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Article

Author(s): Julien Prinet
Editorial office: FVA, Germany
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2. Results of the cartographic survey of water bodies in forests

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Rohr in Luttenbach (Elsass)

Fig. 1: Pipe culvert in Luttenbach (Alsace) (Photo: FVA)

 
Tip: Click on each diagram to enlarge
Aufteilung der Hauptarten der erfassten Feuchtgebiete
Fig. 2: Break down of the main types of surveyed wetland (according to number and area).
 
Durchschnittliche Fläche pro Feuchtgebiets-Haupttyp
Fig. 3: Average size of the main types of wetlands.
 
Feuchtgebietsart
Fig. 4: Spatial structure of the surveyed wetlands.
 
Waldumgebung der erfassten Feuchtgebiete
Fig. 5: Forests surrounding the surveyed wetlands.
 
allgemeine Dynamik, die bei den erfassten Feuchtgebieten beobachtet wurde
Fig. 6: General dynamic observed at the surveyed wetlands.
 
beobachtete Dynamik gemäβ Feuchtgebiets-Hauptart
Fig. 7: Observed formation dynamic according to main wetland type.
 
Deckungsgrad des in den Feuchtgebieten vorkommenden Springkrauts und des Japanknöterichs
Fig. 8: Percentage cover of Himalayan Balsam and Japanese knotweed in wetlands.

In total 1,084 very different wetlands were surveyed. Different in terms of size, function and condition. Apart from the differences in relation to the substrates in which they develop, three “functional” types can be differentiated:

  1. Springs and soakages/ Seepages (Spring areas): commonly occur in the mountains, most are small (<0.5ha) and connected to outcrops and side channels that quickly disappear after forming a stream. They are generally found near the top of bank batters or directly on the batter where seepage emerge. They form point source areas that are often poorly recognised in the landscape.
  2. Floodplains whose occurrence accompanies the dynamics of a stream. The water supply can come from a stream side channel, from floods or overflows (in larger systems) as well as through lateral movement. In the mountains they develop in valley bottoms where they are protected from longitudinal gradient changes in or in moist hollows that are more or less strongly connected to the main channel.
  3. Pools, Peat bogs, Tarns and others independently developed marshes.

A multifaceted typology of cartographically surveyed wetlands was developed for identification purposes. 16 various types can be identified. To simplify the analysis the various wetland types were separated in the following way:

Main area type Sub-type
Willow stands
  • Willow stands in swampland
Alder and Ash-Alder Groves
  • Alder groves in swampland, Alder-Ash forest, planted Alders and Ash-Alders groves
Marsh formations
  • Raised bog, degraded raised bog, tilled raised bog
Associated wetlands
  • Marsh area, high rush thicket, reed belt, large sedge-reeds, herb fields
Springs and soakages/ seepages
  • Springs and soakages/ seepages, tuff areas
Pools, Tarns and Sinkholes
  • Pools, Tarns and Sinkholes

The surveyed wetlands covered 698ha in total. The most common types are the Alder and the Alder-Ash groves, which strongly dominate in terms of number and coverage: 397 Alder groves cover 48% of the surface (Fig. 2). In addition, one finds a large number of springs and soakages/seepages which accompany the water network in valley bottoms of catchments. Although they are common they are generally small (average size under 0.4ha) (Fig 3). Their total surface nevertheless accounts for 19% of the cartographic survey. The least common wetland form is the willow stand, which is little developed in the Vosges mountain range. They occur in the head of valleys on flood terraces and together with Black Alder, close over old marsh areas.

Lastly 114 pools, tarns, or sinkholes were surveyed. About 70% of them develop their own structure while the rest are incorporated into complex wetland areas: into spring areas, Alder-Ash groves or into open wetlands (marsh areas or moors).

Spatial separation and integration of wetlands

Almost 75% of the cases deal only with the homogenous formation of wetlands while the rest are complex or mosaic formations.

Moor development, open wetlands (meadows, reed belts etc.) and willow stands are the most common complex structures formed, meaning where several water influenced areas are unified into one area. This can be partly explained from an ecological perspective; with these areas categorised according to a multifaceted developmental process. So perhaps willow stands are the most common first stage of reforestation, not cultivated marsh areas; and fens arise spontaneously near tilled areas or low lying purple moor-grass meadows.

In contrast, the Alder wetlands and tarns form stable structures whose surfaces are generally very homogeneous in the forest landscape (this is not referring to habitats in a plant-ecology sense).

Some forms are mainly found in paired structures which are connected in an encompassing damp system. This is the case with herb fields, large sedge-reeds, rush thickets and with other wet meadows which one regularly finds in contact with Alder-Ash stands in the heads of valleys.

