Rewetting-Peatlands

 Rewetting-Peatlands

Restoration of ecosystems is increasingly becoming an important tool for mitigating biodiversity loss and safeguarding ecosystem services including climate change mitigation. Peatland restoration involves measures designed to change ecosystems that have been impoverished, damaged or destroyed due to human activity, and also reverting them to a state similar to or as near to their natural state as possible, as well as initiate reestablishing of some of their ecological processes and functions. One of the primary objectives of restoration is to improve the quality of species’ habitats, thus contributing to slowing or halting the rate of biodiversity loss, and at the same time providing a continuous platform for future evolution. The advantages of preserving and restoring peatlands with regard to mitigating climate change is now also widely recognized in international climate and biodiversity policy-making spearheaded by the Ramsar Convention and also recognized by the Convention on Biological Diversity (CBD) and Climate Change Convention (UNFCCC). 

Restoration and re-wetting needs A major factor causing peatland degradation is drainage introducing hydrological changes (including e.g. groundwater abstraction) in adjacent land. Pristine peatlands are still up to this day drained for agriculture, forestry and other uses including e.g. excavation of peat soil for horticulture and fuel. In the north the permafrost peatlands of the arctic and subarctic zone are hardly used. Here human impact is more or less restricted to hunting and gathering, Reindeer Ranifer tarandus grazing and infrastructure (roads, pipelines). Many boreal peatlands are spontaneously forested though. Tree growth is, however, limited by waterlogging. Drainage removes this barrier and stimulates tree growth to allow for economically viable forestry. Re-wetting drained areas may restore ecosystem functions but full recovery of biodiversity and functions will be sometimes difficult. However, the initiation of a process of restoring wetland species habitats and wetland habitat types is important and will in any case enhance the functions and wetland services from the bog.

The most fundamental restoration need for degraded peatlands is to restore their hydrological functions and to restore a hydrological regime which is suitable / optimal for the habitat being restored. Optimised (stabilized) water levels are essential when restoring peatlands to ensure that the right conditions are created for peatland ecological function (e.g. to support climate mitigation outcomes) and for the support peatland biodiversity. Because hydrology often is dependent on a larger catchment a holistic approach will be necessary bearing in mind that just one leak in the hydrological system may cause its drainage. In many cases, hydrology of a peatland will be impaired due to a previous attempt(s) to drain the site through the creation of artificial channels or structures. The purpose of the restoration will therefore be to reverse the effects of these drains: • Drains remove water from the system- sometimes over considerable distances from the drainage channel itself. The purpose of the restoration intervention is to hold back water within the peatland or at least to slow down water loss from the site. • Long-term drainage can lead to changes in soils structure. Physical losses of peat soil occur through oxidation and release of greenhouse gases or erosion causing soil to be washed from the surface. Subsidence and compaction of the soil may also occur. These physical effects cannot be immediately reversed but losses can be prevented while the restoring peatland recovers. • Changes in vegetation occur due to water being less available in a drained site - either lowering water level or water moving more quickly through a site leading to fluctuating water levels and drier periods. Where drainage leads to the loss of peat- forming species, carbon sequestration may also cease. • When drained peatlands are subject to oxidation of the organic soil due to lowering the water table there will be emission CO2 and a cease of emission of methane. In nitrogen loaded peatlands, nitrogen oxide emissions can be expected as well depending on water level. By restoration the water table of bogs the balance of GHG emission will be in favour of reduction of total GHG emissions in the longer term and thereby climate change mitigation depending on the time perspective.

Smaller ditches with a width of up to approximately 2 meters can usually be quite safely blocked using naturally occurring peat. However, peat can also be used successfully for damming larger ditches with a low water column pressure. The peat must be well humified so that it is sufficiently impermeable. However, on sloping ground, peat dams in 2 m width ditches would likely fail and there might be a need to use other materials together with peat to ditch block anything over 1m width. Depending on the needs and finances machinery can be used, however, for small areas or in areas with little funds, the use of machinery may be limited and some measures can be done manually. Peat dams must be constructed using a low pressure excavator or carried out manually in some areas depending on the specific conditions. Machinery will cause damage to the peatland if it is too heavy and manual work will in such cases be a preferred option. The drain spacing, i.e. the intensity of the ditch blocking very much depends on the hydrological assessment of the site, objectives, risks to surrounding land and materials available.Before building of dams, the sides and bottom of the ditch is cleaned by the excavator to remove vegetation and to ensure a peat-to-peat contact. Vegetation is kept for revegetation after the operation. Pure well-humified peat (see also the use of wood in a later section) from the area reachable from the excavator is then compressed across the ditch and keyed into both sides of the ditch. The operation is closed with covering the compressed peat blocking with the left-over vegetation. The length of the peat blocking must be long enough to avoid the water bypassing the blocking and returning back in the downward ditch, slowly causing peat erosion. It should be high enough (at least 0.5 meter higher than the surrounding ground) to allow settlement of the peat. A 2 meter wide ditch will need a blocking of 2-3 meter width and 3-4 meters in length.Blocking ditches with natural occurring peat is a cheap and quick method. A machine operator with high level of expertise in working in bog environments can perform a number of drain blocks per hour when drains are small and close together. Experiences from Scandinavian projects are that by working in highly water saturated and excavated peat often underestimates how much the peat blocking settles subsequently. Additionally, wear and tear from wildlife or domestic animals can be significant because wildlife often choose the blockings when crossing the bog. 

Another way of blocking smaller ditches is by using plates of wood or plastic for piling. However, in large bog areas the transport of large and heavy plates will often be difficult and more expensive than peat blockings. The longevity of wooden dams can be a concern, because the dams become leaky over time. However, since the wooden dams often are used as blockings in smaller ditches and the ditches over time will be blocked due to natural sedimentation and re-filling, the decomposition of the wooden dam is in many cases not a problem. Plywood plates are often produced in standard sizes limiting the size of the ditch that can be blocked by one plate. In wider ditches more plates can be assembled with screws and connecting pieces. Another solution is to build a customized-size plate by assembling wood planks – ideally larch or cedar that will be resistant to decomposition. To block a ditch with plates, the same dimensions are necessary as for peat blockings. As preparation, roots are cut with a pit pincer. Hereafter the plate is piled down with the excavator shovel. A strong U-shaped iron profile was laid on top of the plate to protect the plate from damage during the down-piling. It is important to avoid damages to the wooden plates, as a frayed edge will degrade quickly. For the same reason the plates should preferably be without grooves and ridges.