The role of trees in the geomorphic system of forested hillslopes — A review

Earth-Science Reviews 126 (2013) 250–265

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The role of trees in the geomorphic system of forested hillslopes — A review Łukasz Pawlik ⁎ Department of Geomorphology, Institute of Geography and Regional Development, University of Wrocław, Poland

a r t i c l e

i n f o

Article history: Received 10 January 2013 Accepted 12 August 2013 Available online 27 August 2013 Keywords: Tree Hillslope Biomechanical weathering Biochemical weathering Tree uprooting Biotransport

a b s t r a c t Forested hillslopes form a special geoecosystem, an environment of geomorphic processes that depend strongly on forest ecology, including the growth and decay of trees, changes in structure, disturbances and other fluctuations. Hence, the following various functions of trees are reviewed here: their role in both biomechanical and biochemical weathering, as well as their importance for the hillslope geomorphic subsystem and for transport of soil material via tree uprooting and root growth. Special attention is paid to tree uprooting, a process considered the most efficient and most frequent biogeomorphological indicator of bio-physical activity within forest in complex terrain. Trees have varied implications for soil formation in different environments (boreal to tropical forests) and altitudes. In this paper an attempt has been made to emphasize how trees not only modulate geomorphic processes, but also how they act as a direct or indirect agent of microrelief formation, the most striking example of which being widespread and long-lasting pit-and-mound microtopography. Based on the analyzed literature it seems that some problems attributed to forest ecology can have a fundamental effect on forested hillslope dynamics, a relationship which points to the need for its integration and interpretation within the field of geomorphology. The biology of individual trees has a key influence on the development of e.g. rock faces, weathering front migration and changes in the soil biomantle within upper and lower forest belts. Additionally, forms and sediments depend largely on the horizontal and vertical extent, volume and structure of root systems, as well as on active processes taking place in the root zone and rhizosphere. Furthermore, although trees to a large extent stabilize slope surfaces, their presence can also have a dual effect on slope stability due to tree uprooting, a process which in some circumstances can trigger mass movements (e.g. debris avalanches). So far, several attempts at quantifying the influence of trees on slopes have been made via the use of mathematical equations, enabling researchers to calculate: 1) the root plate volume of uprooted trees, 2) the amount of soil displacement due to tree root growth, and 3) rates of erosion, sedimentation and soil creep. In light of the reviewed literature, the most urgent issue appears to be the need for a thorough study of the interactions and feedbacks occurring between trees and geomorphic systems (e.g. soil mixing and biotransport by trees) in different climate zones, altitudes and time frames, especially in terms of the development of forest ecosystems during the Holocene. © 2013 Elsevier B.V. All rights reserved.

Contents 1. 2.

3. 4.

5. 6.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . Trees and rock weathering . . . . . . . . . . . . . . . . . . . 2.1. Biomechanical weathering . . . . . . . . . . . . . . . . 2.2. (Bio)chemical weathering . . . . . . . . . . . . . . . . Trees, soil mass displacement and soil processes . . . . . . . . . 3.1. Tree uprooting . . . . . . . . . . . . . . . . . . . . . Trees and superficial processes on hillslopes . . . . . . . . . . . 4.1. Trees and accumulation processes . . . . . . . . . . . . 4.2. Trees and slope stability . . . . . . . . . . . . . . . . . Trees and the development of landforms and surface rock structures Geomorphic processes and trees — attempts at quantification . . . 6.1. Calculation of the root plate volume of uprooted trees . . . 6.2. Calculation of soil displacement due to tree root growth . .

