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.