Riparian vegetation communities along watercourse provide specific habitats at the land-water boundary, where water, sediment and energy flows interact with the vegetation itself. River dynamics is at the origin of ecological and hydromorphological processes responsible for the high spatial and temporal variability of environmental factors. Biodiversity varies longitudinally from source to mouth and transversely from open water areas to backwaters periodically flooded by floods. It is also closely dependent on underground flows. As rivers are highly responsive to climate change and human activities affecting watersheds, most rivers have been artificially regulated over the past two centuries. The result has been a significant erosion of biodiversity, the restoration and management of which nowadays requires the search for a sustainable river dynamic and guaranteeing the regular renewal of riverine habitats.
1. Plants adapted to floods
Alluvial plant communities are transitional areas between aquatic and terrestrial environments that are constantly being remodelled by river activity . Under the pressure of imposed constraints, plants have developed adaptive responses that allow them to settle, develop and reproduce in these habitats subject to the influence of water, energy and material flows . Adaptive responses are expressed as:
- highly developed root systems that effectively fix deposits and resist erosion and current;
- an ease of regeneration from the stems and branches buried in the sediments;
- an ability to rapidly colonize favourable environments by vegetative reproduction (rhizomesA more or less elongated perennial underground stem, branched or not, provided with leaves reduced to the state of very small scales, emitting adventitious roots each year and an apical bud that gives rise to an aerial stem, slightly buried in the ground in which it grows horizontally or flush with the surface.) and by dispersing many seeds by water and wind.
- physiological adaptations to withstand prolonged periods of anoxia.
Plants thus resist the stress imposed by floods, whose frequency, intensity, extent and timing of the biological year in which they occur are highly variable. The ability of the river to create new environments through river dynamics explains the high biodiversity because various habitats are then made available. The plants found there, such as willows and poplars for example, are closely dependent on disturbance for their spread and reproduction. Plant development after germination is linked to the presence of exuberant depositsto be exempted: to emerge after a flood and to be stable over periods ranging from the biological year for annual pioneer species to the century for perennial species in mature hardwood alluvial forest.
2. Spatial and temporal organization of the plant landscape
In the alluvial plain, water flows, flows and size of transported sediments and vegetation that will fix the banks, trap the sediments (Figure 1) and then create large debris jams that influence the distribution of flows. The interaction between physical and biotic processes creates a permanent change in the distribution of terrestrial and underground habitats . High biodiversity is observed in these ecosystems where competitive species that monopolize stabilized habitats coexist with fugitive species adapted to fickle habitats that characterize intermediate levels of disturbance . These plant communities are spatially organized in three dimensions.
- Longitudinally. The supply of water, materials and nutrients transported increases from the source to the mouth of the river; however, the slope and speed decrease. As a result, geomorphological models are modified (Figure 2) and it is in the middle sections that the highest biodiversity is found . For example, the floristic procession of white alder groves is the richest in species below 800 metres in the braided piedmont area .
- Laterally. Seasonal variation in water levels leads to lateral channel dynamics and controls alluvial species recruitment and nutrient redistribution  (Figure 3). The impoundment times of the river annexes vary according to the distance to the functional channels.
- Vertically. Various species and communities underground or on land depend on the underflows and depth level of temporary groundwater as well as variations in the permanent groundwater table (Figure 4).
In addition, the plant landscape is still in a state of transition between an old state that can be reconstructed and a future state that is difficult to predict.
3. Ecological and hydromorphological processes
The succession. On stabilized deposits, vegetation changes over time. The plants installed give way to new arrivals which compete for resources. The composition of plant groups – and therefore biodiversity – changes from herbaceous, then shrubby and softwood tree stages to hardwood forest stages (Figure 5). This evolution is explained by changes in internal ecological conditions that are controlled by the plant community (autogenic succession) or created by external factors. In this last model, called allogenic succession, -the most common along dynamic watercourses- floods deposit alluvium that favours the establishment of new species. In this case, an allogeneic succession by facilitation is initiated; it results in an increase in biodiversity during the transitional stages. The terminal stage of mature hardwood forest then evolves through slow autogenic processes that act at the soil level with the participation of fungi and bacteria (Figure 5).
The zoning. The deposits produced by a dynamic watercourse are composed of clay, silt, sand, gravel and pebbles in varying proportions. They have thicknesses and structures that explain the presence of a mosaic of habitats also characterized by surface and subsurface hydrology (see also Figure 4). Species replace and organize themselves according to local environmental conditions. As with succession, the areas are highlighted by one or more dominant species (Figure 6).
Flow, transported load, river style and vegetation. When flows are sufficient and the bottom load is abundant and mobile, the slope increases to ensure downstream transport. However, the stream does not have a flow rate capable of discharging all its bottom load; consequently, its alluvial floor rises. The resulting multiple channels and islands form a braided system (Figures 2, 7a and 8). The constitution and lifespan of islands and channels vary according to the intensity and frequency of flooding, which allows for the existence of various habitats on aerated deposits. The latter particularly host ligneous species that characterize the fast and oxygenated waters such as white alder, black poplar, sea buckthorn, tamarisk and various shrub willows.
When flows are high and background load low, the transport capacity of the watercourse exceeds the energy consumed. He then digs his bed, lengthens his course and lowers his slope by forming meanders. Floods deposit fine materials from the suspended load. This river style is characterized by habitats that support species such as glutinous alder, buckthorn or ash willow, adapted to slow, low-oxygenated waters and compact soils (Figures 2 and 7b).
