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

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 200 km 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 actions that can improve abiotic and biotic function, and ultimately enhancing ecosystem services such biodiversity, habitat, and recreation opportunities.

Trade-offs between carbon stocks and biodiversity in European temperate forests.

Policies to mitigate climate change and biodiversity loss often assume that protecting carbon-rich forests provides co-benefits in terms of biodiversity, due to the spatial congruence of carbon stocks and biodiversity at biogeographic scales. However, it remains unclear whether this holds at the scales relevant for management, and particularly large knowledge gaps exist for temperate forests and for taxa other than trees. We built a comprehensive dataset of Central European temperate forest structure and multi-taxonomic diversity (beetles, birds, bryophytes, fungi, lichens, and plants) across 352 plots. We used Boosted Regression Trees (BRTs) to assess the relationship between above-ground live carbon stocks and (a) taxon-specific richness, (b) a unified multidiversity index. We used Threshold Indicator Taxa ANalysis to explore individual species' responses to changing above-ground carbon stocks and to detect change-points in species composition along the carbon-stock gradient. Our results reveal an overall weak and highly variable relationship between richness and carbon stock at the stand scale, both for individual taxonomic groups and for multidiversity. Similarly, the proportion of win-win and trade-off species (i.e., species favored or disadvantaged by increasing carbon stock, respectively) varied substantially across taxa. Win-win species gradually replaced trade-off species with increasing carbon, without clear thresholds along the above-ground carbon gradient, suggesting that community-level surrogates (e.g., richness) might fail to detect critical changes in biodiversity. Collectively, our analyses highlight that leveraging co-benefits between carbon and biodiversity in temperate forest may require stand-scale management that prioritizes either biodiversity or carbon in order to maximize co-benefits at broader scales. Importantly, this contrasts with tropical forests, where climate and biodiversity objectives can be integrated at the stand scale, thus highlighting the need for context-specificity when managing for multiple objectives. Accounting for critical change-points of target taxa can help to deal with this specificity, by defining a safe operating space to manipulate carbon while avoiding biodiversity losses.

Divergence of riparian forest composition and functional traits from natural succession along a degraded river with multiple stressor legacies.

Prolonged exposure to human induced-stressors can profoundly modify the natural trajectory of ecosystems. Predicting how ecosystems respond under stress requires understanding how physical and biological properties of degraded systems parallel or deviate over time from those of near-natural systems. Utilizing comprehensive forest inventory datasets, we used a paired chronosequence modelling approach to test the effects of long-term channelization and flow regulation of a large river on changes in abiotic conditions and related riparian forest attributes across a range of successional phases. By comparing ecological trajectories between the highly degraded Rhône and the relatively unmodified Drôme rivers, we demonstrated a rapid, strong and likely irreversible divergence in forest succession between the two rivers. The vast majority of metrics measuring life history traits, stand structure, and community composition varied with stand age but diverged significantly between rivers, concurrent with large differences in hydrologic and geomorphic trajectories. Channelization and flow regulation induced a more rapid terrestrialization of the river channel margins along the Rhône River and accelerated change in stand attributes, from pioneer-dominated stands to a mature successional phase dominated by non-native species. Relative to the Drôme, dispersion of trait values was higher in young forest stands along the Rhône, indicating a rapid assembly of functionally different species and an accelerated transition to post-pioneer communities. This study demonstrated that human modifications to the hydro-geomorphic regime have induced acute and sustained changes in environmental conditions, therefore altering the structure and composition of riparian forests. The speed, strength and persistence of the changes suggest that the Rhône River floodplain forests have strongly diverged from natural systems under persistent multiple stressors during the past two centuries. These results reinforce the importance of considering historical changes in environmental conditions to determine ecological trajectories in riparian ecosystems, as has been shown for old fields and other successional contexts.

How maintenance and restoration measures mediate the response of riparian plant functional composition to environmental gradients on channel margins: Insights from a highly degraded large river.

Riparian habitats are transitional zones where strong environmental gradients shape community. To prevent flood risks and channel migration on managed rivers, civil engineering techniques have been widely used. Recently, ecological restoration of rivers has become a major issue. However, given the alteration of natural disturbance regimes induced by human infrastructures, the real added-value of these restoration actions is questionable. Thus, a major challenge is to better understand whether changes in abiotic conditions induced by human activity influence the response of plant communities to environmental gradients. Studying a highly degraded large river, we evaluated the effect of the elevation and soil texture gradients on plant functional composition and assessed whether human-mediated environment gradients, achieved through maintenance and restoration measures, shape community structure. In the summer of 2017, we sampled 17 geomorphic surfaces, mostly gravel bars, along the Rhône River and its tributaries that were either repeatedly cleared (brush clearing vs plowing), newly reprofiled or naturally rejuvenated by high flows. The results show shifts in trait values with elevation and convergence in plant traits with increasing proportion of fine sediments. The co-occurrence of species with contrasting traits was higher in highly disturbed environments, revealing the importance of rejuvenation processes. However, the influence of both environmental gradients was mediated by human activity. For maintenance measures, plowing was better able to promote species diversity and limited biotic homogenization along environmental gradients. Among the three geomorphic surfaces, naturally rejuvenated bars were the most stressful environments, hosting distinct functional assemblages, while communities on newly reprofiled banks were in the same ecological trajectories as repeatedly cleared bars. To promote an effective ecological restoration of riparian zones, (i) a greater variability of the minimum flow is needed, (ii) bedload transport restoration should be a priority and (iii) reprofiled banks should better mimic the landforms of natural river margins.