Add below-ground organic matter before/after planting trees/shrubs: freshwater wetlands

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

Effectiveness

43%
Certainty

30%
Harms

10%

Calculating overall effectiveness category

Background information and definitions

This section involves adding organic matter (i.e. remains or waste products of living organisms) below the ground surface (i.e. by mixing it into the sediment, or placing it into holes) to areas planted with marsh or swamp vegetation.

Organic matter could increase the initial survival and/or growth rate of introduced plants, helping them to establish. Organic matter directly supplies nutrients to growing plants, supplies carbon and energy to soil organisms (which can indirectly increase nutrient availability), helps bind the soil together, retains water during dry periods, and mediates soil temperature (Donahue et al. 1983; Weil & Brady 2016). However, the soil organic matter content of wetland soils may be reduced by disturbance. For example, drainage allows oxygen into the soil, whilst reprofiling removes surface layers rich in organic matter (Bruland et al. 2006). Substances than can be used to add organic matter to wetland soils include compost, sewage sludge, wood chips and seaweed extract.

Note that many studies testing this action do not separate the effects of adding organic matter and disturbing the soil/sediment. We have included such studies, as well as those that do separate the effects of these actions with appropriate controls.

Related actions: Add below-ground organic matter, other than to complement planting; Add inorganic fertilizer before/after planting; Add surface mulch to complement planting.

^{Bruland G.L., Richardson C.J. & Whalen S.C. (2006) Spatial variability of denitrification potential and related soil properties in created, restored, and paired natural wetlands. Wetlands, 26, 1042–1056.}

^{Donahue R.L., Shickluna J.C. & Robertson L.S. (1983) Soils: An Introduction to Soils and Plant Growth, Fifth Edition. Prentice-Hall, Englewood Cliffs, NJ, USA.}

^{Weil R.R. & Brady N.C. (2016) The Nature and Properties of Soils, Fifteenth Edition. Pearson, USA.}

Study locations

Key messages

One study evaluated the effects, on vegetation, of adding below-ground organic matter to freshwater wetlands planted with trees/shrubs. The study was in the USA.

VEGETATION COMMUNITY

Community composition (1 study): One replicated, randomized, paired, controlled study in a created wetland in the USA found that amongst plots planted with tree seedlings, those amended with large amounts compost contained a plant community characteristic of drier conditions, three years later, than the community in unamended plots. The lowest compost dose had no significant effect on this outcome.
Overall richness/diversity (1 study): The same study found that amongst plots planted with tree seedlings, those amended with a large amount of compost had lower plant species richness and diversity, three years later, than unamended plots. Lower compost doses had no significant effect on either outcome.

VEGETATION ABUNDANCE

Overall abundance (1 study): One replicated, randomized, paired, controlled study in a created wetland in the USA found that amongst plots planted with tree seedlings, those amended with compost supported a similar overall vegetation biomass, three years later, to unamended plots.

VEGETATION STRUCTURE

Individual plant size (1 study): One replicated, randomized, paired, controlled study in a created wetland in the USA found that birch Betula saplings were larger, three years after planting seedlings, in plots amended with large amounts of compost than in unamended plots.

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 2002–2005 in a created freshwater wetland in Virginia, USA (Bailey et al. 2007) found that the effects of adding organic matter to plots planted with tree saplings depended on the dose added. Four different doses of organic matter were used. After approximately three years, plots receiving the three highest doses contained a plant community characteristic of drier – but still wetland – conditions than unamended plots (data reported as a wetland indicator index). In plots receiving the two highest doses, birch Betula spp. saplings were significantly larger than in unamended plots (size calculated as an index combining height, stem diameter and crown diameter; data not reported). Plots receiving the highest dose had lower plant species richness (5.3 species/m²) than unamended plots (7.4 species/m²). The same was true for plant diversity (data reported as a diversity index). In all other comparisons, there were no significant differences between amended and unamended plots (see original paper for data). Further, at all four doses, above-ground vegetation biomass did not significantly differ between amended plots (580–790 g/m²) and unamended plots (604 g/m²). Methods: In June 2002, twenty 14-m² plots were established, in four sets of five, on an 8-year-old created wetland. All plots were cleared and tilled. In July, organic matter (dry wood and garden compost) was mixed into the surface of 16 plots (four random plots/set, each with a different dose: 56, 112, 224 or 336 Mg/ha). The remaining four plots (one random plot/set) received no organic matter. In December 2002, ten saplings (five birch, five pin oak Quercus palustris) were planted in each plot and fertilized. Vegetation was surveyed in 2005. Plant species and cover were recorded monthly April–October. Surviving birch saplings were counted in June. Vegetation samples were cut in August, then dried and weighed.
Study and other actions tested
Referenced paper
Bailey D.E., Perry J.E. & Daniels W.L. (2007) Vegetation dynamics in response to organic matter loading rates in a created freshwater wetland in southeastern Virginia. Wetlands, 27, 936-950.

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
Introduce nurse plants to aid focal non-woody plants: brackish/saline wetlands Action Link	Unknown effectiveness (limited evidence)	1	Synopsis Link
Create mounds or hollows before planting non-woody plants: freshwater wetlands Action Link	Unknown effectiveness (limited evidence)	2	Synopsis Link
Reprofile/relandscape (before planting) Action Link	No evidence found (no assessment)	0	Synopsis Link