Biogeomorphic effects of woody vegetation on bedrock streams

Abstract

The dynamic interactions between fluvial processes and vegetation vary in different environments and are uncertain in bedrock settings. Bedrock streams are much less studied than alluvial in all aspects, and in many respects act in qualitatively different ways. This research seeks to fill this lacuna by studying bedrock streams from a biogeomorphic perspective. It aims to identify the impacts of woody vegetation that may be common to fluvial systems and rocky hillslopes in general, or that may be unique to bedrock channels. A review of the existing literature on biogeomorphology – mostly fluvial and rocky hillslope environments – was carried out, and field examples of biogeomorphic impacts (BGIs) associated with fluvial systems of various bedrock environments were then examined to complement the review. Results indicate that bedrock streams exhibit both shared and highly concentrated BGIs in relation to alluvial streams and rocky hillslopes. Bedrock streams display a bioprotective geomorphic form – root banks (when the root itself forms the stream bank) – which is distinctive, but not exclusive to this setting. On the other hand, shared biogeomorphic impacts with alluvial streams include sediment and wood trapping, and bar and island development and stabilization (i.e. bioconstruction/modification and protection). Shared impacts with rocky hillslopes also include bioprotection, as well as displacement of bedrock due to root and trunk growth, and bedrock mining caused by tree uprooting (i.e. bioweathering and erosion). Two BGI triangles were developed to graphically display these relationships. Finally, this paper concludes that bedrock streams exhibit some BGIs that also occur in either alluvial channels or on rocky hillslopes. Therefore, no BGIs were identified that are absolutely unique to bedrock fluvial environments.

Keywords

Biogeomorphology bedrock streams alluvial streams biogeomorphic impacts rocky hillslopes

I Introduction

Very little research has been done on the biogeomorphic effects of woody vegetation in bedrock-controlled streams. The likely reasons for this neglect are threefold. First, the influence of plants on bedrock streams may be assumed to be minimal because bioprotection effects are less important in bedrock owing to greater bed/bank resistance (Miller, 1991; Tinkler and Wohl, 1998; Whipple, 2004). Second, the effects of sediment trapping are less significant because of bedload or dissolved load domination, and bed and banks composed of material that is not readily transportable (Whipple, 2004). Third, some researchers may have assumed insignificant biogeomorphic impacts (BGIs) owing to a lack of vegetation in-channel and less dense vegetation cover on exposed rocks. However, as BGIs are significant in alluvial streams and on the exposed bedrock of hillslopes, the possibility of these impacts in bedrock streams is worth investigating. The purpose of this paper is to identify the impacts of woody vegetation that may be common to fluvial systems and rocky hillslopes in general, or that may be unique to bedrock streams (Figure 1).

Figure 1.

Potential overlap of BGIs on alluvial streams, bedrock streams, and rocky hillslopes.

Fluvial corridors comprise river channels, their margins, and the zone of expansion of frequent floods occupied by riparian vegetation (Corenblit et al., 2010; Gurnell et al., 2016). They are characterized by intense reciprocal adjustments between hydrogeomorphic processes, landforms, and vegetation (Gurnell and Petts, 2006; Gurnell et al., 2005, 2012, 2016; Steiger et al., 2005; Tabacchi et al., 2005; Tockner and Stanford, 2002; Tsujimoto, 1999). Hydrogeomorphic processes greatly affect habitat diversity, vegetation regeneration, and, thus, biodiversity (Hughes et al., 2001). Furthermore, independent hydrologic factors (e.g. flood frequency, flow duration, and period of inundation) exert influence on vegetation patterns (Hack and Goodlett, 1960; Hupp, 1983; Sigafoos, 1961) by affecting most aspects of the life histories of plant species within the fluvial corridors (Hupp and Osterkamp, 1996). Conversely, vegetation – both living and dead – influences fluvial hydrodynamics (Green, 2005), morphogenesis (Hupp and Osterkamp, 1996), and landscape dynamics (Pettit and Naiman, 2005; Ward et al., 2002). Riparian species also play vital roles in ecosystem engineering – that is, modifying the physical characteristics of riparian zones (Gurnell and Petts, 2006). The absence of these species referred to as biogeomorphic ecosystem engineers may limit the diversity of riparian corridors (e.g. Francis et al., 2009). However, while hydrogeomorphic processes and fluvial landforms are important for vegetation establishment, pattern, and diversity, this paper primarily concentrates on the BGIs of vegetation on bedrock streams and how such impacts can initiate and grow into reciprocal effects.

The interactions between fluvial landforms and riparian vegetation respond differently in distinct environmental settings (Gurnell, 2014; Gurnell et al., 2001), and the relationships are uncertain in bedrock settings. Polvi et al. (2014) show that significant differences exist between woody and non-woody vegetation with respect to reinforcing root-associated cohesion and stream bank stability, and indicate a need for future investigation considering different stream bank types at the reach and watershed scales. Furthermore, a recent study by Gurnell et al. (2019) on the differences in root strength between and within species associated with different European river environments indicates that BGIs associated with specific species are variable within and between rivers of different geographical regions, and suggests a need for future research on species in different fluvial environments. By identifying distinctive and shared BGIs associated with bedrock and alluvial fluvial settings, this research contributes to these future research concerns.

Bedrock streams are much less studied than alluvial in all aspects, and in many respects act in qualitatively different ways. This research aims to identify how bedrock river systems are different or similar to alluvial ones and rocky hillslopes from a biogeomorphic perspective. To fulfill the goal of this research, a review of the existing literature on biogeomorphology – mostly fluvial and rocky hillslope environments – was carried out. Field examples of BGIs associated with fluvial systems of six various bedrock environments were then examined to complement the review. Thus, this research identifies the influences of woody vegetation that may be common to fluvial systems in general, or that may be distinctive to bedrock streams.

II Fluvial biogeomorphology and BGIs

Fluvial biogeomorphology studies the bidirectional linkage between hydrogeomorphic and ecological structures and processes. This includes multiple causalities and scale dependencies of the complex emergent patterns along the fluvial corridor linked to active or passive bio-processes (e.g. bioerosion, bioprotection, bioconstruction, and bioturbation) (e.g. Butler, 1995; Corenblit et al., 2007; Naylor et al., 2002; Viles et al., 2008; Wilkinson et al., 2009). The vital components of fluvial biogeomorphology are interactions between the flow regime (flow intensity, duration, and frequency), sediment, and vegetation, particularly those within the riparian zone that can greatly influence the form and dynamics of the river margin. In this context, riparian zones are part of the valley floor that are often inundated by the river, whereas the term valley bottomland has been used to refer to a larger area enclosing all fluvially generated landforms and vegetation, potentially extending from terraces to the channel bed (Hupp and Osterkamp, 1996). Thus, valley bottomlands encompass an enormous diversity of physical configurations and species life forms and assemblages reflecting the regional and local geological, geomorphic, and bioclimatic settings (Corenblit et al., 2015). Riparian and in-channel vegetation responds to and influences fluvial processes. The outcomes of these plant–physical process interactions vary widely across different bioclimatic, biogeographical, and hydrogeomorphological settings. These interactions drive shifting mosaics of landforms and their associated aquatic and terrestrial ecological communities along longitudinal and transverse gradients within fluvial corridors (Gurnell et al., 2016; Poff et al., 1997; Thorp et al., 2010). The interactions between vegetation (both in-stream and riparian) and fluvial geomorphic processes and forms can be expressed with three types of biogeomorphic functions/roles, which are summarized in Table 1.

Table 1.

Biogeomorphic functions/roles, terms, and definitions.

Biogeomorphic function/role	Definition	Examples of biogeomorphic impact
Bioconstruction/modification	Construction of landforms, or net accretion or accumulation associated with direct or indirect biotic effects	Sediment trapping by wood accumulations or plants; dam formation by wood accumulations
Bioprotection	Biotic effects that increase resistance to erosion (and weathering or mass movement)	Vegetation anchoring of bars and islands; root buttressing of banks
Bioweathering and erosion	Biotic effects that result in or facilitate entrainment and removal of rock, soil, and sediment. Biotic effects on weathering and mass wasting are typically included in this category	Soil displacement by tree uprooting; scour induced by wood accumulation; bedrock weathering effects of tree roots

1 BGIs of vegetation on alluvial rivers

Vegetation influences fluvial processes in several ways. These include increasing roughness, flow and mechanical resistance of beds, banks and floodplain surfaces (e.g. Hupp, 1992; Thorne, 1990), trapping sediment in channels and on floodplains (e.g. Gurnell et al., 2001; Gurnell, 2014), and initiating or stabilizing bars and islands (e.g. Page and Nanson, 1982). In addition to riparian plants, large wood (LW) that has been retained in the river channel also intercepts water and sediment, and, thus, can influence fluvial landforms (e.g. islands, bars, etc.) mainly through the process of biostabilization and bioconstruction (Gurnell et al., 2005).

