**Biogeomorphologic Approaches to a Study of Hillslope Processes Using Non-Destructive Methods**

Pavel Raška *Jan Evangelista Purkyně University in Ústí nad Labem, Czech Republic* 

#### **1. Introduction**

20 Studies on Environmental and Applied Geomorphology

Whalley, W. B. (2009), On the interpretation of discrete debris accumulations associated with

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uplands of Great Britain and Ireland in Periglacial and Paraglacial Processes and Environments, edited by J. Knight and S. Harrison, *Geological Society, London, Special*  The aim of this chapter is to present new non-destructive methods and techniques used in the biogeomorphologic study of hillslope processes, particularly sheet erosion and shallow landslides. These processes belong to a broad spectre of natural hazards that have significant impacts on landscape and society and their research represents the fundamental issue for applied geomorphology (Panizza, 1996; Alcántara-Ayala, Goudie eds., 2010). Nondestructive methods are not yet well established in biogeomorphologic research despite their relevance in areas protected under conservation law, in fragile habitats and considering their simple field application. To introduce some of these methods in case studies within the context of biogeomorphology, we first give an introduction to the main concepts regarding landform-biota interactions followed by a focus on hillslope processes. In sections 3 and 4, we present two case studies of the application of non-destructive methods to quantify the bioprotective role of fallen trees (trunk dams, log dams) and to analyse short-term surface stability. In the final section, we suggest possible directions for the future development of non-destructive methods in the biogeomorphology of hillslope processes.

The evolution of Earth's surface in contrast with other planets in the Solar System is characterised by the fundamental role played by organisms, which act directly by creating, modifying and destroying landforms and indirectly by changing other factors that influence surface processes, such as climate and the distribution of energy. Looking at the history of research in this field of expertise, it seems that the significance ascribed to organisms (and particularly to vegetation) within a short history of biogeomorphology grew as rapidly as other fundamental concepts within a hundred year history of geomorphology. Most recently, this trend has led to the assumption that vegetation can indeed be a leading factor in global geomorphic change, and understanding its evolution is crucial to establishing an evolutionary view in geomorphology (Corenblit & Steiger, 2009). From a case study in eastern Kentucky, for instance, Phillips (2009) concluded that if only 0.1 % of net primary production of biomass is assumed to be geologically active, it still exceeds the energy of uplift and denudation. In spite of these results, one can hardly imagine vegetation being

Biogeomorphologic Approaches to

author)

2011).

a Study of Hillslope Processes Using Non-Destructive Methods 23

Fig. 2. The thematic framework of biogeomorphology (Naylor et al., 2002; modified by

The major aims of biogeomorphology were set by Viles et al. (1988) as (i) the influence of landforms on the distribution of organisms and (ii) the influence of organisms on Earth surface processes, but in fact, the two-way linkages were not fully pursued in the book (cf. Wainwright & Parsons, 2010). The "revision" of developments in biogeomorphology presented by Naylor et al. (2002) emphasises three main processes interlinking landforms and organisms: (i) bioconstruction, (ii) bioprotection and (iii) bioerosion. The authors call for studies of the complexity of landform-biota interactions. However, organisms were still considered a static factor, and geomorphologists were often unable to avoid the unidirectional appreciation of the landform-organism (vegetation) relation, which has changed only in past few years (e.g., Marston, 2010; Reinhardt et al., 2010; Corenblit et al.,

The problem of a biogeomorphologic focus on two-way linkages is twofold. First, the development of biogeomorphology was accelerated as geomorphology moved from global and regional research scales aiming at historical interpretations of landscape (Church, 2010) to local scales and individual sites as a result of the quantitative revolution and the establishment of the process approach paradigm. The physical background of the process approach emphasised analyses of Earth surface processes and frequently neglected the feedbacks from organisms. Furthermore, a focus on a local scale did not allow the effective modelling of vegetation changes as a response to geomorphic processes because these changes are also influenced by decision-making effects, which are variable and can be better assessed at a regional scale (see Wainwright & Millington, 2010). One of the possible integrating views to resolve these scale-related problems could be offered by Quaternary landscape ecology, which emphasises evolutionary concepts together with changing patterns of biota and developments in human societies that influence the primary modes of

