4.2. Function of woody plants: biogeomorphological succession
and niche construction
After their establishment on alluvial bars, following the critical
recruitment phase sometimes facilitated by herbaceous formations,
pioneer woody riparian plants act as ecosystem engineers by trapping
large amounts of fine sediments. Studies conducted on the Tech, Garonne
and Allier Rivers have shown that pioneer woody riparian vegetation is a
predominant factor in the stabilization and construction of habitat and
plant succession in dynamic riparian environments.
For example, on the Tech River, Corenblit et al. (2009a) observed a
strong correlation between net annual sedimentation rates and the
intercepted plant biovolume by floods. In the Allier River fluvial
corridor, biotic-abiotic feedbacks also occurred between pioneer woody
vegetation that is strongly influenced by hydrogeomorphological
constraints (sediment transport and deposition, shear stress,
hydrological variability) and fluvial geomorphology which is itself
modulated by established vegetation. Hortobágyi et al. (2018a) studied
16 alluvial bars on the Allier River in the Natural National Reserve of
the Val d’Allier (RNNVA) to assess the ability of three pioneer riparian
engineer species (P. nigra , S. purpurea andS. alba ) to establish and act as ecosystem engineers by trapping
sediments and building/stabilizing fluvial forms on alluvial bars. The
objective was to identify empirically the preferential establishment
window (recruitment and establishment zones) of these three engineer
species, as well as their biogeomorphological feedback window (zones
where these species act significantly on geomorphology). The results
showed that the establishment and feedback windows of the three species
varied significantly along the longitudinal profile of alluvial bars,
namely that relating to the upstream-downstream exposure to
hydrogeomorphological flows, as well as along the transverse gradient,
i.e., the gradient of hydrological connectivity from the main water
channel to the floodplain. At the bar scale, the evolution of fluvial
landforms was controlled differentially according to the morphological
and biomechanical attributes of each of the three species. These latter
exhibited an inherent (genotypical) variation modulated according to age
and degree of exposure on the bar (phenotypic plasticity).
In the current hydrogeomorphological context of the Allier River,P. nigra is the most abundant pioneer woody species and is the
dominant engineer species. It affects the geomorphological dynamics at
the scale of the entire alluvial bars, except in their upstream part
where individuals remain small due to the direct exposure to very high
hydrodynamic forces. S. purpurea established and acted as an
ecosystem engineer in the most exposed locations of the alluvial bars,
i.e., upstream and near the main channel. S. alba established in
the downstream areas of the bars, near the secondary channels, and
affected the geomorphology within patches of mixed vegetation withP. nigra and S. purpurea . The establishment and feedback
windows of the three engineer species also overlapped in places,
especially in the intermediate part of the bars. Thus, the total
establishment and feedback window of the three species corresponds to a
larger spatial extent than that defined individually by each species on
the alluvial bars. This study highlighted the role of the diversity of
functional response and effect traits of riparian engineer species and
the extent of niche construction by these three species along
hydrogeomorphological gradients.
On the Tech River, Corenblit et al. (2009a) observed peaks in specific
diversity and abundance in the seed bank within post-pioneer woody units
that are less directly exposed to hydrogeomorphological flows than
alluvial bars. This was explained by a large amount of floating seeds
that were deposited there during quasi-annual flood peaks following the
low water level, low flow velocities, and high plant roughness.
Floristic distance analyses based on species presence-absences at the
surface and in the seed bank indicated that, unlike the areas
immediately adjacent to the water channels on alluvial bars, a very
small percentage of this diversity contained in the seed bank was
expressed at the surface. The resistant seeds remained in part in
dormancy in the substrate. This result suggests that post-pioneer
biogeomorphological units forming a riparian forest partially
disconnected from hydrogeomorphological perturbations (post-pioneer
fluvial islands and floodplain) potentially fulfil a function as a
reservoir of plant diversity at the scale of the corridor. The storage,
or accumulation of seeds, could contribute to ensure the ecological
resilience of plants following low-frequency, high-amplitude flow
disturbances that cause massive destocking and maximum seed dispersal
within the fluvial corridor.
In a study of the seed bank on the Allier River, Corenblit et al.