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6.3. The damming effect of trees, erosion and exposed roots . . . 6.4. Trees and palaeogeographical reconstructions . . . . . . . . 7. Potential directions for future research — conclusions and final remarks Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction Since their first expansion, trees have remodelled landscapes and have been an important element of the Earth's natural history from at least the Devonian (Meyer-Berthaud et al., 1999). Were early terrestrial plants including trees able to create new niches and habitats? And, in turn, did they help in the evolution of new plant and animal species? This question remains to be answered satisfactorily (Kelly et al., 1998; Gabet and Mudd, 2010). Others, while considering the evolution of trees, seed plants and the colonization of land by forests during the Devonian, have hypothesized that the development of terrestrial vegetation induced a brief late Devonian glaciation by causing a drawdown in atmospheric pCO2 (the partial pressure of CO2) (Algeo and Scheckler, 1998; Berner, 1998, as cited by Goudie and Viles, 2012). Forests currently cover at least 30% of the global land surface (FAO, 2005). As a component of vegetation cover, they form a biological membrane (also known as the Earth's biospheral envelope) that absorbs solar energy (a concept first proposed by Vernadsky, 1926, 1944; see also reviews by Ghilarov, 1995 and Lapo, 2001) and which also acts as a large reservoir of rainwater (Osuch, 1998; Phillips, 2009). Through their growth and decay, trees have a critical and continuous effect on the land surface of the Earth, and, as components of forest ecosystems, their structural development and exchange of species and stands must also be stressed. These natural phenomena occasionally happen in a catastrophic manner, when slow changes are intensified and quickened (e.g. via windstorms). Since at least the end of the 19th century, trees have been considered agents of soil disturbance and mixing (Shaler, 1891; Hack and Goodlett, 1960; see reviews by Johnson, 1993; Wilkinson et al., 2009). However, for a long time the forest environments of the upper and lower montane belts were assumed to be static, or at least with infrequent changes (Jahn, 1989; but see for example Dietrich and Dunne, 1978), and were thus neglected in geomorphic studies (e.g. Klimek and Latocha, 2007). Nevertheless, some exceptions exist. For instance, the above-mentioned remark does not apply to humid tropical denudation systems where “the study of the geomorphology of the humid tropics cannot be divorced from a consideration of the vegetation” (Douglas, 1969, p. 13; also Thomas, 1994). Similarly, interactions between vegetation and hillslope geomorphology have recently been emphasized as crucial to our understanding of linkages between different ecoregions (Marston, 2010). In retrospect, biological influences on geomorphological processes have gained much more attention (see, for example, Table 1 in Phillips, 2009). At present, the need for such an interdisciplinary approach to geomorphology is undeniable, with this approach's character and methods formalized in the subject of biogeomorphology (Viles (Ed.), 1988a, 1988b, 1990; Corenblit et al., 2008; Corenblit and Steiger, 2009). This sub-discipline aims at full integration of the biotic and abiotic aspects of geomorphic systems at all levels of complexity. Forest geomorphology defines the functions of forests and of individual trees, and has already been appreciated as an important discipline within the frame of geomorphology, dealing with many problems typical of mountainous areas (protective forests: FAO, 2005; Sakals et al., 2006). The most important issue currently facing forest geomorphology appears to be the integration of the efforts of different disciplines towards producing an explanation of forested hillslope

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dynamics. This theory will be based on tendencies in forest ecosystem changes and will thus involve integrating knowledge of forest ecology (White, 1979). Similar efforts attempt to bridge the gap between pedology and forest ecology (Šamonil et al., 2010a), as well as between climate change and sediment transport (Constantine et al., 2012). The main aim of this review is to explore, based on existing literature, the role of trees within the forested hillslope domains of mountain geoecosystems, with the most important question arising being: How do trees contribute to the activity and modification of geomorphic processes, and thus also to landscape evolution at different spatial and temporal scales?

2. Trees and rock weathering Vegetation contributes to weathering via various mechanisms over a full range of scales, from the microscale (e.g. the interaction of fine roots with minerals) to the macroscale, where the physical fragmentation of large rocks may take place in the root zone. Trees contribute to weathering processes in many ways and their action can be considered biotic weathering which, as proposed by Selby (1993), is a combination of chemical and physical weathering effects (Fig. 2). Biological weathering is, as proposed by Yatsu (1988), the

Fig. 1. Forest geomorphology as a subdiscipline within biogeomorphology. Only natural processes are included, although Rosenfeld (2004) also considered the effects of forest management activities within the frame of forest geomorphology. See text for explanation (figure not yet published).

