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Volume 38, Issue 1 (2023)
GeoRes 2023, 38(1): 107-119 |
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Received: 2023/02/9 | Accepted: 2023/05/31 | Published: 2023/06/4
Received: 2023/02/9 | Accepted: 2023/05/31 | Published: 2023/06/4
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Darabi Shahmari S, Ahmadabadi A. Determining the Biogeomorphic Feedback Windows Located in the Riparian Area of the Taleqan River, Iran. GeoRes 2023; 38 (1) :107-119
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S. Darabi Shahmari *1, A. Ahmadabadi1
1- Department of Geographical Science, Faculty of Geographical Sciences, Kharazmi University, Tehran, Iran
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Introduction
Hydrogeomorphic conditions including sediment texture, exposure to shear stress, topography, and hydrological variability play a crucial role in determining the dynamics of riparian vegetation. In turn, vegetation can significantly influence water flow, sediment transport, geomorphology, and topographic structure (Gurnell et al., 2005; Corenblit et al., 2007; Bertoldi et al., 2011; Pollen-Bankhead & Simon, 2010). These reciprocal interactions generate strong feedback mechanisms between river landforms and riparian vegetation during the early stages of plant succession.
Over the past two decades, considerable research has been devoted to developing conceptual biogeomorphic models that describe these feedbacks between fluvial geomorphology and vegetation dynamics. In ecology, the concept of ecosystem engineers (Odling-Smee et al., 2003) has been introduced to refer to plant species that substantially modify hydrogeomorphic processes and thereby influence the structure and functioning of riparian ecosystems (Gurnell et al., 2005; Corenblit et al., 2007, 2011, 2015). Dynamic riparian ecosystems, which are frequently exposed to physical disturbances such as floods and exhibit potential ecological feedbacks between geomorphology and engineering plant species, are defined as biogeomorphic ecosystems (Balke et al., 2014; 2015).
Within such ecosystems, the development of fluvial landforms, vegetation succession, and the life cycle of riparian trees are described as biogeomorphic succession (Corenblit et al., 2007). In this context, the concept of the Biogeomorphic Feedback Window (BFW), proposed by Eichel et al. (2015) for mountain environments provides a valuable framework for identifying the spatial and temporal occurrence of feedbacks between geomorphic dynamics and vegetation processes. These authors applied multidimensional scaling and correlation methods to analyze relationships between engineering plant species and geomorphic processes and found that favorable conditions for feedback between specific plant assemblages and slope processes led to solifluction development.
In alluvial environments, the BFW represents the spatiotemporal domain of interactions between riparian willow species and hydrogeomorphic processes that result in strong biogeomorphic feedbacks (Corenblit et al., 2015). Vegetation within these feedback windows enhances the resistance of landforms to hydrogeomorphic disturbances and promotes their recovery after destructive floods, thereby contributing to the stability and ecological resilience of biogeomorphic landforms (Picco et al., 2017). When plant morphological, biomechanical, phenological, and physiological traits are not considered, hydrogeomorphic processes tend to control species composition during the early stages of succession (Richter & Richter, 2000).
Certain woody plant species, such as willows and poplarsexhibit traits that enable their establishment in hydrologically disturbed environments characterized by shear stress, erosion, sediment burial, and drought (Gurnell, 2014). These ecosystem-engineering species can colonize alluvial deposits and exert a potential influence on geomorphology. They develop specific traits in response to hydrogeomorphic disturbances, including adaptations in reproductive strategies and morphological or biomechanical properties to cope with environmental stressors (Karrenberg et al., 2002; Lytle & Poff, 2004).
The resistance and resilience of biogeomorphic ecosystems are closely linked to the feedback windows of engineering plants (Corenblit et al., 2015). Within these windows, vegetation develops adaptive traits that enable survival under harsh conditions while simultaneously influencing landform characteristics and contributing to the stabilization of riverine landforms (Krzeminska et al., 2019). Two major outcomes are associated with the functioning of vegetation within biogeomorphic feedback windows: (a) the inheritance of phenotypic traits within plant communities (ecological inheritance) and (b) the inheritance of landform characteristics. In such coevolutionary feedbacks, plant populations modify their environment, and these environmental changes, in turn, influence their evolutionary trajectories and the structure of plant communities (Post & Palkovacs, 2019; Cahill & McNikle, 2011). In other words, the adaptive responses of engineering plants to disturbances, along with their role in maintaining other species within a colony, lead to the physical stabilization of habitats and the persistence of river landforms (Zhu et al., 2022; Steiger et al., 2001).
