Considering river structure and stability in the light of evolution: feedbacks between riparian vegetation and hydrogeomorphology

River ecological functioning can be conceptualized according to a four‐dimensional framework, based on the responses of aquatic and riparian communities to hydrogeomorphic constraints along the longitudinal, transverse, vertical and temporal dimensions of rivers. Contemporary riparian vegetation responds to river dynamics at ecological timescales, but riparian vegetation, in one form or another, has existed on Earth since at least the Middle Ordovician (c. 450 Ma) and has been a significant controlling factor on river geomorphology since the Late Silurian (c. 420 Ma). On such evolutionary timescales, plant adaptations to the fluvial environment and the subsequent effects of these adaptations on fluvial sediment and landform dynamics resulted in the emergence, from the Silurian to the Carboniferous, of a variety of contrasted fluvial biogeomorphic types where water flow, morphodynamics and vegetation interacted to different degrees. Here we identify several of these types and describe the consequences for biogeomorphic structure and stability (i.e. resistance and resilience), along the four river dimensions, of feedbacks between riparian plants and hydrogeomorphic processes on contrasting ecological and evolutionary timescales. Copyright © 2014 John Wiley & Sons, Ltd.     

The role of soil in vegetated gravelly river braid plains: more than just a passive response?

This paper reviews the role of alluvial soils in vegetated gravelly river braid plains. When considering decadal timescales of river evolution, we argue that it becomes vital to consider soil development as an emergent property of the developing ecosystem. Soil processes have been relatively overlooked in accounts of the interactions between braided river processes and vegetation, although soils have been observed on vegetated fluvial landforms. We hypothesize that soil development plays a major role in the transition (speed and pathway) from a fresh sediment deposit to a vegetated soil‐covered landform. Disturbance (erosion and/or deposition), vertical sediment structure (process history), vegetation succession, biological activity and water table fluctuation are seen as the main controls on early alluvial soil evolution. Erosion and deposition processes may not only act as soil disturbing agents, but also as suppliers of ecosystem resources, because of their role in delivering and changing access (e.g. through avulsion) to fluxes of water, fine sediments and organic matter. In turn, the associated initial ecosystem may influence further fluvial landform development, such as through the trapping of fine‐grained sediments (e.g. sand) by the engineering action of vegetation and the deposit stabilization by the developing aboveground and belowground biomass. This may create a strong feedback between geomorphological processes, vegetation succession and soil evolution which we summarize in a conceptual model. We illustrate this model by an example from the Allondon River (Switzerland) and identify the research questions that follow. Copyright © 2014 John Wiley & Sons, Ltd.     

The biogeomorphological life cycle of poplars during the fluvial biogeomorphological succession: a special focus on Populus nigra L.

Riverine ecosystems are recurrently rejuvenated during destructive flood events and vegetation succession starts again. Poplars (i.e. species from Populus genera) respond to hydrogeomorphological constraints, but, in turn, also influence these processes. Thus, poplar development on bare mineral substrates is not exclusively a one‐way vegetative process. Reciprocal interactions and adjustments between poplar species and sediment dynamics during their life cycle lead to the emergence of biogeomorphological entities within the fluvial corridor, such as vegetated islands, benches and floodplains. Based on a review of geomorphological, biological and ecological literature, we have identified and described the co‐constructing processes between riparian poplars and their fluvial environment. We have explored the possibility that the modification of the hydrogeomorphological environment exerted, in particular, by the European black poplar (Populus nigra L.), increases its fitness and thus results in positive niche construction. We focus on the fundamental phases of dispersal, recruitment and establishment until sexual maturity of P. nigra by describing the hierarchy of interactions and the pattern of feedbacks between biotic and abiotic components. We explicitly relate the biological life cycle of P. nigra to the fluvial biogeomorphic succession model by referring to the ‘biogeomorphological life cycle’ of P. nigra. Finally, we propose new research perspectives based on this theoretical framework. Copyright © 2014 John Wiley & Sons, Ltd.     

