Riparian forest

Floodplain designated a natural plant communities along streams and rivers (see also flood plain ). Riparian forests are azonal forest communities that are heavily affected by flooding and high groundwater levels. Can thus be distinguished from permanently wet, marshy swamp forest lowland forest.

• 6.1 Examples

Description of the biotope

Is the site often or long-lasting, high, and usually quickly passed through (100 to 200 days a year ), a softwood forms. For shorter or less frequent flooding, low-flow farther away from the power center, a hardwood floodplain forms. Extensive riparian hardwood are therefore to be found only in the larger river valleys. For rare and irregular floods, there are forest types that occur outside of the floodplain, often, for example, oak-hornbeam forest. By waterlogging sensitivity of beech otherwise prevailing in Central Europe European beech forests ( missing, however, can according to Ellenberg exception, but also book -rich floodplain forests occur when the soil substrate sandy and the flood frequency is not too high, in which case the groundwater level decreases after the flood very quickly again. such forests are, for example, on the Ems occur ).

At streams and small rivers with mostly narrow floodplains and shorter -lasting flooding can be found instead to the waterline reaching alongside streams alder-ash forests at lower altitudes with alder, in the Alps and the higher mountain ranges replaced by the gray alder. ( Floristisch are these alder-ash forests near the Hartholzauenwäldern. )

Due to the small -scale mosaic of different site conditions, including riparian forests of the richest and most vital habitats in Europe. By the favor of floodplains as their habitat natural floodplain forests in Central Europe have almost disappeared. You are in Europe according to the Habitats Directive, Annex I, protected: " Alluvial forests with Alnus glutinosa and Fraxinus excelsior ( Alno - Padion, Alnion incanae, Salicion albae ) code 91E0 "; " Hartholzauewälder with Quercus robur, Ulmus laevis, Ulmus minor, Fraxinus excelsior or Fraxinus angustifolia ( Ulmenion minoris ) code 91F0 ". The former type of habitat is also protected as a priority. The Member States are obliged under the Directive to establish a network of protected areas of sufficient size for the conservation of these habitats, this should have the name "Natura 2000". According to the Federal Nature Conservation Act include riparian forests to the habitat types that are protected by law under § 30 ( as "natural or semi-natural areas running and standing inland waters, including their banks and the associated shore accompanying natural or semi-natural vegetation as well as its natural or semi-natural silting areas, oxbow lakes and regularly flooded areas " ).

Zonation and characteristics

Riparian forests are dominated by the dynamics of the flowing water. Along the river influences the flow velocity transverse to the flow direction, the height and duration of the flood instrumental vegetation. It is also significant that the flood especially in spring ( in lowland and upland rivers ) or occur in the summer ( at the Alpine rivers ). There are five zones of the river parallel Forests: source drive, the upper, middle overflow, underflow, overflow mouth. The headwaters are in the mountains, varied greatly in the highlands and lowlands.

In addition to this longitudinal division into river sections a more or less distinct transverse structure can be seen, a graduated series of formations, starting from the middle of the river to the edge of the floodplain. Especially in middle and lower reaches of this can be seen. The typical characteristics of the floodplain are the rivers with sediment deposition (in order: gravel - sand-clay ). This reflects the velocity of the water and the resulting different transport force for sediment. The extensive gravel and crushed body of the Central European floodplains have emerged in the ice ages. Only in the immediate vicinity of rivers, the flow rate is high enough that these rearranged today and the gravel is exposed by erosion. A little further away from the waters accumulate at lower flow rates from sandbanks. In 5-10 meters the flow is already strongly decelerated by the vegetation here outweighs ( strongly clay ) Aue- clay. Riparian forests reduce the flow velocity and affect the morphology of the water. When floodplain different successional stages can occur side by side.

Longitudinal structure

At the source drive in the mountains predominate vegetation-free gravel banks because of the high dynamics. Besides willow ( Salix species ), the German tamarisk ( Myricaria germanica ) can sometimes form low-growing shrubs. The floodplain of the sources and source streams in the highlands and lowlands are usually so narrow, that does not form a separate Au or riverside forest. They are simply the adjacent forest inventory with over shields. Special types can be found but in the herb layer. As a " source forests " designated forest types flat - sickerquelliger waterlogging belong to the swamp forest.