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Fig. 2. Traditional division of weathering types, distinguishing biological weathering and its effects (original).

process brought about by living organisms and their decomposition products (p. 287). Similarly, bioweathering covers the range of biological contributions to processes of rock and mineral weathering (Naylor et al., 2002). Undoubtedly, although every plant influences soil properties, as stated by Miles (1986), “trees tend to have greater effects than other plant life forms because of their size and longevity” (p. 55). This last statement refers to the hidden part of trees—their roots. 2.1. Biomechanical weathering Trees bring about physical, chemical, biological and morphological changes to the soils in which they grow (Retzer, 1963). A single tree is a factor of biomechanical weathering influencing soil and regolith properties (Phillips and Marion, 2006; Phillips et al., 2008a, 2008b). There are several ways in which trees and their roots act as agents of biomechanical weathering, with the most important including (Lutz and Griswold, 1939; Retzer, 1963; Schaetzl et al., 1989a; Gabet et al., 2003; Phillips et al., 2005; Phillips and Marion, 2006; Gabet and Mudd, 2010): 1. tree uprooting; 2. physical anchoring of soil on slopes by tree roots (in Johnson, 1993, considered a biomechanical process); 3. physical displacement of soil by root and trunk growth; 4. formation and infilling of stump rot depressions and burned-out stump holes; 5. decay and infilling of former root channels; 6. penetration of bedrock by tree roots (Fig. 3); 7. swaying of trees during high winds which promotes root movement and agitation of soil (causing changes in soil bulk density, porosity and permeability). Direct bedrock disruption is caused by the mining of mineral fragments from the root zone via tree uprooting (Small et al., 1990; Phillips and Marion, 2006; Phillips et al., 2008b). The ability of trees to perform such an activity depends on many factors, such as: 1) bedrock features, 2) characteristics of tree root system (e.g. rooting depth relative to regolith thickness), 3) edaphic conditions and 4) tree species. In many instances, regolith can be locally deepened through tree root growth along bedding planes and penetration into rock fissures; these processes quicken biomechanical weathering and facilitate both moisture migration and water availability. Consequently, chemical weathering can penetrate deeper into solid rock, thus contributing to migration of the weathering front. These tendencies draw attention to bedrock type, especially when the initial stage of slope cover development is considered (Phillips et al., 2008a). Tree roots attack and penetrate the joints or fissures of fractured sandstone, deeply weathered coarse-grained granite

and marble in different ways. Indeed, several studies have underlined the fact that root system architecture, while genetically controlled, is strongly influenced by the characteristics of the substrate (Stone and Kalisz, 1991; Matthes-Sears and Larson, 1995). As well as bedrock features, deep rooting can also be controlled by the presence of a fragipan, excessive stoniness or a clay-rich B horizon (Schaetzl et al., 1989a). Other important biomechanical effects of trees include: (1) physical displacement of soil by root and trunk growth, (2) infilling of stump rot depressions (Fig. 4) and (3) tree uprooting. The above mentioned effects have a considerable influence on forest soils (Retzer, 1963; Phillips and Marion, 2006) and can cause problems in terms of the latter's classification (Retzer, 1963). Although all of these processes are ubiquitous in forested ecosystems, only tree uprooting has received much attention (Šamonil et al., 2010a). The uprooting of trees may contribute to continual mixing of the biomantle and has been recognized as a key factor in soil evolution and forest dynamics (Lutz, 1940; Lyford and MacLean, 1966; Faliński and Falińska, 1986; Scatena and Lugo, 1995; Phillips and Marion, 2006; Šamonil et al., 2009; Lenart et al., 2010; Šamonil et al., 2010b). The physical displacement of soil by expanding tree roots and trunks, although less spectacular, can bring about considerable changes in soil mantle porosity and bulk density which, for instance, during heavy downpours may affect slope stability. However, in most cases roots reinforce hillslopes through root cohesion and their anchoring into bedrock (e.g. Roering et al., 2003). Tree roots can exert an axial and radial pressure as high as 1.45 and 0.91 MPa, respectively (Bennie, 1991, as cited by Gabet et al., 2003 and Gabet and Mudd, 2010), potentially enough to push up a column of soil approximately 100 m thick (Gabet et al., 2003). Birot (1966), as cited by Pitty (1971), provided an example of rock fragment displacement and suggested that a living root 10 cm in width and 1 m in length can move a block weighing 40 tons. Such theoretical assumptions, when added to information regarding potential rooting depth and root zone extent, present a highly impressive view of root power, especially with respect to the wedging and fracturing of rocks. In his excellent book, Yatsu (1988) recalled a classic and, in his view, still unresolved, problem associated with the wedging effect of growing plant roots (p. 365). He stated, after analyzing the radial pressure (instead of the axial pressure, which is frequently wrongly included in theoretical considerations) of the plant root and the tensile strength of a rock block, that such an ability cannot be confirmed and that new data are urgently required in order to solve the problem. This is probably still true in the case of the fracturing of fresh (unweathered) rock by roots, with the process continuing to suffer from a lack of attention and thorough explanation 25 years after Yatsu's statement. However, it is interesting to note that in the case of rock wedging, such an ability had already been ascribed to tree roots in early books on geomorphology. Indeed, Baulig stated as early as 1940 that trees are able to crumble rocks and comminute coarse particles (Baulig, 1958). Scheidegger (1970) later argued that plants penetrate rock fissures and cause their widening (p. 122). However, none of the above-cited authors discussed the issue thoroughly or referred to a dedicated article containing empirical data. It might be possible that the only source of explanation comes from Jackson and Sheldon (1949) and Matthes-Sears and Larson (1995). Investigating a limestone cliff-like valley side in Derbyshire, England, the first authors concluded that woody plants caused recession of the cliff face by “detaching rocks from the edge of the cliff”. However, the authors doubted this was solely a consequence of fissures widening as a result of root thickening and suggested that the action of drainage water was also involved. Although very important, the results of this study were based only on field observations, not measurements of root pressure or long-term monitoring. The second authors found that the roots of an eastern white cedar (T. occidentalis) growing on a limestone cliff penetrated almost exclusively those rock fissures in the softer, more weathered rock layers. Both articles effectively just continue the discussion about rock wedging by tree roots and thus do not allow for a definite rejection of Yatsu's statement. In this context, an interesting

