Add surface mulch: brackish/salt marshes

Overall effectiveness category Unknown effectiveness (limited evidence)
Number of studies: 1
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How is the evidence assessed?

Effectiveness

40%
Certainty

30%
Harms

0%

Calculating overall effectiveness category

Background information and definitions

Organic mulches (i.e. remains or waste products of living organisms) can be placed on the surface of wetlands to stabilize temperatures and humidity, and provide shade to germinating plants. This may create a more hospitable environment for vegetation establishment and growth. Mulches can also help to manage acidification by excluding oxygen from sediments and stimulating microbial processes that neutralize the acidity (Baldwin 2011). Carbon-rich organic matter may also help to shift the competitive balance away from invasive species in polluted environments, enriched in certain nutrients (Reever Morghan & Seastedt 1999; Tilman et al. 1999; Perry et al. 2004). Examples of substances than can be used as mulches include compost, straw, seagrass leaves and seaweed (macroalgae).

Caution: It may be necessary to sterilize mulch before applying it, with heat or radiation, to kill propagules of undesirable plants. Adding organic matter as a mulch may be less labour intensive than mixing it into the soil or sediment, but increases the risk of the material being washed away.

To be summarized as evidence for this action, studies must have added mulch that is largely free from plant propagules. The mulch should be used to help or manage existing vegetation, such as remnant patches of vegetation, or seedlings that germinate from seeds already present.

Related actions: Use covers/barriers to control problematic plants; Add cover other than mulch during marsh restoration/creation; Add surface mulch to complement planting.

^{Baldwin D. (2011) National Guidance for the Management of Acid Sulfate Soils in Inland Aquatic Ecosystems, Environment Protection and Heritage Council and the Natural Resource Management Ministerial Council, Australia.}

^{Perry L.G., Galatowitsch S.M. & Rosen C.J. (2004) Competitive control of invasive vegetation: a native wetland sedge suppresses Phalaris arundinacea in carbon-enriched soil. Journal of Applied Ecology, 41, 151–162.}

^{Reever Morghan K.J. & Seastedt T.R. (1999) Effects of soil nitrogen reduction on nonnative plants in restored grasslands. Restoration Ecology, 7, 51–55.}

^{Tilman E.A., Tilman D., Crawley M.J. & Johnston A.E. (1999) Biological weed control via nutrient competition: potassium limitation of dandelions. Ecological Applications, 9, 103–111.}

Study locations

Key messages

One study evaluated the effects, on vegetation, of using organic mulch to restore or create brackish/salt marshes. The study was in Australia.

VEGETATION COMMUNITY

Overall richness/diversity (1 study): One replicated, paired, controlled study on a sandflat in Australia found that mulched and unmulched plots had similar plant species richness over two years.

VEGETATION ABUNDANCE

Herb abundance (1 study): One replicated, paired, controlled study on a sandflat in Australia found that mulched plots were more likely to contain glasswort Sarcocornia quinqueflora than unmulched plots, after 20 months. However, mulching had no significant effect on glasswort biomass after 20 months, and typically had no significant effect on glasswort cover over two years.

VEGETATION STRUCTURE

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, paired, controlled study in 1999–2001, aiming to create a saltmarsh on a lagoon sandflat in New South Wales, Australia (Chapman & Roberts 2004) found that mulched plots were more likely to contain glasswort Sarcocornia quinqueflora than unmulched plots, but that mulching typically had no significant effect on glasswort biomass, glasswort cover or plant species richness (because of high variability between plots). After 20 months, glasswort was present in a significantly greater proportion of mulched plots (89%) than unmulched plots (56%). However, above-ground glasswort biomass was statistically similar in mulched (9–19 g/units not clear) and unmulched plots (8–11 g/units not clear). Over the two years following intervention, glasswort cover was statistically similar under each treatment in 6 of 10 comparisons (for which mulched: 4–43%; unmulched: 4–19%). In the other comparisons, mulching either increased glasswort cover or had variable effects across the site (see original paper). Meanwhile, plant species richness was similar under each treatment in 11 of 11 comparisons (mulched: 1–4 species/4 m²; unmulched: 1–3 species/4 m²). Methods: In October 1999, forty 4-m² plots were established (in two sets of 20) on almost-bare sediment next to a patchy salt marsh. Twenty plots (10 plots/set) were mulched with seagrass and seaweed (5–10 cm layer). Mulch was reapplied when it thinned or was blown away. Plant species and cover were surveyed 10–11 times over two years after mulching. In June 2001, glasswort shoots were cut from 12 plots, then air-dried and weighed. Roughly half way through the study, some plots were damaged by motorbike riders (further details not reported).
Study and other actions tested
Referenced paper
Chapman M.G. & Roberts D.E. (2004) Use of seagrass wrack in restoring disturbed Australian saltmarshes. Ecological Management & Restoration, 5, 183-190.

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?

List of journals searched by synopsis

All the journals searched for all synopses

This Action forms part of the Action Synopsis:

Marsh and Swamp Conservation

Related Actions

Actions	Effectiveness	Studies	Category
Add cover other than mulch Action Link	No evidence found (no assessment)	0	Synopsis Link
Reprofile/relandscape: brackish/saline swamps Action Link	Likely to be beneficial	5	Synopsis Link
Raise water level to restore/create brackish/salt marshes from other land uses Action Link	Unknown effectiveness (limited evidence)	2	Synopsis Link