4.1. Function of herbaceous plants: facilitation and resilience
Fieldwork carried out on the Tech River, South France, that is, in a Mediterranean-mountainous torrential context, has highlighted the functional role of herbaceous formations on alluvial bars (Corenblit et al., 2009a). According to the hydrosedimentary regime, herbaceous units may be able to induce a facilitation effect for the recruitment and survival of herbaceous and woody seedlings and promote the resilience of plant biodiversity on exposed alluvial bars through habitat stabilization and seed trapping. In the case of the Tech River, vegetation starting at the herbaceous stage on highly exposed gravel surface, had a very significant impact on surface sediment dynamics and the seed bank, i.e., the set of seeds contained in the substrate, which may contribute to the ecological and biogeomorphological resilience of the river system.
Dense herbaceous structures colonizing the most exposed areas of alluvial bars protected fine substrate very effectively from erosion during floods. Once established, living herbaceous structures, even senescent or dead, contributed recurrently, every year, to the deposition of new layers of sandy-silty sediments (on average 3 cm yr-1). These deposits occurred mainly at the edge of the main channel and secondary channels during quasi-annual morphogenic torrential floods where the erosion process is normally dominant on coarse-grained surfaces. The structure of the seed bank had been described in detail under a binocular microscope in the laboratory LEFE, Toulouse, from numerous sediment samples taken in the deposition zones. The deposition of very numerous seeds (sometimes up to 1,000 seeds and 50 species per 10 cm3 sample) accompanied this process of stabilization and retention of fine sediments at the top surface of the gravel bars near the water resource. Erosion (on average 20 cm yr-1) measured in the adjacent areas with low plant cover suggested that the herbaceous cover locally represented a critical parameter which controlled the threshold of mobilization of coarse sediment, surface deposition, and the stabilization of fine sediment.
The action of herbaceous structures on sediment dynamics had so far been neglected in high energy fluvial contexts. However, it turned out to play an important ecological role in promoting the resilience of plant biodiversity on alluvial bars that are regularly swept by floods and strongly constrained in summer by possible severe temperatures and hydric stress. This process is similar to the development of a regeneration niche under strong biotic control (Lavorel and Chesson, 1995; Grubb, 1997; Silvertown, 2004). It seems appropriate to establish here the relationship with the facilitation process in constrained environments as described by Bertness and Callaway (1994) and Bruno et al. (2003).
The facilitation process self-sustains itself as long as a large torrential flood does not destabilize the entire vegetation-sediment complex. In some locations on alluvial bars, a few native herbaceous species (e.g., Polygonum lapathifolium , Echinocloa crus-galli ) and exotic species (e.g., Cyperus eragrostis ,Bidens frondosa , and Paspalum paspalodes ) with high abundance alone fulfilled the function of stabilizing and retaining fine sediments and seeds. However, in most cases, this function was ensured at the community scale by a multitude of opportunistic herbaceous species with low abundance that form a heterogeneous, dense, and complex mat structure due to its architectural and biomechanical characteristics. This latter observation suggests that the combination of different morphological groups and biomechanical abilities of herbaceous plants, or even life histories, within the same plant unit positively influenced the resistance and resilience of the entire vegetation structure to hydraulic constraints and the effectiveness of sediment and seed retention.
The areas subjected to the effects of herbaceous formations also represented a favourable support for the initiation of the niche construction process by pioneer woody vegetation as well as the associated herbaceous plant succession. These biogeomorphological units with an herbaceous cover and an associated layer of fine sediments offered more favourable habitat conditions for the summer survival of seedlings of P. nigra , Salix spp., and Alnus glutinosa . Herbaceous units on the edge of the channel showed a tendency to evolve in less than three years – at least in the absence of a too large flood – towards dense pioneer shrub formations dominated on the Tech by the three pioneer riparian species mentioned above. These mixed herbaceous/shrub pioneer units (between 2 and 3 m in average height three years after their recruitment) have a very high surface roughness and remained perfectly in place during the flash floods via a very high biomechanical resistance of the woody structures. In addition, they showed a strong resilience of the herbaceous floristic structure constituting the lower layer, i.e., the local seed bank and new seed inputs during floods permitting recovery.