(2016a) demonstrated that pioneer woody individuals of P. nigraand Salix spp. that establish on the alluvial coarse-grained bars
of this high-energy river promote the retention of fine sediment during
quasi-annual floods, within their stands and just downstream in the form
of sediment tails, i.e. obstacle marks. The retention of fine sediment
is accompanied by the deposition of a large number of seeds transported
by the flow, i.e., by hydrochory. Here, the hypothesis was validated
that, on exposed alluvial bars, pioneer riparian trees, either isolated
or growing in dense groups, form obstacles to flow and predominantly
control the deposition of seeds within sediment tails composed of fine
sands and silts depositing on a gravel surface. The structure of the
seed bank was described in detail at the binocular microscope in the
laboratory and from a germination test performed in a greenhouse. It was
compared between samples collected in the sediment tails and control
samples collected on the bare surfaces of the alluvial bars. At the
surface (at 2 cm depth), the abundance of seeds and species richness
were significantly higher in areas subjected to the tree obstacle
(number of seeds N: 693 ± 391; number of species S: 17 ± 3) than on bare
surfaces (N: 334 ± 371; S: 13 ± 5). Surface and sub-surface samples (at
20 cm depth) were also significantly different, with the sub-surface
samples being almost devoid of seeds (N: 514 ± 413, S: 15 ± 5 and N: 3 ±
6; S: 1 ± 2).
These results suggest a biogeomorphological feedback loop between
sediment dynamics controlled by riparian trees, the retention of
associated seeds, and the growth of herbaceous formations from these
seeds. This feedback loop is complex and involves, in a first step, the
process of hydrochory and sediment retention by trees; in a second step,
the growth of herbaceous vegetation on sediment tails and the local
dispersion of seeds by barochory; and in a final step, the contribution
of herbaceous formations to the stabilization and construction of the
sediment tail during quasi-annual floods. Corenblit et al. (2016b) have
demonstrated that this feedback loop promotes the resistance and
resilience of herbaceous plant diversity on the bars of the Allier
River.
Altogether, the results of these in situ studies on the Tech,
Garonne and Allier Rivers, targeting vegetation on the surface and in
the seed bank, highlight a characteristic and recurring correlation
between geomorphological adjustments and growth, physiognomy and plant
composition on the surface and in the seed bank. The floristic structure
in the seed bank and on the surface, the plant physiognomy and the
distribution of strategies along the longitudinal and transversal
gradients of disturbance do not result from a unidirectional effect of
the hydrogeomorphological compartment, but rather from a feedback loop
with the hydrogeomorphological components under biotic control.
The process of sediment accretion within pioneer plant structures leads
to a transition from a phase dominated by hydrogeomorphological
processes, in which seeds and seedlings are unidirectionally subjected
to physical constraints (submersion, erosion, sedimentary burial, water
table fluctuations), to a phase where abiotic-biotic interactions are
multiple, intense, and bidirectional. These pioneer units dominated by
opportunistic strategies evolve into more robust post-pioneer tree units
in just a few years (between 4 and 5 years), reach an average height of
7 m and develop on topographically raised surfaces due to the rapid
accretion of sand and silt. This phase, which is generally transient, is
characterized by strong landscape heterogeneity and high plant diversity
at the scale of the bar. In just a few years and the absence of
destruction caused by an exceptional flood or lateral erosion due to
channel displacement, abiotic-biotic feedbacks drive biogeomorphological
units towards a more stable and ecologically mature phase, which is more
resistant to hydrogeomorphological disturbance events, physiological
stress and invasion by exotic species (Corenblit et al., 2014b). Within
these units, partially disconnected from frequent flood events, sediment
deposits become negligible for quasi-annual floods. At this stage,
competitive strategies take over in terms of floristic structure and
plant diversity decreases sharply due to the exclusion of many
opportunistic herbaceous species. Within the cycle of fluvial landform
construction and plant succession, this stage represents a
biogeomorphological “attractor”. It coincides with the formation of
post-pioneer and mature fluvial islands (sensu Gurnell et al.,
2018a, 2002) and alluvial floodplains supporting softwood species and,
possibly at the end of the cycle, hardwood species.