Populus nigra L. establishment and fluvial landform construction: biogeomorphic dynamics within a channelized river

Populations of the riparian pioneer species Populus nigra L. which establish on alluvial bars within river channels modulate sediment dynamics and fluvial landforms. Dense cohorts of P. nigra have colonized gravel point bars along the channelized River Garonne, France, during the last 20 years and have enhanced the vertical, lateral and longitudinal development of the bars. For this period, the geomorphic characteristics of two wooded point bars on this laterally stable river are closely linked to the spatial distribution and intensity of establishment and resistance of different cohorts of P. nigra. Furthermore, P. nigra colonization dynamics were controlled by engineer effects of this same species. This relationship is illustrated by a significant correlation between key geomorphic and biological variables measured in situ and characterized with a set of four aerial photographs taken between 2000 and 2010. The development of wooded point bars, which are discrete biogeomorphic units, over the studied period, appear to result from a specific biogeomorphic positive feedback of matter aggregation and vegetation establishment related to sediment trapping and stabilization by pioneer engineer plants. We propose a conceptual model of biogeomorphic unit construction for channelized, lateral stable rivers. We consider the resultant biogeomorphic units as functional from an ecological point of view because P. nigra enhances at the cohort scale (i) its own inherent capacity to resist hydrogeomorphic disturbances, and (ii) its resilience capacity as a result of successful colonization, especially downstream of mature poplar stands. Copyright © 2016 John Wiley & Sons, Ltd.     

Monitoring and reconstructing past biogeomorphic succession within fluvial corridors using stereophotogrammetry

Past fluvial biogeomorphic succession dynamics, i.e. reciprocal interactions and adjustments between vegetation growth and fluvial landform construction, were monitored and reconstructed using stereophotogrammetry. The four‐dimensional spatio‐temporal stereophotogrammetric analyses were based on the use of archival analogue and digital aerial photographs. First, we tested the relevance of the technique to produce floodplain digital terrain models (DTMs) and cover height models (CHMs) of the dynamic River Allier, France, and compared the models derived from photogrammetric procedures to field measurements for CHMs and to LiDAR data for DTMs. Automatic photogrammetric procedures tended to create inaccurate digital models with production of outliers, incomplete sectors and areas of confusion especially for analogue stereo‐pairs. Expert correction using stereoscopic viewing improved the vertical accuracy of the digital models, but the vegetation height tended to be underestimated: approximately 0.50 m for vegetation heights less than 10 m, up to 1.50 m for tree heights higher than 25 m. Second, we applied this method to a wooded point bar located on the channelized River Garonne, France. At the scale of the point bar, accurate biogeomorphic maps that show terrain and vegetation height changes in all three spatial dimensions were produced and accurate vegetation growth curves from the early stages of establishment until maturity were extracted. Assuming that a set of conditions is satisfied (e.g. spatial scale of investigation, quality of the photographs), our results show that the photogrammetric method applied in this research can be used operationally to detect and quantify present fluvial biogeomorphic dynamics (i.e. changes of topography and vegetation canopy height) within fluvial corridors of temperate rivers with satisfactory accuracy. Copyright © 2016 John Wiley & Sons, Ltd.     

Quantification of biogeomorphic interactions between small-scale sediment transport and primary vegetation succession on proglacial slopes of the Gepatschferner,Austria

Proglacial slopes provide suitable conditions for observing the co-development of abiotic and biotic systems. The frequency and magnitude of geomorphic processes and plant composition govern this interplay, which is described in the model of biogeomorphic succession. In high mountain environments, this model has only been tested in a limited number of studies. The study aimed to quantify small-scale sediment transport via erosion plots along a plant cover gradient and to investigate the influence of sediment transport on plant communities. We aimed to generate quantitative data to test existing biogeomorphic models. Small-scale biogeomorphic interactions were investigated on 30 test plots of 2 × 3 m size on proglacial slopes of the Gepatschferner (Kaunertal) in the Austrian Alps during the snow-free summer months over three consecutive years. The experimental plots were established on slopes along a plant cover gradient. A detailed vegetation survey was carried out to capture biotic conditions, and specific sediment yield was measured at each plot. Species abundance and composition at each site reflected successional stages. Additional environmental parameters, such as terrain age, geomorphometry, grain size distribution, soil nutrients, and precipitation, were also included in the analyses. We observed two pronounced declines in geomorphic activity on plots with both above 30% and above 75% plant cover. Nonmetric multidimensional scaling showed distinct clusters of vegetation composition that mainly followed a successional gradient. Sites that were affected by high-magnitude geomorphic events showed different environmental conditions and species communities. Quantified process rates and observed species composition support the concept of biogeomorphic succession. The findings help to narrow down a biogeomorphic feedback window.     