The water network notion

Apart from wetland point sources one should also consider the network forming structure of these wetlands. In a river-like floodplain, the wetlands of the floodplain are clearly connected in an encompassing network, which because of the strong fragmentation of the environment (settlements, cultivated surfaces etc.) must be given even greater consideration. In mountainous forest, the less precise the classification of these small water systems is, the more the forest is presented as a very homogenous and stable situation (less fragmentation). The research showed that 56% of wetlands are not connected to other wet formations, while 36% are constantly connected and 8% temporarily connected. If a connection exists, they are mostly created via a stream (77% of the cases) and secondly via ditches that accompany site infrastructure (forest streets, lanes etc).

In the research area the occurrence and functionality of wetlands was closely tied to the water network.

The forested valleys of the Vosges and Alsatian Jura ranges therefore form a large water network structure. They contain complex wetlands which support the formation of flood plains. Likewise, geomorphology determines the character of a functioning network which is more or less closely connected through wetlands from its source, at the head of a catchment, to the lower valley. The forest worker should integrate these spaces in to his or her administrative scheme as unique features which shape the catchment.

Wetland surroundings

80% of the surrounding forested areas could be called “natural” areas (with over 70% native species Fig. 5). 10% of the cases dealt with unnatural conditions (with over 70% exotic species). Lastly the value of wetlands within plantations (mainly spruce and Douglas Fir) must be emphasised. They make up 10% of the total wetlands.

Natural development of wetlands

This type of development mainly relates to those areas noted as stable (Fig. 6) and whose functionality appears undisrupted. The most commonly observed dynamic is the increasing stocking by hard woods which can accompany degradation of the area. This situation is particularly apparent in open areas (especially in moors where 38% are stable and 47% already overgrown) (Fig. 7).

These dynamics can be classified into natural ecological processes. In situations where aggravating factors favour canopy closure, the forester needs to strive for potential interventions which open it up again (e.g. strong dynamics of spruces in some spruce forests surrounding moors)

A number of tarns demonstrate progressive infilling. Depending on the problem, clearance or sludge removal can be carried out in places.

Exotic and invasive plant species

The most common species is the Himalayan balsam (Impatiens grandiflora), which occurs in 6.9% of the surveyed areas. The second most common species is the Japanese Knotweed (Fallopia japonica) (1.5% of the surveyed areas). One comes across both species particularly in Alder and Alder-Ash stands and in wetlands which accompany watercourse networks (apart from springs in catchment headwaters).

Disturbances associated with exotic invasive plants are little discussed globally and don’t appear to be an important problem in forested areas. Particularly because they don’t form a continuous ground cover in over ¾ of the cases (self sown or individual plants). The Himalayan balsam occurs more commonly because it can form a continuous ground cover. It mainly occurs in wetlands associated with rivers where it appears as a dominant species in herb fields or amongst the tall grasses of Alder-Ash stands. However, its occurrence doesn’t prevent, or only to a small extent, the expression of the local flora.

As a rule, the occurrence of these species indicates a loss of naturalness of an area. In forested areas, the seed sources are strongly tied to human activities: maintenance of the site infrastructure (forest lanes, streets, ditches etc.), and brining in backfill or other material from outside the area.

Himalayan balsam Japanese Knotweed Canadian Goldenrod Prunus serotina (Black cherry) False Acacia
Alder forest 12,7 % 3,1 % 1 % Traces 0,8
Forest shrubbery 10,9 % 0 % 2,2 % 0 0 %
Associated wetlands (Meadows etc.) 10,3 % 0 % 0,8 % 0 1,6 %
Pools, Tarns and Sinkholes 2,6 % 0 % 0 % 0 0 %
Springs and soakages/ seepages 2,2 % 0,8 % 0 % Traces 0,6 %
Moor 0 % 1,1 % 0 % 0 0 %
TOTAL 7,1 % 1,4 0,5 % 0 0,65 %

Impacts of forest management on wetlands

Forest management measures can have various levels of direct or indirect impacts on wetlands:

  • On the quality and quantity of water: at the catchment level its influence varies according to the makeup of the stand, the area and the harvest method or locally through pollution associated with chemical fire fighting measures;
  • On the functionality of the area (water balance): directly through sewerage systems or landfills in wetlands and indirectly through roading systems (site development etc.);
  • On the ecological quality of habitats: the naturalness, representativeness of biological habitats etc. are aspects which forest managers can worsen through their choices with respect to stand development, planting, development of invasive species, hunting regime etc. The forest manager can also influence the micro-climatic conditions of wetland biotopes. This can result from light levels, the stocking grade (density) or the vertical layering of the stand etc.