The second problem is methodological and emerges from the different backgrounds of the two disciplines integrated within biogeomorphology: geomorphology and ecology. While geomorphologic conclusions are frequently drawn from theoretical considerations combined with detailed field surveys and measurements, ecological conclusions usually result from statistical analyses of extensive datasets (Haussmann, 2011). The prevailing geomorphologic approach, then, only anticipates the real situation, where biota is frequently understood as a static factor or as a source of proxy data for the study of Earth surface processes. Fig. 2 shows the thematic framework of biogeomorphology emerging from

decision making (e.g., Delcourt & Delcourt, 1988).

responsible for creating the global geomorphic patterns comprising everything from mountain ranges to valleys (cf. Scheidegger, 2007) even though the absence or presence and character of vegetation can modify the rate of landform evolution by protecting the Earth's surface (factor being limited by extent of rhizosphere) or by changes in energy diversification (assuming land cover pattern scale, which is sufficient to influence continental to global climate). Moreover, the role of vegetation in different time horizons (i.e., scale dependency) is unclear. The classic work of Schumm & Lichty (1965) shows how vegetation changes from a dependent to independent factor through time. More recently, Phillips (1995), at a more local scale, has shown that vegetation may in fact be a dependent variable rather than a controlling factor, which has also been discussed by Raska & Orsulak (2009).

Fig. 1. Major publications devoted to the interaction of organisms and geomorphology shown in historical perspective.

The abovementioned uncertainties in geomorphic significance of organisms are partly caused by the development of the scientific background of biogeomorphology, the conceptualisation of which as a young subdiscipline of geomorphology is still developing. An overview of the main achievements in biogeomorphology as represented by fundamental publications is shown in Figure 1. The first works to discuss the landformorganism relation date back to the 19th century; however, it was not until 1960s that biogeomorphology was established as a research approach by Hack & Goodlett (1960). The very first attempt to give an overview of the field was in the 1980s, when Viles (in Viles et al. 1988) defined biogeomorphology as a "concept of an approach to geomorphology, which explicitly considers role of organisms". Since then, biogeomorphology has been increasingly pursued in studies across varying environments. According to the Web of Science database (Thomson-Reuters), the annual number of works with biogeomorphology as a focal topic increased from less than five to more than 15 during the last 20 years (Fig. 1). This trend is also apparent in dendrogeomorphologic studies, while the number of zoogeomorphologic papers remains consistently low. This pattern indicates the prevailing appreciation of vegetation, the role of which can be studied at different scales and can be generalised. In contrast to the increasing number of biogeomorphologic papers, it is somewhat surprising that the amount of biogeomorphologic content has decreased in textbooks, being lowest in 1970s (see Stine & Butler, 2011).

responsible for creating the global geomorphic patterns comprising everything from mountain ranges to valleys (cf. Scheidegger, 2007) even though the absence or presence and character of vegetation can modify the rate of landform evolution by protecting the Earth's surface (factor being limited by extent of rhizosphere) or by changes in energy diversification (assuming land cover pattern scale, which is sufficient to influence continental to global climate). Moreover, the role of vegetation in different time horizons (i.e., scale dependency) is unclear. The classic work of Schumm & Lichty (1965) shows how vegetation changes from a dependent to independent factor through time. More recently, Phillips (1995), at a more local scale, has shown that vegetation may in fact be a dependent variable rather than a controlling

Fig. 1. Major publications devoted to the interaction of organisms and geomorphology

The abovementioned uncertainties in geomorphic significance of organisms are partly caused by the development of the scientific background of biogeomorphology, the conceptualisation of which as a young subdiscipline of geomorphology is still developing. An overview of the main achievements in biogeomorphology as represented by fundamental publications is shown in Figure 1. The first works to discuss the landformorganism relation date back to the 19th century; however, it was not until 1960s that biogeomorphology was established as a research approach by Hack & Goodlett (1960). The very first attempt to give an overview of the field was in the 1980s, when Viles (in Viles et al. 1988) defined biogeomorphology as a "concept of an approach to geomorphology, which explicitly considers role of organisms". Since then, biogeomorphology has been increasingly pursued in studies across varying environments. According to the Web of Science database (Thomson-Reuters), the annual number of works with biogeomorphology as a focal topic increased from less than five to more than 15 during the last 20 years (Fig. 1). This trend is also apparent in dendrogeomorphologic studies, while the number of zoogeomorphologic papers remains consistently low. This pattern indicates the prevailing appreciation of vegetation, the role of which can be studied at different scales and can be generalised. In contrast to the increasing number of biogeomorphologic papers, it is somewhat surprising that the amount of biogeomorphologic content has decreased in textbooks, being lowest in

factor, which has also been discussed by Raska & Orsulak (2009).

shown in historical perspective.