Conditions for feedbacks between geomorphic and vegetation dynamics on lateral moraine slopes: a biogeomorphic feedback window

Little Ice Age lateral moraines represent one of the most important sediment storages and dynamic areas in glacier forelands. Following glacier retreat, simultaneous paraglacial adjustment and vegetation succession affect the moraine slopes. Geomorphic processes (e.g. debris flows, interrill erosion, gullying, solifluction) disturb and limit vegetation development, while increasing vegetation cover decreases geomorphic activity. Thus, feedbacks between geomorphic and vegetation dynamics strongly control moraine slope development. However, the conditions under which these biogeomorphic feedbacks can occur are insufficiently understood and major knowledge gaps remain. This study determines feedback conditions through the analysis of geomorphic and vegetation data from permanent plots in the Turtmann glacier foreland, Switzerland. Results from multivariate statistical analysis (i) confirm that Dryas octopetala L. is an alpine ecosystem engineer species which influences geomorphic processes on lateral moraines and thereby controls ecosystem structure and function, and (ii) demonstrate that biogeomorphic feedbacks can occur once geomorphic activity sufficiently decreases for D. octopetala to establish and cross a cover threshold. In the subsequent ecosystem engineering process, the dominant geomorphic processes change from flow and slide to bound solifluction. Increasing slope stabilization induces a decline in biogeomorphic feedbacks and the suppression of D. octopetala by shrubs. We conceptualize this relationship between process magnitude, frequency and species resilience and resistance to disturbances in a ‘biogeomorphic feedback window’ concept. Our approach enhances the understanding of feedbacks between geomorphic and alpine vegetation dynamics on lateral moraine slopes and highlights the importance of integrating geomorphic and ecological approaches for biogeomorphic research. Copyright © 2015 John Wiley & Sons, Ltd.     

Formation and dynamics of vegetated fluvial landforms follow the biogeomorphological succession model in a channelized river

Feedback between hydrogeomorphological processes and riparian plants drives landscape dynamics and vegetation succession in river corridors. We describe the consequences of biogeomorphological feedback on the formation and dynamics of vegetated fluvial landforms based on observations from the channelized Isère River in France. The channel was laterally confined with embankments and mostly straightened. From the beginning of the 1970s to the end of the 1990s, alternate bars were progressively but heavily colonized by vegetation. This context presented an exceptional opportunity to analyse temporal adjustments between fluvial landforms and vegetation succession from bare gravel bars to mature upland forest as the consequence of biogeomorphological interactions. Based on a GIS analysis of aerial photographs (between 1948 and 1996), we show that the spatiotemporal organization of vegetated bars within the river channel observed in 1996 resulted from a bioconstruction and biostabilization effect of vegetation and interactions between bars of varying age, size and mobility. Field measurements in 1996 reflected how a strong positive feedback between sedimentary dynamics and riparian vegetation succession resulted in the construction of the vegetated bars. A highly significant statistical association of geomorphological and vegetation variables (RV of co-inertia analysis = 0.41, p < 0.001) explained 95% of the variability in just one axis, supporting the existence of very strong feedback between geomorphological changes (i.e. the transformation of small bare alternate bars to fluvial landforms covered by mature upland forest, and vegetation succession). Such dynamics reflect the fluvial biogeomorphological successions model, as described by the authors earlier. © 2020 John Wiley & Sons, Ltd.     

