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

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 trees or trunks.

#### **3.1 The concept**

26 Studies on Environmental and Applied Geomorphology

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

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

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

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

international journals between 2001 and 2011.

sampling techniques in the field.

detected event.

section.

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.

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

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

Biogeomorphologic Approaches to

section 3.3); C - structure of a trunk dam body.

mosses and especially fungi (e.g., Bader et al., 1995).

estimation of the volume of accumulated material (VAM).

trunk above the surface.

a Study of Hillslope Processes Using Non-Destructive Methods 29

Fig. 5. Cross-section through the trunk dam. A - real (grey) and theoretical (black dashed) geometry of trunk dam with parameters measured in the field (lower case) and computed values (upper case); B - simplified trunk dam geometry used for spatial modelling (see

iii. Cover: Trunk dams that have been stabilised long enough are covered by plants. The dynamic habitat of trunk dams is suitable for pioneer species. The results of a field survey in the study area show that the plants inhabiting trunk dams often belong to invasive species (e.g., *Impatiens glandulifera*). The speed of colonisation and the relative importance of species depend on the type of environment in which the trunk dam is located (McCullough, 1948). The fast colonisation of a trunk dam surface accelerates the stabilisation of its body through plant roots. While the body of a trunk dam is colonised mostly by herbs, the trunk itself in different stages of decay represents a habitat for

One of the aims of our previous research was to design a simple non-destructive method for estimating the volume of material accumulated by trunk dams. After initial attempts, an MS Excel Spreadsheet was designed. The EVAM (Estimation of Volume of Accumulated Material v1.0, available at <http://lsru.geography.ujep.cz>) approximates the cross-section of a trunk dam to a simple triangle-like shape (Fig. 5A), and it enables an accurate

The first results, which compared the accumulation and denudation rates in two sampling sites (Raska & Orsulak, 2009) showed that in terms of the current dynamics of a hillslope surface, the accumulation rate may be as important as the denudation rate. The overall results for 26 trunk dams analysed at three sites within the studied catchment revealed certain relations between the individual geometric parameters of a trunk dam (Fig. 6A). The highest correlation is between accumulation width "a" and VAM, but it will be shown later that there are certain difficulties in the generalisation of this relation. By contrast, a weak correlation was revealed between trunk radius "r" and VAM. However, the trunk radius and geometry of a trunk may partly influence other values, such as "a" and the height of a

The height of a trunk above the surface represents one of the most important factors influencing VAM values, as will be shown in relations between the width of accumulation, the slope inclination and VAM (Figs. 6B and 6C). Fig. 6B shows how VAM grows with increasing slope inclination. The model assumes that the height of a trunk above the surface is a dependent variable calculated from other input parameters. By contrast, the model in Fig. 6C assumes the trunk dam is in contact with the surface and, thus, is an independent

variable. In this case, the higher is the slope inclination, the lower is the VAM.

autochthonous. The trunk dam forms as the tree or its parts fall and are stabilised by obstacles on the surface. Depending on topography, density, the total length and other characteristics of trunk dams in the area, they can play a significant role in the reduction of the volume of material that is transported downslope. The collective (2007) shows that the amount of dead wood can be higher than 8 m3.ha-1. Vallauri et al. (2003) lists examples from European forests, which show that the volume of dead wood varies from 0.6 to almost 20 m3.ha-1. The total volume of fallen trees depends on the density and diversity of forest cover and on its ecological integrity.

Fig. 4. Trunk dam in advanced stage of plant succession (left), trunk dam breach (right).

#### **3.2 Structure and dynamics**

A trunk dam is a biogeomorphologic system compound of five components, three of which form the mass of the trunk dam and two of which (base surface, barriers of embedment of a trunk) create its boundaries. The three inherent components are the following (Figs. 4, 5C):


autochthonous. The trunk dam forms as the tree or its parts fall and are stabilised by obstacles on the surface. Depending on topography, density, the total length and other characteristics of trunk dams in the area, they can play a significant role in the reduction of the volume of material that is transported downslope. The collective (2007) shows that the amount of dead wood can be higher than 8 m3.ha-1. Vallauri et al. (2003) lists examples from European forests, which show that the volume of dead wood varies from 0.6 to almost 20 m3.ha-1. The total volume of fallen trees depends on the density and diversity of forest

Fig. 4. Trunk dam in advanced stage of plant succession (left), trunk dam breach (right).