The importance of these concepts lies in the fact that, in the absence of major disturbances, particularly human interventions, the interactions between riparian ecosystems and river geomorphology promote the absorption of disturbances and the stabilization of both riverine ecosystems and landforms. Therefore, minimizing human interventions and preserving riparian vegetation communities are critical components of river management, as they enhance both ecological and geomorphic stability. The objective of this study is to analyze the spatial extent of biogeomorphic feedback windows along the Taleghan River over the period 1991–2021.
Methodology
This field-based research utilized four sets of satellite images and aerial photographs. Initially, all images were geometrically corrected and mosaicked using distinct and common ground features. Subsequently, the satellite images and aerial photographs were georeferenced in the ENVI software environment using ground control points (GCPs) obtained via GPS and topographic maps. For each pair of UltraCam images, between 120 and 220 control points (a combination of field-based and topographic map points) were applied to minimize RMS error. Similarly, for each pair of aerial photographs, between 30 and 50 control points were used to achieve precise georeferencing.
The Taleghan River was divided into three sections of upstream, midstream, and downstream based on variations in slope and elevation derived from the longitudinal profile extracted from the digital elevation model (DEM) created using Cartosat images. The average elevations of these sections were 1936 m, 1875 m, and 1793 m, respectively, with a very low slope difference ranging from 0.009 to 0.013 between the river’s source and mouth. Within each section, specific reaches were identified for geomorphic analysis following the GUS method [Darabi Shahmari et al., 2023].
The spatial extent of biogeomorphic feedback windows (BFWs) was examined within each reach of the Taleghan River. These mosaics represent areas where constructive forces dominate over destructive ones, allowing vegetation to maintain ecological resilience and resistance. Despite temporal fluctuations in the surface area of these vegetation colonies over the study period (1991–2021), the core zones composed mainly of mature woody species remained stable.
To identify the locations of BFWs composed of woody and shrubby vegetation across the four image sets, the corresponding mosaicked images were overlaid, and vegetation colonies with approximately consistent positions and surface areas were extracted. Only canopy areas exceeding 500 m² were considered; smaller canopies were excluded due to fragmentation and discontinuous ground cover.
Following the spatial identification of stable vegetation colonies during the 1991–2021 period, a field survey was conducted in September 2021. The geographical locations of the BFWs were used to assess ecological (stem density, stem diameter, vegetation height, and ground cover), morphometric (extent of vegetation damage), and biogeomorphic (presence of depositional landforms such as sediment tongues on the leeward side of colonies) characteristics.
In evaluating geomorphic processes, three main categories were considered: erosion, transport, and deposition.
Findings
The positions of biogeomorphic feedback windows (BFWs) in each of the five reaches of the Taleghan River were determined using satellite images and aerial photographs. The location of relatively stable vegetation colonies along the river was identified according to the river planform.
According to the findings, 54.1% of riparian colonies along the studied river were located on the right bank, and 45.1% on the left bank. In 26.8% of the studied vegetation colonies, severe damage was observed, in 34.5% minor damage, and in 37.8% no significant damage was noted. Damaged vegetation colonies were mostly located in the upstream reaches and often on the immediate riverbank. Factors such as vegetation establishment on the channel side (alluvial terrace), bed scour due to high hydraulic stress, reduced channel width, and lateral flow intrusion contributed to this. In other sections, changes in turbulence intensity and shear stress caused maximum values to shift away from the channel edges. The highest number of BFWs relative to reach length was observed in the upstream reach.
Among a total of 82 relatively stable riparian vegetation colonies, approximately 70.7% showed the formation of longitudinal and transverse sediment tails and accumulative biogeomorphic landforms. These biogeomorphic accumulations were observed along longitudinal, transverse, or both slopes. Longitudinal sediment tails (30.5%) were influenced by the channel’s longitudinal slope, while transverse sediment tails (69.3%) were affected by the reverse slope. Significant differences were observed between upstream and downstream vegetation mosaics based on the longitudinal slope. Upstream, the sediment matrix was coarser, while downstream and in the channel center, sand-dominated matrices prevailed. The reverse slope had a stronger influence on stabilizing sediment tails at the front (lee side) and end of the vegetation colonies. Transverse biogeomorphic accumulations occurred in colonies with greater height and more advance toward the channel center; otherwise, they were more common in mid-channel colonies. In some sections, sediment tails were stabilized under the influence of both longitudinal and transverse slopes (32.9%).