The floodplain is usually formed at the upper reaches only as a narrow, river -long tape. Typical are the river and creek accompanying alder and ash forests here. In the mountains the gray alder, here is the dominant tree species, according to their gray alder riparian forest is named ( Alnetum incanae ). In deeper layers of the gray alder from alder and the ash is replaced. Characterizing forest community is the alder-ash forest, the Alno Fraxinetum (according to other authors of bird cherry, Prunus padus, also Pruno - Fraxinetum called ), along with a number of similar and closely related companies.

In the middle reaches, the flow rate decreases significantly, the sedimentation nutrient-rich fine material ( Auenlehms ) predominates. In addition to the vegetation of stony banks now form of soft wood and riparian hardwood.

A significant decrease in the gap and increasing meander characterize the situation in the lower reaches of the river. The flow rate decreases, consequently sink even the smallest airborne particles ( clays and silts ) down. By periodically or occasionally floods occur, depending on the parent rock or substrate nutrient- rich soils. Characteristic soil type is the brown Auenboden or Vega, of Alpine rivers frequently Black Auenboden or Tschernitza. Softwood and hardwood floodplain are here in the state of nature is often available in a large-scale mosaic of bayous, Brüchern and riparian zones.

At the mouth of the run vegetation development is dependent on whether the tides affect the water level with. Is this, as in most central European rivers in the case of special form gewässernah reedbeds. These salt- affected sites are therefore inherently mostly forest free. In slightly higher margin areas of the floodplain, the tidal reed beds will continue to be accompanied by hardwood floodplain forest.

The salinity of the Tide is a limiting factor dar. example of a Tideauwald is the nature reserve Heuckenlock in Hamburg- William Castle.

Transverse structure

The ideal typical series of steps in the middle and lower reaches follows this structure:

• Riverbed
• Amphibious shore area ( sand and gravel bars ) gravel-bed vegetation ( in the middle reaches of alpine Schwemmlingen ) and pioneer communities ( German tamarisk, willow pioneer )
• River reeds and fringe communities in sandy areas ( or the flood basins ) ( baffle small cattail )
• Softwood ( willows )
• Hartholzaue (Au (s) forest )
• Flood- free area: often ( glacial ) gravel terraces

Mosaic embedded in the underflow: oxbow lakes ( cut-off meander loops and side channels ) converted to backwaters of various types, usually only in the deepest layers of swamp forests (eg March )

The typical lowland forest type of the agent runs is the gray alder forest, which requires periodic flooding (at least 1x per 10 years). The root of the gray alder have the most direct contact with the riverine floodplains Grundwasser.Typisch for the agent runs is the Aulehm. The riparian forests naturally belong mainly in the foothills area even forests that are extremely dry soil when the floodplain is composed mainly of gravelly or sandy material. This is because at low tide the water table in the gravel (which is always equal to the river surface ) can drop so much that the water for the roots of many plants is hardly achievable. Especially in the floodplains of the rivers coming from the Alps in Bavaria ( Lech! ) or schott reach the top levels of the floodplain, there are extremely dry soil Schneeheide pine forests. Local can dry out as much of the ground that (semi-) natural lean grass ( Brenne or hot border called ) are scattered throughout the forest.

Adjustments to the water

The plants of the floodplain forest in the root zone can show special adaptations to the changing water level. The elm changes in age their root systems from a taproot to a heart root, probably to through roots as much soil above the water table.

In year-round water-saturated and virtually oxygen-free soils with only small and short-term flooding the alder is found that supplies their roots through breathing holes in the trunk with oxygen.

Alders and willows have their roots in relatively large air-filled spaces between the cells ( intercellular spaces ), in which oxygen is transported. This can aboveground over Korkwarzen ( lenticels ) are picked up and transported by diffusion and thermal osmosis to the roots. Some pastures spread preferred over broken Aststücke landing and take root (eg Crack Willow ). The high growth rate is typical of the softwood trees, since the natural flow constantly changes the morphology of the bank area.

Importance and threat of lowland forests

There are many riparian forests cut down and converted to pasture. The desire to regulate the rivers and to make as navigable throughout the year, then, especially in Central Europe, only remnants of the original floodplain deposit left over ( about 300 km ² floodplain and of which 60 km ² close to nature ), such as the Leipzig floodplain forest. Since then, the frequent and severe river flooding back in the public eye and suggest a negative development in hydraulic engineering (see torrents, Gewässerkorrektion ), is a reversal of this trend in progress. You renatured especially smaller rivers and river sections (eg Isar, National Park Donau-Auen ).