Living vegetation provides resistance to the forces of fluvial processes, and this role of vegetation as a vital mediating agent of hydrogeomorphic processes within the fluvial corridor has been acknowledged in several studies (Gurnell and Petts, 2006; Gurnell et al., 2001, 2012) (Table 2). Furthermore, living vegetation increases cohesion via root mass, and therefore increases resistance of vegetated landforms (Gran et al., 2015). McKenney et al. (1995) quantified the Manning’s roughness coefficient incorporating vegetation, and showed how vegetation roughness and resistance affect fluvial hydrodynamics and morphogenesis in gravel-bed streams. Vegetation also has substantial influence in trapping and stabilizing fluvially transported sediment. These impacts can foster the construction of distinct landforms and accelerate the development of larger landforms such as river banks, vegetated islands, and floodplains (e.g. Gurnell et al., 2016).

Table 2.

Example studies demonstrating biogeomorphic feedbacks associated with alluvial streams.

Type of study or evidence	Example references
Bar and island formation and evolution	Abbe and Montgomery (1996); Corenblit et al. (2011); Fetherston et al. (1995); Francis et al. (2009); Francis (2009); Gurnell (2014); Gurnell and Petts (2006); Gurnell et al. (2001, 2005, 2012); Harwood and Brown (1993); Nanson (1981); Page and Nanson (1982).
Floodplain–wood dynamics	Collins et al. (2012); Gurnell et al. (2001); Wohl (2013); Wohl and Scott (2017).
Hotspot zones of species modifying landforms – ecosystem engineering	Collins et al. (2012); Gurnell (2014).
Fluvial biogeomorphic succession	Corenblit et al. (2007, 2009a, 2010).

Many studies have focused on deposited wood and its influence on process–form dynamics along the fluvial corridor (Table 2). Gurnell et al. (2005) discussed the role of wood, particularly when the deposited trees are able to sprout and anchor themselves to bar surfaces, in relation to the formation and dynamics of island-braided rivers. Reviews of wood–sediment dynamics along river corridors include Gurnell et al. (2001), Wohl (2013), and Wohl and Scott (2017). Gurnell et al. (2001) incorporated both the impact of wood and living vegetation in their conceptual model of island development within fluvial systems. Studies have also identified the association of dead wood with the initiation of specific types of fluvial landforms, such as reinforcing the core of scroll bars (Nanson, 1981), or building bar apex jams (Abbe and Montgomery, 1996) and lateral jams (Fetherston et al., 1995) behind which sediment and organic matter accumulate to provide a substrate on which riparian trees may establish. Kramer and Wohl (2015) describe an extreme example of this phenomenon, driftcretion, where large concentrations of driftwood contribute to sedimentation, influencing shoreline morphology and evolution by interacting with vegetation. Furthermore, Fetherston et al. (1995) suggested that dead wood plays a vital role in reducing mean boundary shear stress, and thus protects the surfaces and margins of islands and bars. However, while LW plays a key role in promoting landform protection and stability, they can also destabilize fluvial landforms by promoting erosion. For example, a study of forested and grassed stream banks by Trimble (1997) suggested that forested stream banks, relative to grassed ones, can destabilize stream channels by promoting erosion. Mature forests produce LW, which may destabilize streams locally by affecting the distribution of stream power via diverting flow against banks (Gregory and Davis, 1992; Gurnell and Gregory, 1995).

Finally, there are studies on the effects of fluvial hydrodynamics and forms on vegetation germination and their successful establishment, growth, survival, and distribution. These are based on the details of how boundary conditions for vegetation – including flow regime, substrate, and channel geometry – are likely to govern vegetation distribution and their influences within fluvial corridors (e.g. Bendix, 1998, 1999; Corenblit et al., 2007, 2009b; Hupp and Osterkamp, 1985, 1996; Hupp and Rinaldi, 2007; McBride and Strahan, 1984; Shafroth et al., 1998). However, no comparable studies of these dynamics have been conducted on fluvial systems that are characterized by bedrock-controlled channels.

2 BGIs of vegetation on hillslopes

BGIs of vegetation on hillslopes are similar to those of fluvial corridors in a number of cases. Many biogeomorphic and pedologic studies have emphasized the importance of tree root systems in which roots play a primary role in soil development, regolith disturbance, bedrock mining by tree uprooting, and soil displacement by growing roots (Pawlik et al., 2016). Pawlik (2013) specified three BGIs of trees on bedrock hillslopes that can also be potentially important for bedrock streams: (i) growing root systems – these disintegrate rock fragments and widen fissures in bedrock; (ii) growing trunks – the physical displacement of bedrock; and (iii) tree uprooting – direct bedrock disruption via mining (see Table 3 for example studies). Growing root systems can have an immense impact on physical and chemical weathering. The radial pressure exerted by tree root systems can reach 0.91 MPa and axial pressures as high as 1.45 MPa (Bennie, 1991), which is sufficient to break up bedrock. The roots inevitably increase in length and girth and split the rocks apart slowly (Matthes-Sears and Larson, 1995). Phillips (2015) showed that about 90% of the examined trees of his study conducted on limestone bedrock hillslopes exhibited evidence of: i) joint widening both horizontally and vertically by root penetration; ii) mechanical displacement of bedrock along bedding planes; and iii) root exposure indicating the removal of material at the tree base (Table 3). Phillips (2016) further explained how the widening of joints can promote chemical weathering in such karst-associated bedrock environments. A combination of root growth in joints, trunk expansion, and the development of basal flares near the tree–ground interface can displace rock fragments both vertically and horizontally (Phillips, 2015). Thus, trees can promote weathering of bedrock and displace mass via root and trunk growth (Gabet and Mudd, 2010; Lutz and Griswold, 1939) (Table 3).

Table 3.

Example studies demonstrating biogeomorphic impacts associated with rocky or thin-soil hillslopes.

Type of study or evidence	References
Displacement and movement of soil and rock fragments by tree uprooting	Lutz (1960); Martin (2006); Pawlik (2013); Phillips and Marion (2006); Phillips et al. (2008); Schaetzl et al. (1989); Small et al. (1990); Šamonil et al. (2010).
Bedrock mining associated with tree uprooting	Pawlik (2013); Phillips (2015); Phillips (2016).
Displacement of bedrock by root and trunk growth	Birot (1966); Gabet and Mudd (2010); Gabet et al. (2003); Jackson and Sheldon (1949); Lutz and Griswold (1939); Matthes-Sears and Larson (1995); Phillips (2015); Roering et al. (2003).
Accelerated weathering along joints and bedding planes	Bormann et al. (1998); Gabet et al. (2003); Marion (2005, 2006); Phillips and Phillips (2015); Yatsu (1988).

Uprooting of trees usually occurs during storms with strong winds, ice storms, or excessive rainfall. Uprooted trees can break down bedrock, transport soil downslope, and hinder soil horizonation (Gabet et al., 2003). In bedrock settings, uprooting results in bedrock mining as opposed to thicker soils where bioturbation is the key consequence. Uprooting has been characterized as one of the primary mechanisms of downslope mass movement process (Schaetzl et al., 1989; Small et al., 1990) (Table 3), which, in turn, promotes weathering and erosion of exposed bare soil/rock and slope destabilization (e.g. Phillips et al., 2017).

Finally, the infilling of stump holes and trapping of sediments from upslope are distinctive BGIs within rocky hillslopes (Pawlik, 2013; Phillips, 2015; Shouse and Phillips, 2016) as bedrock stream environments have limited potential to display such impacts. Additionally, within hillslope environments, tree growth may enclose (or partly enclose) rock fragments and prevent the downslope movement of sediments until the death of the tree and wood decomposition (Phillips, 2015). However, this may also occur along fluvial environments.