Approximately 26.8% of vegetation mosaics, particularly colonies located at the immediate channel margin, were heavily damaged with no ground cover. The occurrence of accumulative biogeomorphic landforms in more distant locations from the channel was observed. Near the immediate channel margin, due to proximity to maximum hydraulic stress and lower resistance thresholds, sediment accumulation in edge colonies was mostly temporary and resulted in unstable depositional clusters. Riparian trees near the main channel were younger and smaller, with stem diameters less than 4–8 cm, compared to trees located farther from the riverbed, which had stem diameters greater than 12 cm.
Woody vegetation with greater height and stem diameter was widely established in positions farther from the channel, in sorted sediments. The proportion of flood-induced damage in these trees was lower, and dense ground cover was present in these colonies. The likelihood of biogeomorphic accumulative landforms occurring in colonies increased with the presence of sediment tails. Sediment tails serve as anchoring points for seeds and pioneer plants. The lowest occurrence of sediment deposition was observed in reach 4 (72.7%) and reach 2 (25.9%), while the highest was in reaches 3 (84.5%) and 1 (75.1%). The presence or absence of biogeomorphic accumulative landforms was strongly associated with other biotic parameters. In creating accumulative biogeomorphic landforms, vegetation older than one year, stem diameter, and stem density were important factors. Transverse slope had more influence than longitudinal slope in controlling the occurrence of accumulative biogeomorphic landforms.
The relationship between biological tails in the lee of vegetation colonies and other biotic parameters (stem diameter, density, height, ground cover, and damage level) indicated that sediment tail formation was affected by all these parameters. Damage level had a significant effect (p=0.004) on biotic density, as higher damage reduced the probability of stabilizing biogeomorphic accumulations. In the central section of sediment bars deposited along the transverse and longitudinal edges of vegetation colonies, plants with flexible colonies and extensive biomass trapped sediments as a strong engineering effect. Stem diameter had the greatest effect on the plant’s sediment retention capacity. When vegetation was exposed to less sediment, other morphological traits, particularly ground cover and stem density, determined the effectiveness of sediment trapping.
Longitudinal deposits located on the right edge of the BFW, which is the bank directly exposed to river flow, were more prone to detachment and erosion compared to longitudinal deposits on the lee side of the BFW. Therefore, longitudinal accumulations in the lee had greater frequency and larger scale compared to those exposed to flow.
Based on the occurrence of longitudinal and transverse tails in BFWs, two types of biogeomorphic feedback windows along the riverbank were observed. In the first type, because the BFWs were not aligned along the riverbank (due to slope differences in bank colonies and BFW advancement toward the mid-channel), both longitudinal and transverse sediment tails were observed. In the second type, the occurrence of longitudinal sediment tails was less likely. In these colonies, the riverbank BFWs were aligned with slight slope differences and uniform channel progression, so there was minimal advance toward the channel center, limiting the opportunity for sediment accumulation in the mid-channel section and formation of biogeomorphic deposits.
Riparian woody vegetation can be considered ecosystem engineers because they increase sediment deposition, form sediment tails, and consequently create landforms. The engineering effect of these riparian plants occurs primarily in locations with physical positions favorable for sedimentation related to flow characteristics. River engineer species can develop different skills or responses to hydrogeomorphic constraints, allowing establishment in an unstable and dynamic geomorphic environment. Biogeomorphic feedback windows are located at a distance from the channel edges because stability of vegetation and sediment trapping is limited at immediate channel margins. When suitable establishment occurs, the height of the landform increases, gradually forming a stable site for vegetation, with seed lodging and depositional landform formation occurring near the center of longitudinal and transverse sediment tails.
Discussion
The aim of this study was to identify the location and characteristics of biogeomorphic feedback windows (BFWs) along the banks of the Taleghan River. In rivers, trees and shrubs influence hydrogeomorphic processes and lead to the formation of alluvial landforms. These trees often form dense colonies, creating concentrated stations and developing vegetative anchors, as plants in colonies are less prone to uprooting than solitary individuals. These colonies also act as ecosystem engineers by trapping sediments, organic matter, and nutrients. The formation of biogeomorphic landforms represents a habitat niche structure that, through facilitative processes among riparian plants, enhances survival capacity, resource storage, and maturation between destructive flood events.
