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Dynamic riverine landscapes: The role of ecosystem engineers Harvey, GL; Bertoldi, W
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Information about this research object was correct at the time of download; we occasionally make corrections to records, please therefore check the published record when citing. For more information contact [email protected] 1 This is the final pre-publication version of: 2 3 Harvey, G.L. and Bertoldi, W. (2015) Dynamic riverine landscapes: the role of 4 ecosystem engineers. Earth Surface Processes and Landforms 40: 1701– 5 1704. doi: 10.1002/esp.3757. 6
7 Dynamic riverine landscapes: the role of ecosystem engineers
8 Gemma L Harvey1 and Walter Bertoldi2
9
10 1School of Geography, Queen Mary University of London, London, UK
11 2Department of Civil, Environmental and Mechanical Engineering, University of
12 Trento, Trento, Italy
13
14 Abstract
15 An important and highly active research agenda has developed at the interface
16 of fluvial geomorphology and ecology that addresses the capacity for vegetation
17 and animals to act as ecosystem engineers within fluvial systems. This paper
18 briefly introduces this research domain and describes the fifteen papers that
19 contribute to the special issue on ‘Dynamic riverine landscapes: the role of
20 ecosystem engineers’. The papers illustrate the breadth of research activity at
21 this interface, investigating the influence of a range of ecosystem engineering
22 organisms through a combination of field study, laboratory experiments,
23 numerical simulation and analysis of remotely sensed data. Together, the
24 papers address a series of key themes: conceptual frameworks for feedbacks
25 between aquatic biota, hydraulics, sediment dynamics and nutrient dynamics
26 and their quantification through experimental and field research; the potential
27 contribution of ecosystem engineering species to assist river recovery and
1
1 restoration; and the contribution of riparian vegetation to bank stability and
2 morphodynamics across a range of spatio-temporal scales.
3 Keywords: biogeomorphology, ecosystem engineering, fluvial processes
4
5
6 The ecosystem engineering role of aquatic and riparian biota in driving
7 morphodynamics, habitat complexity and biodiversity at a variety of spatio-
8 temporal scales represents an important and rapidly advancing research
9 agenda in river science. The term ‘ecosystem engineer’ refers to organisms that
10 directly or indirectly modulate the availability of ecosystem resources and hence
11 modify, maintain or create habitat (Jones et al., 1994). Within river corridors, a
12 range of organisms including invertebrates, fish, mammals and aquatic and
13 riparian vegetation are influenced by fluxes of water, sediment and nutrients.
14 These organisms, in turn, modify fluvial processes across scales ranging from
15 individual sediment grains to river-floodplain systems (Reinhardt et al., 2010;
16 Bertoldi et al., 2011; Rice et al., 2012a; Gurnell, 2014).
17
18 Biogeomorphological interactions can initiate and maintain morphological
19 complexity in pristine or semi-natural systems and have the capacity to assist
20 the geomorphological recovery and restoration of degraded rivers in a way that
21 minimises management intervention (Palmer et al., 2005; Beechie et al., 2010;
22 Gurnell et al., 2012). In contrast, invasive species acting as ecosystem
23 engineers can represent a system disturbance with potential for negative
24 impacts on the delivery of ecosystem services (e.g. Harvey et al., 2011;
25 Greenwood and Kuhn, 2014). As a result, ecosystem engineering plays a
2
1 critical role in the functioning and management of dynamic riverine landscapes.
2 A deeper understanding of bi-directional interactions between the biotic and
3 abiotic components of fluvial systems within a range of environmental contexts
4 is crucial, particularly in light of the significant and increasing pressures arising
5 from climatic change, management interventions and invasive species.
6
7 This special issue arose from a session at the European Geosciences Union
8 General Assembly in 2012, which addressed the role of ecosystem engineers
9 in driving fluvial processes and landform dynamics. The session and special
10 issue contribute to a highly active area of research at the interface between
11 geomorphology and ecology (e.g. Darby, 2010; Wheaton et al., 2011; Butler
12 and Sawyer, 2012; Rice et al., 2012b). Rice et al. (2012b), for example, present
13 a special issue of Earth Surface Processes and Landforms focusing on
14 disturbance regimes at this interface and note a substantial increase in papers
15 on biogeomorphology published by the journal in the six years previous. This
16 trend has continued. For instance in a ‘State of Science’ themed issue of ESPL
17 in January 2015 three out of seven papers focus on the importance of
18 feedbacks between vegetation and geomorphological processes in fluvial
19 systems and (at the time of writing) five out of the twelve most cited papers
20 published in the journal since 2012 address the impact of vegetation or animals
21 on sediment dynamics and river evolutionary trajectories (Osterkamp et al.,
22 2012; Polvi and Wohl, 2012; Stoffell et al., 2012; Fryirs, 2013; Gurnell, 2014).
