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.