Riparian vegetation life stages control the impact of flood sequencing on braided river morphodynamics

With riverine flooding set to be more frequent in many parts of the world as a result of climate change, the interactions between fluvial morphodynamics and riparian vegetation may depend in part on the sequence of flood events. This paper describes a laboratory study of the geomorphic adjustment of a braided river to sequences of floods across five different strengths of braidplain vegetation. By using alfalfa as a proxy for braidplain vegetation, the differing plant life stages were used to represent the varying strengths of biogeomorphic feedbacks across the floods. Boundary conditions were constrained by sets of experimental runs with both equilibrium sediment loads and deficit loads. Changes in bed topography were monitored and assessed using a detailed digital elevation model, digital imagery and continuous monitoring of the transported sediment. Results demonstrate that in absence of plant colonization, vegetation placed the rivers in a non-equilibrium condition, in which riparian vegetation encouraged the development of new channels, increased the system channel width and enhanced topographic irregularity, these effects being more noticeable during the low-flow periods. The morphodynamics was found to be less sensitive to variations in flood discharges as the vegetation influence (strength) increased from minimum to maximum, until vegetation began to die back and the impacts of flood sequences became yet again evident. Although the overall sediment transport rate was reduced under full-grown vegetation conditions, the presence of the mature plants across the braid bars resulted in the greatest channel scour depths. Results are considered in light of expected changes in flood frequency with climate and likely morphodynamic responses of river systems as a result.     

Contrasting biogeomorphic processes affecting salt‐marsh development of the Mokbaai,Texel, The Netherlands

The growth and decline of salt marshes may be the result of various interacting biogeomorphic processes and external factors. We present a case study of the Mokbaai on the Wadden island of Texel, where we assess the relative importance and the interaction between the biogeomorphic processes and various disturbances. We analysed changes in vegetation composition in the salt marsh and sedimentation–erosion patterns of the adjoining intertidal flat over a 30‐year period. Vegetation underwent regression in the lower parts of the marsh, i.e. the low marsh zone changed into pioneer zone. Comparing elevation measurements from 2013 and 1983 showed that the adjoining intertidal flats eroded 15–25 cm. Maintenance dredging of a nearby harbour might negatively impact the sediment balance indicating that the regression of the lower parts of the salt marsh is caused by a lack of sediment. Simultaneously, a change in the local hydrology led to vegetation succession into high and brackish salt marsh, increased organic sediment production and consequently cliff formation. The results from this case study show that, even in a relatively small salt marsh, changes in external factors may set in motion a series of biogeomorphic processes and feedbacks, leading to locally contrasting trends in spatiotemporal development. © 2016 The Authors. Earth Surface Processes and Landforms Published by John Wiley & Sons Ltd.     

Controls on the sexual and asexual regeneration of Salicaceae along a highly dynamic, braided river system

Salicaceae are key pioneer riparian tree species that have the ability to reproduce sexually and asexually. Recent research has suggested that Salicaceae act as ‘ecosystem engineers’, modifying hydrological and geomorphological processes, resulting in the stabilisation and growth of landforms. Understanding these interactions requires knowledge of the controls on Salicaceae regeneration. This paper describes a study of Salicaceae establishment and growth along a reach of a highly dynamic, island-braided river. The sexual and asexual regeneration of three species were investigated using experimental planting of cuttings and observation of seedlings. Plots were located at a range of elevations, in different habitats associated with the established riparian vegetation and in contrasting sediment types. Survival and growth were monitored over two growing seasons. Asexual regeneration was more successful than sexual regeneration, with cuttings demonstrating faster growth rates and tolerance of broader environmental conditions than seedlings. Cutting survival and growth was highest in sediments with a relatively high organic content and in plots located between patchy Salicaceae stands or in the lee of islands. Seedling mortality was extremely high due to fluvial disturbance, although seedlings in habitats that were protected from fluvial disturbance survived. Seedling growth showed preferences for particular sedimentary conditions, which varied between species. The major control on regeneration was the upstream presence of established Salicaceae, particularly on islands, which provided open sites that were protected from fluvial disturbance and suitable for regeneration. Thus, asexual regeneration facilitated sexual regeneration by rapidly colonising sites that provided habitats protected from fluvial disturbance for seedling establishment. This supports previous work suggesting that Salicaceae can act as ‘ecosystem engineers’.     