1970s (see Stine & Butler, 2011).

Fig. 2. The thematic framework of biogeomorphology (Naylor et al., 2002; modified by author)

The major aims of biogeomorphology were set by Viles et al. (1988) as (i) the influence of landforms on the distribution of organisms and (ii) the influence of organisms on Earth surface processes, but in fact, the two-way linkages were not fully pursued in the book (cf. Wainwright & Parsons, 2010). The "revision" of developments in biogeomorphology presented by Naylor et al. (2002) emphasises three main processes interlinking landforms and organisms: (i) bioconstruction, (ii) bioprotection and (iii) bioerosion. The authors call for studies of the complexity of landform-biota interactions. However, organisms were still considered a static factor, and geomorphologists were often unable to avoid the unidirectional appreciation of the landform-organism (vegetation) relation, which has changed only in past few years (e.g., Marston, 2010; Reinhardt et al., 2010; Corenblit et al., 2011).

The problem of a biogeomorphologic focus on two-way linkages is twofold. First, the development of biogeomorphology was accelerated as geomorphology moved from global and regional research scales aiming at historical interpretations of landscape (Church, 2010) to local scales and individual sites as a result of the quantitative revolution and the establishment of the process approach paradigm. The physical background of the process approach emphasised analyses of Earth surface processes and frequently neglected the feedbacks from organisms. Furthermore, a focus on a local scale did not allow the effective modelling of vegetation changes as a response to geomorphic processes because these changes are also influenced by decision-making effects, which are variable and can be better assessed at a regional scale (see Wainwright & Millington, 2010). One of the possible integrating views to resolve these scale-related problems could be offered by Quaternary landscape ecology, which emphasises evolutionary concepts together with changing patterns of biota and developments in human societies that influence the primary modes of decision making (e.g., Delcourt & Delcourt, 1988).

The second problem is methodological and emerges from the different backgrounds of the two disciplines integrated within biogeomorphology: geomorphology and ecology. While geomorphologic conclusions are frequently drawn from theoretical considerations combined with detailed field surveys and measurements, ecological conclusions usually result from statistical analyses of extensive datasets (Haussmann, 2011). The prevailing geomorphologic approach, then, only anticipates the real situation, where biota is frequently understood as a static factor or as a source of proxy data for the study of Earth surface processes. Fig. 2 shows the thematic framework of biogeomorphology emerging from

Biogeomorphologic Approaches to

sheet erosion - detection, dating

sheet erosion

gully erosion - detection, dating

shallow landslides - effect of roots on slope

shallow landslides protective function of



protective effect of fallen

disturbances by animals

advances in the field of expertise)

vegetation

stability

trees

landslides

uprooting - effect on soils

debris flows - detection, dating


soil and regolith


effect of fallen trees - shaping debris flows

rockfall

trees


Aim (process or effect) Studied object

(approach)

vegetation (modelling)

roots (anatomy analyses)

roots (anatomy analyses)

forest cover (modelling)

survey)

trunk (tree ring analyses)

roots, trunk (field

trunk (scars, tree ring analyses)

trunk (scars, tree ring analyses)

different species (field survey and experiments)

Table 1. Overview of selected biogeomorphologic approaches to a study of hillslope processes (references are selected to show the different approaches and methodological

The second group of approaches can be called non-destructive. These approaches focus on measurements and analyses of visible features that represent past or current interactions between vegetation and hillslope processes. To analyse rockfall activity and patterns in protected forests, Stoffel (2005) performed analyses of the distribution and visibility of scars on trees. Attention was also given to log jams shaping debris flows trajectories (Lancaster et

modelling)

trunk (field survey,

a Study of Hillslope Processes Using Non-Destructive Methods 25

based on physical laws, which are quite often calibrated by the results of limited field surveys or laboratory measurements. These approaches are applied mostly in the modelling of changes in vegetation patterns and the related influence on sediment supply, and they often exploit specially developed GIS-based modelling software, such as MIKE 11 or HEC-RAS, although the incorporation of vegetation parameters into these models is limited.