During the establishment of BFWs, intra-group interactions among trees occur, promoting colony survival and growth, as trees and young saplings protect each other against hydraulic stress. In addition to improving the capacity for trapping mineral and organic matter, canopy-forming species can support a network connecting plants, soil, and groundwater, which influences nutrient transfer, cycling, and storage within the habitat structure, as well as nutrient uptake and exchange. In this way, climax conditions can be reached in riparian and mid-channel river ecosystems.
Factors influencing variations in the structure of BFWs, based on characteristics associated with the establishment of accumulative biogeomorphic landforms, include the ratio of sediment accumulation, damage level of sediment tails, vegetation height, vegetation density, stem diameter, and ground cover. The initial stage of the complex process of establishing a biogeomorphic accumulative landform is the creation of a primary physical habitat. Longitudinal and transverse sediment accumulations along the river margin or mid-channel represent the first step for vegetation establishment and the development of BFWs or in the early stages of a BFW, and this varies across different BFWs. Without this, BFWs cannot establish, and if they do, they cannot maintain their habitat structure due to insufficient conditions for development. Thus, a key feature in examining differences between BFWs is the establishment of relatively stable sediment accumulations, while other features associated with maintaining and developing these accumulations are also considered.
The relationship between the ratio of BFWs or sediment accumulations and other factors, such as vegetation height, stem diameter, density, and ground cover, was assessed using the chi-square test. Results indicated significant relationships between damage level, vegetation height, stem diameter, and stem density. The establishment of biogeomorphic accumulative landforms was directly associated with stem diameter, height, and vegetation density, and inversely related to the damage level of BFWs, while ground cover showed no significant effect on the establishment of sediment accumulations or BFWs.
Riverine geomorphic units played a significant role in establishing a primary physical habitat. However, the richness of geomorphic units in terms of base channel or floodplain presence plays a greater role than their density.
The findings of this study indicated that due to the density of relatively stable woody vegetation along the riverbank and the consequent stability of BFWs over a 30-year period, phenotypic changes in plants and the ecological inheritance of these changes to subsequent generations are plausible. Maintaining riparian ecosystems is important not only for enhancing landform stability but also for improving water quality, preserving animal habitats, and maintaining micro- and macro-scale habitat dimensions within the riverbed.
Consistent with these results, Corenblit et al. studied BFWs in three willow species along a 20 km stretch of the Eure River in France, showing that all three species acted as ecosystem engineers. The BFWs of these species were influenced by upstream–downstream longitudinal gradients and the reverse slope toward the floodplain. Similarly, in this study, consistent with the influence of longitudinal and transverse sediment accumulation on BFW development, two types of BFWs were observed. In the first type, due to BFW expansion toward the mid-channel, both longitudinal and transverse sediment trapping occurred, providing opportunities for landform development. In the second type, due to limited expansion toward the mid-channel, sediment accumulation occurred only transversely. Therefore, the combined effect on channel narrowing and BFW expansion is evident in the first type.
The highest occurrence of longitudinal and transverse biogeomorphic accumulations was observed in reaches 1 and 3 of the Taleghan River. Moreover, the stability of BFWs in reach 1 and the upstream river sections was due to lower levels of human intervention and favorable conditions for vegetation establishment. Consequently, feedbacks between geomorphic processes and BFWs, and the maintenance of riparian ecosystems, were more pronounced upstream, particularly in reaches 1 and 2, compared to other reaches. The lowest sediment accumulation, which provides opportunities for vegetation colony development, was observed in reaches 4 and 5, primarily due to human interventions, increased bank erosion, and fewer BFWs. Therefore, suitable opportunities for vegetation colony development were observed upstream, particularly in reaches 1–3. Downstream, in reaches 4 and 5, due to increased human interventions, especially river channelization, the potential for vegetation habitat development and positive effects on river landform stability was considerably reduced.
Conclusion
In managing the Taleghan River, attention should be given to restoring riparian ecosystems by reducing human interventions and creating favorable conditions for the development of biogeomorphic feedback windows (BFWs). The stable establishment of BFWs ensures the persistence of plant habitats containing ecosystem engineer species, which contributes to habitat preservation and development as well as the stability of river landforms.
Acknowledgments: Not applicable.
Ethical permission: Not applicable.
Conflict of Interest: The authors declare no conflict of interest.
Authors’ Contributions: Darabi Shahmari S (first author), Principal Researcher/Introduction Writer/Statistical Analyst (60%); Ahmadabadi A (second author), Assistant Researcher/Discussion Writer/Methodologist (40%)
Funding: The funding required for this research was provided from personal income.