Biogeomorphic keystones and equivalents: Examples from a bedrock stream

Biogeomorphic keystone species profoundly impact landscapes, such that their introduction or removal would cause fundamental changes in geomorphic systems. This paper explores the concept of biogeomorphic keystone species by examining the general vs. species-specific biogeomorphic impacts (BGIs) of trees on a limestone bedrock-controlled stream, Shawnee Run, in central Kentucky. Field investigation identified three strong BGIs: (i) biogeomorphic pool formation via bioweathering; (ii) root bank-associated bioprotection; and (iii) avulsion-originated island development linked to bioprotection. This research evaluates these impacts in the context of keystone or other biogeomorphic roles. A field survey was conducted on nine stream reaches, each consisting of 10–12 hydraulic units of riffle, pool, and run. Results suggest that American sycamore (Platanus occidentalis) plays a keystone role by promoting the development of ~42% of pools in the study area. While geomorphic pools are formed by fluvial process–form linkages, these biogeomorphic pools are developed by sycamore root-induced channel bed bioweathering. Only American sycamore and chinquapin oak (Quercus muehlenbergii) exhibited root-bank development amongst 15 different species identified – and thus play a vital role in bank bioprotection. Lastly, trees can promote avulsion-originated island formation by creating erosion-resistant bioprotective patches. Mature trees (in terms of size), particularly large American sycamore and chinquapin oak, dominate Shawnee Run islands with a mean diameter at breast height (DBH) > 40 cm. However, other trees can provide comparable bioprotection, particularly at mature stages. Because its absence would result in fundamentally different stream morphology, sycamore can be considered a biogeomorphic keystone species in Shawnee Run. © 2020 John Wiley & Sons, Ltd.     

Perspectives on biogeomorphology, ecosystem engineering and self-organisation in island-braided fluvial ecosystems

Using the River Tagliamento, Italy, as an example, we examine the role of self-organisation in the formation and dynamics of vegetated islands in fluvial ecosystems. We consider how various biogeomorphic processes, such as feedbacks between tree growth and sedimentation, influence island self-assembly, as well as the potential influences of island landforms on resource distribution and shifts in ecosystem state. Despite the abundance of island landforms of different sizes and ages in island-braided reaches along the River Tagliamento, island formation is only found within a specific hydrological and sedimentary envelope, and depends upon a delicate balance of biotic-abiotic feedbacks. As a result, island landforms tend to be lost when river functioning is altered by human interventions. We argue that the specific biogeomorphic processes and self-organisation associated with river island dynamics offer an example of biogeomorphic inheritance, in which reciprocal feedbacks between species and geomorphic processes favour engineer species and promote the future development of the landforms. Thus, islands represent extended phenotypes – or external expressions of genetic traits – of key riparian ecosystem engineers. This capacity to modify the physical environment has important implications for landform evolution and riparian biodiversity. In conclusion, we propose several topics that merit investigation to improve our understanding of the biogeomorphology and self-organisation of river island systems.     

Effects of riparian vegetation on stream bank subaerial processes in southwestern Virginia,USA