3 Reciprocal interactions between vegetation and geomorphic processes

Vegetation within fluvial corridors influences the flow hydraulics and landforms by increasing shear strength, retaining sediment, and affecting the hydraulic and mechanical properties of the substrate. Similarly, fluvial dynamics, water availability, and sediment erosion, transportation, and deposition determine the distribution and vigor of many species (Gurnell, 2014). Thus, vegetation and fluvial processes and forms are connected with each other via reciprocal effects that grow or diminish by biogeomorphic feedbacks. For example, Francis et al. (2009) explained how biogeomorphic feedbacks between vegetation growth and sedimentation influence island formation and self-assembly. Another example illustrated how feedback relationships between pioneer species and a high-magnitude disturbance (i.e. flood) lead to the development of a highly resilient fluvial landscape. Landform accretion, vegetation succession, and increasing geomorphic stability governed the development of such resilient landforms (Corenblit et al., 2010). In a related context, Gurnell (2014) introduced the idea of hotspots (Table 2). Hotspots are environmental envelopes within which ‘engineer’ plant species interact strongly with fluvial processes. They are enclosed within areas where fluvial processes or interspecies competition dominate. The location of hotspots shifts through time, corresponding to periods of relatively higher or lower fluvial disturbance. Within the hotspots, certain ‘engineer’ species are able to interact with fluvial processes by retaining and reinforcing sediments to build landforms (riverbanks, islands, floodplains) and habitat that are then colonized by other plant species. All these examples indicate that the relationships between riparian vegetation and hydrogeomorphic processes are driven by complex feedback mechanisms, which determine the spatial structure and dynamics of riparian ecosystems (Hastings et al., 1993). Moreover, Bendix and Cowell (2010) discussed the effects of wood accumulation on channel hydraulics and morphogenesis, where wood accumulation was triggered by post-wildfire flooding events. Thus, they showed how riparian ecology and fluvial geomorphology are causally connected with bidirectional influences.

Hillslope environments also exhibit biogeomorphic-feedback-associated interactions. One example of positive biogeomorphic feedbacks includes the development of dissolutional grooves at the root–limestone bedrock interface. Dissolutional activity is enhanced along the roots that penetrate joints and extend across boulder and exposed bedrock surfaces. Thus, root growth promotes the further development of solutional grooves in many karst environments (Phillips, 2015). Phillips (2015) also showed how root penetration along vertical and horizontal joints can enhance weathering and moisture flux, and increase the susceptibility to bedrock mining. This leads to locally thicker regolith. Literature on root–rock interactions suggests that locally deepened regolith provides favorable sites for future tree establishment, and root channels and root-widened fissures are favored sites for future root penetration (Estrada-Medina et al., 2013; Martin, 2006; Phillips and Lorz, 2008; Shouse, 2014; Stone and Kalisz, 1991). Thus, biogeomorphic effects can extend beyond the lifetime of a single tree, and repeated reoccupation can lead to continued localized modification. Crowther (1987), in karst systems in peninsular Malaysia, also found that most chemical activity is associated with bedrock in contact with roots, which indicates the presence of the positive feedback relationships discussed above. Further, in a similar bedrock environment, Phillips (2016) showed how Chinquapin oak roots exert direct impacts on the surrounding trees by creating dissolutional grooves and channels, and lifting and displacing rock plates. Thus, he illustrated the reciprocal interactions between vegetation and hillslope processes from an ecosystem engineering perspective.

Biogeomorphic feedbacks influence two aspects of fluvial and hillslope ecosystems: i) state transitions; and ii) diversity of landforms and plant species.

3.1 Ecosystem state transitions

Biogeomorphic interactions may result in ecosystem state transitions (Dent et al., 2002; Francis, 2009). In the case of natural fluvial ecosystems, state transitions are influenced not only by the disruption of key hydrogeomorphological drivers, but also feedbacks between flow regimes and sediments, and vegetation dynamics (Francis, 2009). These feedbacks result in characteristic biogeomorphic patterns and strongly affect ecosystem functioning and biodiversity. The fluvial biogeomorphic succession model suggested by Corenblit et al. (2007) is one example. This model illustrates how riparian plant communities and landforms co-evolve via bidirectional linkages associated with feedback mechanisms. It comprises four phases – geomorphic, pioneer, biogeomorphic, and ecologic – characterized by progressive changes in the relative dominance of hydrogeomorphic and ecological processes. The first stage is characterized by geomorphic systems that are exclusively driven by interactions between flow and sediment or substrate, with a successive amplification of vegetation influence in the next three phases. The model by Gurnell et al. (2016) is another example in this context, which conceptualized the nature of vegetation–hydrogeomorphology interactions in the absence of human influences for different European biogeographical settings. This model is founded upon some hydrogeomorphologically centered prior models, most importantly the island development model (Gurnell et al., 2001), the large-wood cycle concept (Collins et al., 2012) and the fluvial biogeomorphic succession model (Corenblit et al., 2007). The model by Gurnell et al. (2016) explains how hydrogeomorphological constraints vary spatially and temporally within fluvial corridors, giving rise to five distinct lateral zones where particular subsets of plant–physical processes prevail. However, because this model considers the distribution of these zones according to valley confinement (i.e. longitudinal variability from confined headwaters to unconfined floodplain reaches) and river types, it is, therefore, potentially relevant to bedrock streams. In addition to these models, Van Dyke (2016) explicitly discussed biogeomorphic-feedback-associated channel adjustment and consequent evolution under the framework of a state-and-transition model. His study established that the complex evolutionary pattern of a fluvial corridor is a function of the interactions between bio-hydro-geomorphic fluxes and landscape that vary across space and time. Other biogeomorphic studies demonstrating reciprocal linkages include Bertoldi et al. (2009), Corenblit et al. (2009a, 2009b, 2015), Gurnell et al. (2001, 2005, 2012), and Stoffel and Wilford (2012). In the context of forested hillslopes, Phillips et al. (2017) suggested that biogeomorphic succession may be more varied than the linear sequential fluvial biogeomorphic succession model, and may include pathways where biogeomorphic feedbacks are more persistent.

3.2 Diversity of landforms and species

Biogeomorphic feedbacks may also influence ecosystem diversity within fluvial and hillslope environments. For example, in the case of fluvial systems, the engineering activity of some riparian species rooted into the bank toe can develop ‘hotspot’ zones (see Section II.3), which may promote the future colonization of other plant species (Gurnell, 2014). Gurnell et al. (2005) discussed how water and sediment interception by wood during floods can foster landform diversity by initiating the physical creation, modification, or maintenance of habitats (e.g. islands, bars, etc.), which, in turn, can increase biodiversity. Hupp and Rinaldi (2007) denoted riparian zones as the potentially most diverse ecosystems worldwide, where species richness substantially increases along the transverse gradient from channel bed to terraces. Other studies related to the idea of biogeomorphic feedbacks and biodiversity include Gurnell and Petts (2006), Gurnell et al. (2007), Bertoldi et al. (2009), Francis et al. (2009), etc. Moreover, Shouse and Phillips (2016) showed an instance of increasing diversity of geomorphic forms for a non-fluvial hillslope environment. Here, they discussed how vegetation-induced regolith thickening, driven by mechanisms associated with root penetration in bedrock, can promote landform diversity.

Biogeomorphic feedbacks and associated ecosystem engineering by plants do not always increase diversity in geomorphic forms or in the plant species that are present (e.g. An et al., 2007; Fei et al., 2014; Tickner et al., 2001). For example, Tamarix – a riparian invasive species – can negatively affect two aspects of fluvial systems: i) channel geometry (e.g. Graf, 1978); and ii) diversity of in-stream landforms (e.g. Busby and Schuster, 1973). First, Tamarix species foster aggradation and build stable floodplains and riverbanks by increasing roughness to hydraulic shear, trapping and stabilizing transported sediment and debris (Birkeland, 1996). Aggradation, in turn, leads to a narrowing of the river channel (Tickner et al., 2001). A similar study on the Green River, Utah, showed that invasion of the same species promoted an average reduction in channel width of 27% (Graf, 1978). Second, Busby and Schuster (1973) identified a negative relationship between Tamarix invasion and the extent of sandbar and gravel cover within streams in Texas. Thus, in addition to channel geometry, Tamarix can adversely influence the diversity of landforms within the channel. Moreover, Tamarix can also affect species diversity by deteriorating the habitat characteristics for other species. The phreatophytic nature of Tamarix species and their rapid establishment along river margins can significantly depress riparian water table levels in arid regions. The depletion of water tables is caused by the ability of Tamarix to root directly into the groundwater (Loope et al., 1988; Vitousek, 1990). Thus, Tamarix can decrease species diversity by reducing the available water for other species.

III Biogeomorphology of bedrock streams

1 Bedrock versus alluvial streams

Whipple (2004) defined bedrock streams as channels that lack continuous cover of alluvial sediments, even at low flow, and exist only where transport capacity exceeds bedload sediment flux over a long period of time. Tinkler and Wohl (1998) characterized a bedrock channel as one with 50% bedrock exposed in the bed and banks, or covered by an alluvial veneer which is largely mobilized during high flow events such that the underlying bedrock geometry strongly influences patterns of flow hydraulics and sediment movement. Channels that are not confined by bedrock or terraces, but are flanked by floodplains, are called alluvial channels (Schumm, 2007). Alluvial channels are those that have formed their channel in bed and bank sediment that the stream can readily entrain and transport for a wide range of flows (Leopold and Maddock, 1953; Schumm, 1977).

