5.3. Allier River landscape: dominance of physical forcing in a context with a more regular disturbance regime
The biogeomorphological dynamics of the Allier River at the landscape scale contrast with those of the Tech and Isère Rivers, where vegetation act as an important factor of stabilisation in the active tract. On the Allier River, hydrogeomorphological processes still seem to be dominant within the active tract over the effect of vegetation on the structuring of the habitat mosaic, due to a more regular high disturbance regime and strong lateral bank erosion dynamics ensured by the translations of the main channel during bankfull floods (Garófano-Gómez et al., 2017). However, the hydrogeomorphological conditions in the Allier River are presently undergoing changes.
At the scale of the fluvial corridor, Hortobágyi et al. (2018b) showed that the vegetation classes with the lowest height are concentrated hundred meters away from the main channel and at higher elevations, about 3-4 m. These results do not apparently match with the theoretical FBS model (Corenblit et al., 2007), in which the age of the vegetation increases with increasing distance from the main channel and relative elevation. On the Allier River, the development of low shrub vegetation hundred meters away from the main channel is the result of a secondary “dry” succession dominated by Prunus spp. on floodplain surfaces today disconnected from frequent floods that are also subject to grazing. The fact that mature large-sized tree vegetation is located near the main channel suggests that the preferential areas for rejuvenation of the plant succession and associated fluvial forms are located along the point bars in the meander concavities.
The tallest vegetation classes are located further upstream on the bars, near the main channel. This is explained by the fact that the channel is migrating very actively and progressively towards the opposite bank and the bars are migrating downstream. Surprisingly, the difference between the average elevation of bare and vegetated bar surfaces is not significant. The vegetation cover is concentrated around 0.5 m above the low water level, while the bare substrate is concentrated below 0.5 m and at around 1 m of elevation. The occurrence of these open bare surfaces at higher elevations relative to the low water level seems to reflect an unsuccessful recruitment of pioneer riparian woody vegetation, including P. nigra , on these surfaces too much disconnected from the surface and alluvial water pools.
At the bar scale, on the Allier River, the biogeomorphological succession model was rather supported by these results. The classes with highest vegetation reflecting the mature stage within the vegetation succession were indeed located at a higher altitude. However, the average elevation of the vegetation height classes of 5-10 m and > 10 m do not vary and therefore do not have a very specific topographic signature. The maximum sediment trapping effect is probably reached when the height of the vegetation is around 5-10 m, or about 4-8 years after recruitment. These observations are in good agreement with the observations on the Garonne River for P. nigracohorts. The most important physiognomic changes in P. nigra(loss of the multi-stemmed shrub architecture in favour of a single large trunk) occur during this rather early growth phase. From 15 m in height, the density of P. nigra stems decreases drastically and their diameter increases very significantly. These physiognomic changes result from the reduction of the exposure of individual plants to hydrogeomorphological constraints, partly thanks to biotically controlled sediment accretion and the protective effect of individuals against each other. The maximum occurrence of tall arboreous structures at the scale of the bar is currently located on the Allier from 0.5 to 1 m above the low water level. The observed elevation of the classe with the highest vegetation on the Allier River is consistent with observations on other high-energy rivers, such as the Tagliamento River in Italy (Bertoldi et al., 2011) and the Santa Maria River in the United States (Kui et al., 2014; Manners et al., 2015). In these rivers, the highest topographic levels are associated with intermediate canopy height classes (1-5 m and 5-10 m). However, on the Allier River, the highest topographic levels are associated with canopy heights of 5-10 m and > 10 m. A possible explanation of this distributional pattern of vegetation height classes is that the rate of sediment accumulation by the pioneer vegetation varies depending on the type of river, sediment load, and hydrogeomorphological regime. The Allier River has a substantial sediment load, which could result in a significant sediment trapping effect caused by pioneer vegetation in the form of successive parallel lines along the water channel in the Allier River. This phenomenon could potentially account for the increased elevation in the 5-10 m and > 10 m height classes.
However, Garófano-Gómez et al. (2017) analysed the vegetation succession based on eight sets of aerial images of the riparian corridor spanning from 1967 to 2014 and its relation with the Allier River flow regime. The results indicated that between 1967 and 2005, the study area exhibited a shifting habitat pattern. After 2005, there was a significant decrease in progression and retrogression processes, leading to increased stability, with no more channel migration and patches no longer experiencing either progression or retrogression. This study highlights a distinct turning point in succession processes at the beginning of the 21st century with a contraction of the active tract resulting from a decrease in flood frequency and magnitude. The biogeomorphological interaction between vegetation and geomorphology is likely to adapt in the near future, with the increasing influence of plants on landform stabilization and development.