Johannes Steiger

Implications of biological and physical diversity for resilience and resistance patterns within Highly Dynamic River Systems

Abstract.The structure and function of alluvial Highly Dynamic River Systems (HDRS) are driven by highly variable hydrological disturbance regimes, and alternate between resistant, metastable states and resilient, transitional states. These are in turn subject to influences of feedback loops within hydrogeomorphic and biological processes. Here we consider how resistance and resilience largely determine HDRS ecosystem trajectories and how these characteristics can be modified by natural and anthropogenic processes. We review the mechanisms by which biodiversity can affect both resistance and resilience and introduce a conceptual framework that incorporates some unique HDRS characteristics. We suggest that resilient and resistant patterns frequently coexist in the active tract of these river systems, and that this coexistance promotes the return of metastable states after major disturbances. In contrast, highly resistant and poorly resilient patterns dominate at their external boundaries. The loss of these natural dynamics resulting from direct and indirect human impacts causes deviations to resistance and resilience patterns and therefore to HDRS trajectory. We propose that understanding the role of interactions between biological and physical processes that control resistance and resilience is crucial for system restoration and management.

The emergence of an 'evolutionary geomorphology'?

Earth surface processes and landforms are modified through the actions of many microorganisms, plants and animals. As organism-driven landform modifications are sometimes to the advantage of the organism, some of these landform features have become adaptive functional components of ecosystems, concurrently affecting and responding to ecological and evolutionary processes. These recent eco-evolutionary insights, focused on feedback among geomorphologic, ecological and evolutionary processes, are currently leading to the emergence of what has been called an ‘evolutionary geomorphology’, with explicit consideration of feedbacks among the evolution of organisms, ecosystem structure and function and landform organization at the Earth surface. Here we provide an overview in the form of a commentary of this emerging sub-discipline in geosciences and ask whether the use of the term ‘evolutionary geomorphology’ is appropriate or rather misleading.

Historical fluvial palaeodynamics and multi-proxy palaeoenvironmental analyses of a palaeochannel, Allier River, France

A palaeoenvironmental study of a palaeochannel within the lower alluvial floodplain of the Allier River, France, has been carried out at a high temporal resolution. Research was based upon a multi-proxy approach using different sedimentological (magnetic susceptibility, sediment texture and loss on ignition) and palaeoecological (pollen, non-pollen palynomorphs (NPPs), diatoms – indicators). The palaeochannel was dated from six radiocarbon dates and spans from the second to the eleventh century AD. The cultural landscape history of this floodplain and the progressive hydrological disconnection of the palaeochannel between the Antiquity and the early Middle Ages could be reconstructed according to several characteristic environmental phases. Co-inertia analysis linking pollen, NPPs and diatom corroborated the significant co-structure of these proxies and allowed to distinguish functional palaeoecological groups in terms of landscape openness, on-site aquatic and hygrophilous vegetation, riparian woodland, soil moisture and grazing pressure. Results obtained underpin the importance of grazing as a major agent of landscape change in floodplains, especially since the ninth century AD, when the clearance of the riparian woodland is attested. Gallo-Roman and Late-Antiquity land-use systems, on the contrary, allowed for the preservation of riparian fluvial landscapes.

Autochthonous Versus Allochthonous Organic Matter in Recent Soil C Accumulation Along a Floodplain Biogeomorphic Gradient: An Exploratory Study

Abstract The mechanisms controlling soil succession in floodplains remain much less studied than in uplands due to the complexity that flooddriven erosion and sedimentation bring into soil development processes. The amount of organic matter and C generally grows with soil ageing and is controlled by multiple and interacting allogenic and autogenic factors, but to what extent the production of organic matter by in situ vegetation contributes to soil formation in floodplains remains unknown. The objective of this work was to explore the importance of autochthonous organic matter versus allochthonous organic matter in organic C accumulation of floodplain forest soils along a vegetation succession and hydrogeomorphic connectivity gradient. Physicochemical analyses of sediment collected after one single flood event in a large Mediterranean floodplain (Middle Ebro, a 9th order regulated river reach in NE Spain) were used to estimate the proportion of organic C found in the topsoil (first 10 cm) samples of young (<25 yr), mature (25-50 yr) and old (>50 yr) floodplain forests that had an allochthonous (i.e., % of organic C deposited by floods) or autochthonous (i.e., % of organic C produced in situ by vegetation) source. Results of this exploratory study showed that the accumulation of autochthonous organic C in the floodplain topsoil only occurred in floodplain forests older than 50 year-old, but even then, it was more than six-fold less abundant than that with an allochthonous origin. Moreover, a linear mixed effect model showed that, although autochthonous organic C accumulation was mainly explained by the forest structure, a small proportion of it was also controlled by an allogenic factor, the groundwater table depth. Then, groundwater table depth variations could be partly controlling autochthonous organic matter production and decomposition in this Mediterranean floodplain. Although flow regulation and embankment has dramatically limited the hydrogeomorphic dynamics of the river, allogenic overbank sedimentation during flood events still controls floodplain soil succession and organic C accumulation in the floodplain.

Exploring the key drivers of riparian woodland successional pathways across three European river reaches

Climate change and river regulation are negatively impacting riparian vegetation. To evaluate these impacts, process‐based models are preferred over data‐driven approaches. However, they require extensive knowledge about ecohydrological processes. To facilitate the implementation of such process‐based models, the key drivers of riparian woodland successional pathways across three river reaches, in Austria, Portugal, and Spain, were explored, employing two complementary approaches. The principal component analyses highlighted the importance of the physical gradients determining the placement of the succession phases within the riparian and floodplain zones. The generalized additive models revealed that the initial and pioneer succession phases, characteristic of the colonization stage, appeared in areas highly morphodynamic, close in height and distance to the water table, and with coarse substrate, whereas elder phases within the transitional and mature stages showed incremental differences, occupying less dynamic areas with finer substrate. The Austrian site fitted well the current successional theory (elder phases appearing sequentially further up and distant), but at the Portuguese site, the tolerance of the riparian species to drought and flash flood events governed their placement. Finally, at the Spanish site, the patchy distribution of the elder phases was the remnants of formative events that reshaped the river channel. These results highlight the complex relationships between flow regime, channel morphology, and riparian vegetation. The use of succession phases, which rely on the sequential evolution of riparian vegetation as a response to different drivers, may be potentially better reproducible, within numerical process‐based models, and transferable to other geographical regions.