23
24 The papers in this special issue explore the interactions between fluvial
25 processes and a variety of engineering organisms (including animals, aquatic
3
1 vegetation and riparian vegetation) achieved through a combination of literature
2 review, development of conceptual models, laboratory experiments, numerical
3 simulations and the analysis of remotely sensed data. The papers address a
4 series of key themes: frameworks for conceptualising the feedbacks between
5 aquatic biota, hydraulics, sediment dynamics and nutrient dynamics and their
6 quantification through experimental and field research; the potential
7 contribution of ecosystem engineering species to assist river recovery and
8 restoration within degraded systems; and the contribution of riparian vegetation
9 to bank stability and morphodynamics across a range of spatio-temporal scales.
10
11 The rapid advancement of this interdisciplinary research domain necessitates
12 the development of new conceptual frameworks within which biogeomorphic
13 interactions can be hypothesised and tested. Based on a detailed review of the
14 literature, Corenblit et al. (2014) present the biogeomorphological life cycle of
15 the European black poplar (Populus nigra); a conceptual model that links the
16 biological development of poplars, from seed deposition to mature tree, with the
17 processes by which they modify the hydrogeomorphological environment. The
18 model outlines four phases, across which the nature and intensity of bi-
19 directional interactions vary as the plant and fluvial landform co-evolve. The
20 geomorphological and pioneer phases are dominated by fluvial processes,
21 while reciprocal biogeomorphic interactions are strongest during the third,
22 biogeomorphological phase. During the ecological phase, interactions between
23 P. nigra and fluvial processes are limited to high magnitude flow events. The
24 authors hypothesise that the ways in which P. nigra modifies its environment
4
1 results in positive niche construction and outline the cutting-edge research
2 agenda required to test their hypothesis.
3
4 Evidence for feedbacks between plant traits and the abiotic river environment
5 is presented by Schoelynck et al. (2014) who explore the role of plant
6 morphological plasticity in vegetation-sediment-flow interactions. Focusing on
7 the floating-leaved, rooted aquatic macrophyte Nuphar lutea, their field
8 measurements reveal higher numbers of submerged leaves and larger leaf
9 areas in a lotic stream habitat compared to an adjacent oxbow lake. Sediments
10 retained around N. lutea patches in flowing water were finer and richer in
11 organic material with higher nutrient content than unvegetated patches while
12 vegetated and unvegetated patches in the still water habitat showed no
13 significant difference in sediment properties. The findings indicate important
14 feedbacks between plant morphology and the hydrodynamic environment and
15 an ecosystem engineering role of N. lutea with potential implications for local
16 sedimentation processes and nutrient cycling.
17
18 Building on the work of Corenblit et al. (2014), Bätz et al. (2015) consider the
19 role of soil in biogeomorphic succession within the geomorphically active
20 environment of river braid plains. The authors propose that within such dynamic
21 systems, the conventional mode of soil development requires adaptation in
22 order to incorporate disturbances and supply of resources by erosional and
23 depositional processes. A conceptual model for the co-evolution of braided river
24 morphodynamics is presented, emphasising the critical role played by soil
25 evolution in the early stages of fluvial landform development which can improve
5
1 local environmental conditions and facilitate the co-evolution of vegetation and
2 landforms. The model is tested using field data from a braided river-terrace
3 system, highlighting the relatively rapid development of soils even within more
4 geomorphically active areas as a result of these feedbacks.
5
6 Feedbacks induced by a geomorphic agent (Pacific salmon), known to modify
7 bed material and nutrient delivery to streams, are explored experimentally by
8 Rex et al. (2014). Outdoor flume experiments are used to simulate and to
9 quantify the formation and delivery to the streambed of salmon-based flocs,
10 comprising fine sediment and salmon organic matter, during active spawning
11 and post-spawn periods. Interactions between inorganic and organic material
12 through floc formation resulted in increased delivery of salmon organic matter
13 to the river bed under simulated active spawning conditions, and substantial
14 growth of flocs within the bed following infiltration which is attributed to microbial
15 activity. The findings indicate a feedback loop whereby the sequestration of
16 salmon organic matter and hence marine-derived nutrients within the
17 streambed may influence stream productivity.
18
19 The influence of fish on river bed material properties is explored further by
20 Pledger et al. (2014) who use laboratory flume experiments to quantify the
21 impact of a benthic-feeding fish (Barbus barbus) on gravel bed sediment
22 structures, entrainment and bedload fluxes. Benthic-feeding fish are
23 widespread in rivers and disturb bed material through foraging behaviours but
24 little is known of their impact on sediment dynamics. The flume simulations
25 reveal that substrates exposed to fish feeding were associated with higher
6
1 microtopographic roughness, and increased grain entrainment counts and
2 bedload flux which are attributed to the alterations to bed material organization
3 and structure. Given that benthic foraging is common among a large number of
4 species, and is spatially widespread and temporally persistent the authors
5 encourage further work to quantify these hitherto largely unexplored impacts on
6 sediment dynamics.
