Figure 1. Number of peer reviewed journal articles per year
using the combined terms ”riparian vegetation” + ”geomorphology” +
”river*”, as archived on Clarivate Web of Knowledge (all databases)
(https://www-webofscience-com.inee.bib.cnrs.fr/). A.M. Gurnell’s first
article using these terms was published in 1997 (see vertical black
arrow) and her name comes out first with more than 12% of all articles
authored or co-authored. Accessed on 11th November
2023.
The objective of this article is to present a set of studies and results
from the past 20 years obtained by the authors and many collaborators,
including Angela M. Gurnell, on a panel of French rivers: Tech, Garonne,
Isère and Allier Rivers. In particular, feedbackmechanisms between fluvial morphodynamics and riparian vegetation
dynamics were investigated directly in the field and used high
resolution remote sensing at the scale of individual plants,
populations, communities and landscapes as well as during
semi-controlled ex situ experiments at the scale of individual
plants. Collectively, the authors’ research contributed to elucidate
some key aspects of feedback dynamics between the lowest and highest
levels of the riparian ecosystem organisation. This article presents and
discusses those key aspects.
Feedbacks between vegetation and fluvial geomorphology at
nested spatiotemporal scales
Riparian ecosystems are among the most dynamic and variable environments
on Earth’s surface in terms of structure and function, making them a
target of many ecological restoration projects (Amoros and Petts, 1993;
Naiman and Décamps, 1997; Muñoz-Mas et al., 2017; Gonzalez et al., 2018;
González, et al. 2022; O’Briain et al., 2023). They represent land-water
interface zones where matter and energy are concentrated, transit, are
stored, and transformed. The flows of water, sediment, organic matter –
from dissolved organic matter to large woods –, and seeds that pass
through, both cyclically (hydrological regime) and stochastically
(floods), contribute to maintain a high degree of heterogeneity in the
mosaic of habitats (Townsend, 1989; Gurnell et al., 2005). This
spatiotemporal heterogeneity generates strong ecological gradients and
sometimes exceptional levels of biodiversity, including a variety of
survival and reproduction strategies in a shifting mosaic (Ward, 1989;
Schnitzler et al., 1992; Ward et al., 2002; Stanford et al., 2005;
Bornette et al., 2008).
Hydroecological theoretical models are based on the postulate that the
structure and functioning of aquatic and riparian communities can be
explained by considering the links between life history traits of
species and the geomorphological structures and processes that govern
flows of matter and energy (Vannote et al., 1980; Newbold et al., 1982;
Pickett and White, 1985; Amoros et al., 1987; Junk et al., 1989;
Townsend, 1989; Ward, 1989; Tockner et al., 2000). Models such as the
River continuum concept, Flood pulse and Flow pulse, Nutrient
spiralling, Patch dynamics, were nevertheless developed according to a
type of Physical habitat template approach (sensu Southwood,
1977), that is, focusing strictly on the a priori unidirectional
effect of the physicochemical heterogeneity on biodiversity and
ecosystem functioning. Previous research on riparian ecosystems has not
sufficiently considered how and to what extent the three spatial
dimensions of riverine ecosystems are controlled by riparian vegetation.
Certain plant species and assemblages have a profound and lasting impact
on the structure and function of riparian ecosystems by modulating
matter and energy fluxes and modifying substrate cohesion (Gurnell et
al., 2012). Riparian plant species (e.g., the black poplar Populus
nigra L. in the Northern hemisphere) that significantly affect
hydrogeomorphological processes and fluvial landforms have been
identified as ecosystem engineers (sensu Jones et al., 1994),
keystone species (sensu Paine, 1966), and, more recently, as
potential niche constructors sensu Odling-Smee et al. (2003)
(Edwards et al., 1999; Gurnell and Petts, 2006; Corenblit et al., 2009b;
Francis et al., 2009).