This section is concerned with the observed impacts on the structure and functionality of wetlands or on the mapped impacts related to forest management. The appendix is a qualitative description without a deeper analysis.

Afforestation

7.8% of the surveyed wetlands within the research area were impacted by coniferous tree planting (across all types of wetlands). The intensity of these damages varied according to the size of the affected surface in relation to the total wetland. In most of the surveyed cases, these plants took up between 15 and 50% of the total surface. As a rule this related to afforestation of valleys or open areas that were earlier considered unproductive and where during the 60s & 70s there was a widespread goal of improving these sites’ financial value in a socio-economic context through reforestation. This situation led to a change in vegetation and hence an imbalance between water function and occurring plant communities.

Disruptions related to forest use

Timber harvesting carried out in wetlands or in the immediate surrounds directly influence the functionality of an area.

The intensive light increases following timber harvesting generates perturbations whose intensity progressively declines with replanting. According to the cartographic survey, this especially affects open wetlands like tarns and moors which react sensitively to sudden changes in plant coverage. To reduce disturbances in these cases, one should strive to maintain a planted buffer (woody and herbaceous species) directly adjoining the wetland. This should act as a buffer against damaging side effects associated with timber harvesting. In ¼ of the surveyed wetlands brushwood was left after cultivation. The actual impacts however were limited. The forester’s attention should rather be directed to carrying out appropriate harvesting. Complete clearing of the felling area is not necessary, however care must be taken that the left over wood doesn’t completely cover the wetland (suppressing vegetation, eutrophication etc.)

Ruts (soil compaction) are also prominent. They were surveyed in 8.5% of the wetlands, especially in moors, and it is therefore necessary to apply careful management in these types of areas.

Vorhandene Erschlieβungswege oder Fahrspuren von forstlicher Tätigkeit im Umfeld von oder in Feuchtgebieten
Fig. 9: Forest development roads or lanes present in wetlands or their surrounds.

The Vosges are criss-crossed by a large network of skidding tracks and forest lanes, measured at over 4.2km/100 ha of forest (Patzelt 2003). One commonly encounters forest lanes in the immediate surroundings of wetlands (Fig. 9). This situation mainly arises because of topographic conditions as the distribution of site development roads is much more extensive in areas with less steep gradients. Only a few measures can cause a reduction of these adverse impacts, which in a number of cases can lead to a significant change in watercourse function. Machinery movement (excluding skidding tracks and forest lanes) in wetlands or their immediate vicinity must also be monitored. Wetlands are very commonly characterized by hydromorphic soils with low load capacities which react sensitively to compaction and trafficking. Trafficking in these areas must be avoided to maintain soil function.

The organisation of the site opening up is an aspect over which the owner and forest manager has direct influence. The entire disturbance (direct and indirect influences) which site development roads have on watercourses is encompassed by the organisation. Restoration measures such as the removal of surplus site opening roads in valleys and favouring non-permanent lanes on hillsides, can also be aimed for.

Impacts on water discharge

Drainage systems were established in 16% of the cases. The still working drainage systems were no longer maintained and their functioning was progressively reduced through sedimentation. These types of drainage systems were most commonly observed in open landscapes with high moisture levels (moors, marshes etc.) on which farming operations were possible in the past. Here, selected measures should slowly reduce these drainage effects. Forest roads and lanes in contact with wetlands can also bring about a change in water discharge in that they function as drains. But they can also reduce discharge of water. No analysis of this type was carried out under the terms of this research project.

Störungsquellen für Feuchtgebiete
Fig. 10: Sources of wetland disturbance.

Infilling wetlands also causes changes to the water hydrology of these areas. Infilling doesn’t affect the whole wetland and are mostly traced back to opening up measures (road tracks, storage areas etc.). Wetlands in valley floors are most affected.

Disturbances from sources other than forest management

There are only a few sources of forest wetland disturbance apart from forest work and management. Activities and operations in natural areas, and therefore in wetlands, are already regulated under numerous legal frameworks. In most cases no other sources of disruptions were established. Only a few individual cases were recorded - like uncontrolled dumps (in most cases no longer used). Approximately 10% of the cases dealt with permanent feed lots or lures (corn, beets etc.), salt licks and treated beech wood. The rest are mainly accounted for by raised stands (raised hides) or game meadows. Large fauna (ungulates and wild pigs) particularly value the refuge possibilities and nutrient sources offered in the forested areas of natural wetlands. Their presence alone poses no problem, only the surplus population which can arise from excessive food supplies, are a source of disturbance to wetlands (excess browsing, soil damage etc.).

Article series: Cartographic survey of water bodies in the Vosges and Alsatian Jura ranges by the ONF

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