7
8 The impact of ecosystem engineering animals on the physical environment may
9 be amplified where individuals are present in large numbers as is often the case
10 for invasive species. Harvey et al. (2014) explore the impact of invasive signal
11 crayfish (Pacifastacus leniusculus) on fine sediment dynamics within river
12 channels. Laboratory mesocosm experiments demonstrate the ability of signal
13 crayfish to mobilise pulses of fine sediment by burrowing into constructed clay
14 banks and bed substrates, while field data reveal similar pulsed fine sediment
15 events and an increase in ambient turbidity levels. The findings indicate that
16 signal crayfish have the potential to influence suspended sediment yields in
17 rivers, with potential implications for morphological change, physical habitat
18 quality and the transfer of nutrients and pollutants.
19
20 In contrast to the potentially deleterious impacts of invasive species, Curran
21 and Cannatelli (2014) discuss an example of beavers as a tool in the restoration
22 of degraded river systems. The paper reports on changes in channel
23 morphology following beaver dam construction in a low-gradient, fine-sediment
24 dominated channel that was adjusting to the breaching of a downstream dam.
25 The beaver dams concentrated flow into a single channel and encouraged
7
1 deposition at channel margins, contributing to increased channel stability and
2 sediment storage. The authors suggest that in cases where dam removal can
3 lead to bank erosion and channel migration, beaver dams may be used to
4 enhance lateral stability and support the removal of structures.
5
6 Aquatic plants can also contribute to the recovery of channels from human
7 interventions through their role in retaining fine sediments and building fluvial
8 landforms. Gurnell et al. (2013) use a large national data set combined with
9 field survey and germination trials to explore the distribution and geomorphic
10 impact of the linear emergent macrophyte Sparganium erectum. Results
11 demonstrate that S. erectum is widespread across river types but achieves
12 significant cover and hence has greater potential for landform development in
13 low gradient, low energy stretches with relatively fine bed material. Sediment
14 retention by S. erectum is primarily influenced by the size and density of the
15 plant stand, rather than the size of the individual plants, with tightly packed
16 stands retaining more sediment than low density stands. The retained
17 sediments create landforms that emerge as benches and trap large numbers
18 of viable seeds, generating a terrestrialising marginal habitat that can promote
19 channel narrowing in over-widened reaches.
20
21 River restoration may increase cover of instream vegetation and hence
22 potential for fine sediment retention. Within this context, the paper by Gibbs et
23 al. (2014) highlights the importance of accounting for sediment-associated
24 contaminant mobilisation and storage in restoration design in order to optimise
25 the benefits of restoration for ecosystem and human health. Their field
8
1 research highlights the potential for retention of heavy metals within vegetated
2 and unvegetated sediment patches in urban river reaches. High concentrations
3 of Cu, Pb and Zn in excess of sediment quality guidelines for ecological and
4 human health were found in gravel patches, and in both vegetated and
5 unvegetated fine sediments. The fine sediments were also associated with
6 greater bioavailability of metals, reflecting the smaller grain size and higher
7 organic matter content of these patches. The results contribute to the scientific
8 basis for river restoration design, particularly in relation to enhancing outcomes
9 for urban river restoration projects.
10
11 Riparian vegetation has wide ranging impacts on fluvial processes and river
12 behaviour across multiple scales, from the stabilising effect of individual roots
13 in river banks, to the effects of large wood on reach-scale river morphology, to
14 the influence of floodplain vegetation development river planform. At the
15 microscale, Edmaier et al. (2014) use laboratory experiments to quantify the
16 uprooting characteristics of seedlings of Medicago sativa and Avena sativa.
17 These species are used to represent riparian vegetation in physical models as
18 a result of their simple root architectures and ability to grow in sandy substrates.
19 The experiments demonstrate positive relationships between total root length
20 and uprooting force/work and a higher resistance for the multi-root system (A.
21 sativa) compared to the single-root system (M. sativa). Sediment particle size
22 and moisture content also influence the ability of a seedling to withstand
23 uprooting, with smaller forces required to uproot seedlings growing in wetter
24 and coarser sediments. The results contribute to the understanding of plant
9
1 uprooting resistance and will aid the design of ecomorphodynamic flume
2 experiments.