Knowledge from alluvial and gravel-bed systems cannot be directly transferred to bedrock rivers (Tinkler and Wohl, 1998) as such attempts have already fallen into difficulties (e.g. Tinkler and Parish, 1998; Vaughn, 1990). Key differences with respect to alluvial streams are often attributable to slower change (Schumm and Chorley, 1983; Whipple, 2004), unidirectional change (Tinkler and Wohl, 1998), greater role of bed/bank resistance, more direct influence of lithology and structure (Miller, 1991; Tinkler and Wohl, 1998; Whipple, 2004), and an enhanced role of processes such as dissolution, abrasion, and plucking (Tinkler and Wohl, 1998; Whipple et al., 2000; Wohl and Ikeda, 1998). Bedrock channels occur mainly, but not exclusively, in actively incising portions of landscapes where channels are cut into resistant rock units (Whipple, 2004). This explains greater influence of lithology and structure, greater role of bed/bank resistance, and, therefore, the dominant erosion processes and slower change of bedrock channels than that of alluvial rivers. The bed and banks of bedrock rivers are not composed of transportable sediments, but are erodible (Whipple, 2004). As bedrock streams often do transport appreciable sediment, some BGIs observed in alluvial streams are likely important in bedrock systems too, such as sediment trapping and initiating or anchoring bars and islands. While the role of vegetation in enhancing bed and bank resistance might be minimal, vegetation could still influence flow hydraulics, and work on tree–bedrock interactions in terrestrial settings indicates vegetation could be important in weathering and the reduction of resistance of bedrock (Pawlik et al., 2016). This suggests that BGIs on bedrock banks and channels could be significant and need to be recognized.

In this section, I will discuss the BGIs of woody vegetation on bedrock streams from the context of different biogeomorphic roles (i.e. bioconstruction and modification, bioprotection, and bioweathering and erosion) (see Table 1) played by vegetation. In addition, I will address which impacts are common to fluvial systems and rocky hillslopes in general, and which are unique to bedrock channels.

1.1 Bioconstruction/modification

The effects of vegetation related to the role of bioconstruction and habitat modification are widely documented in the fluvial biogeomorphic literature, but from the alluvial stream perspective (see Table 2). However, examples of these BGIs can also be found in bedrock streams (Figures 2 and 3).

Figure 2.

Tree growing in limestone bedrock channel, trapping sediment and wood, Shawnee Run, KY (left); tree growing in sandstone bedrock stream, trapping sediment and wood, Ouachita Mountains, AR (right).

Figure 3.

Island formation, anchoring, and modification in Shawnee Run, KY.

Sediment can be transported in bedrock channels and subsequently trapped by vegetation. Again, an alteration of flow hydraulics can facilitate riparian vegetation establishment and survival, which, in turn, can reinforce sediment trapping in bedrock streams (see, e.g., Auble et al., 1994). Many bedrock streams, such as Shawnee Run (Figures 2 (left) to 4), are mixed bedrock and alluvial (cobble, gravel, boulders). Riparian and in-channel plants and LW associated with bedrock streams have the potential to trap these sediments and, thus, can create local alluvial reaches. However, these bioconstructive roles played by vegetation and wood are also common in alluvial reaches exemplified in several studies referred to in Table 2 (floodplain–wood dynamics, hotspot zone studies, etc.) Furthermore, in-channel sediment trapping can lead to the development of bars and islands (Figures 2 and 3) both in bedrock and alluvial streams. The process of bar and island formation, stabilization, and modification in bedrock streams can be hypothesized in two ways:

Initiation, stabilization, and development of bars (Figure 2) and islands via deposition caused by in-channel live vegetation or log jams (Fetherston et al., 1995, Gurnell and Petts, 2006; Page and Nanson, 1982).

High flow or secondary channels parallel to the main channel can develop during floods and sometimes expand. More resistant patches with larger trees between the secondary and main channel are not eroded and may remain as islands if the secondary channel persists and grows.

In addition to landform construction, sediment and wood trapping by riparian vegetation has the potential to modify the characteristics of the stream bed, riparian zones, and floodplains. Examples of vegetation-induced landform modification in alluvial streams can be found in many biogeomorphic literature, including Gurnell and Petts (2002), Wohl and Scott (2017), and Gurnell (2014). However, sediment and wood trapping by vegetation in bedrock streams also exhibits comparable biogeomorphic outcomes – for example, substrate modification by vegetation-induced sediment trapping and subsequent deposition (see Figure 4, right).

Figure 4.

Tree growing in-channel (right) and at bank edge (left) in a limestone bedrock channel, trapping sediment and wood, Shawnee Run, KY.

LW in the channel and on the floodplain also contributes to bioconstruction and modification (Table 4). Evidence of these BGIs is also found in bedrock rivers (Figures 5 and 6), many of which have reaches with small floodplains. LW contributes to flow dynamics via flow diversion, backwater effects, and substrate modification and construction via sediment and wood trapping. For example, LW pieces or log jams have the potential to alter bedrock reaches into alluvial ones. They can reduce the differences in elevation (thus decreasing slope), which, as a result, can reinforce deposition and modify substrate characteristics (Massong and Montgomery, 2000). Thus, bedrock reaches can be forced into alluvial ones by LW (Figure 5 (left)).

Table 4.

Evidence of biogeomorphic impacts and roles of vegetation on fluvial geomorphic processes and forms in bedrock environments.

Forms of biogeomorphic impacts	Processes	Biogeomorphic roles
Wood accumulation in channel	Flow diversion, backwater effects, sediment and wood trapping, potential for channel narrowing, habitat creation, island and bar formation	Bioconstruction Bioerosion
Wood accumulation – sediment trapping – on floodplain	Habitat creation and modification, and potential for channel narrowing	Bioconstruction
Live vegetation – wood and sediment trapping – on floodplain	Habitat creation and modification, and potential for channel narrowing	Bioconstruction
Live vegetation – wood and sediment trapping – at bank edge	Development of bars, potential for channel narrowing	Bioconstruction
Live vegetation in-channel	Flow diversion, backwater effects, sediment and wood trapping, potential for habitat creation in the form of bars or islands	Bioconstruction Bioprotection Bioerosion
Live vegetation – bar/island anchoring	Reinforced deposition, habitat creation and modification, and increasing resistance to erosion	Bioprotection Bioconstruction
Root banks/buttresses	Increasing roughness to hydraulic shear and mechanical resistance, wood and sediment trapping, and potential for accelerated biochemical weathering	Bioprotection Bioprotection Bioweathering
Live vegetation – in channel and on floodplain	Displacement of bedrock by root and trunk growth Accelerated weathering along joints and bedding planes	Bioweathering and erosion
Uprooting of vegetation –bank edge, in-channel, and on floodplain	Bedrock mining	Bioweathering

Figure 5.

Large wood in-channel (left) associated with an alluvial reach; large wood on floodplain (middle); large wood at bank edge (right) in a limestone bedrock channel, Shawnee Run, KY.

Figure 6.

Root bank in limestone bedrock channels: Raven Run, KY (left); San Marcos River, TX (right).

1.2 Bioprotection

The bioprotective role of vegetation is well documented in the biogeomorphic literature. However, bedrock streams intrinsically have greater bed/bank resistance than alluvial channels (Miller, 1991; Tinkler and Wohl, 1998; Whipple, 2004). Therefore, the role of bioprotection is ambiguous for bedrock streams to some extent as they are inherently resistant.

Bioprotective functions in bedrock streams were detected in the form of root banks – when the root itself creates the stream bank (Figure 6 and Table 4). Hydraulic shear does not seem to be capable of eroding root banks, just as it cannot erode intact bedrock. Thus, where root banks occur directly overlying bedrock, as has been observed in the field, there may be little or no increase in resistance. Nevertheless, root banks can entrap fine sediment and lead to the formation of extensive fine sediment benches. In such cases, root banks will protect an extended area surrounding them from fluvial erosion, and, thus, can considerably contribute to bioprotection. On the other hand, root banks along alluvial banks in a bedrock-controlled or alluvial stream considerably increase bank resistance.

While the root bank may be physically bioprotective, in bedrock-controlled streams they may enhance chemical and biomechanical weathering of the underlying rock. Therefore, the roots undoubtedly affect the resistance of the banks in the form of protection while the tree is alive, but exposing more weathered and less resistant rock when the root bank is gone. This suggests that the relative importance of bioprotection along bedrock versus alluvial streams, and the protective versus erosive effects of root banks, needs further investigation.

1.3 Bioweathering and erosion

Bioweathering and erosion, to some extent, have received less attention in the fluvial biogeomorphic literature. However, widespread evidence of this biogeomorphic role were identified in bedrock streams (Figures 7 to 9). Even though the effects of bioweathering and erosion are widely overlooked in fluvial (more specifically, alluvial) biogeomorphic studies, they are frequently addressed in studies associated with the hillslopes of rocky environments (see Section II.2). Examples of bioweathering and erosion associated with bedrock streams are discussed in Section III.2.