Hydrogeomorphic conditions including sediment texture, exposure to shear stress, topography, and hydrological variability play a crucial role in determining the dynamics of riparian vegetation. In turn, vegetation can significantly influence water flow, sediment transport, geomorphology, and topographic structure (Gurnell et al., 2005; Corenblit et al., 2007; Bertoldi et al., 2011; Pollen-Bankhead & Simon, 2010). These reciprocal interactions generate strong feedback mechanisms between river landforms and riparian vegetation during the early stages of plant succession.
Over the past two decades, considerable research has been devoted to developing conceptual biogeomorphic models that describe these feedbacks between fluvial geomorphology and vegetation dynamics. In ecology, the concept of ecosystem engineers (Odling-Smee et al., 2003) has been introduced to refer to plant species that substantially modify hydrogeomorphic processes and thereby influence the structure and functioning of riparian ecosystems (Gurnell et al., 2005; Corenblit et al., 2007, 2011, 2015). Dynamic riparian ecosystems, which are frequently exposed to physical disturbances such as floods and exhibit potential ecological feedbacks between geomorphology and engineering plant species, are defined as biogeomorphic ecosystems (Balke et al., 2014; 2015).
Within such ecosystems, the development of fluvial landforms, vegetation succession, and the life cycle of riparian trees are described as biogeomorphic succession (Corenblit et al., 2007). In this context, the concept of the Biogeomorphic Feedback Window (BFW), proposed by Eichel et al. (2015) for mountain environments provides a valuable framework for identifying the spatial and temporal occurrence of feedbacks between geomorphic dynamics and vegetation processes. These authors applied multidimensional scaling and correlation methods to analyze relationships between engineering plant species and geomorphic processes and found that favorable conditions for feedback between specific plant assemblages and slope processes led to solifluction development.
In alluvial environments, the BFW represents the spatiotemporal domain of interactions between riparian willow species and hydrogeomorphic processes that result in strong biogeomorphic feedbacks (Corenblit et al., 2015). Vegetation within these feedback windows enhances the resistance of landforms to hydrogeomorphic disturbances and promotes their recovery after destructive floods, thereby contributing to the stability and ecological resilience of biogeomorphic landforms (Picco et al., 2017). When plant morphological, biomechanical, phenological, and physiological traits are not considered, hydrogeomorphic processes tend to control species composition during the early stages of succession (Richter & Richter, 2000).
Certain woody plant species, such as willows and poplarsexhibit traits that enable their establishment in hydrologically disturbed environments characterized by shear stress, erosion, sediment burial, and drought (Gurnell, 2014). These ecosystem-engineering species can colonize alluvial deposits and exert a potential influence on geomorphology. They develop specific traits in response to hydrogeomorphic disturbances, including adaptations in reproductive strategies and morphological or biomechanical properties to cope with environmental stressors (Karrenberg et al., 2002; Lytle & Poff, 2004).
The resistance and resilience of biogeomorphic ecosystems are closely linked to the feedback windows of engineering plants (Corenblit et al., 2015). Within these windows, vegetation develops adaptive traits that enable survival under harsh conditions while simultaneously influencing landform characteristics and contributing to the stabilization of riverine landforms (Krzeminska et al., 2019). Two major outcomes are associated with the functioning of vegetation within biogeomorphic feedback windows: (a) the inheritance of phenotypic traits within plant communities (ecological inheritance) and (b) the inheritance of landform characteristics. In such coevolutionary feedbacks, plant populations modify their environment, and these environmental changes, in turn, influence their evolutionary trajectories and the structure of plant communities (Post & Palkovacs, 2019; Cahill & McNikle, 2011). In other words, the adaptive responses of engineering plants to disturbances, along with their role in maintaining other species within a colony, lead to the physical stabilization of habitats and the persistence of river landforms (Zhu et al., 2022; Steiger et al., 2001).
The importance of these concepts lies in the fact that, in the absence of major disturbances, particularly human interventions, the interactions between riparian ecosystems and river geomorphology promote the absorption of disturbances and the stabilization of both riverine ecosystems and landforms. Therefore, minimizing human interventions and preserving riparian vegetation communities are critical components of river management, as they enhance both ecological and geomorphic stability. The objective of this study is to analyze the spatial extent of biogeomorphic feedback windows along the Taleghan River over the period 1991–2021.