Reference

Gärtner et al. (2001), Bodoque et al. (2005), Gärtner (2007), Rubiales et al. (2008)

Dorren et al. (2004), Vanacker et al. (2007)

Preston & Crozier (1999), Schwarz et al.

Sidle et al. (1985), Bathurst et al. (2010)

Schaetzel et al. (1989), Phillips & Marion

Perret et al. (2006), Stoffel & Perret (2006)

Bollschweiler et al. (2008), Stoffel (2010),

Trimble & Mendel (1995), Gover & Poesen

Fantucci (1999), Gers et al. (2001)

Šilhán & Pánek (2010)

trunk (field survey) Lancaster & Hayes (2003), Matyja (2007)

Raska & Orsulak (2009)

(1998), Hall & Lamont (2003)

Vandekerckhove et al. (2001)

roots (modelling) Wu et al. (1979), Abe & Ziemer (1991),

(2010)

(2006)

Naylor et al. (2002) and modified to emphasise the mutual linkages between biota and landforms.

## **2. Biogeomorphology of hillslope processes: Main themes and recent advances**

The abovementioned constraints of a study of landform-biota interactions are quite apparent in the research of hillslope processes, as these represent one of the primary focuses of geomorphologists, and the evolution of biogeomorphology was tightly connected with advances in hillslope studies. Hillslopes represent the most common landforms across environments varying from periglacial to tropic regions (e.g., Anderson & Brooks, 1996 eds.), and they were a key landform in designing fundamental models of landscape evolution, ranging from the classic models of W.M. Davis, W. Penck and L.C. King (for overview see Summerfield, 1991) to modern ones, such as backwearing model by R.V. Ruhe, R.B. Daniels and J.G. Cady, nine-unit model by A.J. Conachre and J.B. Dalrymple, COSLOP by F. Ahnert, and many others. Much of the Earth's surface represented by hillslopes is covered by vegetation, whether a sparse cover of lichens, mosses, grass or less or a more continuous cover of shrubs and trees. Furthermore, the hillslopes are habitats for various animals, some of which prefer gentle slopes covered with a deep soil layer and others that developed a preference for rock-mantled slopes (see Butler, 1995, for overview). At the same time, hillslopes belong among the most dynamic landforms of the Earth's surface, enabling the occurrence and acceleration of different natural hazards, such as sheet and gully erosion, landslides, rockfalls, rock avalanches, debris flows, snow avalanches, and others, which have impacts on the manmade objects and human activities in a landscape (Alcántara-Ayala & Goudie, 2010 eds.). The enormous share of hillslopes on the Earth's surface together with their dynamics implies the necessity of intense applied research as well as opportunities for the development of new, effective research techniques. Finally, considering the abovementioned distribution of organisms on hillslopes, these techniques frequently draw upon analyses of organisms, whose distribution, activity or physiognomic modifications serve as proxy indicators of hillslope processes, and the primary attention is devoted to vegetation in this respect.

The first work on dendrogeomorphologic responses to geomorphic processes in terms of their chronology was by Alestalo (1971). Since then, many studies have focused mainly on mass movements and erosion. Concerning research on erosion, Thorns (1985) summarised not only the state of the art but also established a framework to understand feedbacks between vegetation and erosion, thus extending the traditional unidirectional approach in biogeomorphology; this was also enabled by his attention to non-linear dynamic systems in biogeomorphology. Most recently, Marston (2010) presents a comprehensive overview of research on hillslope-vegetation linkages, including research history, main functions of vegetation, feedbacks between vegetation and landforms within the disturbance regimes, and suggestions for future research directions.