Figure 7.

Bedrock weathering due to trunk growth along the bank of limestone bedrock rivers: Raven Run, KY (left); Dix River, KY (right).

Figure 8.

Bedrock weathering due to root growth along the bank of bedrock rivers: Granite bedrock, Union County, SC (left); limestone bedrock, Shawnee Run, KY (middle and right).

Figure 9.

Bedrock mining due to tree uprooting along the bank of a limestone bedrock river, Shawnee Run, KY.

2 Bedrock streams versus rocky hillslopes

BGIs of vegetation on rocky hillslopes are similar to those of bedrock-dominated fluvial corridors in a number of cases (Section II.2 and Table 3). All of these impacts eventually contribute to the role of bioweathering and erosion.

Root and trunk growth causes weathering and subsequent erosion in bedrock river systems, and the identified examples are analogous to those of rocky hillslopes. The displacement and disintegration of bedrock via root and trunk growth, accelerated weathering along joints, and bedding plains are common BGIs of vegetation on bedrock streams (Figures 7 and 8). These processes can promote the supply of sediment in bedrock streams, which, in turn, can affect channel morphogenesis.

The impacts of tree uprooting on bedrock rivers are similar to those of rocky hillslopes. Biogeomorphic and pedologic studies have emphasized the importance of tree uprooting, in which roots play a significant role in soil development, regolith disturbance, and bedrock mining (Pawlik et al., 2016). Tree uprooting in bedrock-controlled streams primarily causes the disintegration and mining of bedrock (Figure 9). In addition, tree uprooting can potentially weaken the contiguous joints and bedding planes along stream banks, and, thus, can promote further bank erosion. I observed one case of uprooting and bedrock mining within a channel bed, but it is unknown whether this is common in bedrock fluvial environments.

IV Fluvial BGIs

1 Distinct versus shared BGIs: the BGI triangles

BGIs associated with bedrock streams can be highly concentrated or common in other environmental settings (e.g. alluvial and hillslope settings). Here, highly concentrated refers to those BGIs that are not unique to alluvial streams, bedrock streams, or rocky hillslope environments, but are uncommon in the other settings. Figure 10 illustrates BGIs that are strongly associated with a specific environmental setting and, thus, highly concentrated in either alluvial streams, bedrock streams, or rocky hillslopes. In contrast, Figure 11 illustrates shared BGIs – that is, impacts that are not restricted to a specific environmental setting and are likely to occur in all three environments of bedrock streams, alluvial streams, and rocky hillslopes.

Figure 10.

Examples of BGIs concentrated in bedrock or alluvial stream or rocky hillslope settings.

Figure 11.

Examples of BGIs common in multiple environmental settings.

The top corner of the BGI triangle in Figure 10 illustrates the BGI that is highly concentrated in bedrock streams – development of root banks along fluvial corridors. Evidence of root banks in bedrock streams was identified during field reconnaissance surveys. Root banks are more common in bedrock streams than alluvial ones, and the likely reason for this is primarily attributable to the geological contrasts between bedrock and alluvial streams. The bed and banks of bedrock rivers are more resistant than alluvial ones (Tinkler and Wohl, 1998; Whipple, 2004) and are not composed of transportable sediments (Whipple, 2004). Thus, bank roots are less likely to be either exposed by erosion or covered by deposition in bedrock streams. Again, owing to the greater resistance of bedrock stream banks, the bank line probably largely controls the trees and the root growth in contrast to alluvial streams where roots gradually evolve with tree growth and stabilize the bank. Thus, in the bedrock case, roots become exposed on the river bank partly to stabilize the tree and partly to spread the root system so that side roots can penetrate gaps in the rock to find water and nutrients. All these indicate that root banks are likely to be more concentrated in bedrock streams than alluvial ones. Field reconnaissance work on bedrock streams in Kentucky shows that root banks commonly occur where bedrock is exposed, whereas alluvial banks of the same streams rarely offer any evidence of this feature. However, though root banks may be highly concentrated in bedrock streams, they also occur along alluvial stream banks.

The left corner of the triangle (Figure 10) shows the BGIs concentrated in alluvial streams. The impacts include initiation and development of bars and islands (bioconstruction), and root-reinforced deposition of sediment and wood within channels and on floodplains (bioconstruction and protection). Although evidence of these BGIs can also be found in bedrock streams, mid-channel island and bar creation owing to the presence of live vegetation or wood-reinforced deposition are more concentrated in alluvial streams (e.g. Gurnell, 2014; Gurnell and Petts, 2002, 2006; Gurnell et al., 2001, 2012). The right corner of the triangle indicates BGIs that are highly concentrated in bedrock hillslopes. Infilling of stump holes and trapping of sediments from upslope (bioconstruction and modification) (Pawlik, 2013, Shouse and Phillips, 2016) are distinctive BGIs within rocky hillslopes, while fluvial environments have limited potential to display such impacts.

Conversely, the trapping of sediments and rock fragments on floodplain and in-channel by vegetation (i.e. bioconstruction/modification) is common in all fluvial systems. Therefore, these impacts can be placed in both the top (i.e. bedrock streams) and the left (i.e. alluvial streams) corners of the BGI triangle in Figure 11. Again, while evidence of bedrock displacement owing to tree root and trunk growth, and bedrock mining caused by tree uprooting (examples of bioweathering and erosion), were identified within bedrock fluvial environments, they are also common in bedrock hillslopes (see Table 3). As a result, such impacts fit at both the top (bedrock streams) and the right (rocky slopes) corners of the BGI triangle. It is noteworthy that the most common biogeomorphic role played by vegetation is bioprotective in nature (referred to as inherent bioprotection in Figure 11). In fluvial systems and hillslope environments, vegetation stabilizes and protects landforms from erosion via root cohesion and sediment trapping and deposition. Thus, these BGIs are common in all three geomorphic settings (i.e. bedrock streams, alluvial streams, and rocky hillslopes).

Bedrock streams share BGIs both with alluvial streams and rocky hillslopes. Shared BGIs of bedrock and alluvial streams (that do not occur in bedrock hillslopes) are caused by the nature of geomorphic work done by fluvial systems and biota regardless of the environmental settings. On the other hand, bedrock streams and rocky hillslopes exhibit common BGIs (that do not occur in alluvial streams) owing to the comparable geological controls maintained in these settings. It is noteworthy that no such BGIs have been identified so far that are common in alluvial streams and rocky hillslope environments, but not present (at least potentially) in bedrock fluvial systems. Further, shared BGIs associated with all three environmental settings (see Table 5 and Figure 11) indicate entangled relationships among vegetation, geomorphic process–form linkages, and environmental settings. While the BGIs of vegetation associated with these three different settings are not similar in all cases, the biogeomorphic roles (i.e. bioconstruction, bioprotection, bioerosion) played by them are analogous.

Table 5.

Shared and highly concentrated biogeomorphic impacts.

Environmental settings	Biogeomorphic impacts
Bedrock	Root banks
Alluvial rivers	Island and bar creation by reinforcing deposition
Hillslopes	Infilling of stump holes; triggering and reinforcing mass movement caused by bioweathering
Alluvial and bedrock rivers	Sediment and wood trapping at bank edge, in-channel, and floodplain; island and bar stabilization by reinforced deposition; island and bar creation and stabilization causing channel avulsion
Bedrock hillslopes and bedrock rivers	Displacement of bedrock due to root and trunk growth; bedrock mining caused by tree uprooting
Alluvial rivers and bedrock hillslopes	No common impacts that do not also occur on bedrock rivers
Bedrock rivers, alluvial rivers and hillslopes	Trapping upslope sediments and wood; resisting erosion via increasing cohesion

2 Fluvial BGIs and channel forms and processes

While very few studies specifically address BGIs of vegetation in bedrock streams, some reasonable speculations can be made about the channel form and process dynamics facilitated by them. The possible scenarios for bedrock streams are summarized in Table 6, with explanations discussed below:

Root-associated bioweathering of bedrock channel bed and banks can potentially influence channel incision and widening, whereas such effects may be insignificant in alluvial streams.

Bedrock mining caused by tree uprooting can locally influence channel widening and deepening in bedrock streams; however, effects will largely depend on whether the trees are located on the bank or in-channel. Other impacts – that is, channel narrowing, aggradation, and flow divergence – will vary not only by the location of the uprooted tree/s but also by their extent to which roots and wood impede flow and block channels. However, these impacts occur in all fluvial systems.

Vegetation-induced sediment trapping on banks and in-channel can potentially contribute to channel narrowing for all fluvial systems. Further, in-channel sediment trapping can promote aggradation and the subsequent development of islands or mid-channel bars. Thus, these impacts and processes can change single-thread channels to multiple-thread ones. Furthermore, sediment trapping on floodplain surfaces can increase the bank height, which can alter the channel geometry by lowering the width/depth ratio.

