Facilitate tidal exchange before/after planting non-woody plants: brackish/saline wetlands

Overall effectiveness category Unknown effectiveness (limited evidence)
Number of studies: 2
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How is the evidence assessed?

Effectiveness

40%
Certainty

30%
Harms

0%

Calculating overall effectiveness category

Background information and definitions

This action involves increasing the frequency or duration of flooding by tides in areas planted with marsh or swamp vegetation. Tidal exchange could be facilitated by breaching sea walls or embankments, installing or widening culverts, excavating tidal creeks, or opening/closing sluice gates. Tidal wetlands may be brackish/saline (e.g. mangroves, coastal marshes) or freshwater (e.g. at the upstream end of estuaries, as in the Mississippi, Yangtze, and Elbe rivers; Baldwin et al. 2009).

^{Baldwin A.H., Barendregt A. & Whigham D. (2009) Tidal Freshwater Wetlands. Backhuys Publishers, Lieden.}

Study locations

Key messages

Two studies evaluated the effects, on vegetation, of facilitating tidal exchange in brackish/saline wetlands planted with emergent, non-woody plants. Both studies were in the same estuarine site in the USA.

VEGETATION COMMUNITY

VEGETATION ABUNDANCE

VEGETATION STRUCTURE

Height (1 study): One replicated, controlled study in a salt marsh in the USA found that planted California cordgrass Spartina foliosa reached a similar height, after three growing seasons, in areas with an excavated tidal creek and areas without a tidal creek.
Individual plant size (1 study): One replicated, controlled study in a salt marsh in the USAfound that planted salt marsh herbs reached a similar overall size, after 1–2 growing seasons, in areas with an excavated tidal creek and areas without a tidal creek.

OTHER

Survival (2 studies): Two replicated, controlled studies in a salt marsh in the USA found that planted salt marsh herbs typically had similar survival rates, after 1–2 growing seasons, in areas with an excavated tidal creek and areas without a tidal creek.

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 replicated, controlled study in 1999–2002 in an estuary in California, USA (O’Brien & Zedler 2006) found that excavating tidal creeks before planting salt marsh plants typically had no significant effect on their survival or size. Over the first year after initial planting, dead plants were replaced by new plants of a similar age. The number of replacements needed per plot, and for four of five species, was statistically similar in catchments with or without a tidal creek (data not reported). Over the second year of the study, plot-level survival was statistically similar under both treatments (creek: 3.3; no creek: 2.9 survivors/plot). The survival rate was similar under each treatment for three of five planted species (creek: 24–80%; no creek: 37–70%) but higher in catchments with a creek for the other two species (creek: 63–93%; no creek: 48–74%). Across both years, surviving plants were a similar size (combination of height and lateral extent) in catchments with and without tidal creeks. This was true for both plot- and species-level comparisons (data not reported). Methods: In winter 1999/2000, an area of estuarine sediment was reprofiled to intertidal elevations. A tidal creek was dug in three of six catchments within the site. In December 2000, 90 greenhouse-reared salt marsh plants were planted in each catchment (five plants, each a different species, in each of eighteen 2.24-m² plots/catchment). Some plots had also been tilled and/or amended with kelp compost. Colonizing vegetation was removed until October 2001. Until December 2001, dead planted vegetation was replaced. Replacements were counted. In August 2002, final survival, height and lateral spread of planted vegetation were recorded.
Study and other actions tested
Referenced paper
O’Brien E.L., & Zedler J.B. (2006) Accelerating the restoration of vegetation in a southern California salt marsh. Wetlands Ecology and Management, 14, 269-286.
A replicated, controlled study in 1999–2002 in an estuary in California, USA (O’Brien & Zedler 2006) found that excavating tidal creeks before planting California cordgrass Spartina foliosa did not significantly affect cordgrass density or height. After three growing seasons, the density of California cordgrass stems was statistically similar in catchments with or without a tidal creek. The same was true for the average height of those stems. No data were reported. Methods: In winter 1999/2000, twelve 15 x 30 m plots were established during the excavation of a salt marsh. Six plots were within the catchments of three excavated tidal creeks. The other six plots were in three catchments without tidal creeks. Kelp compost was also added to half of the plots. In February 2000, plugs of California cordgrass (range 50–100 cm tall) were dug from a nearby marsh and planted (2 m apart) in the plots. In August 2002, cordgrass stems were counted and measured in four 0.25-m² quadrats/plot (each with ≥15 stems).
Study and other actions tested
Referenced paper
O’Brien E.L., & Zedler J.B. (2006) Accelerating the restoration of vegetation in a southern California salt marsh. Wetlands Ecology and Management, 14, 269-286.

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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This Action forms part of the Action Synopsis:

Marsh and Swamp Conservation

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