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piece of evidence was collected in underground archaeological monuments in Rome, Italy, by Caneva et al. (2009), who documented several examples of damage caused by growing roots. Gabet et al. (2003) illustrated the “mechanical disruption of bedrock” (Fig. 6, p. 261), with Gabet and Mudd (2010) later supporting their own biogeomorphic model of soil production through the statement that “a tree's roots can penetrate bedrock and split it apart”. It seems that such processes are indeed active but only in conjunction with: 1) rock wedging (facilitated by chemical weathering, as well as nutrient and water availability along fissures) and 2) tree throw during which a part of fractured bedrock is extracted, evidence of which can be easily observed in the root plates of fallen trees. Direct mechanical fracturing of rock faces by tree roots is rather unrealistic; root growth is such a slow process that it can be overlapped by other physical effects such as frost wedging. The above-described issue may have a greater meaning when the spatial development and extent of tree roots are taken into account. Although the reported average maximum depth of rooting is 3 m for temperate deciduous forest and 4 m for temperate coniferous forest (Gregory, 2006), it is estimated that merely 1 to 10% of a tree's total root length occurs in soil below 1 m depth (Crow, 2005; see also Schenk and Jackson, 2002). As a result, root wedging is frequently absent and physical and chemical activity of roots is limited to the topsoil and subsoil. Finally, whereas tree roots are able to penetrate rock fissures as small as 100 μm (Zwieniecki and Newton, 1995), individual trees may create different patterns in soils at horizontal scales of 5–15 m (Binkley and Giardina, 1998). In conclusion, the pedological effects of trees are related to at least three types of mechanisms of change: 1) biomechanical, 2) hydrological and 3) mechanical (Phillips and Marion, 2005). According to various authors, the pedological influence of trees represents an important control on local soil variability, especially in terms of changes due to tree throw. A similar conclusion was drawn by Šamonil et al. (2011) after studying soils within pit-and-mound microtopography in the Novohradské Mts., Czech Republic. Hydrological mechanisms of change are beyond the scope of this review, but a synthesis can be found for instance in Ohte and Tokuchi (2011). The first order mechanism in this case is rainwater transmission through concentrated stem flow and infiltration along main living roots (Selby, 1993), with tree root channels (macropores) also facilitating water infiltration and percolation (Gabet et al., 2003). 2.2. (Bio)chemical weathering Chemical weathering caused by biota is potentially much more effective than weathering under abiotic conditions (e.g. Berner, 1998). As recently reviewed by Lucas (2001), plants directly control water dynamics, weathering and the chemistry of weathering solutions (Kelly et al., 1998). Biota, from microbes through to vascular plants, also enhance chemical weathering through: 1) stabilizing soil, 2) producing humic and other organic and inorganic acids and chelating agents

