Forest rewetting in fens – the potential for climate, peatland and forest

Peatlands are areas with excess water. With the exclusion of air, submerged dead plant matter cannot decompose. Over the course of thousands of years it eventually turns into peat. Bavaria is home to a wide range of peatland types, as listed and explained in the Bavarian Peatland Type Catalogue. They can be divided roughly into two main types:

Raised bogs, which are raised above the surroundings hourglass-like – hence their name - can be imagined as being like sponges. In these nutrient-poor, highly acidic environments, only highly specialised peat mosses and certain types of sedge (Cyperaceae) are able to grow. The “sponge” formed by the peat mosses captures the rainwater, allowing the peat mosses to grow upward, while the layers underneath successively die off, in the process forming new layers of peat, 1 milimeter per year. The restoration of such raised bogs aims to retain the rainwater in the “sponge” again as it falls - by closing off existing water drainage ditches.

The situation in the fens is quite different. They are very diverse, ranging from very acidic and nutrient-rich fens to those that tend to develop from spring sources (known as spring fens) or in depressions without drainage (swamp fens) or from the silting up of a body of water (silt fens). What they have in common is the origin of their excess water, which is the groundwater.

In the case of fens, too, the water balance must be restored in order to restore them as ecosystems and habitats. In contrast to raised bogs however, fens do not work like a sponge, but like a bathtub: the water runs out at the lowest point. If we want to rewet them, we must fill them with water from the bottom, i.e. raise the groundwater level. Since the large fens are often located in intensively used valley landscapes, there is considerable potential for land users and neighbouring land owners to be affected in the prozess.

There are tried and tested guidelines for the restoration of raised bog areas, as well as experience from and evaluations of numerous projects. Today we know that the long-term impermeability of the dams often depends on important details when they are being built. Manually installed structures are generally unable to provide such long-term impermeability, even when installed with great care. The relevant procedures therefore had to be developed further. Increasingly, special construction methods and small excavators with little ground pressure and traction are commonly being used. With these, the ditches are not only divided up into so-called chambers by the dams, but also filled in – be that with peat or with other substrates, such as mixtures of wood shavings and sawdust. But even here, the devil is in the detail if the water is to be prevented from trickling through the bottom layer of the ditch and thus still running out of the bog, unnoticed by the observer.

And as far as the forest is concerned, the restoration of raised bogs has also had to evolve. There is now a growing realisation that gradual rewetting, which is also more compatible with the preservation of the peripheral bog forest and the parts of the forest suitable for rewetting, will achieve the (same) goal more mildly and more quickly. Ultimately, it is thus a question of restoration WITH the forest rather than AGAINST the forest. Bog-edge forests and sparse bog forests have a stabilising effect on the bog’s water balance and micro-climate. Formerly unfortunately widespread practices such as the precautionary clearing of the bog-edge forests to avoid potential problems with bark beetles in the boggy, poorly accessible bog-edge forest were thus counterproductive for the restoration of the bog.

After the clearing measure, the areas often dried out more than before, due to the effects of sunlight and wind, and dense regeneration of alder buckthorn, downy birch and silver birch as well as Norway spruce and Scots pine often ensued . The hoped-for effects of the measure on the rewetting of the area through the loss of the “pump-effect” then did not materialise. In principle, the water balance regulates the trees – not the other way round. 

The growth of vigorous, closed-canopy forests on peatland sites is always dependent on a functioning drainage system. If the drainage system is no longer maintained, it will silt up and fill up by itself as the edges of the ditches collapse, peat moss grows and the soil subsides. This often leads to an uncontrolled and gradual, but at least partial rewetting of these forests. In the medium to long term, forests on peat can often become bog forests again “all by themselves”, as Kaule & Peringer (2015) found on a considerable area in Bavaria. This process should be encouraged, by blocking off ditches and implementing carefully considered forestry measures – not through radical deforestation.

The conditions in fens are completely different to those in raised bogs: Here the groundwater is often already lowered due to other factors that are not originally related to the fen, but often because of infrastructure measures such as road and regulated rivers. In addition to the targeted drainage of the fen body through the digging of ditches and measures to straighten and deepen the channels and rivers, climate change, and the extraction of drinking water also have a negative effect on the groundwater level.

After this essential introduction to the current state of knowledge and the requirements specifically for fens, we now turn our attention to the Griessenbacher Moos fen, a typical river valley fen area in the lower Isar valley. Despite its typical fen history, including drainage, utilisation and grassland and peat extraction, a peatland has been preserved that is significant in terms of the thickness of its peat layer, its state of (little) decomposition and its surface area, and thus suitable for restoration. Covering an area of approx. 65 ha, its core is forested. Its current growing stock has developed almost entirely as a result of planting. In addition to spruce stands, near-natural deciduous forest stands also cover an extensive area. The stands of black alder, downy birch and some aspen (Populus tremula) that have been planted over the last 20–40 years represent the natural tree species of these fen sites, and they thus also formed part of the original vegetation of the fen in varying proportions.

