Long-term river management legacies strongly alter riparian forest attributes and constrain restoration strategies along a large, multi-use river

Abstract

Many terrestrial ecosystems have undergone profound transformation under the pressure of multiple human stressors. This may have oriented altered ecosystems toward transient or new states. Understanding how these cumulative impacts influence ecosystem functions, services and ecological trajectories is therefore essential to defining effective restoration strategies. This is particularly the case in riverine ecosystems, where the profound alteration of natural disturbance regimes can make the effectiveness of restoration operations questionable. Using the case study of legacy dike fields, i.e., area delimited by longitudinal and lateral dikes, along the regulated Rhône River, we studied the impacts of long-term channelization and flow regulation on environmental conditions and riparian forests attributes along a 200km climatic gradient. We characterized the imprint of human stressors on these forests by comparing the dike field stands to more natural stands in both young and mature vegetation stages. Across four reaches of the river between Lyon and the Mediterranean Sea, we found that channelization consistently promoted high rate of overbank sedimentation and rapid disconnection of dike field surfaces from the channel. The rapid terrestrialisation of dike field surfaces, i.e., the process by which former aquatic areas transition to a terrestrial ecosystem as a result of dewatering or sedimentation, fostered a pulse of riparian forest regeneration in these resource-rich environments that differs from more natural sites in structure and composition. Within the dike fields, older pre-dam stands are dominated by post-pioneer and exotic species, and post-dam stands support large, aging pioneer trees with a largely exotic understory regeneration layer. These patterns were associated with differences in the relative surface elevation among dike fields, whereas species shifts generally followed the river's longitudinal climate gradient. To enhance the functionality of these human-made ecosystems, restoration strategies should target the reconnection of dike fields to the river by dismantling part of the dikes to promote lateral erosion, forest initiation and community succession, as well as increasing minimum flows in channels to improve connection with groundwater. However, since a river-wide return to a pre-disturbance state is very unlikely, a pragmatic approach should be favoured, focusing on local 3 actions that can improve abiotic and biotic function, and ultimately enhancing ecosystem services such biodiversity, habitat, and recreation opportunities.

The accumulation of anthropogenic stressors induces a progressive shift in the ecological preferences and morphological traits shared by riparian plant communities

“…Furthermore, the cover of the ‘flood‐tolerant riparian herbs’ guild was shown to increase in response to prolonged flooding conditions in China (Su et al, 2020). This combination of traits suggests that flow regulation and stabilisation associated with limited sediment supply conditions have promoted less stressful, more stable, and more fertile abiotic conditions (Franklin et al, 2009; Janssen et al, 2021) and the development of medium to tall, often clonal, perennial species (e.g., Chen et al, 2017; Minchinton & Bertness, 2003), better adapted to competitive interactions because less subject to environmental selection. Although these results can be partially explained by differences in gravel bar topography and soil texture between the five river reaches studied, they also reflect differences and adaptations that can be more directly attributed to anthropogenic environmental changes.…”

“…Finally, and from an applied point of view, our results showed that the restoration actions targeting a single stressor will not be sufficient to reorient riparian plant communities towards an ecological state closer to reference systems. For example, on the Rhône River, the accumulation of human‐induced stressors over time, i.e., a correction phase during the 19th century followed by a derivation phase in the second half of the 20th century, have deeply altered the flow and sediment regime of this large river (Tena et al, 2020; Vázquez‐Tarrío et al, 2019) as well as the associated riparian communities (Janssen et al, 2021). Thus, even if lateral bank erosion will certainly be enhanced by actions at the site scale, such as with the removal of lateral dikes, the reactivation of the fluvial dynamics allowing the recreation of pioneer habitats will require a more integrated approach based on the restoration of hydrological, morphological, and ecological processes at the reach scale.…”

Can vegetation clearing operations and reprofiling of bars be considered as an ecological restoration measure? Lessons from a 10‐year vegetation monitoring program (Loire River, France)

“…In highly regulated and channelized gravel bed rivers, where flow and sediment regimes are largely altered but also where restoration of pioneer riparian communities is a major issue, as is the case for the Rhone (e.g. Janssen et al 2021) and the Rhine (e.g. Baptist et al 2004), there is an urgent need to better document the response and dynamics of riparian communities to artificially rejuvenated surfaces.…”

A dynamic riparian forest structure model for predicting large wood inputs to meandering rivers

“…effectively to investigate succession patterns in a number of biomes (Balian & Naiman, 2005;Cline & McAllister, 2012;Fierke & Kauffman, 2005;González et al, 2010;Scott et al, 1997). In rivers heavily modified by flow regulation and channelization, these assumptions do not always hold, and riparian community dynamics often diverge from more natural systems (Janssen et al, 2020b).…”

“…More generally, information on the structure, composition, and dynamics of floodplain forests is also fundamental for assessing how changes in river management affect riparian ecosystems (Janssen et al, 2020b;Johnson et al, 2012). Management actions that are relevant in this context include horticultural restoration, constraints on meander migration from installation of revetment, modifications of river hydraulics from levee construction, and flow regime changes caused by dams, diversions, and groundwater pumping.…”