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.