Bar stabilization by woody vegetation in all fluvial systems can influence channel widening and narrowing both positively and negatively. These impacts will largely depend on two factors: i) the relative magnitudes of bar width versus erosion of adjacent banks triggered by the bars; and ii) the location of the bars – that is, whether a bar is attached to the bank or in mid-channel.

Bioprotection plays a negative role on channel widening and narrowing in all river systems. It amplifies bank resistance via root cohesion, and aids energy dissipation via roughness effects. Thus, the bioprotective impacts of vegetation offset hydraulic stresses.

Vegetation-induced hydraulic effects, most importantly flow diversion and turbulence, can result in heterogeneous impacts on channel forms and processes – for example, island stabilization and/or expansion by inducing deposition, or channel incision by triggering local scour of the channel bed. These impacts will largely depend on the environmental settings and the boundary conditions of the fluvial systems.

Vegetation can stimulate island formation by promoting in-channel sediment trapping and aggradation. Island formation accompanied by channel splitting further has the potential to foster bank erosion caused by island-associated flow deflection. Thus, vegetation can passively promote channel widening.

LW accumulations or wood dams/jams have a positive influence on channel widening, aggradation, and divergence. For example, a partial blockage of the channel can lead to flow divergence and subsequent channel widening. Such blockages can also induce turbulence-associated local scour (Thompson, 2006), which can lead to pool formation (i.e. channel deepening). A complete blockage of the channel by wood dam can reduce the local slope, and, thus, can reinforce channel aggradation (Massong and Montgomery, 2000).

Table 6.

Potential biogeomorphic impacts of woody vegetation on bedrock streams.

Biogeomorphic impacts of woody vegetation	W	N	I	A	D
Root bioweathering of bedrock banks	+	0	0	0	0
Root bioweathering of bedrock bed	0	0	+	0	0
Uprooting	+	+	+	+	+
Sediment trapping – bank	–	+	0	0	0
Sediment trapping – channel	0	+	–	+	+
Sediment trapping – floodplain surface	0	0	+	–	0
Bar stabilization	+, –	+, –	0	0	+, 0
Bioprotection (increased bank resistance)	–	0, –	0	0	0
Bioprotection (energy dissipation via roughness effects)	–	0, –	0	0	0
Hydraulic effects (flow diversion, turbulence)	+, 0, –	+, 0, –	+, 0, –	+, 0, –	0
Island formation	+	–	0	0	+
Large wood dams/jams	+	–	0	+	+

W: channel widening; N: channel narrowing; I: channel incision; A: channel bed aggradation; D: flow divergence (channel splitting).

+, –, 0: positive, negative, and no direct impacts, respectively.

The discussion noted above suggests that while bedrock and alluvial fluvial systems exhibit comparable biogeomorphic influences in most cases, they are dissimilar in terms of processes related to bioweathering and erosion. However, as the scenarios discussed are largely inferential, future field-based research should explore bedrock fluvial systems from biogeomorphic perspectives.

V Summary and future research

Bedrock streams are understudied compared to alluvial ones in many aspects. This research seeks to fill this lacuna by studying bedrock streams from a biogeomorphic context. It shows that bedrock streams exhibit both shared and highly concentrated BGIs (defined in Section IV) in relation to alluvial streams and bedrock hillslope environments (Table 5). The relations are graphically illustrated via two biogeomorphic triangles (Figures 10 and 11). Analysis reveals that bedrock streams display a bioprotective geomorphic form – root banks (when the root itself forms the stream bank), which is distinctive, but not exclusive, to this setting. On the contrary, shared BGIs include: i) sediment and wood trapping, and bar and island development and stabilization – that is, bioconstruction/ modification with alluvial streams; ii) displacement of bedrock due to root and trunk growth, and bedrock mining caused by tree uprooting – that is, bioweathering and erosion with bedrock hillslopes. This study concludes that bedrock streams exhibit some BGIs that also occur in either alluvial channels or on rocky hillslope environments. Therefore, no BGIs were identified that are absolutely unique to bedrock fluvial environments. Further, this research brings forth some important research queries related to bioprotection and bioweathering/erosion. Field evidence shows that where bedrock is exposed within the channel or along the bank, the bioprotective roles are minimal at best, while bioweathering and erosion-related impacts are probably more prominent (e.g. bedrock displacement by root and trunk growth). On the other hand, where bedrock is not exposed, the role of bioprotection associated with bedrock streams appears to be analogous to that of alluvial streams (except for the root bank case). However, further field-based investigations are required to understand these relationships by answering the following research questions:

What is the relative importance of bioprotection along alluvial and bedrock streams, as bedrock ones are quite resistant anyway?

What is the role of bioweathering and erosion along stream banks in bedrock channel evolution?

Finally, future research needs to look at larger samples of bedrock rivers, including the alluvial–bedrock transitional streams, which are influenced by different types of geology. The following aspects of bedrock streams are worthy of further investigation:

The ideas presented in this research are relevant to reinforced (human-controlled) river channels where woody vegetation may colonize hard reinforcement such as concrete, laid brick, and stone riprap. Therefore, future work related to stream restoration and river bank protection should address these ideas, most importantly bioprotection and bioweathering/erosion.

BGIs and related processes associated with bedrock streams almost certainly vary spatially and temporally. Future studies should attempt to quantify these variations for different types of bedrock streams.

This research will allow some assessment of the contrasting BGIs across soil covered versus bedrock/thin soil hillslopes, and bedrock – transitional – alluvial channels in different biogeographical and energy environments.

Finally, bedrock channels are present from deserts to wet tropics, with a broad range of tree species that exhibit different growth rates, resilience to mechanical disturbance, and tolerances for inundation. Therefore, future research should explore the following questions:

Are there some biomes or hydroclimatic regions where woody vegetation is more likely to influence bedrock channel processes or forms?

Does the influence of vegetation depend on factors associated with boundary conditions such as lithology, joint geometry, flow regime, and channel geometry that limit the ability of trees to germinate and survive?

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Tasnuba Jerin

References

Abbe

Montgomery

(1996) Large woody debris jams channel hydraulics and habitat formation in large rivers. Regulated Rivers Research & Management 12(23): 201–221.

Zhou

, et al. (2007) Spartina invasion in China: Implications for invasive species management and future research. Weed Research 47(3): 183–191.

Auble

Friedman

Scott

(1994) Relating riparian vegetation to present and future streamflows. Ecological Applications 4(3): 544–554.

Bendix

(1998) Impact of a flood on southern California riparian vegetation. Physical Geography 19(2): 162–174.

Bendix

(1999) Stream power influence on southern Californian riparian vegetation. Journal of Vegetation Science 10(2): 243–252.

Bendix

Cowell

(2010) Fire, floods and woody debris: Interactions between biotic and geomorphic processes. Geomorphology 116(3): 297–304.

Bennie

(1991) Growth and mechanical impedance. In: Waisel

Eshel

Kafkafi

(eds) Plant Roots: The Hidden Half. New York: Marcek Dekker, 393–414.

Bertoldi

Gurnell

Surian

, et al. (2009) Understanding reference processes: Linkages between river flows sediment dynamics and vegetated landforms along the Tagliamento River Italy. River Research and Applications 25(5): 501–516.

Birot

(1966) General Physical Geography. London: George G Harrap & Co Ltd.

10.

Birkeland

(1996) Riparian vegetation and sandbar morphology along the lower Little Colorado River, Arizona. Physical Geography 17(6): 534–553.

11.

Bormann

Wang

Bormann

, et al. (1998) Rapid plant-induced weathering in an aggrading experimental ecosystem. Biogeochemistry 43: 129–155.

12.

Busby

Schuster

(1973) Woody phreatophytes along the Brazos River and selected tributaries above Possum Kingdom Lake. Texas Water Development Board Report 168. Austin, TX: TWDB.

13.

Butler

(1995) Zoogeomorphology: Animals as Geomorphic Agents. New York: Cambridge University Press.

14.

Collins

Montgomery

Fetherston

, et al. (2012) The floodplain large-wood cycle hypothesis: A mechanism for the physical and biotic structuring of temperate forested alluvial valleys in the North Pacific coastal ecoregion. Geomorphology 139: 460–470.

15.

Corenblit

Baas

Balke

, et al. (2015) Engineer pioneer plants respond to and affect geomorphic constraints similarly along water–terrestrial interfaces world-wide. Global Ecology and Biogeography 24(12): 1363–1376.

16.

Corenblit

Baas

Bornette

, et al. (2011) Feedbacks between geomorphology and biota controlling Earth surface processes and landforms: A review of foundation concepts and current understandings. Earth-Science Reviews 106(3): 307–331.