4. Human influence over the long term
River metamorphoses and biodiversity. Alpine rivers drain watersheds subject to climatic hazards and changes in use. Very reactive, they have undergone several river style metamorphoses in the past . For example, during the Small Ice AgeA period marked by significant winter cooling and short summers in Europe and North America. It lasted for more than 400 years, from the early 15th century to the mid-19th century., the excess load, due to increased precipitation and erosion on bare slopes, was at the origin of the development of braiding in the piedmont area. If there is still one model among others in the Alps, this braided landscape with vegetated islands (Figure 8) probably had a high biodiversity. This assumption, confirmed on unbounded rivers , is consistent with the hypothesis of intermediate disturbance .
19th century: modifications to the structure of the alluvial landscape. In the 19th century, braided rivers were dammed and flows (water and sediment) were controlled by diversion, artificial clogging and drainage works that altered the alluvial off-digue space .
The simplification of the system  affects biodiversity at the landscape level . The habitat areas characteristic of braided systems are reduced by 80%. On the other hand, outside the dyke, alkaline lowland habitats are expanding. In the rising artificial bed, the succession of sills (shoals) and wetting (deeper areas) leads to the installation of islets (Figure 9). These mobile lateral deposits, specific to dammed rivers with steep slopes, are the last refuges for plants characteristic of braided systems.
20th century: erosion of biodiversity. In watersheds, declining precipitation and the vegetalization of bare land have had an impact on river dynamics since the beginning of the 20th century. Reduced runoff on stabilized slopes limits erosion; in addition, much of the sediment is stored upstream in hydroelectric reservoirs. The result is a coarse load deficit that results in bed sinking, accentuated by aggregate extraction from rivers. There is then a stabilization of lateral deposits. The annual floods, capped by dams, are no longer sufficient to mobilize the bottom load that can renew hydromorphological forms. On the other hand, decadal floods during the vegetative period create spectacular elevations, increasing the stability of landings. This results in a loss of biodiversity by homogenization of vegetation at the forest stages at the end of succession. Early succession plants such as the small cattail, linked to dynamic watercourses, are threatened with extinction. The same applies to willows (six species present), tamarisk, sea buckthorn, black poplar and white alder, a tree typical of the Alpine foothills. Heterogeneously structured alluvial communities, composed of native species with natural self-regeneration, are giving way to homogeneous, stable and highly exotic species-impregnated communities.
21st century: revitalizing watercourses. Afforestation of alluvial banks reduces the capacity of the dammed channel, which is no longer able to provide transit during exceptional floods. The protection of highly anthropized alluvial plains requires appropriate development. Where possible, the river’s freedom of movement can be extended or retention basins can be created outside the dike in areas reserved for alluvial forest, poplar plantations or grasslands and marshes. However, deforestation and islet clearing remains a necessary step to ensure the safety of people and property and the conservation of biodiversity. Within the channel, biological richness effectively depends on the creation of bare spaces where the initial phases of plant succession will begin. Large trees that can generate ice jams are prohibited.
21st century: managing flows to conserve biodiversity. Water and material flows must be reserved to ensure the creation of habitats favourable to the dispersal, germination, survival and reproduction of alluvial species. For example, the ten-year flood of May 2010 was positive for the recolonization of the islets of the Isère that had been levelled the previous winter. In July, black poplar, several species of willows, myricaria and various herbaceous plants were planted, including the small cattail. This rare species then spread as the myrtle groves flourished (Figure 10). As self-regulation of the restored system is recommended, to encourage regular renewal of the shapes, the manager will be able to use two levers: solid flows (coarse bottom load) and liquid flows. Flow manipulations must be tested in synergy with the managers of hydroelectric facilities. At the same time, modelling work is also required to determine which parameters of artificial disturbance to create, in terms of amplitude (instream flows ), frequency, extent and biological period for species with similar adaptive responses .
5. Reference state for biodiversity restoration
Climate, alpine colonization and development of braiding. In the case of alpine rivers, there has been a significant increase in the volume of coarse materials transported during the Iron Age (2700/2400 BP), the post-Roman period (500-700 AD) and the Early Ice Age (1550-1850). With the increase in rainfall, human activities, such as deforestation and the development of agriculture on the slopes, were the main causes. These changes were then at the origin of the braiding process proved by the paleochannels inscribed in the levels dated from these eras .
Old maps and representation of a reference state. During the Little Ice Age, the piedmont rivers presented the entire variety of habitats (about forty plant communities) associated with the “braided system with vegetated islands”. This diversity represented by the 18th and 19th century maps (see Figure 8 and ) lends the braided system the characteristics of the reference model  for the renaturation of the Alpine alluvial plains. Returning to a natural state is highly debatable here because many communities with high biodiversity are the result of human exploitation of plants . Moreover, for socio-economic reasons, restoring the landscape prior to containment is utopian.
However, by punctually restoring more space of freedom to the river, we significantly increase biological richness. This application of the “rosary” technique  confirms that it is not necessary to redevelop the entire river corridor to restore most of its ecological potential. Through a river dynamic found in a few sections, we obtain a regular renewal of habitats. These sites are reservoirs of biodiversity. Present on rivers such as the Arc (Figure 11), they guarantee beautiful populations of small cattails , sea buckthorn and tamarisk on the restored islets of the Isère downstream. There are also many species usually found at high altitudes (so-called alpine species).
Future of alpine alluvial biodiversity. One of the major issues related to global warming will be water management and its impacts on river ecology. If the policy of renaturation of the alluvial plains by increasing the reserved flows  was not pursued, it would be necessary to expect a significant change in the spatial organization and biodiversity of alluvial plant landscapes.
References and notes
Cover image. Vegetation of a braided stream: Tagliamento, Italy. [Source: © Photo J. Girel]
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To cite this article: GIREL Jacky (2019), Alpine alluvial landscapes and biodiversity, Encyclopedia of the Environment, [online ISSN 2555-0950] url : https://www.encyclopedie-environnement.org/en/life/alpine-alluvial-landscapes-and-biodiversity/.
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