Methodology
This field-based research utilized four sets of satellite images and aerial photographs. Initially, all images were geometrically corrected and mosaicked using distinct and common ground features. Subsequently, the satellite images and aerial photographs were georeferenced in the ENVI software environment using ground control points (GCPs) obtained via GPS and topographic maps. For each pair of UltraCam images, between 120 and 220 control points (a combination of field-based and topographic map points) were applied to minimize RMS error. Similarly, for each pair of aerial photographs, between 30 and 50 control points were used to achieve precise georeferencing.
The Taleghan River was divided into three sections of upstream, midstream, and downstream based on variations in slope and elevation derived from the longitudinal profile extracted from the digital elevation model (DEM) created using Cartosat images. The average elevations of these sections were 1936 m, 1875 m, and 1793 m, respectively, with a very low slope difference ranging from 0.009 to 0.013 between the river’s source and mouth. Within each section, specific reaches were identified for geomorphic analysis following the GUS method [Darabi Shahmari et al., 2023].
The spatial extent of biogeomorphic feedback windows (BFWs) was examined within each reach of the Taleghan River. These mosaics represent areas where constructive forces dominate over destructive ones, allowing vegetation to maintain ecological resilience and resistance. Despite temporal fluctuations in the surface area of these vegetation colonies over the study period (1991–2021), the core zones composed mainly of mature woody species remained stable.
To identify the locations of BFWs composed of woody and shrubby vegetation across the four image sets, the corresponding mosaicked images were overlaid, and vegetation colonies with approximately consistent positions and surface areas were extracted. Only canopy areas exceeding 500 m² were considered; smaller canopies were excluded due to fragmentation and discontinuous ground cover.
Following the spatial identification of stable vegetation colonies during the 1991–2021 period, a field survey was conducted in September 2021. The geographical locations of the BFWs were used to assess ecological (stem density, stem diameter, vegetation height, and ground cover), morphometric (extent of vegetation damage), and biogeomorphic (presence of depositional landforms such as sediment tongues on the leeward side of colonies) characteristics.
In evaluating geomorphic processes, three main categories were considered: erosion, transport, and deposition.
- Stem density was classified into three groups based on inter-stem spacing: Low (>1.5 m), moderate (1–1.5 m), and high (<1 m).
- Stem diameter was divided into four categories: Very small (<4 cm), small (4–8 cm), medium (8–12 cm), and large (>12 cm).
- Vegetation height was categorized as very short (<0.5 m), short (0.5–1.5 m), medium (1.5–3 m), and tall (>3 m).
- Ground herbaceous cover was visually estimated as absent, low, or dense.
- Vegetation damage level was classified as low, high, or undamaged.
Findings
The positions of biogeomorphic feedback windows (BFWs) in each of the five reaches of the Taleghan River were determined using satellite images and aerial photographs. The location of relatively stable vegetation colonies along the river was identified according to the river planform.
According to the findings, 54.1% of riparian colonies along the studied river were located on the right bank, and 45.1% on the left bank. In 26.8% of the studied vegetation colonies, severe damage was observed, in 34.5% minor damage, and in 37.8% no significant damage was noted. Damaged vegetation colonies were mostly located in the upstream reaches and often on the immediate riverbank. Factors such as vegetation establishment on the channel side (alluvial terrace), bed scour due to high hydraulic stress, reduced channel width, and lateral flow intrusion contributed to this. In other sections, changes in turbulence intensity and shear stress caused maximum values to shift away from the channel edges. The highest number of BFWs relative to reach length was observed in the upstream reach.
Among a total of 82 relatively stable riparian vegetation colonies, approximately 70.7% showed the formation of longitudinal and transverse sediment tails and accumulative biogeomorphic landforms. These biogeomorphic accumulations were observed along longitudinal, transverse, or both slopes. Longitudinal sediment tails (30.5%) were influenced by the channel’s longitudinal slope, while transverse sediment tails (69.3%) were affected by the reverse slope. Significant differences were observed between upstream and downstream vegetation mosaics based on the longitudinal slope. Upstream, the sediment matrix was coarser, while downstream and in the channel center, sand-dominated matrices prevailed. The reverse slope had a stronger influence on stabilizing sediment tails at the front (lee side) and end of the vegetation colonies. Transverse biogeomorphic accumulations occurred in colonies with greater height and more advance toward the channel center; otherwise, they were more common in mid-channel colonies. In some sections, sediment tails were stabilized under the influence of both longitudinal and transverse slopes (32.9%).