An overview of the basic approaches and techniques used in the biogeomorphologic study of hillslope processes is presented in Table 1 along with references to some of the major papers published within this scope. Regarding the methods used, these approaches could be divided into three groups. The first one has in common the use of modelling techniques

Naylor et al. (2002) and modified to emphasise the mutual linkages between biota and

The abovementioned constraints of a study of landform-biota interactions are quite apparent in the research of hillslope processes, as these represent one of the primary focuses of geomorphologists, and the evolution of biogeomorphology was tightly connected with advances in hillslope studies. Hillslopes represent the most common landforms across environments varying from periglacial to tropic regions (e.g., Anderson & Brooks, 1996 eds.), and they were a key landform in designing fundamental models of landscape evolution, ranging from the classic models of W.M. Davis, W. Penck and L.C. King (for overview see Summerfield, 1991) to modern ones, such as backwearing model by R.V. Ruhe, R.B. Daniels and J.G. Cady, nine-unit model by A.J. Conachre and J.B. Dalrymple, COSLOP by F. Ahnert, and many others. Much of the Earth's surface represented by hillslopes is covered by vegetation, whether a sparse cover of lichens, mosses, grass or less or a more continuous cover of shrubs and trees. Furthermore, the hillslopes are habitats for various animals, some of which prefer gentle slopes covered with a deep soil layer and others that developed a preference for rock-mantled slopes (see Butler, 1995, for overview). At the same time, hillslopes belong among the most dynamic landforms of the Earth's surface, enabling the occurrence and acceleration of different natural hazards, such as sheet and gully erosion, landslides, rockfalls, rock avalanches, debris flows, snow avalanches, and others, which have impacts on the manmade objects and human activities in a landscape (Alcántara-Ayala & Goudie, 2010 eds.). The enormous share of hillslopes on the Earth's surface together with their dynamics implies the necessity of intense applied research as well as opportunities for the development of new, effective research techniques. Finally, considering the abovementioned distribution of organisms on hillslopes, these techniques frequently draw upon analyses of organisms, whose distribution, activity or physiognomic modifications serve as proxy indicators of hillslope processes, and the primary attention is devoted to

The first work on dendrogeomorphologic responses to geomorphic processes in terms of their chronology was by Alestalo (1971). Since then, many studies have focused mainly on mass movements and erosion. Concerning research on erosion, Thorns (1985) summarised not only the state of the art but also established a framework to understand feedbacks between vegetation and erosion, thus extending the traditional unidirectional approach in biogeomorphology; this was also enabled by his attention to non-linear dynamic systems in biogeomorphology. Most recently, Marston (2010) presents a comprehensive overview of research on hillslope-vegetation linkages, including research history, main functions of vegetation, feedbacks between vegetation and landforms within the disturbance regimes,

An overview of the basic approaches and techniques used in the biogeomorphologic study of hillslope processes is presented in Table 1 along with references to some of the major papers published within this scope. Regarding the methods used, these approaches could be divided into three groups. The first one has in common the use of modelling techniques

**2. Biogeomorphology of hillslope processes: Main themes and recent** 

landforms.

**advances** 

vegetation in this respect.

and suggestions for future research directions.

based on physical laws, which are quite often calibrated by the results of limited field surveys or laboratory measurements. These approaches are applied mostly in the modelling of changes in vegetation patterns and the related influence on sediment supply, and they often exploit specially developed GIS-based modelling software, such as MIKE 11 or HEC-RAS, although the incorporation of vegetation parameters into these models is limited.


Table 1. Overview of selected biogeomorphologic approaches to a study of hillslope processes (references are selected to show the different approaches and methodological advances in the field of expertise)

The second group of approaches can be called non-destructive. These approaches focus on measurements and analyses of visible features that represent past or current interactions between vegetation and hillslope processes. To analyse rockfall activity and patterns in protected forests, Stoffel (2005) performed analyses of the distribution and visibility of scars on trees. Attention was also given to log jams shaping debris flows trajectories (Lancaster et

Biogeomorphologic Approaches to

trees or trunks.

