Ducks Unlimited NZ
Wednesday, 18 August 2021 22:09

Wetland restoration under the microscope

Written by Shannon Bentley
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Shannon Bentley, right, and a colleague, Nicki Papworth, taking plant measurements in one of the wetlands. Shannon Bentley, right, and a colleague, Nicki Papworth, taking plant measurements in one of the wetlands.

Restored wetlands on private farms deliver ecosystem services and increase diversity, with varied results, a recent study has found. Shannon Bentley, a master’s student at Victoria University and the first recipient of a Wetland Care Scholarship funded by Ducks Unlimited NZ, is studying how wetland restoration on farms changes plant, soil, and microbial characteristics. In 2018-2019, as a part of the research group, Wetlands for People and Place, she sampled 18 privately restored wetlands and paired unrestored wetlands on farms in the Wairarapa.

For her master’s thesis, she analysed the wetland plant communities, soil physiochemical characteristics, and soil microbial communities to understand how they change with restoration. She found that wetland restoration on private property shifts plant, soil, and microbial characteristics towards desirable remnant wetland conditions. She also showed that the outcomes of wetland restoration varied within and between wetlands.

More than 40 per cent of New Zealand is held in private ownership, and private property holds huge potential for wetland restoration, containing 259,000km of stream length (Daigneault, Eppink, & Lee, 2017). Additionally, wetland degradation is extensive in lowland environments which are primarily in private ownership.

The outcomes of wetland restoration undertaken by landowners’ own prerogatives are poorly tracked. Private restoration is driven by personal preferences and finances, so the extent and form of restoration are varied. For example, the 18 wetlands sampled in Shannon’s study were restored in many different contexts and using many different techniques.

Wetlands differed in time since initial restoration (6 months to 42 years ago), size (0.4ha to 33.7ha), upstream watershed area (4ha to 2,263ha), dominant plant community (woody v herbaceous), and the number of restoration techniques used (2 to 8).Wetland restoration is beneficial for a number of reasons, but particularly for regaining wetland ecosystem services and increasing native biodiversity. Ecosystem services are ecological processes and functions that have beneficial outcomes for humans.

Compared with all other ecosystems, wetlands produce the highest levels of ecosystem services per unit area. This high production of ecosystem services is due to wetlands’ unique biology and geology resulting from their position at the interface of water and land. Wetlands are particularly effective at producing the ecosystem services of water purification, flood abatement and climate regulation through carbon sequestration.

Shannon found that with restoration, soils regained wetland traits, providing more ecosystem services. The restored soils had higher carbon content, lower bulk density, and lower plant-available phosphorous. Increased soil carbon content shows the carbon sequestration potential of soil expands with restoration.

Additionally, reduced plant-available phosphorous indicates restored wetlands can take up phosphorous and improve downstream water quality.

And finally, with increased carbon and reduced bulk density, water moves through the wetland soils slower to reduce peak flood heights.

Shannon found wetland soil microbes increased in biomass and fungal dominance after restoration. These changes, in part, explained the increase in capacity for wetlands to deliver ecosystem services.

Soil microbes are responsible for decomposition and nutrient cycling. A greater mass of microbes means restored wetlands have more capacity for biogeochemical cycling and decomposition, which can accelerate processes such as carbon burial, as
seen with the increased carbon in restored wetland soils compared with unrestored soils.

The presence of arbuscular mycorrhizal fungi (AMF) also increased after restoration. AMF help plants survive and reduce phosphorous in soils, thus contributing to cleaner waterways.

Each restored wetland showed a lot of variability of soil and microbial responses within and across wetlands. Because wetlands are found along a hydrological gradient between saturated soils and drier, upland soils, the microbial and soil properties differed according to the landscape position.

Soils close to the water’s edge, as a result of being saturated, had more carbon, microbial biomass, and more fungal biomass, and were less dense. By examining the plant communities, Shannon found that, after restoration, plant diversity increased within the plot and across the landscape. This means that with restoration, habitat heterogeneity increases, a beneficial outcome that increases ecosystem stability and the number of ecosystem services produced.

Additionally, she found that wetland soils and plant and microbial communities showed different levels of recovery that were not consistent with the length of time that they had been restored.

Some projects’ soil and microbial characteristics recovered faster than others. The main difference between fast and slower-recovery wetlands was the hydrological regime.

Restoration projects that occurred on isolated hydrological systems such as depressional, rainwater-fed wetlands took far longer to re-establish remnant wetland conditions. Projects undertaken on flowing hydrological systems such as springs and streams underwent successional processes faster to establish wetland conditions that have a higher production of ecosystem services.

The study has shown that restoring wetlands on private farms increases the ecosystem’s ability to simultaneously produce multiple different ecosystem services and support more biodiversity.

As food production demands continue to rise simultaneously as land becomes more scarce, agricultural systems are becoming more industrialised and intensive. This is placing pressure on natural ecosystems, as seen by reduced water quality, native habitat, and changing landscapes from carbon sinks to sources.

There is increasing recognition that we need mixed agroecosystems so food production does not compromise other ecosystem services.

Wetland restoration is gaining significant traction as a solution to issues surrounding water quality, climate regulation, flooding, and loss of native habitat, and this study has shown that private restoration is effective tool to do just this.

Shannon concludes her report by saying: “I would like to thank all the landowners that generously allowed us to sample their wetlands; each site we visited was so beautiful and unique.

“I thank Ducks Unlimited for the funding that allowed me to do this research. I also thank the Holdsworth Foundation, the Sir Hugh Kawharu Foundation, Wairarapa Moana Trust, and Victoria University for the financial support I have received.

“Finally, I would like to thank my supervisors Dr Julie Deslippe and Dr Stephanie Tomscha for their guidance and help in producing this research.”

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