A trunk dam is a biogeomorphologic system compound of five components, three of which form the mass of the trunk dam and two of which (base surface, barriers of embedment of a trunk) create its boundaries. The three inherent components are the following (Figs. 4, 5C): i. Trunk: This component usually appears as a stem with large branches, but sometimes it can also be present in the form of a complete tree or as individual large branches. Depending on surface roughness and trunk geometry, the height of its position above

ii. Body: The main body of a trunk dam is formed by a mixture of sediments of varying fractions (from clay to angular scree and rounded boulders) and organic components including decomposed parts of plants, litter, etc. Its top can vary from flattened (cf. Matyja, 2007) to rough and inclined at locations, where the base surface has a higher inclination and where the trunk dam is subject to further impacts of rolling and bouncing clasts. The internal structure of the body depends on the evolutionary history of each trunk dam and the type of material, which is delivered from the upper parts of the hillslope. The bodies of the newly formed trunk dams include an unsorted mixture of material, but when the trunk dam is stabilised for longer time, the body can display stratification as a result of the sequence of events that delivered the material to the trunk dam and/or as a result of a sieve effect and sorting within the trunk dam body. An example of a trunk dam with such a partly stratified body as detected during the

field survey (Raska & Orsulak, 2009) is shown in Fig. 5C.

cover and on its ecological integrity.

**3.2 Structure and dynamics** 

the surface can vary.

Fig. 5. Cross-section through the trunk dam. A - real (grey) and theoretical (black dashed) geometry of trunk dam with parameters measured in the field (lower case) and computed values (upper case); B - simplified trunk dam geometry used for spatial modelling (see section 3.3); C - structure of a trunk dam body.

iii. Cover: Trunk dams that have been stabilised long enough are covered by plants. The dynamic habitat of trunk dams is suitable for pioneer species. The results of a field survey in the study area show that the plants inhabiting trunk dams often belong to invasive species (e.g., *Impatiens glandulifera*). The speed of colonisation and the relative importance of species depend on the type of environment in which the trunk dam is located (McCullough, 1948). The fast colonisation of a trunk dam surface accelerates the stabilisation of its body through plant roots. While the body of a trunk dam is colonised mostly by herbs, the trunk itself in different stages of decay represents a habitat for mosses and especially fungi (e.g., Bader et al., 1995).

One of the aims of our previous research was to design a simple non-destructive method for estimating the volume of material accumulated by trunk dams. After initial attempts, an MS Excel Spreadsheet was designed. The EVAM (Estimation of Volume of Accumulated Material v1.0, available at <http://lsru.geography.ujep.cz>) approximates the cross-section of a trunk dam to a simple triangle-like shape (Fig. 5A), and it enables an accurate estimation of the volume of accumulated material (VAM).

The first results, which compared the accumulation and denudation rates in two sampling sites (Raska & Orsulak, 2009) showed that in terms of the current dynamics of a hillslope surface, the accumulation rate may be as important as the denudation rate. The overall results for 26 trunk dams analysed at three sites within the studied catchment revealed certain relations between the individual geometric parameters of a trunk dam (Fig. 6A). The highest correlation is between accumulation width "a" and VAM, but it will be shown later that there are certain difficulties in the generalisation of this relation. By contrast, a weak correlation was revealed between trunk radius "r" and VAM. However, the trunk radius and geometry of a trunk may partly influence other values, such as "a" and the height of a trunk above the surface.

The height of a trunk above the surface represents one of the most important factors influencing VAM values, as will be shown in relations between the width of accumulation, the slope inclination and VAM (Figs. 6B and 6C). Fig. 6B shows how VAM grows with increasing slope inclination. The model assumes that the height of a trunk above the surface is a dependent variable calculated from other input parameters. By contrast, the model in Fig. 6C assumes the trunk dam is in contact with the surface and, thus, is an independent variable. In this case, the higher is the slope inclination, the lower is the VAM.