Approximately 26.8% of vegetation mosaics, particularly colonies located at the immediate channel margin, were heavily damaged with no ground cover. The occurrence of accumulative biogeomorphic landforms in more distant locations from the channel was observed. Near the immediate channel margin, due to proximity to maximum hydraulic stress and lower resistance thresholds, sediment accumulation in edge colonies was mostly temporary and resulted in unstable depositional clusters. Riparian trees near the main channel were younger and smaller, with stem diameters less than 4–8 cm, compared to trees located farther from the riverbed, which had stem diameters greater than 12 cm.
Woody vegetation with greater height and stem diameter was widely established in positions farther from the channel, in sorted sediments. The proportion of flood-induced damage in these trees was lower, and dense ground cover was present in these colonies. The likelihood of biogeomorphic accumulative landforms occurring in colonies increased with the presence of sediment tails. Sediment tails serve as anchoring points for seeds and pioneer plants. The lowest occurrence of sediment deposition was observed in reach 4 (72.7%) and reach 2 (25.9%), while the highest was in reaches 3 (84.5%) and 1 (75.1%). The presence or absence of biogeomorphic accumulative landforms was strongly associated with other biotic parameters. In creating accumulative biogeomorphic landforms, vegetation older than one year, stem diameter, and stem density were important factors. Transverse slope had more influence than longitudinal slope in controlling the occurrence of accumulative biogeomorphic landforms.
The relationship between biological tails in the lee of vegetation colonies and other biotic parameters (stem diameter, density, height, ground cover, and damage level) indicated that sediment tail formation was affected by all these parameters. Damage level had a significant effect (p=0.004) on biotic density, as higher damage reduced the probability of stabilizing biogeomorphic accumulations. In the central section of sediment bars deposited along the transverse and longitudinal edges of vegetation colonies, plants with flexible colonies and extensive biomass trapped sediments as a strong engineering effect. Stem diameter had the greatest effect on the plant’s sediment retention capacity. When vegetation was exposed to less sediment, other morphological traits, particularly ground cover and stem density, determined the effectiveness of sediment trapping.
Longitudinal deposits located on the right edge of the BFW, which is the bank directly exposed to river flow, were more prone to detachment and erosion compared to longitudinal deposits on the lee side of the BFW. Therefore, longitudinal accumulations in the lee had greater frequency and larger scale compared to those exposed to flow.
Based on the occurrence of longitudinal and transverse tails in BFWs, two types of biogeomorphic feedback windows along the riverbank were observed. In the first type, because the BFWs were not aligned along the riverbank (due to slope differences in bank colonies and BFW advancement toward the mid-channel), both longitudinal and transverse sediment tails were observed. In the second type, the occurrence of longitudinal sediment tails was less likely. In these colonies, the riverbank BFWs were aligned with slight slope differences and uniform channel progression, so there was minimal advance toward the channel center, limiting the opportunity for sediment accumulation in the mid-channel section and formation of biogeomorphic deposits.
Riparian woody vegetation can be considered ecosystem engineers because they increase sediment deposition, form sediment tails, and consequently create landforms. The engineering effect of these riparian plants occurs primarily in locations with physical positions favorable for sedimentation related to flow characteristics. River engineer species can develop different skills or responses to hydrogeomorphic constraints, allowing establishment in an unstable and dynamic geomorphic environment. Biogeomorphic feedback windows are located at a distance from the channel edges because stability of vegetation and sediment trapping is limited at immediate channel margins. When suitable establishment occurs, the height of the landform increases, gradually forming a stable site for vegetation, with seed lodging and depositional landform formation occurring near the center of longitudinal and transverse sediment tails.
Discussion
The aim of this study was to identify the location and characteristics of biogeomorphic feedback windows (BFWs) along the banks of the Taleghan River. In rivers, trees and shrubs influence hydrogeomorphic processes and lead to the formation of alluvial landforms. These trees often form dense colonies, creating concentrated stations and developing vegetative anchors, as plants in colonies are less prone to uprooting than solitary individuals. These colonies also act as ecosystem engineers by trapping sediments, organic matter, and nutrients. The formation of biogeomorphic landforms represents a habitat niche structure that, through facilitative processes among riparian plants, enhances survival capacity, resource storage, and maturation between destructive flood events.
