3.3. Effect of Populus nigra at the scale of the alluvial bar
Corenblit et al. (2016b) established on alluvial bars of the Garonne River: (i) the modality of variation of annual sedimentation and topographic aggradation rates within P. nigra cohorts of different ages, and (ii) the physiological response (growth rate, production of adventitious roots, suckers, and surface stem density) associated with hydrogeomorphological modifications underP. nigra control. Within the strongly channelized river section of the Garonne River downstream of Toulouse studied (Steiger et al., 2000, 2001), populations of black poplars establish on alluvial bars and predominantly modulate the dynamics of fine sediments and fluvial landforms. Following the cessation of massive gravel extraction in the main channel during the 1980s, successive dense cohorts ofP. nigra have successfully colonized the entire alluvial bars since mid of the 1990s. These very dense cohorts, which settle from upstream to downstream on point bars in successive meander loops, induce very important sedimentary accretion and landform stabilization.
Over a period of thirty years, the geomorphological characteristics of wooded alluvial bars have evolved in close relationship with the dynamics of cohort establishment and plant growth. The geomorphological characteristics of these bars (topography, thickness, and texture of sediments) are statistically linked to the spatial distribution and intensity of establishment by the successive cohorts of P. nigra . In return, geomorphological changes strongly influence the physiognomy of cohorts (density, diameter, size, and architecture of individuals) as well as the floristic structure of the herbaceous layer, with the transition from open units with high plant biodiversity of opportunistic annual and biannual species to closed units under mature black poplar cover dominated in the understory strata by Rubus , Urtica ,Convovulus , and Galium spp. This reciprocal causal relationship between geomorphological dynamics and plant dynamics is suggested by a strong correlation between key geomorphological and ecological variables measured in situ and from aerial photographs taken by the IGN between 2000 and 2010 (Corenblit et al., 2016b). In addition, this study found that recruitment and establishment dynamics of P. nigra are controlled by the engineer effects of this species through a facilitation effect linked to the obstacle and protection effect offered to new seedlings and saplings by upstream cohorts.
The construction of the biogeomorphological units “wooded point bars” is the result of a positive biogeomorphological feedback of stabilization and closure of the environment linked to the massive trapping of sand by dense P. nigra cohorts and the vertical growth and to a lesser extend to lateral expansion of vegetation on the landforms under construction. In situ root excavations demonstrated the systematic formation of numerous adventitious roots that explore the new layers of sediment (Corenblit et al., 2018) – with a tropism linked to the higher organic level as observed on the Allier River by Ding (2014). At the same time the dense network of adventitious roots combined to the structural roots increase the resistance of individuals to uprooting. In the current context, the recurrent massive trapping of fine sediments seems to improve the anchoring, growth and resistance performance of P. nigra populations to floods with a return period of twenty years.
No significant vegetation destruction was observed on the Garonne following vicennial floods since the 1990s. These biogeomorphological units are functional from an ecological point of view because at the population scale P. nigra is able (i) to strengthen its own inherent ability to resist hydrogeomorphological disturbances and thus is able to grow to sexual maturity in favourable conditions, and (ii) to strengthen its resilience capacity resulting from successful sexual reproduction through the facilitation effect offered by the upstream more mature cohorts.
However, the very strong spatial expansion by P. nigra , in a context of channelization that no longer allows the channel to sufficiently erode its banks, has led, after thirty years, to a very imminent situation of blocking of the biogeomorphological succession in the post-pioneer and mature stages with reducing possibility of rejuvenation and regeneration of open alluvial surfaces (Corenblit et al., 2016b). It was suggested that only a very exceptional flood with a return period of about 100 years may be able to reset the succession today.
The observations of P. nigra population and landform dynamics on the Garonne River are concordant with the concept of biogeomorphological life cycle (BLC) proposed by Corenblit et al. (2014). The BLC model applies at the population level and aligns with the concept of positive niche construction sensu Odling-Smee et al. (2003). Natural selection acting on riparian trees over their life cycle would, over the long term, lead to an increased probability of survival and sexual reproduction in individuals exhibiting suitable trait variations (Corenblit et al., 2009b, 2020b). The fluvial environment, characterized by strong physical constraints, exerts a predominant selective pressure on riparian trees. However, the influence of riparian trees on the hydrogeomorphological components of their environment can modulate this pressure throughout the life cycle. Indeed, the life history of poplars in a riparian context consists of a succession of characteristic developmental phases where physical components of the hydrogeomorphological environment and biological characteristics of riparian trees interact (Corenblit et al., 2014a). The establishment of a population of P. nigra on exposed sediment surfaces is not an exclusively unidirectional process in which seedlings solely undergo physical selection pressures. The feedback dynamics betweenP. nigra trait values and population physiognomy and geomorphology during its life cycle lead to the emergence of biogeomorphic entities in the river corridor, such as vegetated fluvial islands, bars and wooded floodplains. The BLC describes the fundamental phases of propagule dispersal, recruitment, and establishment up to the sexual maturity of P. nigra , outlining the hierarchy of interactions and feedbacks between biotic and abiotic components during these phases. This model elucidates how the modification of the hydrogeomorphological environment by the black poplar enhances its fitness (determined by its survival, growth, and sexual reproduction), resulting in a positive niche construction.