Biogeomorphologic Approaches to

stabilisation by plants), and

trunk dam body).

wood characteristics (strength,

geometry and stage of decomposition)

structure of material within the trunk

Table 2. Factors influencing the stability of a trunk dam.

can be performed by analysing the following indicators:

where:

text).

dam body

a Study of Hillslope Processes Using Non-Destructive Methods 31

The fundamental point in the evaluation of the protective effect of trunk dams is the time scale in which they can effectively protect the surface against erosion. The stability of a trunk dam depends on several factors and can be expressed by the following equation:

• E ... external factors (s - slope inclination, e - embedment, d - disturbations, p -

• I ... internal factors (p - position of a trunk dam, w - wood characteristics, including strength, geometry and stage of decomposition, m - structure of material within the

The role of these factors is summarised in Table 2. The persistence of a trunk dam is limited by the total decomposition of the trunk and emptying of the accumulated material or by the breach of a trunk dam. Nevertheless, in some cases, the body of a trunk dam can be stabilised by plants to such an extent that even the total decomposition of a trunk will not result in the destruction of a trunk dam (see the three-way development model in further

slope inclination lower slope inclination higher slope inclination

trees

disturbances no impacts rockfall, zooturbation stabilisation by plants with plant cover without plant cover

stratified,

base surface

Related to the stability issues is the necessity to evaluate the persistence of the protective effect of trunk dams over time. It emerges from the very nature of the problem that the protection given by fallen trees, which are subject to gradual decomposition, will be effective only at small time scales, i.e., from the fall of the tree to its total decomposition or breach (Fig. 4). According to the results of the field survey, the age of individual trunk dams varied from a few days (newly fallen trees) to more than a year. The dating of a trunk dam

• the stage of trunk decomposition - large pieces of wood decay relatively slowly. Harmon et al. (1986) has shown that in temperate regions, half-time decay can vary from 25 yr (*Quercus*) to 150 yr (*Pseudotsuga*). Schowalter et al. (1992) conducted an

large trunk radius, straight trunk, early stage of decomposition

interconnected with

Factor Stability Instability

position of a trunk dam transverse downslope

embedment embedded by standing

S = f.(Es,e,d,p , Ip,w,m) (1)

embedded by surface

small trunk radius, curved trunk, late stage of decomposition

discontinuity between trunk dam body and base

not stratified,

surface

roughness

Fig. 6. A - relationship between measured parameters and the volume of accumulated material (VAM) in the dataset from a field survey; B, C - theoretical values of VAM and "a" for varying and for stable (zero) height of trunk above the surface (see text for further explanation).

As we discussed in section 3.1, the reason for the biogeomorphologic study of trunk dams is their ability to decelerate the velocity of rolling and bouncing clasts and their protective role against sheet erosion. The protective function against clasts, which move downslope as a result of rockfall events, is enabled by the trunk acting as a barrier and by the deceleration of clasts at the flattened top of a trunk dam (Raska, 2007). The bioprotective effect against sheet erosion is more complex, and its significance varies in the different parts of a trunk dam. In general, the bioprotective effects against sheet erosion could be direct and indirect. The direct effects consist of the accumulation of allochthonous material in the body of a trunk dam for a certain time. The indirect effects emerge from local feedbacks between trunk dam ecology and surface dynamics. As trunk dams are colonised by plants, they reduce soil erosion by the following:


However, the surface below the trunk dam often displays higher rates of denudation, as the trunk dam body retains all the sedimentary material. Also, the overland flow, which continues downslope, has higher erosive energy (see Fig. 4).

The fundamental point in the evaluation of the protective effect of trunk dams is the time scale in which they can effectively protect the surface against erosion. The stability of a trunk dam depends on several factors and can be expressed by the following equation:

$$\mathbf{S} = \mathbf{f}. (\mathbf{E}\_{\text{s,c,d,p}}, \mathbf{I}\_{\text{p,w,m}}) \tag{1}$$

where:

30 Studies on Environmental and Applied Geomorphology

Fig. 6. A - relationship between measured parameters and the volume of accumulated material (VAM) in the dataset from a field survey; B, C - theoretical values of VAM and "a" for varying and for stable (zero) height of trunk above the surface (see text for further

• interception and deceleration of the velocity of raindrops falling on the ground; • enhancing infiltration through root systems and increasing soil aggregate stability;

• increasing the volume of organic substances in the soil (Trimble, 1988; Gyssels et al.,

However, the surface below the trunk dam often displays higher rates of denudation, as the trunk dam body retains all the sedimentary material. Also, the overland flow, which

• transpiration of soil water, thus decreasing water content in soil;

• increasing surface roughness by roots, fallen leaves, etc.;

continues downslope, has higher erosive energy (see Fig. 4).