Riparian vegetation is frequently used for stream bank stabilization, but the effects of vegetation on subaerial processes have not been quantified. Subaerial processes, such as soil desiccation and freeze–thaw cycling, are climate‐related phenomena that deliver soil directly to the stream and make the banks more vulnerable to fluvial erosion by reducing soil strength. This study compares the impact of woody and herbaceous vegetation on subaerial processes by examining soil temperature and moisture regimes in vegetated stream banks. Soil temperature and water tension were measured at six paired field sites in southwestern Virginia, USA, for one year. Results showed that stream banks with herbaceous vegetation had higher soil temperatures and a greater diurnal temperature range during the summer compared to forested stream banks. Daily average summer soil water tension was 13 to 57 per cent higher under herbaceous vegetation than under woody vegetation, probably due to evapotranspiration from the shallow herbaceous root system on the bank. In contrast to summer conditions, the deciduous forest buffers provided little protection for stream banks during the winter: the forested stream banks experienced diurnal temperature ranges two to three times greater than stream banks under dense herbaceous cover and underwent as many as eight times the number of freeze–thaw cycles. During the winter, the stream banks under the deciduous forests were exposed to solar heating and night time cooling, which increased the diurnal soil temperature range and the occurrence of freeze–thaw cycling. Study results also indicated that freeze–thaw cycling and soil desiccation were greater on the upper stream bank due to thermal and moisture regulation of the lower bank by the stream. Therefore, subaerial erosion and soil weakening may be greater on the upper stream banks. Additional research is needed on the influence of subaerial processes on both subaerial and fluvial erosion. Copyright © 2006 John Wiley & Sons, Ltd.     

A bivariate model for estimating reciprocal causality within biogeomorphic ecosystems: The cross-lagged panel model

Biogeomorphic ecosystems (e.g. rivers, salt marshes, mangroves and coastal dunes) are shaped by feedbacks between geomorphology and engineer plants that occur at various spatiotemporal scales. The classical bivariate and multivariate statistical methods currently used in biogeomorphology do not permit clear identification of reciprocal causality between geomorphic and biological variables. The aim of this article is to present the potential of the cross-lagged panel model (CLPM) to estimate reciprocal associations (causality) between one geomorphic and one biological variable over time. This tool, which originates from behavioural, social, medical and educational sciences, has clear potential as a novel approach to causal analysis in the context of biogeomorphic ecosystems. We provide a case study of the application of CLPM for analysing biogeomorphic feedbacks between topography and Populus nigra L. physiognomy on a wooded point bar of the Garonne River, France. © 2018 John Wiley & Sons, Ltd.     

Interactions between river flows and colonizing vegetation on a braided river: exploring spatial and temporal dynamics in riparian vegetation cover using satellite data

Field, laboratory, and numerical modelling research are increasingly demonstrating the potential of riparian tree colonization and growth to influence fluvial dynamics and the evolution of fluvial landforms. This paper jointly analyses multi‐temporal, multispectral ASTER data, continuous river stage and discharge data, and field observations of the growth rates of the dominant riparian tree species (Populus nigra) along a 21 km reach of the Tagliamento River, Italy. Research focuses on the period 2004–2009, during which there was a bankfull flood on 24 October 2004, followed by 2 years with low water levels, nearly 2 years with only modest flow pulses, and then a final period from 15 August 2008 that included several intermediate to bankfull flow events. This study period of increasing flow disturbance allows the exploration of vegetation dynamics within the river's active corridor under changing flow conditions. The analysis demonstrates the utility of ASTER data for investigating vegetation dynamics along large fluvial corridors and reveals both spatial and temporal variations in the expansion, coalescence, and erosion of vegetated patches within the study reach. Changes in the extent of the vegetated area and its dynamics vary along the study reach. In sub‐reaches where riparian tree growth is vigorous, the vegetated area expands rapidly during time periods without channel‐shaping flows, and is subsequently able to resist erosion by bankfull floods. In contrast, in sub‐reaches where tree growth is less vigorous, the vegetated area expands at a slower rate and is more readily re‐set by bankfull flood events. This illustrates that the rate of growth of riparian trees is crucial to their ability to contribute actively to river corridor dynamics. Copyright © 2011 John Wiley & Sons, Ltd.     

