Create mounds or hollows before planting non-woody plants: freshwater wetlands

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

Effectiveness

30%
Certainty

32%
Harms

10%

Calculating overall effectiveness category

Background information and definitions

This action involves creating discrete mounds (e.g. by adding blocks of soil, bundles of sticks, other coarse woody debris) or hollows (e.g. by excavation) before planting marsh or swamp vegetation. The scale of this action falls somewhere between reprofiling/relandscaping (large-scale landscape features, tens of metres wide) and disturbing the ground surface (which may create small scale mounds or hollows, millimetres or a few centimetres wide/deep).

Often, this action aims to mimic the natural microtopography of marshes or swamps, which can be created by sediment accumulation, erosion, tree fall, root growth or animal activity (Vivian-Smith 1997, Bruland & Richardson 2005). Microtopography can increase plant diversity, because the different microclimates or microelevations may support different species (Vivian-Smith 1997). Planting into mounds can be useful if seedlings would otherwise be flooded too deeply or for too long (Zamith & Scarano 2010). Large woody debris will also add nutrients and organic matter to a site as it decomposes.

Studies that examine the effects of planting into existing microtopographic features (e.g. mounds), even if they compare effects between different kinds of features, are not summarized as evidence here (e.g. Raulings et al. 2007; Sleeper & Ficklin 2016).

Related actions: Create mounds or hollows, other than to complement planting; Reprofile/relandscape before planting; Disturb soil/sediment surface before planting without creating discrete mounds and/or hollows.

^{Bruland G.L. & Richardson C.J. (2005) Hydrologic, edaphic, and vegetative responses to microtopographic reestablishment in a restored wetland. Restoration Ecology, 13, 515–523.}

^{Raulings E.J., Boon P.I., Bailey P.C., Roache M.C., Morris K. & Robinson R. (2007) Rehabilitation of swamp paperbark (Melaleuca ericifolia) wetlands in south-eastern Australia: effects of hydrology, microtopography, plant age and planting technique on the success of community-based revegetation trials. Wetlands Ecology and Management, 15, 175–188.}

^{Sleeper B.E. & Ficklin R.L. (2016) Edaphic and vegetative responses to forested wetland restoration with created microtopography in Arkansas. Ecological Restoration, 34, 117–123.}

^{Vivian-Smith G. (1997) Microtopographic heterogeneity and floristic diversity in experimental wetland communities. Journal of Ecology, 85, 71–82.}

^{Zamith L.R. & Scarano F.R. (2010) Restoration of a coastal swamp forest in southeastern Brazil. Wetlands Ecology and Management, 18, 435–448.}

Study locations

Key messages

Two studies evaluated the effects, on vegetation, of creating mounds or hollows in freshwater wetlands before planting emergent, non-woody plants. Both studies were in the same wetland in the USA, but used different experimental set-ups.

VEGETATION COMMUNITY

VEGETATION ABUNDANCE

Individual species abundance (2 studies): Two replicated, randomized, paired, controlled studies in a wetland in the USA found that tussock sedge Carex stricta cover was typically similar across plots, after two growing seasons, whether sedges were planted into created mounds or hollows, or planted into flat ground.

VEGETATION STRUCTURE

Individual plant size (2 studies): Two replicated, randomized, paired, controlled studies in a wetland in the USA found that planting tussock sedges Carex stricta into created mounds or hollows had no significant effect on their individual biomass, after 1–2 growing seasons, when compared to planting into flat ground.

OTHER

Survival (2 studies): Two replicated, randomized, paired, controlled studies in a wetland in the USA found that planting tussock sedge Carex stricta into created mounds or hollows did not improve, and typically reduced, its survival rate compared to planting into flat ground. Survival was monitored after 1–2 growing seasons.
Growth (2 studies): The same studies found that planting tussock sedge Carex stricta into created mounds or hollows typically had no significant effect on its growth rate, over 1–2 growing seasons, compared to planting into flat ground.

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, randomized, paired, controlled study in 2012–2013 in a freshwater wetland in Wisconsin, USA (Doherty & Zedler 2015) found that creating mounds before planting tussock sedge Carex stricta did not improve survival rates, and typically had no significant effect on sedge growth, biomass or cover. After two growing seasons, survival rates were lower for sedges planted in mounds than on flat ground in seven of eight comparisons (for which mounds: 27–93%; flat: 100%). There was typically no significant difference between treatments in sedge growth rate (11 of 16 comparisons; see original paper for data). In three of the other five growth rate comparisons, all in the second growing season after planting, sedges grew faster in mounds (0.021–0.028 mm/mm/day) than on flat ground (0.013 mm/mm/day). In most cases, there was also no significant difference between treatments for final above-ground sedge biomass (four of four comparisons, for which mounds: 5–34 g/plant; flat: 7–39 g/plant) and final sedge cover (three of four comparisons, for which mounds: 11–46%; flat: 38–62%). Methods: In spring 2012, thirty 1-m² plots were established, in six sets of five, in a wetland undergoing restoration. Soil mounds were built in 24 of the plots (five random plots/set). Mounds were either 8 cm tall, 16 cm tall, 16 cm tall with 50% woodchip, or 32 cm tall. The other six plots were left as flat ground. Five nursery-reared tussock sedges were planted into each plot (one plant/mound in plots with mounds) then regularly watered and weeded. Survival and above-ground biomass of planted sedges, and total tussock sedge cover, were surveyed in June–August 2013. Biomass was dried before weighing. Growth rates were calculated from leaf lengths measured in 2012 and 2013. This study used the same site as (2), but a different experimental set-up.
Study and other actions tested
Referenced paper
Doherty J.M. & Zedler J.B. (2015) Increasing substrate heterogeneity as a bet-hedging strategy for restoring wetland vegetation. Restoration Ecology, 23, 15-25.
A replicated, randomized, paired, controlled study in 2013 in a freshwater wetland in Wisconsin, USA (Doherty & Zedler 2015) found that creating mounds or hollows before planting tussock sedge Carex stricta typically had no significant effect on sedge growth, biomass or cover, and reported that creating hollows reduced survival rates. After one growing season, sedges planted in hollows had a lower survival rate (63%) than sedges planted on flat ground (≥90%; data for mounds not reported; statistical significance not assessed). The treatments had no significant effect, compared to planting in flat ground, on sedge growth rate (mounds: 0.026–0.028 mm/mm/day; hollows: 0.032–0.035 mm/mm/day; flat: 0.027–0.035 mm/mm/day), final above-ground sedge biomass (g/plant; data not reported), or final sedge cover (six of six comparisons, for which mounds: 11–38%; hollows: 3–11%; flat: 15%). Methods: In spring 2013, twenty-four 1-m² plots were established, in six sets of four, in a wetland undergoing restoration. Soil mounds (8 cm tall or 16 cm tall) were built in 12 of the plots (two random plots/set). Square hollows (10 cm deep; 15 cm across) were dug in six of the plots (one random plot/set). The final six plots were left as flat ground. Five nursery-reared tussock sedges were planted into each plot (one plant/mound or hollow where relevant). Survival and above-ground biomass of planted sedges, and total tussock sedge cover, were surveyed in June–August 2013. Biomass was dried before weighing. Growth rates were calculated from leaf lengths measured in 2013. This study used the same site as (1), but a different experimental set-up.
Study and other actions tested
Referenced paper
Doherty J.M. & Zedler J.B. (2015) Increasing substrate heterogeneity as a bet-hedging strategy for restoring wetland vegetation. Restoration Ecology, 23, 15-25.

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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