Actively manage water level before/after planting non-woody plants: freshwater wetlands

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

Source countries

Key messages

  • Three studies evaluated the effects, on vegetation, of actively managing water levels in freshwater wetlands planted with emergent, non-woody plants. All three studies were in the USA. Two studies used the same experimental wet basins but planted different species.

VEGETATION COMMUNITY

  • Overall richness/diversity (1 study): One study in a freshwater marsh in the USA found that amongst plots amended with wetland soil, those flooded for longer contained fewer emergent plant species over the rest of the growing season following drawdown.
  • Characteristic plant richness/diversity (1 study): The same study found that amongst plots amended with wetland soil, those flooded for longer contained fewer wetland-characteristic plant species over the rest of the growing season following drawdown.

VEGETATION ABUNDANCE 

  • Overall abundance (1 study): One study in a freshwater marsh in the USA found that amongst plots amended with wetland soil, those flooded for longer developed more submerged vegetation biomass before drawdown, but developed less emergent vegetation (biomass and stem density) over the rest of the growing season after drawdown.
  • Individual species abundance (2 studies): Two studies quantified the effect of this action on the abundance of individual plant species. For example, one controlled, before-and-after study in wet basins in the USA found that the effect of mimicking a natural (falling) water regime on lake sedge Carex lacustris biomass and density, in the three years after planting, depended on the year and various environmental factors (e.g. planting density, elevation and weeding of competitors).

VEGETATION STRUCTURE

  • Height (2 studies): Two controlled studies in wet basins in the USA examined the effect of mimicking a natural (falling) water regime, compared to a stable or rising regime, on the height of sedges over three years after planting. One of the studies found no significant effect on the height of tussock sedge Carex stricta in three of three years. The other study found that the effect on the height of lake sedge Carex lacustris varied within and between years.

OTHER

  • Survival (2 studies): Two controlled studies in wet basins in the USA examined the effect of mimicking a natural (falling) water regime, compared to a stable or rising regime, on the survival of sedges Carex over three years after planting. The precise effect depended on the year and/or plot elevation. In the first year, sedge survival was typically lower under the falling regime.

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 study in 1990 in a created freshwater marsh in Texas, USA (McKnight 1992) found that the length of flooding and drawdown on shelves amended with wetland soil affected the abundance, richness and composition of submerged and emergent vegetation. The longer plots were flooded, the more submerged vegetation biomass they contained (five months flooding: 191; three months: 46; one month: 0 g/m2). In contrast, for emergent vegetation that matured after drawdown, plots flooded for longer contained less above-ground biomass (five months: 99; three months: 134; one month: 769 g/m2), fewer stems (five months: 1,126; three months: 340; one month: 1,851 stems/m2), fewer species (five months: 17; three months: 17; one month: 29 species/0.25 m2) and fewer species that “prefer wet or semi-wet soils” (five months: 10; three months: 8; one month: 13 species/0.25 m2). The duration of flooding also affected the biomass of individual plant species (see original paper for data). Methods: The study used a created marsh containing three shelves of differing height. In late February 1990, wetland soil was added to all shelves and then they were flooded. The water level was then drawn down in stages, exposing one shelf after one month, one after three months and one after five months. Vegetation was surveyed from 11–20 plots/shelf (each 0.25 m2). Submerged vegetation was collected immediately before drawdown. Emergent vegetation was collected once “mature”. Vegetation was dried before weighing.

    Study and other actions tested
  2. A controlled, before-and-after study in 1995–1997 in three recently excavated wet basins in Minnesota, USA (Budelsky & Galatowitsch 2000) found that the effect of simulating a naturally falling water level on survival, abundance and height of planted lake sedge Carex lacustris varied across time and/or environmental conditions. For example, in the first year after planting, sedge survival was lower under a falling water regime (82%) than under a rising water regime (97%). Water regime did not significantly affect survival rates in the second and third year after planting. In contrast, sedge biomass and stem density were not significantly increased by a falling water regime in the first year after planting (e.g. falling: 20; stable: 29; rising: 57 g/m2 biomass) but were higher under a falling water regime by the third year (e.g. biomass falling: 953; stable: 536; rising: 573 g/m2 biomass). In the third year, sedges in plots under a falling regime were at least as tall (average: 55–100 cm; maximum: 88–158 cm) as sedges under a stable or rising regime (average: 27–102 cm; maximum: 54–147 cm). Methods: The study used three wet basins (same as in Study 3), each of which was managed with a different water regime: falling, stable or rising throughout the growing season. The falling regime was most similar to natural conditions in local depressional wetlands (deepest flooding at start of growing season). In May 1995, nursery-reared lake sedge was planted into 48 bare, 5-m2 plots (16 plots/basin; 10 or 45 plants/plot). The plots were situated at four different elevations, and half of the plots in each basin were weeded (colonizing plants removed) throughout the study. Vegetation was surveyed through the 1995, 1996 and 1997 growing seasons.

    Study and other actions tested
  3. A controlled, before-and-after study in 1995–1997 in three recently excavated wet basins in Minnesota, USA (Budelsky & Galatowitsch 2004) found that found simulating a naturally falling water level had no significant effect on the height of planted tussock sedge Carex stricta, and that effect on sedge survival depended on other factors. In each of three years, the height of planted sedges was statistically similar under a falling, rising or stable water regime (data not reported). Sedge survival was significantly affected by water regime in the first and second years after planting (but not the third), but the effect depended on plot elevation. For example, first-year survival was >98% under all water regimes in higher/drier plots, but ranged from 47% (falling regime) to 96% (rising regime) in lower/wetter plots. The study also reported data on biomass/plant and shoot number/plant. The effect of water regime on these metrics depended on time since planting, elevation and/or weeding (see original paper). Methods: The study used three wet basins (same as in Study 2), each of which was managed with a different water regime: falling, stable or rising throughout the growing season. The falling regime was most similar to natural conditions in local depressional wetlands (deepest flooding at start of growing season). In May 1995, nursery-reared tussock sedge was planted into 48 bare, 5-m2 plots (16 plots/basin; 10 or 45 plants/plot). The plots were situated at four different elevations, and half of the plots in each basin were weeded (colonizing plants removed) throughout the study. Vegetation was surveyed through the 1995, 1996 and 1997 growing seasons.

    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.

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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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