How, and to what extent, the structure and function of riparian
ecosystems are controlled by abiotic-biotic feedbacks across various
spatiotemporal scales has become an important issue in fluvial
biogeomorphology. This question must be addressed from the fine scale of
the individual plant, the microhabitat and single hydrological event to
the coarse scale of plant community mosaic, the fluvial corridor and the
hydrosedimentary regime. In particular, the mechanisms by which engineer
plants respond to, and shape, the mosaic of riparian habitats are not
fully understood. Some evolutionary adaptations of plants in response to
the hydrogeomorphological disturbance regime such as floating seeds,
synchronization of reproduction with the flood regime (Junk et al.,
1986; Guilloy-Froget et al., 2002), vegetative reproduction in addition
to sexual reproduction strategies (Barsoum, 2002; Francis and Gurnell,
2006; Moggridge and Gurnell, 2009), and biomechanical stem flexibility
in order to avoid uprooting during flood events (Karrenberg et al.,
2002; Lytle and Poff, 2004; Bornette et al., 2008), help riparian plants
to survive despite high physical constraints. These adaptations,
combined with phenotypic plasticity, allow engineer plants to
mechanically resist to drag force in the flow and to influence their
riverine environment. By trapping sediments, organic matter, and seeds,
engineer plants can create, build, and maintain riparian habitats that
are beneficial to other plant species, including other engineer plants.
In addition to the role of vegetation assemblages (i.e., community
scale) in shaping the hydrogeomorphic features of riverine ecosystems,
specific life history, morphological and biomechanical traits related to
distinct engineer species also play an important role. In the absence of
large, destructive floods, the short-range actions of plants – within
their own stands and immediately downstream – typically result under
adequate conditions in the accumulation of fine sediment, nutrients,
organic matter, which improve habitat quality. In addition, with the
deposition of seeds (e.g., Goodson et al., 2001, 2002; Gurnell et al.,
2004, 2006; Corenblit et al., 2009a, 2016a), the improved habitat
quality, in turn, promotes the establishment and growth of new plant
species, including other engineer species. This process can increase the
likelihood of pioneer woody engineer species such as P. nigrareaching sexual maturity (Corenblit et al., 2014a, 2018).
The synchronized process of fluvial landform construction and plant
establishment/growth/succession can lead to the emergence of
characteristic biogeomorphological landforms such as fluvial islands
that are the result of engineering effects of riparian plants (Gurnell
et al., 2001). These positive abiotic-biotic feedback dynamics which
potentially affect the fluvial style (e.g., island braided riverssensu Gurnell et al. 2019) are consistent with the action of
ecosystem engineers (sensu Jones et al., 1994; Gurnell and Petts,
2006) and, potentially, niche constructors (sensu Odling-Smee et
al., 2003; Corenblit et al., 2015). In turn, the effects of riparian
engineer plants on the hydrogeomorphological components of riverine
ecosystems involve a set of responses in these plants, from the
individual to the community mosaic level and vice versa. This
abiotic-biotic feedback is organized in three stages (Fig. 2):
(1) Initial response of plants to hydrogeomorphological constraints: at
the beginning of growth and succession, plants must respond to a variety
of hydrogeomorphological constraints, including mechanical forces (e.g.,
stream flow, erosion and sediment burial) and physiological stress
factors (e.g., prolonged immersion and drought). These constraints act
as a powerful filter for the colonisation, establishment and growth of
engineer plants in exposed areas on alluvial bars. However, they can
also stimulate morphological and biomechanical responses that increase
plant resistance to these constraints.
(2) Effects of engineer plants on hydrogeomorphological components: the
mechanical resistance allows plants to act passively on their
hydrosedimentary and geomorphological environment. Once established,
engineer plants can modify the hydrogeomorphological components of their
environment in a variety of ways. For example, their roots can increase
substrate cohesion, which can help to stabilize bars and banks and
prevent erosion. Their trunks, branches and leaves can promote trapping
of sediment and organic matter, which can help to build up islands,
floodplains and create new habitats.
(3) Plant response to changes in the hydrogeomorphological environment:
as engineer plants modify their environment, they can create positive
feedback loops that further enhance their impact and growth (Corenblit
et al., 2009a, 2014, 2018) as well as negative feedback loops translated
into the transition toward stabilized and mature stages of the
vegetation succession on raised alluvial surfaces (Bendix and Hupp,
2000; Corenblit et al., 2007; Corenblit et al., 2009a). For example, the
increased stability of river bars and banks and their vertical accretion
provided by engineer plants can reduce erosion and sediment burial,
which can create more favourable conditions for the establishment of
other competitive plants. This can lead to the development of complex
and diverse riverine ecosystems with increased beta diversity at the
landscape scale (Gurnell et al., 2005).