% cover) with some vegetation characteristic of broadleaf marshes (12–30% cover). Over the first 4–6 years after raising water levels, one marsh remained dominated by wetland-characteristic grasses (18–50% cover). The other became dominated by broadleaf marsh vegetation (11–68% cover). In subsequent years, both marshes were dominated by a mix of Peruvian water primrose Ludwigia peruviana (9–70%) and broadleaf marsh vegetation (4–34% cover). Total vegetation cover and species richness were variable over time, but often lower after intervention (49–97% cover; 7–20 species/100 m2) than before (77–93% cover; 16–26 species/100 m2). Plant diversity was statistically similar before and after intervention in both marshes (data not reported). Methods: Between 1999 and 2001, the water level was raised in two degraded marshes in Sections A and C of the Kissimmee River floodplain. This was achieved by “eliminating” a drainage ditch (one marsh) and dechannelizing the river (other marsh). Plant species and their cover were surveyed before intervention (from summer 1998) and for approximately seven years after (until summer 2008), in three 100-m2 plots/marsh.

    (Summarised by: Nigel Taylor)

  2. Actively manage water level: freshwater marshes

    A before-and-after study in 1984–1990 of two marshes on a floodplain in Florida, USA (Toth 2010) reported that following restoration of seasonal water level fluctuations, plant species richness and diversity decreased in both marshes, but the abundance of individual plant species responded differently in each marsh. Unless specified, statistical significance was not assessed. Plant species richness declined in both marshes, from 6.1–7.3 species/m2 in the year before intervention to 5.0–5.4 species/m2 after five years of fluctuating water levels – although this decline was only statistically significant in one marsh. Species diversity also declined in both marshes (data reported as a diversity index). The frequency of individual plant species, including those characteristic of broadleaf marshes, did not always respond consistently in both marshes. For example, pickerelweed Pontederia cordata frequency declined in one marsh (from 65% to 21% of quadrats) but was stable in the other (87% before and after). Methods: From September 1985, water level fluctuations were restored to two degraded marshes in Section B of the Kissimmee River floodplain, by actively managing the amount of water released into the river. Before intervention the marshes were nearly permanently flooded. After intervention, there were clear wet and dry seasons, with and without standing water. Plant species and their cover were surveyed before intervention (summer 1984–1985) and for five years after (until September 1990), in 1-m2 quadrats along two transects (37–48 quadrats/transect).

    (Summarised by: Nigel Taylor)

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