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.