3
4 Polvi et al. (2014) explore the functional role of different vegetation types in
5 stabilising river banks through a combination of field measurement and
6 numerical modelling. Differences in tensile strength, density and morphology of
7 riparian root systems were quantified for four functional groups (trees, shrubs,
8 graminoids and forbs) using field measurements, while numerical modelling
9 was used to predict the additional cohesion provided by each plant species for
10 different bank material textures. Woody vegetation (tree and shrub groupings)
11 was associated with greater tensile strength, root diameter and lateral root
12 extent as well as added cohesion in comparison to the non-woody vegetation
13 (graminoids and forbs). The paper offers a framework that can be used to
14 explore the functional role of a wider range of vegetation types in bank
15 stabilisation and is of direct relevance to the management and restoration of
16 river corridors.
17
18 Riparian vegetation becomes an important component of the instream
19 environment when vegetated margins are eroded and vegetation (e.g. large
20 wood) is supplied to the channel. Bertoldi et al. (2014) use flume experiments
21 to explore wood dynamics in braided streams. Patterns of wood deposition and
22 remobilization are investigated using wooden dowels in three parallel flumes
23 filled with uniform sand. The experiments show that wood is dispersed on bar
24 tops, generally in small accumulations containing fewer than five logs. Turnover
25 rates of deposited logs are very high as a result of the highly dynamic evolution
10
1 of the channel network and bar locations, and do not depend on the wood input
2 rate or the presence of roots. In these model runs, the presence of large wood
3 alone does not affect the morphodynamics of the braided system, highlighting
4 the role of vegetative regeneration and fine sediments in creating stable wood
5 jams and associated fluvial landforms.
6
7 Finally, three papers in this special issue explore how interactions between
8 vegetation development and hydromorphological processes (stream power,
9 flow regime and channel migration) control morphodynamic behaviour and
10 evolutionary trajectories in braided rivers. Perona et al. (2014) present a 1D
11 ecomorphodynamic model developed to predict vegetation distribution in river
12 reaches with converging banks. 1D equations for flow and sediment transport
13 were modified to include representation of vegetation dynamics in order to
14 predict the minimum channel width below which vegetation is expected to
15 disappear (the vegetation ‘front’). The mathematical model is tested through
16 laboratory flume experiments using a sand bed uniformly seeded with Avena
17 sativa. The analysis confirms the role of stream power in setting suitable
18 conditions for vegetation development and the longitudinal position of the
19 vegetation front by increasing uprooting capacity along the convergent reach.
20 The experiments also demonstrate the importance of hydrological timescales
21 (time between flood events) relative to biological timescales (vegetation growth)
22 in controlling the extent of vegetation colonisation.
23
24 The relationship between vegetation colonization and hydrological regime is
25 explored in detail by Surian et al. (2015) who analyse aerial images of the
11
1 Tagliamento River (Italy) for a 60 year period. In this very dynamic, high energy
2 river the turnover of riparian vegetation is high. Half of the vegetated areas
3 persist for less than 5-6 years, and less than 10% persists for more than 20
4 years. The analysis demonstrates that riparian vegetation can be significantly
5 eroded by relatively frequent, low magnitude flood events (recurrence interval
6 1–2.5 years), with more densely vegetated areas showing a higher threshold.
7 These findings suggest the ecosystem engineering effect of riparian vegetation
8 is strongly dependent on the hydrological regime and the available stream
9 power and the authors highlight additional controls on vegetation dynamics
10 including occurrence of erodible vegetated margins (for generating wood
11 supply) that influence the development of new islands.
12
13 Gran et al. (2015) analyse the evolution of feedbacks between riparian
14 vegetation and channel dynamics in an active braided system where sediment
15 loads are decreasing following a volcanic eruption (Mount Pinatubo,
16 Philippines). Following a highly dynamic post-eruption phase, vegetation is
17 now able to persist year-round and actively influence sediment dynamics.
18 Results from a cellular model informed by field data illustrate the importance of
19 the ratio between biological (vegetation development) and morphodynamic
20 (channel migration) timescales in controlling the capacity of vegetation to
21 modify river morphology. In addition, local effects such as strong seasonality
22 of precipitation and sediment load, as well as groundwater level fluctuations,
23 may affect the ability of vegetation to colonise sediment bars and hence
24 determine the evolutionary trajectory of the morphology-vegetation interactions.
25
12
1 The 15 papers in this special issue demonstrate the diversity and breadth of
2 research at the interface between geomorphology and ecology. The findings
3 reported deepen our understanding of the bi-directional interactions between
4 biotic and abiotic components of riverine landscapes, but also make significant
5 contributions to the scientific basis of sustainable river management and
6 restoration. Many of the papers identify key questions requiring further
7 investigation, providing opportunities for novel interdisciplinary collaborations
8 that employ the wide variety of research approaches illustrated in this issue.
9
10 Acknowledgements
11 We would like to thank the reviewers for all of the papers included this special
12 issue for their helpful and constructive comments. We also thank Stuart Lane
13 and Fiona Kirkby for all their help and support with co-ordination of the special
14 issue.
15
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