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(Yatsu, 1988), 3) acting as a sink for the nutrients freed by weathering and 4) contributing to physical weathering through the microfracture of mineral grains (Schwartzman and Volk, 1989). Factor 1 points to the ability of soil to act as a high surface area reservoir in which water is stored and used as a medium for acid attack (Schwartzman and Volk, 1989). Other factors which control rates of biotic weathering include: 1) biomass, 2) surface area of contact and 3) the capacity of organisms such as plants (roots) and mycorrhizal fungi to interact physically and chemically with minerals (Taylor et al., 2009). The role of vegetation has been considered, for instance, in silicate weathering phenomena (Goudie and Viles, 2012), while lichens have gained special attention in studies examining biological (biochemical) weathering (Viles, 1995; Lee and Parsons, 1999; Chen et al., 2000; Lisci et al., 2003; Goudie and Viles, 2012). Higher plants (vascular plants) may have a more significant impact on soil (see Cochran and Berner, 1996 and Berner, 1998) because they develop high potential root systems that control soil properties (e.g. soil pH; Gruba, 2009). Berner (1998) argued that during the Devonian, higher plants were able to affect the chemical weathering of silicate rocks because of deep rooting and good drainage. This effect can be also attributed to trees, which have been proven to enhance chemical weathering thanks to: 1) the extent of their root systems, 2) biochemical processes in the rhizosphere in which alteration of minerals takes place (Yatsu, 1988) and 3) the transmittance of water and moisture along root channels. The rhizosphere has been defined as “the volume of soil surrounding the roots which is affected by it” (Gregory, 2006; Calvaruso et al., 2009). Studies investigating Hawaiian basalts (USA) have shown that chemical alteration is much more advanced under higher-plant communities than at unvegetated or lichen-encrusted sites (Cochran and Berner, 1996). When compared to abiotic conditions, root activity is also likely to increase weathering rates of basaltic rocks by up to 5 times, as was proved experimentally by Hinsinger et al. (2001). Other experimental data indicate that under artificially planted red pines (Pinus resinosa Aiton), weathering rates of primary minerals are more than 10 times higher than those in a non-vegetated environment (Bormann et al., 1998). However, it should be remembered that “weathering rates determined in the laboratory are generally orders of magnitude greater than field predictions” (Kelly et al., 1998, p. 33). Nevertheless, tree roots play a very important role because they contribute to both physical and biochemical weathering. The latter contributes to a greater extent within the upper soil horizons and rhizosphere for two reasons. First, the rapid turnover of fine roots affects all the soil material via the action of rhizospheric processes. Second, variation in biological activity is greater in the upper soil horizons (Lucas, 2001). For instance, Yatsu (1988) recalled the results of experiments conducted by Spyridakis et al. (1967) which showed that the rhizospheric activity of coniferous and deciduous tree seedlings led to the transformation of biotite to kaolinite (most effectively by the white cedar Thuja occidentalis) and biotite to vermiculite (by the Monterey pine Pinus radiata). Moreover, it is interesting to note that in the rhizosphere, invading plant roots

Fig. 3. Examples of the biomechanical influence of trees on bedrock, rock faces and individual blocks (original).

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Fig. 4. A conceptual model of the displacement of rock fragments and soil mass by root system growth. After complete decomposition of a stump, some rock fragments fall into the stump hole and root channels are infilled. Figure inspiration drawn from Phillips and Marion (2006).

affect mineral grains both mechanically and chemically, with some part of this work attributed to root-associated microorganisms such as fungi and bacteria (Bonneville et al., 2009; Calvaruso et al., 2009 and references cited therein). Fungi and bacteria produce organic acids which promote chemical weathering (Cochran and Berner, 1996); recent data have revealed that the most important for biotic weathering is the combined activity of roots and mycorrhizal fungi driven by carbon uptake from photosynthetic organisms (Taylor et al., 2009). This set of complex interactions can lead for instance to the formation of root groves, which are common in karst areas (Wall and Wilford, 1966; also “root karst”, see Viles, 1988a, 1988b) and are also known in sandstone regions (Dorn et al., 2013). Root groves form mainly as a result of biochemical influences, although biophysical processes have also been mentioned (e.g. enlargement of microfractures; Viles (Ed.), 1988a, 1988b; Dorn et al., 2013). Gabet et al. (2003) argued that, through breaking bedrock, tree roots can cause a six-fold increase in the surface area available for chemical weathering (by creating spaces for water and different weathering factors; Gabet and Mudd, 2010). Although this view (see Fig. 6, p. 261, Gabet et al., 2003) is really only a simplification made for modelling purposes, to support it the authors also referred to the work of Lutz (1960) who “noted many instances of fresh rock torn out of bedrock” (p. 261). For comparison, Schaetzl and Anderson (2005) state that as rocks are fractured due to physical weathering factors other than tree roots, their surface area increases geometrically. Tree root exploration is also considered a primary mechanism of the downward extension of the ecosystem boundary (Bormann et al., 1998). Important data at the watershed scale

