Ecosystem ecology during degradation and regeneration

The degeneration processes of alkaline fens in Poland can be natural, but more often they are triggered by human activity (Ilnicki 2002). Most frequently they result from distortion of hydrological conditions, agricultural land use and the fragmentation of habitats (Herbichowa & Wołejko 2004). The remaining distur-bances, e.g., succession, changes in species composition or eutrophication, are secondary to those mentioned above but no less important and lead to serious consequences for sedge-moss fen vegetation.

Favourable conservation status of alkaline fens, by definition, excludes the existence of drainage systems. No drainage ditches existed in natural sedge-moss fens and natural drainage system, if existing, developed freely. In some situations (especially in the mountains or more sloped areas), water erosion processes in undisturbed fens progressed spontaneously without any human impact, leading either to worsening of habitat status or even to its disappearance. It is also visible in many lowland fens where the habitat disappears along the water outlet (most often the river) but remains properly developed in the central or edge parts (e.g., Jabłońska et al. 2011). In spite of this, in a majority of cases humans were responsi-ble for the degradation of the alkaline fens. Most of all it was caused by the need to increase plant production, and the drainage infrastructure was the principal way to achieve this goal (Ilnicki 2002, van der Linden 1982, van Diggelen et al. 2006).

Photo 76: Drainage ditch in the mire near the Księże Lake (photo by E. Gutowska).

The subsidence of the water table is dependent on the distance between the ditches and their depth (Braekke 1983). If the drainage system facilities are pres-ent in the fen, by definition the habitat status should be considered as distorted.

The impact zone of the drainage ditches extends from several to several tens of meters, which is determined by the ecological conditions (Okruszko 1969a, Il-nicki 2002). For instance, if a network of drainage ditches is present in the alkaline fen, even when the ditches are several hundred meters apart, the water table will be lowered and the drainage will be accelerated, especially along the ditches and channels. It can be manifested by development of tall herb communities or rush type vegetation along the ditch and a reduction of the contribution of characteris-tic habitat 7230 species. Moreover, due to the availability of oxygen (Mannerkoski 1985) and biogenes from decomposed peat, such areas often began to be over-grown by shrubs or trees (Jeglum 1974).

Overgrowth of drainage ditches does not always eliminate their impact. The peat structure in overgrown ditches is very often less compact than in the remaining parts of the peat bed, which results from the fact that peat depositing over several tens of years will be more permeable than that accumulated over hundreds or thousands of years (see Baden & Eggelsman 1963, Ilnicki 2002). As a result, despite ditch overgrowth, the water will still be able to flow faster to the outflow than through the remaining part of the peat bed.

Photo 77: Willow thickets in Pakosław Mire (photo by E. Gutowska).

The hydrological conditions can be changed not only by the increased outflow but also by a reduced groundwater inflow to alkaline fens. The most common cause of this situation is a significant groundwater abstraction in the catchment where the fen is located. If in the fen catchment area there are, for instance, green-houses utilizing large amounts of water abstracted from the groundwater, or if the fen is located within the reach of a cone of depression, such as a large city or a mine, it can lead to reduced water supply to the fen. As a consequence, the fen lacks an adequate amount of water to remain sufficiently hydrated and conse-quently the peat surface layers dry out.

Draining of the peat bed changes the air and water conditions of the upper peat layer (Ilnicki 2002). The availability of oxygen triggers peat decomposition and its transformation into moorsh which is accompanied by a release of nutri-ents to the environment. Moorsh, which has an aggregated structure, unlike peat having a fibrous structure, cannot transmit water by capillary rise as efficiently as undecomposed peat (Ilnicki 2002), which aggravates water deficit in the upper peat bed layer and prevents the development of species specialized at living at low oxygen availability (plants, animals, fungi and other microorganisms). As a result, the vegetation changes, and sedges and species that need wet and basic soil to grow disappear while – depending on the ecosystem characteristics – rush or meadow species emerge and, if agricultural use has been abandoned, shrub and tree species also follow (van Diggelen et al. 2006). All of them can function only

Photo 78: Changes in fen vegetation structure caused by dewatering: encroachment of meadow species (photo by E. Gutowska).

when the soil fertility is greater than that found in alkaline fens, which addition-ally limits the availability of light at the fen bottom. Next, low species typical of alkaline fen withdraw (e.g., Liparis loeselii, Carex limosa, Carex lasiocarpa), and the moss layer almost completely perishes (Kotowski & van Diggelen 2004).