The “GRIMO” project was established to collect baseline data and to carry out a feasibility study for the option of rewetting this fenland. The aim of this first project was thus to describe the peatland area in terms of its starting conditions for a more bog-compatible use, and to quantify this initial situation in as many regards as possible. This serves both to create a basis for the future-oriented decision-making for the forest owner, and to establish reference points for quantifying and measuring the future development of the forest and peat body.

The surveys on site recorded on the one hand the peatland parameters (peat thickness, peat types and condition, peatland water balance, peat and water chemistry) and the peatland-typical biological diversity (peatland-typical indicator species from the species groups of vascular plants and ground beetles). On the other hand, operational aspects were considered, with increment measuring tapes installed in representative stands. The forest growth was then examined and evaluated in order to determine important economic parameters for future stocking. It was also necessary to consider forest accessibility from the point of view of wetter future site conditions. 

The effects of rewetting on the different forest stands were calculated in a peatland study that considered three rewetting scenarios. These variants differ in terms of which part of the Griessenbach Moos fen will effectively become wetter as a result of ditch closure and the raising of the water table; and the distance that must be maintained from the adjacent areas for these measures. Depending on the scenario, water levels of 30 or 50 cm below the surface of the ground are reached as an annual average on different parts of the fenland. Although only water levels of 30 cm and less completely preserve the peat, even 50 cm below ground level is considerably better than 80 cm or 100 cm and hence a “step in the right direction.”

Standard calculation methods were used to calculate greenhouse gas emissions and carbon storage respectively in the peatland soil and in the growing stands, both for the outset and for the three scenarios. The calculation methods were based on the modelled water levels below the ground surface (using hydrological modelling of different damming variants in GIS with subsequent calculation of the climate-relevant emissions according to the PEP model (Drösler et al. 2013)), as well as the timber volume, growth and yield (using the corresponding calculation models of the carbon calculator of the [German Forestry Council] DFWR). Both calculations allow us to consider the climate relevance - currently a highly valued socio-political aspect. The effects on forestry management were also calculated in detail. The forest infrastructure network and timber extraction costs are considered and quantified economically from the point of view of a management regime adapted to wet sites.

The result shows that the thickness of the peat layer and peat condition still make a rewetting option for this fenland possible, which in this case can be rather simply achieved by damming ditches and thus likely and sufficiently raising the water table to sustain wetter forests, depending of course of the future climate development. Potentially peat-forming bog sedges that would benefit from rewetting are still present particularly in the ditches and depressions in the terrain, as a starting point for the proliferation of peatland-typical vegetation – as are characteristic inhabitants of these peatland habitats on the part of the fauna (Figure 5). In both species groups, however, there are only small residual populations of the peatland-typical species. It is important to promote these through the measures taken, so that they are restored as dominant species in a restored fen – as they were in the original fen.

The existing deciduous forest stands of black alder and downy birch (and aspen) are suitable for the modelled rewetting, meaning that would not suffer diebacks or in other ways be significantly adversely affected in their growth behaviour by such rewetting. In some cases, their growth would in fact be improved by the increased water supply. They therefore continue to supply wood as a valuable raw material, and also store carbon in it. These stands also contain a significant amount of potential high-grade timber - sufficient for exploitation and a forest management based on the production of such sawwood and veneer quality timber.

However, there are also considerable areas of Norway spruce, sycamore maple and black walnut stands of low stem quality (e.g. Figure 6). These are not suitable for rewetting, so that, depending on the extent of the rewetting on site, the rewetting variant, and the position within the area, a decline in growth or even diebacks are to be expected. These stands would therefore need to be converted in conjunction with a rewetting of the stands. However, due to their poor quality and in some cases also poor prospects in the light of climate change, there a change in the growing stock in these stands in the medium term was to be expected.

In an economic analysis, the annuities of the different stocking and rewetting variants in the light of economically important parameters of the possible set of measures regarding rewetting, stocking and harvesting were compared. The analysis shows that the more naturally stocked stands and moist to wet variants are overall less profitable than the current management variant. This is because of the necessary investments and the lower growth performance, and in spite of the expected value development of the stands. However, the spruce stands in particular are subject to considerably higher silvicultural risks, and are not sustainable in terms of soil protection (peat preservation). They therefore do not represent a viable future alternative, even before taking into account the effects of climate change.

The existing forest road and skidding path infrastructure and the extraction methods would also have to be adapted to wet management. However, even without the modelled rewetting, the current state of preservation of the forest road on the “soft soils” is not favourable, and requires continuous improvement and material input.