Sediment transmission and storage: the implications for reconstructing landform development

The late Holocene (last 3000 years) development of the lower Ribble valley (northwest England) displays evidence for a complex response to a sediment recharge event forced by land‐use change induced increases in erosion and sediment delivery. The deposition of fluvial sediments during the late Holocene was restricted to a series of reaches or depocenters separated by zones with no sediment accumulation constrained by older glacial and fluvial terrain. Apparent reach‐wide correlations of fluvial terraces break down under the scrutiny applied by comprehensive and extensive radiocarbon control. Bayesian testing of relative order models show that large‐scale geomorphological changes, e.g. the progression from one terrace level to another, were time transgressive between different depocenters. The different histories of sediment delivery and storage are probably a function of local‐ and process‐scale variations in these depocenters, and reflect (dis)connectivity relationships within a reach in propagating a basin‐scale change (superslug) in the sediment regime. Disconnectivity in the depositional regime through a fluvial reach limits what we can reconstruct in terms of sediment budgets, but radiocarbon dating of multiple palaeochannels offers considerable potential for landform‐based research to uncover rates of change within individual depocenters. Copyright © 2009 John Wiley & Sons, Ltd.     

Streambank sediment loading rates at the watershed scale and the benefit of riparian protection

Streambank erosion is a pathway for sediment and nutrient loading to streams, but insufficient data exist on the magnitude of this source. Riparian protection can significantly decrease streambank erosion in some locations, but estimates of actual sediment load reductions are limited. The objective of this research was to quantify watershed‐scale streambank erosion and estimate the benefits of riparian protection. The research focused on Spavinaw Creek within the Eucha‐Spavinaw watershed in eastern Oklahoma, where composite streambanks consist of a small cohesive topsoil layer underlain by non‐cohesive gravel. Fine sediment erosion from 2003 to 2013 was derived using aerial photography and processed in ArcMap to quantify eroded area. ArcMap was also utilized in determining the bank retreat rate at various locations in relation to the riparian vegetation buffer width. Box and whisker plots clearly showed that sites with riparian vegetation had on average three times less bank retreat than unprotected banks, statistically significant based on non‐parametric t‐tests. The total soil mass eroded from 2003 to 2013 was estimated at 7.27 × 10⁷ kg yr.^?1, and the average bank retreat was 2.5 m yr.^?1. Many current erosion models assume that fluvial erosion is the dominant stream erosion process. Bank retreat was positively correlated with stream discharge and/or stream power, but with considerable variability, suggesting that mass wasting plays an important role in streambank erosion within this watershed. Finally, watershed monitoring programs commonly characterize erosion at only a few sites and may scale results to the entire watershed. Selection of random sites and scaling to the watershed scale greatly underestimated the actual erosion and loading rates. Copyright © 2016 John Wiley & Sons, Ltd.     

Biogeomorphic feedback between plant growth and flooding causes alternative stable states in an experimental floodplain

It is important to understand the mechanisms of vegetation establishment on bare substrate in a disturbance-driven ecosystem because of many valuable ecosystem services. This study tested for empirical indications of local alternative stable states controlled by biogeomorphic feedbacks using flume experiments with alfalfa: (1) single flood experiments different in flood intensity and plant growth, (2) long-term evolution experiments with repeated flooding and seeding. We observed: (1) a combination of thresholds in plant growth and flooding magnitude for upgrowing seedlings to survive; (2) bimodality in vegetation biomass after floods indicating the existence of two alternative states, either densely vegetated or bare; (3) facilitation of vegetation establishment by the spatial pattern formation of channels and sand bars. In conclusion, empirical indicators were demonstrated for local alternative stable states in a disturbance-driven ecosystem associated with biogeomorphic feedbacks, which could contribute to the protection and restoration of vegetation in such ecosystems.     

Earth Surface Exchanges (ESEX) Commentary on ‘Plants as river system engineers’ by A. Gurnell. Earth Surface Processes and Landforms 39: 4–25, 2014. DOI 10.1002/esp.3397

In a review of the role of plants in river systems, Gurnell (2014) explains how living riparian vegetation can moderate and manipulate river environments by trapping sediment and promoting longer‐term stability. Although the review concentrates on perennial plants in the humid temperate zone, this commentary acts as a reminder that some plants in other kinds of fluvial environment do not act in this way. This is done by describing how Impatiens glandulifera (Himalayan Balsam), a highly invasive annual plant that is now found in many countries on three separate continents, may significantly increase soil erosion along riverbanks and the riparian zone of inland watercourses. Copyright © 2014 John Wiley & Sons, Ltd.