**3.1 The concept** 

a Study of Hillslope Processes Using Non-Destructive Methods 27

In this section, we will define and characterise trunk dams in terms of their structure, dynamics and protective effects. Then, we will present the recent advances in implementing the results of field research to regional models of the bioprotective effect of trunk dams at a catchment scale using the geographic information systems (GIS). The aim of the developed model is to transfer the local information about the protective effect of individual trunk dams to the regional scale and to estimate the total volume of material retained by fallen

The major recent focus of biogeomorphologic (dendrogeomorphologic) studies of hillslope processes is on the protective effects of trees and shrubs. The research is especially carried out in protection forests. The delimitation of protection forests emerges from their general ability to control or modify the natural hazards connected with Earth's surface dynamics (Berger & Rey, 2004). Different national approaches and nomenclatures have been adopted for the zonation of protection forests and for their management, the difficulty of which emerges from the requirement to maintain both the ecosystem's integrity and the protective function of these forests (Dorren et al., 2004). The protection forests in mountainous areas are predominantly determined for rockfall impact reduction. Most protection forests in highlands and slightly undulating terrains without rock faces have a protective function against soil erosion. In the Czech Republic, where the present study was performed, these forests are called soil-protecting forests (Collective, 2007). In many studies and applied works on protection forests, the focus is on standing trees and shrubs, although Dorren et al. (2007), for instance, also explicitly mention the significance of lying trunks for increasing surface roughness and reducing the velocity of bouncing and rolling clasts of rock. The overall number of biogeomorphologic papers dealing with fallen trees, however, is small,

and the issue has been much more in the focus of forest ecologists and biologists.

formerly was the removal of all woody debris (cf. Harmon, 2002).

In forest ecology and management, fallen trees and their large parts are referred to as coarse woody debris (CWD), woody detritus, or downed wood, and a significant ecological role is ascribed to the process of tree death and decomposition (Masser et al., 1984; Harmon et al., 1986; Franklin et al., 1987). Lying trees and their parts increase the habitat diversity in forests, thus increasing biodiversity, as they offer good conditions for the development of mosses, lichens, plants and fungi communities as well as insect (*Insecta*) and other organisms. Furthermore, the importance of fallen trees lies in their influence on channel diversity in water streams, in the increase of the production function of forests and in the positive influence on the carbon cycle in forests. The ecological significance of fallen trees has led to changes in the traditional forest management approach, the aim of which

The trunk dam concept presented here was originally described in Raska & Orsulak (2009), resulting from the typology of the bioprotective effects of trees at rock-mantled slopes in the Ceske stredohori volcanic mountain range in the NW Czech Republic (Raska, 2007, 2010). In contrast to studies of allochtonous log jams, which often form as a result of mass movements on hillslopes, the present concept emphasises the role of individual trunks (or logs), which, depending on the local surface roughness and density of standing trees, are rather

**3. Structure, dynamics and protective effects of trunk dams** 

al., 2003; Matyja, 2007). However, most of these techniques are usually combined with sampling techniques in the field.

The third and most exploited group of approaches, beginning with the classic work of Alestalo (1971), draws upon extensive field research and sampling using different strategies. The samples are usually taken as cores, wedges or cross-sections from stems, stumps and roots (for nomenclature see Gschwantner et al., 2009). Sampling and tree ring analyses are frequently supported by measurements of stem or root deformation, and studies based on this approach have already offered good results regarding the spatiotemporal patterns of rockfall (e.g., Stoffel & Perret, 2006), debris flow (Bollschweiler et al., 2008) and landslide (e.g., Fantucci, 1999) activity. The application of the approach encounters several problems, however. These problems consist of the number and suitability of tree species for cross dating (Grissino-Mayer, 1993), the availability of reference datasets and other issues (see Stoffel & Perret, 2006). The number of samples differs according to the extent of the area under research, the time span and type of process being studied (Fig. 3), which result in the varying efficiency of the sampling strategy as expressed by the number of samples per one detected event.

Fig. 3. Number of samples used for the detection and dating of debris flows, landslides and rockfall events in relation to the length of the monitored period in years and to the number of species analysed. Source: based on own review of 25 selected papers published in international journals between 2001 and 2011.

In some areas, however, it is not possible or suitable to take cross-sections and wedges from roots and stems or to measure root geometry deformations after uncovering the soil layer. In these cases, one has to employ non-destructive techniques, which enable the detection of processes and the estimation of their rates from the current positions and visible deformations of vegetation in the field. Moreover, several of the abovementioned destructive approaches focus the enormous potential of information recorded in vegetation species as proxy indicators, which oftentimes leads to maintaining the unidirectional approach to a study of landform-biota relations as discussed in the first section.