17.

Corenblit

Gurnell

Steiger

, et al. (2008) Reciprocal adjustments between landforms and living organisms: Extended geomorphic evolutionary insights. Catena 73(3): 261–273.

18.

Corenblit

Steiger J Gurnell

, et al. (2009a) Control of sediment dynamics by vegetation as a key function driving biogeomorphic succession within fluvial corridors. Earth Surface Processes and Landforms 34(13): 1790–1810.

19.

Corenblit

Steiger

Gurnell

, et al. (2009b) Plants intertwine fluvial landform dynamics with ecological succession and natural selection: A niche construction perspective for riparian systems. Global Ecology and Biogeography 18(4): 507–520.

20.

Corenblit

Steiger

Tabacchi

(2010) Biogeomorphologic succession dynamics in a Mediterranean river system. Ecography 33(6) 1136–1148.

21.

Corenblit

Tabacchi

Steiger

, et al. (2007) Reciprocal interactions and adjustments between fluvial landforms and vegetation dynamics in river corridors: A review of complementary approaches. Earth-Science Reviews 84(1): 56–86.

22.

Crowther

(1987) Ecological observations in tropical karst terrain, West Malaysia. III. Dynamics of the vegetation-soil-bedrock system. Journal of Biogeography 14(2): 157–164.

23.

Dent

Cumming

Carpenter

(2002) Multiple states in river and lake ecosystems. Philosophical Transactions of the Royal Society of London B: Biological Sciences 357(1421): 635–645.

24.

Estrada-Medina

Santiago

Graham

, et al. (2013) Source water, phenology and growth of two tropical dry forest tree species growing on shallow karst soils. Trees 27(5): 1297–1307.

25.

Fei

Phillips

Shouse

(2014) Biogeomorphic impacts of invasive species. Annual Review of Ecology Evolution and Systematics 45: 69–87.

26.

Fetherston

Naiman

Bilby

(1995) Large woody debris physical process and riparian forest development in montane river networks of the Pacific Northwest. Geomorphology 13(1–4): 133–144.

27.

Francis

(2009) Ecosystem prediction and management. In: Castree

Demeritt

Liverman

, et al. (eds) A Companion to Environmental Geography. London: Blackwell, 421–441.

28.

Francis

Corenblit

Edwards

(2009) Perspectives on biogeomorphology ecosystem engineering and self-organisation in island-braided fluvial ecosystems. Aquatic Sciences 71(3): 290–304.

29.

Gabet

Mudd

(2010) Bedrock erosion by root fracture and tree throw: A coupled biogeomorphic model to explore the humped soil production function and the persistence of hillslope soils. Journal of Geophysical Research 115: F04005.

30.

Gabet

Reichman

Seabloom

(2003) The effect of bioturbation on soil processes and sediment transport. Annual Review of Earth and Planetary Sciences 3: 249–273.

31.

Graf

(1978) Fluvial adjustments to the spread of tamarisk in the Colorado Plateau region. Geological Society of America Bulletin 89(10): 1491–1501.

32.

Gran

Tal

Wartman

(2015) Co-evolution of riparian vegetation and channel dynamics in an aggrading braided river system Mount Pinatubo Philippines. Earth Surface Processes and Landforms 40(8): 1101–1115.

33.

Green

(2005) Modelling flow resistance in vegetated streams: Review and development of new theory. Hydrological Processes 19(6): 1245–1259.

34.

Gregory

Davis

(1992) Coarse woody debris in stream channels in relation to river channel management in woodland areas. Regulated Rivers Research & Management 7(2): 117–136.

35.

Gurnell

(2014) Plants as river system engineers. Earth Surface Processes and Landforms 39(1): 4–25.

36.

Gurnell

Gregory

(1995) Interactions between semi-natural vegetation and hydrogeomorphological processes. Geomorphology 13(1–4): 49–69.

37.

Gurnell

Petts

(2002) Island-dominated landscapes of large floodplain rivers, a European perspective. Freshwater Biology 47(4): 581–600.

38.

Gurnell

Petts

(2006) Trees as riparian engineers: The Tagliamento River Italy. Earth Surface Processes and Landforms 31(12): 1558–1574.

39.

Gurnell

Bertoldi

Corenblit

(2012) Changing river channels: The roles of hydrological processes plants and pioneer fluvial landforms in humid temperate mixed load gravel bed rivers. Earth-Science Reviews 111(1): 129–141.

40.

Gurnell

Corenblit

García de Jalón

, et al. (2016) A conceptual model of vegetation–hydrogeomorphology interactions within river corridors. River Research and Applications 32(2): 142–163.

41.

Gurnell

Goodson

Thompson

, et al. (2007) The river-bed: A dynamic store for plant propagules? Earth Surface Processes and Landforms 32(8): 1257–1272.

42.

Gurnell

Holloway

Liffen

, et al. (2018) Plant root and rhizome strength: Are there differences between and within species and rivers? Earth Surface Processes and Landforms 44(1): 389–392.

43.

Gurnell

Petts

Hannah

, et al. (2001) Riparian vegetation and island formation along the gravel-bed Fiume Tagliamento Italy. Earth Surface Processes and Landforms 26(1): 31–62.

44.

Gurnell

Tockner

Edwards

, et al. (2005) Effects of deposited wood on biocomplexity of river corridors. Frontiers in Ecology and the Environment 3(7): 377–382.

45.

Hack

Goodlett

(1960) Geomorphology and forest ecology of a mountain region in the central Appalachians. United States Geological Survey Professional Paper 347, Washington, D.C.: United States Government Printing Office.

46.

Harwood

Brown

(1993) Fluvial processes in a forested anastomosing river: flood partitioning and changing flow patterns. Earth Surface Processes and Landforms 18(8): 741–748.

47.

Hastings

Hom

Ellner

, et al. (1993) Chaos in ecology: Is mother nature a strange attractor? Annual Review of Ecology and Systematics 24(1): 1–33.

48.

Hughes

Adams

Muller

, et al. (2001) The importance of different scale processes for the restoration of floodplain woodlands. River Research and Applications 17(4–5): 325–345.

49.

Hupp

(1983) Vegetation pattern on channel features in the Passage Creek Gorge Virginia. Castanea 48: 62–72.

50.

Hupp

(1992) Riparian vegetation recovery patterns following stream channelization: A geomorphic perspective. Ecology 73(4): 1209–1226.

51.

Hupp

Osterkamp

(1985) Bottomland vegetation distribution along Passage Creek Virginia in relation to fluvial landforms. Ecology 66(3): 670–681.

52.

Hupp

Osterkamp

(1996) Riparian vegetation and fluvial geomorphic processes. Geomorphology 14(4): 277–295.

53.

Hupp

Rinaldi

(2007) Riparian vegetation patterns in relation to fluvial landforms and channel evolution along selected rivers of Tuscany (Central Italy). Annals of the Association of American Geographers 97(1): 12–30.

54.

Jackson

Sheldon

(1949) The vegetation of magnesian limestone cliffs at Markland Grips near Sheffield. Journal of Ecology 37: 38–50.

55.

Kramer

Wohl

(2015) Driftcretions: The legacy impacts of driftwood on shoreline morphology. Geophysical Research Letters 42(14): 5855–5864.

56.

Leopold

Maddock

(1953) The hydraulic geometry of stream channels and some physiographic implications. Professional Paper 282A US Geological Survey, Washington, D.C.: United States Government Printing Office.

57.

Loope

Sanchez

Tarr

, et al. (1988) Biological invasions of arid land nature reserves. Biological Conservation 44: 95–118.

58.

Lutz

(1960) Movement of rocks by uprooting of forest trees. American Journal Science 258(10): 752–756.

59.

Lutz

Griswold

(1939) The influence of tree roots on soil morphology. American Journal Science 237(6): 389–400.

60.

McBride

Strahan

(1984) Establishment and survival of woody riparian species on gravel bars of an intermittent stream. American Midland Naturalist 112(2): 235–245.

61.

McKenney

Jacobson

Wertheimer

(1995) Woody vegetation and channel morphogenesis in low-gradient gravel-bed streams in the Ozark Plateaus Missouri and Arkansas. Geomorphology 13(1–4): 175–198.

62.

Martin

(2006) Effects of forest and grass vegetation on fluviokarst hillslope hydrology Bowman’s Bend Kentucky. PhD Dissertation, University of Kentucky, USA.

63.

Massong

Montgomery

(2000) Influence of sediment supply, lithology, and wood debris on the distribution of bedrock and alluvial channels. Geological Society of America Bulletin 112(4): 591–599.

64.

Matthes-Sears

Larson

(1995) Rooting characteristics of trees in rock: A study of Thuja occidentalis on cliff faces. International Journal of Plant Sciences 156: 679–686.

65.