During the establishment of BFWs, intra-group interactions among trees occur, promoting colony survival and growth, as trees and young saplings protect each other against hydraulic stress. In addition to improving the capacity for trapping mineral and organic matter, canopy-forming species can support a network connecting plants, soil, and groundwater, which influences nutrient transfer, cycling, and storage within the habitat structure, as well as nutrient uptake and exchange. In this way, climax conditions can be reached in riparian and mid-channel river ecosystems.
Factors influencing variations in the structure of BFWs, based on characteristics associated with the establishment of accumulative biogeomorphic landforms, include the ratio of sediment accumulation, damage level of sediment tails, vegetation height, vegetation density, stem diameter, and ground cover. The initial stage of the complex process of establishing a biogeomorphic accumulative landform is the creation of a primary physical habitat. Longitudinal and transverse sediment accumulations along the river margin or mid-channel represent the first step for vegetation establishment and the development of BFWs or in the early stages of a BFW, and this varies across different BFWs. Without this, BFWs cannot establish, and if they do, they cannot maintain their habitat structure due to insufficient conditions for development. Thus, a key feature in examining differences between BFWs is the establishment of relatively stable sediment accumulations, while other features associated with maintaining and developing these accumulations are also considered.
The relationship between the ratio of BFWs or sediment accumulations and other factors, such as vegetation height, stem diameter, density, and ground cover, was assessed using the chi-square test. Results indicated significant relationships between damage level, vegetation height, stem diameter, and stem density. The establishment of biogeomorphic accumulative landforms was directly associated with stem diameter, height, and vegetation density, and inversely related to the damage level of BFWs, while ground cover showed no significant effect on the establishment of sediment accumulations or BFWs.
Riverine geomorphic units played a significant role in establishing a primary physical habitat. However, the richness of geomorphic units in terms of base channel or floodplain presence plays a greater role than their density.
The findings of this study indicated that due to the density of relatively stable woody vegetation along the riverbank and the consequent stability of BFWs over a 30-year period, phenotypic changes in plants and the ecological inheritance of these changes to subsequent generations are plausible. Maintaining riparian ecosystems is important not only for enhancing landform stability but also for improving water quality, preserving animal habitats, and maintaining micro- and macro-scale habitat dimensions within the riverbed.
Consistent with these results, Corenblit et al. studied BFWs in three willow species along a 20 km stretch of the Eure River in France, showing that all three species acted as ecosystem engineers. The BFWs of these species were influenced by upstream–downstream longitudinal gradients and the reverse slope toward the floodplain. Similarly, in this study, consistent with the influence of longitudinal and transverse sediment accumulation on BFW development, two types of BFWs were observed. In the first type, due to BFW expansion toward the mid-channel, both longitudinal and transverse sediment trapping occurred, providing opportunities for landform development. In the second type, due to limited expansion toward the mid-channel, sediment accumulation occurred only transversely. Therefore, the combined effect on channel narrowing and BFW expansion is evident in the first type.
The highest occurrence of longitudinal and transverse biogeomorphic accumulations was observed in reaches 1 and 3 of the Taleghan River. Moreover, the stability of BFWs in reach 1 and the upstream river sections was due to lower levels of human intervention and favorable conditions for vegetation establishment. Consequently, feedbacks between geomorphic processes and BFWs, and the maintenance of riparian ecosystems, were more pronounced upstream, particularly in reaches 1 and 2, compared to other reaches. The lowest sediment accumulation, which provides opportunities for vegetation colony development, was observed in reaches 4 and 5, primarily due to human interventions, increased bank erosion, and fewer BFWs. Therefore, suitable opportunities for vegetation colony development were observed upstream, particularly in reaches 1–3. Downstream, in reaches 4 and 5, due to increased human interventions, especially river channelization, the potential for vegetation habitat development and positive effects on river landform stability was considerably reduced.
Conclusion
In managing the Taleghan River, attention should be given to restoring riparian ecosystems by reducing human interventions and creating favorable conditions for the development of biogeomorphic feedback windows (BFWs). The stable establishment of BFWs ensures the persistence of plant habitats containing ecosystem engineer species, which contributes to habitat preservation and development as well as the stability of river landforms.
Acknowledgments: Not applicable.
Ethical permission: Not applicable.
Conflict of Interest: The authors declare no conflict of interest.
Authors’ Contributions: Darabi Shahmari S (first author), Principal Researcher/Introduction Writer/Statistical Analyst (60%); Ahmadabadi A (second author), Assistant Researcher/Discussion Writer/Methodologist (40%)
Funding: The funding required for this research was provided from personal income.
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