3.1. Responses of aerial and underground parts of riparian
plants to hydrogeomorphological in situ constraints
When individuals of P. nigra establish on alluvial bars in
dynamic rivers, they can strongly affect fluvial geomorphology by
trapping fine sediments on alluvial bars during floods, and thus build
new biogeomorphological landforms (Gurnell, 2014; Hortobágyi et al.,
2018a). The engineer effect of P. nigra on alluvial bars depends
on its architecture, biomass, and exposure to mechanical stress. The
species has a strong phenotypic plasticity that allows to adjust its
physiological, morphological and biomechanical traits according to local
hydrogeomorphological conditions. The quantification of the
morphological and biomechanical variation of response traits withinP. nigra populations which depend on exposure to mechanical
constraints related to flow is essential to better understand why, to
what extent, and where the black poplar is able to modulate
geomorphology and thus gain in its survival, growth, and reproduction.
In an empirical study conducted on the Allier River, Hortobágyi et al.
(2017) quantified the variation in response traits of P. nigrawithin a population that was two years old, based on three levels of
exposure to mechanical constraints. In highly exposed areas, at the head
of an alluvial bar, saplings developed response traits that allow for
greater resistance, including a reduced size (avoidance strategy; see
Bornette and Puijalon, 2011; Puijalon et al., 2011), a flexible and
inclined stem, and also a more robust root system (tolerance strategy;
see Bornette and Puijalon, 2011; Puijalon et al., 2011). In a highly
exposed context, avoidance morphological and biomechanical responses led
to a decrease in drag during floods. These responses that increase the
resistance of young individuals nevertheless limit the engineer effect,
i.e., their ability to trap fine sediment and nutrients in relation with
increased surface biomass and roughness. This limitation of the engineer
effect of black poplar in the most exposed areas, at the head of the
bar, is therefore translated by a limitation of sediment trapping and
niche construction at the very early stage of establishment.
This observation suggests the existence of a functional compromise in
the expression of traits between resistance to floods (i.e., improvement
of survival) and niche construction (i.e., improvement of habitat
conditions under biotic control and therefore improvement of resource
acquisition and growth). In less exposed areas of alluvial bars – in
the middle part and downstream of the bars as well as within near
secondary channels – young poplars developed a wider, longer, less
flexible, and less inclined stem, as well as larger leaves. This
expresses a prioritization of resource acquisition and competition
abilities and at the same time the possibility for niche construction
during ordinary floods. The total length (underground part + aerial
part) of the plants is maximal at the tail of the bar with a notable
effect on the trapping of fine sediments.
Complementary observations of individual roots system on the same
alluvial bars showed that plant burial by fine sediment systematically
stimulates the production of adventitious roots and aerial biomass,
which in turn increases sediment trapping (Ding, 2014). This was
interpreted as a positive biogeomorphological feedback of sedimentary
accretion and plant growth. The construction of fluvial landforms by
black poplar cohorts seems to correspond to a positive niche
construction strategy leading to the progressive reduction of mechanical
constraints and to a modification of the habitat that positively
influences the survival, growth, and ultimately the sexual reproduction
of poplars (Corenblit et al., 2016b, 2018, 2020a,b). Biogeomorphological
processes occurring at a small scale caused by different morphological
and biomechanical plant traits therefore may have an influence on the
processes of fluvial landform construction at a larger spatial scale
(Merritt, 2013; Corenblit et al., 2015; O’Hare et al., 2016; Diehl et
al., 2017).