Examples of complete damage of alkaline fens due to drainage have been de-scribed. The Wizna fen was overgrown by mossy sedge communities with Hermin-ium monorchis before drainage only a half a century ago, while now it is covered by birch forest with nettle undergrowth (Kołos 2004). Tomaszewski (1998) described the destruction of a spring fen near Gostyń in Wielkopolska. In recent years, hab-itat 7230 on the Całowanie fen in the Mazovia has vanished despite conservation attempts (see Chapter 8.2.2). Hundreds of other alkaline fens were destroyed with-out any documentation of this process.

Another impact which significantly affects the status of alkaline fens is related to their use. In the natural state, owing to the ecological conditions, these ecosys-tems did not need agricultural use for maintenance. However, after artificial drainage systems were constructed in fens in order to improve management of wetlands and increase crop production, without extensive use, especially mowing, habitat 7230 dis-appears as a result of succession. Pratotechnical measures have positive effect on the vegetation but they cause compaction of peat (Schipper et al. 2007), sometimes they can also damage sward and unify the fen surface structure, which happens especially when heavy equipment is used (e.g. adapted groomer-like equipment) (Kotowski et

Photo 79: Changes in vegetation structure caused by peat soil degradation:

encroachment of nitrophilous plants (photo by E. Gutowska).

al. 2013). Peat compaction during land use can lead to the formation of the com-pressed layer which in some cases can disturb the chemistry of the water available to plants (Schot et al. 2004). It hinders access of alkali-rich underground water to surface peat layers, and it is replaced by nutrient-poor precipitation water. This is followed by oligotrophization and acidification processes which can be aggravated by the presence of drainage ditches lowering the groundwater table.

The next phenomenon contributing to the degeneration of alkaline fens is re-lated to habitat fragmentation. Apart from the physical damage of parts of sedge moss fens, habitat fragmentation facilitates the penetration of undesired species (including alien ones) and is important for proper fen hydrology. For instance, the construction of a road through the Rospuda river valley mires would cause irreversible habitat damage along the road, and would additionally alter water flow directions within the whole ecosystem which could lead to disadvantageous changes in the sedge moss vegetation.

The above-described phenomena lead to the degradation of habitat 7230. Sub-sequent implementation of conservation measures is very difficult and requires vast knowledge on the functioning of a particular ecosystem. As mentioned above, the disruption of the hydrological relations is of key importance to dis-turbances in sedge moss fens, and it primarily should be remedied because it can even disarrange the effect of agricultural use (Kołos & Banaszuk 2018). Habitats only slightly transformed, where sedge moss vegetation still exists, require an

im-Photo 80: Ruts resulting from mowing with heavy equipment – the Upper Biebrza River Basin (photo by E. Gutowska).

provement of the hydrological conditions by, for instance, blockage of ditches and removal of trees and shrubs every several years and/or the elimination of unde-sired species (Mälson et al. 2010). If conservation measures are properly chosen, disadvantageous phenomena should fade in parallel with an improvement of the conservation status of the habitat. In heavily degraded habitats, such measures are insufficent and, in addition to improvement of hydrological conditions, it will probably be necessary to remove the surface moorsh layer and decomposed peat that are a source of biogenes hindering the development of low, light-preferring species associated with alkaline fens (Stańko et al. 2018 and references cited there-in). It appears that it is required to remove the decomposed peat from the whole fen area because otherwise problems with eutrophication in the surrounding ar-eas can affect the restored patch. Such a situation can be described as a “reset” of the fen development and a restoration of its status from several hundred years ago.

Unfortunately, this procedure is very expensive and not always successful.

Photo 81: Fen mowing with light equipment prevents sward damage – the Upper Biebrza River Basin (photo by E. Gutowska).

6. HABITAT 7230