Managing the legacy of historical drainage ditches in boreal peatlands: exploring the effect on water quality and greenhouse gas fluxes

Managing the legacy of historical drainage ditches in boreal peatlands: exploring the effect on water quality and greenhouse gas fluxes

A northern story of peatland drainage

Wetlands represent one of the world's most vital ecosystems due to their important physical, chemical, and biological functions that play a critical role for climate change, biodiversity, hydrology, and human health. At northern latitudes, most wetlands are peatlands with seasonally water logged conditions, long-term carbon sequestration functions, and unique biodiversity. Historically, to increase acreage for agriculture or forestry, extensive peatland areas were artificially drained by digging drainage ditches. This resulted in many peatlands ceasing to be wetlands as the ditch-networks effectively removed the water-logged characteristic. In Sweden, the extent of peatlands has declined by up to 2 million hectares due to drainage and a total of approximately 1 million km of a century old artificial drainage channels still exist in this mostly forested landscape.

But, what happens to a peatland when it is drained?

When a peatland is drained, the groundwater level is lowered, increasing the aeration of formerly saturated peat soil and increasing decomposition, leading to changes in peat properties, increased microbial activity, larger carbon (C) and nitrogen (N) losses in water and greenhouse gas fluxes, resulting in ecosystem-scale changes that likely influence plant and animal communities and biodiversity. In many cases, productive forest has been established on the drained peatland, if the drainage was thorough enough that the water-logged characteristics disappeared and the nutrients available in the peatland were suitable. In other cases, the peatland remains unproductive, primarily because too nutrient poor conditions. Nevertheless, today, around 14% of Swedish productive forest is established in drained peatlands.

So, what should we do with drainage ditches today?

In Sweden, the decision for what to do with the large number of historical drainage ditches remains an open question. Most commonly, the management alternatives are either: 1) rewetting by filling and/or blocking the drainage ditches to return the water table to historic levels and bring back more pristine environmental conditions; or 2) cleaning the drainage ditches that have infilled with sediment and vegetation to keep the water table low and maintain the potential for forest biomass production.

We found the perfect setting for our experiment, the Trollberget Experimental Area

To determine how these management alternatives affect greenhouse gas fluxes and water quality, we found the perfect setting and established the Trollberget Experimental Area (TEA). Here, in 2018 we launched a unique research infrastructure with a fully replicated ditch management experiment applied at the catchment scale. The TEA consists of six side-by-side treated experimental catchments, two unproductive historically drained catchments that never established a productive forest due to nutrient limitations, and four productive historically drained catchments with established productive forest. The nearby Svartberget research infrastructures also provide reference conditions and contl catchments.

The Trollberget Experimental Area unproductive catchments before restoration works started (left) and, (right) one of the productive historically drained catchments with the ditch infilled with sediments. Photos: Virginia Mosquera (left) and Eliza Maher Hasselquist (right).

And, how did we use the TEA?

Two unproductive peatlands were used to test the effects of rewetting the peatland through ditch blocking. To do this, the sparse tree cover in the peatland was cut and removed and a crawling excavator took peat and trees from on-site to fill in the 100-year-old ditch. We monitored the effect in water quality parameters for more than two years before the restoration work was done in 2021 and we measured greenhouse gas fluxes after the rewetting. Alongside, two of the forested catchments were used to test the effect of cleaning and the other two the effect of not cleaning the drainage ditches after the forest had been clear-cut. Again, a crawling excavator was used, but in these catchments the purpose was to clean the ditches from sediments and vegetation and maintain the drainage of the site. Here, after the forest clear-cut was finished we monitored the pre-treatment conditions for about a year before the ditch cleaning in mid-September 2021 and we monitored for over a year after the treatment.

The crawling excavator filling the ditches (left) in the restoration catchment and cleaning the ditches in the clear-cut catchments (right) in the TEA. Photo: Andreas Palmén (left) and Virginia Mosquera (right).

Cleaning or blocking drainage ditches quickly changes the water table

The blocking of the ditches to restore the peatland resulted in a significant increase in the water table depth compared to the pre-restoration period, rising from 19 cm to 8 cm below the ground level after restoration. On the contrary, but as intended, cleaning ditches lowered the water table in relation to the control sites from 32 cm to 43 cm below the ground level.

After blockage (left) the water table had a significant increase while the drainage capacity was restores in the ditches that were cleaned (right). Photo: Shirin Karimi (left) and Virginia Mosquera (right).

Well, opposite water table management seems to have opposite effects

The rise in water table after rewetting increased the concentration of dissolved organic carbon, mercury and nutrients in the stream outlet of the restored peatland. Furthermore, we believe the increase in water table activates shallower hydrological pathways that also lead to an increase in methane and carbon dioxide emissions. In contrast, lowering the water table by ditch cleaning had the opposite effects in water quality, where dissolve organic carbon, mercury and nutrient concentration were lower in the ditches of the catchments where the ditches were cleaned. While ditch cleaning in general seems to have had a mitigating influence on the negative effects of the clear-cut for most variables, a much higher increase in sediment load is potentially detrimental to downstream fish habitats and spawning grounds. In terms of greenhouse gasses, the ditch cleaned catchments were sources of carbon dioxide, but after the decrease in water table there was a slight decrease in emissions. Additionally, after the cleaning of the ditches the catchment remained as a small sink for methane.

To really understand, we do need to continue monitoring the effects

A key take home point is that our results are important findings of the initial ecosystem responses to blocking or cleaning drainage ditches as we conducted our study over a relatively short period. Some effects, like changes in water quality, might be temporary and, other effects, such as carbon emissions, may take years to several decades to stabilize as the ecosystem adjusts. Our results offer a warning signal of the high uncertainty around the effects on water quality and carbon fluxes of these management alternatives and show that in order to provide a more solid base for future management decisions we need to continue monitoring the long-term effects of blocking to cleaning the ditches. In the meantime, these management decisions should be made more cautiously and carefully given the little published information we have on their outcomes in a northern boreal context.

Eddy Covariance tower installed in the restored (left) and ditch cleaned (right) catchments to monitor greenhouse gasses fluxes Photo: Andreas Palmén (left) and Alisa Krasnova (right).

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Boreal Ecology
Life Sciences > Biological Sciences > Ecology > Boreal Ecology

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