3.2. Responses of the aerial and underground parts to
hydrogeomorphological constraints in experimental ex situconditions
As observed in the field, specific morphological and biomechanical
response traits of avoidance and tolerance (resistance traits) allowP. nigra to effectively colonize alluvial bars and thus to
modulate hydrogeomorphological processes (Fig. 3). Quantitative
experimental studies of the modulation of riparian plant morphology and
biomechanics induced by hydrogeomorphological constraints permit to
better understand under semi-controlled conditions how plants can
colonize a physically highly exposed and naturally disturbed fluvial
landform. These modulations of individual plants potentially translate
the development of a resistance or avoidance strategy, or both, in these
plants, which ultimately can induce changes in fluvial morphodynamics at
the landscape scale.
Experimental research should particularly provide a quantitative answer
to two key questions such as: How does drag force caused by stream flow
affect the growth of P. nigra ? How do successive sediment burials
and the accumulation of sediment influence the growth ofP. nigra ? The response of P. nigra to these two categories
of mechanical constraint determines both the efficiency of the growth of
individuals to sexual maturity and the evolution of fluvial landforms.
Garófano-Gómez et al. (2018) conducted an ex situ experiment in
semi-controlled conditions at the GEOLAB laboratory in Clermont-Ferrand,
France, to dissociate the effects of hydrogeomorphological factors
acting on anchorage and plant growth. The experiments targeted the
morphological and biomechanical responses of young P. nigraindividuals. One hundred and twenty-eight P. nigra cuttings of
Jean Pourtet variety were planted in one hundred and twenty-eight
flexible bags of 0.6 m3 filled with Allier River sand
with an automatic irrigation system adapted to a filtering substrate.
This system included a microporous ring for automatic irrigation for
each bag. The young P. nigra individuals were subjected to two
types of constraints during a full growing season (from March to
November), i.e., three treatments in total: (1) no constraint (this
population without any treatment served as a control); (2) sediment
burial with a calibrated grain texture; (3) mechanical constraint
reproducing a calibrated hydraulic constraint repeated at a certain
frequency and intensity; (4) the two constraints combined on a
population of cuttings.
The growth of the aerial and root parts was precisely quantified by
measuring numerous morphological and biomechanical traits of the
underground and aerial parts: morphological and architectural soft
traits (e.g., maximum height, cumulative length of stems, growth rate,
stem diameter, root branching rate, root depth, ratio of elongation and
biomass between the aerial and underground parts, number of structural
roots, ratio of biomass between fine roots and structural roots, root
growth orientation, specific leaf area); anatomical and biomechanical
traits (flexural stiffness, stem breakage resistance, and cell size).
Hard traits were also measured at the end of the experiment, including
resistance to pulling out with a winch test. The measurement of
morphological and architectural parameters of the aerial part was
carried out continuously on the experimental site during growth. The
measurements of the root part were performed in the laboratory at the
end of the growing season. 2D and 3D modelling techniques, including
multi-image photogrammetry, were used to automatically reconstruct and
measure plant architecture.
Results indicated a significant effect of the treatments on the size,
position, and type of roots. Sediment burial (SB) induced the production
of numerous fine adventitious roots, and the application of stem bending
(DF) induced the production of more solid basal structural roots.
Morphological responses were more pronounced in the sediment burial
treatment than in the stem bending treatment. The control (C) showed
reduced aerial biomass compared to the treatments, but a root-to-shoot
biomass ratio in favour of the roots. The combination of bending and
sediment burial (DFSB) produced a positive surface growth response. DF
showed a significantly higher biomass of basal roots than SB and DFSB,
while SB had a higher biomass of lateral roots.
These findings are consistent and complementary with the in situobservations (Ding, 2014; Hortobágyi et al., 2017; Corenblit et al.,
2020b). They provide evidence of the high morphological and
biomechanical plasticity of the black poplar, which ensures its
establishment in various conditions of exposure to shear stress and
sediment burial and thus its capacity in high density to collectively
affect sediment transport.