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.