As we discussed in section 3.1, the reason for the biogeomorphologic study of trunk dams is their ability to decelerate the velocity of rolling and bouncing clasts and their protective role against sheet erosion. The protective function against clasts, which move downslope as a result of rockfall events, is enabled by the trunk acting as a barrier and by the deceleration of clasts at the flattened top of a trunk dam (Raska, 2007). The bioprotective effect against sheet erosion is more complex, and its significance varies in the different parts of a trunk dam. In general, the bioprotective effects against sheet erosion could be direct and indirect. The direct effects consist of the accumulation of allochthonous material in the body of a trunk dam for a certain time. The indirect effects emerge from local feedbacks between trunk dam ecology and surface dynamics. As trunk dams are colonised by plants, they reduce soil

explanation).

erosion by the following:

2005).


The role of these factors is summarised in Table 2. The persistence of a trunk dam is limited by the total decomposition of the trunk and emptying of the accumulated material or by the breach of a trunk dam. Nevertheless, in some cases, the body of a trunk dam can be stabilised by plants to such an extent that even the total decomposition of a trunk will not result in the destruction of a trunk dam (see the three-way development model in further text).


Table 2. Factors influencing the stability of a trunk dam.

Related to the stability issues is the necessity to evaluate the persistence of the protective effect of trunk dams over time. It emerges from the very nature of the problem that the protection given by fallen trees, which are subject to gradual decomposition, will be effective only at small time scales, i.e., from the fall of the tree to its total decomposition or breach (Fig. 4). According to the results of the field survey, the age of individual trunk dams varied from a few days (newly fallen trees) to more than a year. The dating of a trunk dam can be performed by analysing the following indicators:

• the stage of trunk decomposition - large pieces of wood decay relatively slowly. Harmon et al. (1986) has shown that in temperate regions, half-time decay can vary from 25 yr (*Quercus*) to 150 yr (*Pseudotsuga*). Schowalter et al. (1992) conducted an

Biogeomorphologic Approaches to

a Study of Hillslope Processes Using Non-Destructive Methods 33

Fig. 7. Model of the protective effect of trunk dams within the experimental catchment. A slope inclination derived from DEM (1:10000); B - land cover classes CORINE 2006; C hypothetic calculated volume of accumulated material; D - calculated volume of accumulated material weighted by slope inclination (pixel size 20x20 m). Data sources: CORINE 2006 (European Environmental Agency), digital elevation data (CUZK).

The cross-section of a body of a trunk dam was assumed to be in contact with a trunk and having the shape of a rectangular triangle (Fig. 5B). Thus, the only necessary values for the calculation of VAM were the accumulation width "a", the length of trunk "L" and the slope inclination "α". Instead of trunk radius "r", we set the average "x" value (Fig. 5B) calculated from the field survey results. The variability of this value depends on the forest structure and age within the modelled catchment and on the surface roughness influencing height of a trunk above the surface. In the present study, the structure and age of forests is relatively uniform (Fig. 7B). There are no significant differences in trunk radius variations among the studied sites, and therefore, we set the uniform "x" value (0.35 m). Similarly, the length of trunks was set as an average from the field survey, which was 2500 m.ha-1. Thus, the results of the simplified VAM model will correspond only to the expression of slope inclination. The slope inclination was derived from a digital elevation model (DEM; Fig. 7A). The pixel

experiment to assess the decomposition process of four species (*Pseudotsuga*, *Tsuga*, *Abies* and *Thuja*) and concluded that the decomposition is significantly influenced by the initial wood chemistry and by the colonisation pattern, especially the penetration of the bark barrier and colonisation by wood fungi. Harmon et al. (2000) have presented a new method for estimating biomass loss by wood decay. Their results have shown that in most cases, the biomass loss has negative exponential progress, while in one case (*Pinus sylvestris*), the regression trend was polynomial, displaying different phases of decomposition. The relative dating of trunk decomposition is usually made by distinguishing categories of (i) hard timber, (ii) edge soft - centre hard, (iii) edge hard centre soft and (iv) totally rotten (e.g., Collective, 2007);


Based on the field survey, Raska & Orsulak (2009) proposed a hypothetical model of the three-way development of a trunk dam in the mid-segment of the hillslope. The three ways are the following: (i) stabilisation of the trunk dam by vegetation cover, (ii) dam breach and formation of a new trunk dam and (iii) denudation after the dam breach without the formation of a new trunk dam.