Miller

(1991) The influence of bedrock geology on knickpoint development and channel-bed degradation along downcutting streams in south-central Indiana. The Journal of Geology 99(4): 591–605.

66.

Nanson

(1981) New evidence of scroll-bar formation on the Beatton River. Sedimentology 28(6): 889–891.

67.

Naylor

Viles

Carter

NEA

(2002) Biogeomorphology revisited: Looking towards the future. Geomorphology 47(1): 3–14.

68.

Page

Nanson

(1982) Concave-bank benches and associated floodplain formation. Earth Surface Processes and Landforms 7(6): 529–543.

69.

Pawlik

(2013) The role of trees in the geomorphic system of forested hillslopes—a review. Earth-Science Reviews 126: 250–265.

70.

Pawlik

Phillips

Samonil

(2016) Roots rock and regolith: Biomechanical and biochemical weathering by trees and its impact on hillslopes - A critical literature review. Earth-Science Reviews 159: 142–159.

71.

Pettit

Naiman

(2005) Flood-deposited wood debris and its contribution to heterogeneity and regeneration in a semi-arid riparian landscape. Oecologia 145(3): 434–444.

72.

Phillips

(2015) Hillslope degradation by trees in central Kentucky. Available at: https://www.researchgate.net/profile/Jonathan_Phillips4/publication/279189727_Hillslope_Degradation_by_Trees_in_Central_Kentucky/links/558d4de008ae18cfc19e1d62/Hillslope-Degradation-by-Trees-in-Central-Kentucky.pdf (accessed 15 April, 2019).

73.

Phillips

(2016) Biogeomorphology and contingent ecosystem engineering in karst landscapes. Progress in Physical Geography 40(4): 503–526.

74.

Phillips

Lorz

(2008) Origins and implications of soil layering. Earth-Science Reviews 89(3): 144–155.

75.

Phillips

Marion

(2005) Biomechanical effects lithological variations and local pedodiversity in some forest soils of Arkansas. Geoderma 124: 73–89.

76.

Phillips

Marion

(2006) The biomechanical effects of trees on soils and regoliths: Beyond treethrow. Annals of the Association of American Geographers 96: 233–247.

77.

Phillips

Marion

Turkington

(2008) Pedologic and geomorphic impacts of a tornado blowdown event in a mixed pine-hardwood forest. Catena 75: 278–287.

78.

Phillips

Šamonil

Pawlik

, et al. (2017) Domination of hillslope denudation by tree uprooting in an old-growth forest. Geomorphology 276: 27–36.

79.

Poff

Allan

Bain

, et al. (1997) The natural flow regime. BioScience 47(11): 769–784.

80.

Polvi

Wohl

Merritt

(2014) Modeling the functional influence of vegetation type on streambank cohesion. Earth Surface Processes and Landforms 39(9): 1245–1258.

81.

Roering

Schmidt

Stock

, et al. (2003) Shallow landsliding root reinforcement and the spatial distribution of trees in the Oregon Coast Range. Canadian Geotechnical Journal 40: 237–253.

82.

Šamonil

Král

Hort

(2010) The role of tree uprooting in soil formation: A critical literature review. Geoderma 157: 65–79.

83.

Schaetzl

Burns

Johnson

, et al. (1989) Tree uprooting: Review of impacts on forest ecology. Vegetation 79: 165–176.

84.

Schumm

(1977) The Fluvial System. New York: John Wiley and Sons.

85.

Schumm

(2007) River Variability and Complexity. New York: Cambridge University Press.

86.

Schumm

Chorley

(1983) Geomorphic controls on the management of nuclear waste. Washington, DC: US Nuclear Regulatory Commission NUREG/CR-3276.

87.

Shafroth

Auble

Stromberg

, et al. (1998) Establishment of woody riparian vegetation in relation to annual patterns of streamflow, Bill Williams River, Arizona. Wetlands 18(4): 577–590.

88.

Shouse

(2014) Biomechanical effects of trees and soil thickness in the cumberland plateau. PhD Dissertation, University of Kentucky, Lexington, KY.

89.

Shouse

Phillips

(2016) Soil deepening by trees and the effects of parent material. Geomorphology 269: 1–7.

90.

Sigafoos

(1961) Vegetation in relation to flood frequency near Washington DC. US Geological Survey Professional Paper 424, Washington, D.C.: United States Government Printing Office, pp. 248–249.

91.

Small

Schaetzl

Brixie

(1990) Redistribution and mixing of soil gravels by tree uprooting. The Professional Geographer 42: 445–457.

92.

Steiger

Tabacchi

Dufour

, et al. (2005) Hydrogeomorphic processes affecting riparian habitat within alluvial channel–floodplain river systems: A review for the temperate zone. River Research and Applications 21(7): 719–737.

93.

Stoffel

Wilford

(2012) Hydrogeomorphic processes and vegetation: Disturbance process histories dependencies and interactions. Earth Surface Processes and Landforms 37(1): 9–22.

94.

Stone

Kalisz

(1991) On the maximum extent of tree roots. Forest Ecology and Management 46(1–2): 59–102.

95.

Tabacchi

Planty-Tabacchi

Roques

, et al. (2005) Seed inputs in riparian zones: Implications for plant invasion. River Research and Applications 21: 299–313.

96.

Thompson

(2006) The role of vortex shedding in the scour of pools. Advances in Water Resources 29(2): 121–129.

97.

Thorne

(1990) Effects of Vegetation on River Bank Erosion and Stability. In: Thornes

(ed) Vegetation and Erosion: Processes and Environments. New Jersey: John Wiley & Sons, pp. 125–144.

98.

Thorp

Thoms

Delong

(2010) The Riverine Ecosystem Synthesis: Toward Conceptual Cohesiveness in River Science. Boston, MA: Elsevier.

99.

Tickner

Angold

Gurnell

, et al. (2001) Riparian plant invasions: Hydrogeomorphological control and ecological impacts. Progress in Physical Geography 25(1): 22–52.

100.

Tinkler

Parish

(1998) Recent adjustments to the long profile of Cooksville Creek an urbanized bedrock channel in Mississauga Ontario. In: Tinkler

Wohl

(eds) Rivers Over Rock: Fluvial Processes in Bedrock Channels. Washington, DC: American Geophysical Union, 167–187.

101.

Tinkler

Wohl

(1998) A primer on bedrock channels. In: Tinkler

Wohl

(eds) Rivers Over Rock: Fluvial Processes in Bedrock Channels. Washington, DC: American Geophysical Union, 1–18.

102.

Tockner

Stanford

(2002) Riverine flood plains: Present state and future trends. Environmental Conservation 29(3): 308–330.

103.

Trimble

(1997) Stream channel erosion and change resulting from riparian forests. Geology 25(5): 467–469.

104.

Tsujimoto

(1999) Fluvial processes in streams with vegetation. Journal of Hydraulic Research 37(6): 789–803.

105.

Van Dyke

(2016) Nature’s complex flume—Using a diagnostic state-and-transition framework to understand post-restoration channel adjustment of the Clark Fork River Montana. Geomorphology 254: 1–15.

106.

Vaughn

(1990) Flood dynamics of a concrete-lined urban stream in Kansas City Missouri. Earth Surface Processes and Landforms 15(6): 525–537.

107.

Viles

Naylor

Carter

NEA

, et al. (2008) Biogeomorphological disturbance regimes: Progress in linking ecological and geomorphological systems. Earth Surface Processes and Landforms 33(9): 1419–1435.

108.

Vitousek

(1990) Biological invasions and ecosystem process: Towards an integration of population biology and ecosystem studies. Oikos 57: 7–13.

109.

Ward

Tockner

Arscott

, et al. (2002) Riverine landscape diversity. Freshwater Biology 47(4): 517–539.

110.

Whipple

(2004) Bedrock rivers and the geomorphology of active orogens. Annual Review of Earth and Planetary Sciences 32: 151–185.

111.

Whipple

Hancock

Anderson

(2000) River incision into bedrock: Mechanics and relative efficacy of plucking abrasion and cavitation. Geological Society of America Bulletin 112(3): 490–503.

112.

Wilkinson

Richards

Humphreys

(2009) Breaking ground: Pedological geological and ecological implications of soil bioturbation. Earth-Science Reviews 97(1): 257–272.

113.

Wohl

(2013) Floodplains and wood. Earth-Science Reviews 123: 194–212.

114.

Wohl

Ikeda

(1998) Patterns of bedrock channel erosion on the Boso Peninsula Japan. The Journal of Geology 106(3): 331–346.

115.

Wohl

Scott

(2017) Wood and sediment storage and dynamics in river corridors. Earth Surface Processes and Landforms 42(1): 5–23.

116.

Yatsu

(1988) The Nature of Weathering: An Introduction. Tokyo: Sozosha.