were published by Moulton et al. (2000), who calculated a 4 times greater weathering release of Mg2+ from forested areas and proved pyroxene weathering to be 9 to 10 times higher there than in barren but geologically similar areas. Additionally, when considering individual forest communities, different tree species have varying effects on both weathering and soil formation, e.g. podsolization. For instance, a spruce ecosystem was found to cause a threefold increase in calcium denudation from monzonite granite in comparison to a beech coppice on the same parent material (Bormann et al., 1998). Conifers, especially spruces: 1) accelerate podsolization, 2) cause surface acidification and 3) cause higher rates of organic matter accumulation. In contrast, broadleaved species are associated with: 1) different soil fauna which may decrease podsolization via physical intermixing of the A and B horizons, 2) a reduction in soil acidity and 3) mull-humus formation (Miles, 1986). The first of these processes can refer to the soil homogenization sequence proposed by Hole (1961). In their analysis of chemical weathering and denudation in a forest ecosystem based on a 20-year “sandbox” experimental study, Balogh-Brunstad et al. (2008) concluded that disturbance can be an important factor controlling chemical denudation rates (in this case a rapid increase in denudation rates was observed after tree harvesting). Another important issue is linked to palaeogeographical reconstructions and chemical weathering. Chemical weathering is thought to have been a dominant process under forest cover in the Holocene (Starkel, 1977; Bieroński et al., 1992), in contrast to the dynamic mechanical denudation which took place during cold phases in the Pleistocene (e.g. Migoń, 2006). The postglacial succession of forest led to the stabilization of slope mantles which then became a potential source of

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Fig. 5. Sequence of pit and mound pair development due to tree uprooting caused by strong winds. Note: years taken from Šamonil et al. (2009). Author's original work.

weathering material. Slope mantle volume in the Sudety Mts., SW Poland, has been calculated to be at least 1,000 000 m3 km-2 (Klimek and Latocha, 2007). 3. Trees, soil mass displacement and soil processes 3.1. Tree uprooting Although the significance of the tree uprooting process has been underlined in several research articles (Phillips et al., 2008b; Šamonil

Fig. 6. Boulder trapped in the root system of a fallen tree in an advanced stage of decomposition. An example found in the lower belt forest of the Karkonosze Mts., Sudety, SW Poland.

et al., 2010a), review publications (Schaetzl et al., 1989a, 1989b, 1990; Viles, 1990; Johnson, 1993; Gabet et al., 2003; Marston, 2010; Šamonil et al., 2010b) and scientific books (Huggett, 1995; Schaetzl and Anderson, 2005), an appreciation of its importance is not equally shared among the different Earth Science disciplines. Nevertheless, the process has been thoroughly studied from the point of view of forest ecology (Putz, 1983; Jonsson and Dynesius, 1993; Everham and Brokaw, 1996; Ulanova, 2000; Šamonil et al., 2009; Šebková et al., 2012), forestry (Steven, 1953; Vicena et al., 1979; Brázdil, 1998; Brázdil et al., 2004) and soil science (in terms of soil production: Gabet and Mudd, 2010; Šamonil et al., 2010a, 2010b; soil chemistry: Liechty et al., 1997; Šamonil et al., 2008a, 2008b; and soil disturbance and pit-and-mound microrelief: Lutz and Griswold, 1939; Lutz, 1940; Denny and Goodlett, 1956; Lyford and MacLean, 1966; Schaetzl, 1986; Lenart et al., 2010; Šamonil et al., 2010a). For the most recent and comprehensive review considering tree uprooting vs. soil interactions, see Šamonil et al. (2010a). Although a number of papers focus on the geomorphic consequences of the tree uprooting process (Lutz, 1960; Kotarba, 1970; Schaetzl and Follmer, 1990; Norman et al., 1995; Embleton-Hamann, 2004; Osterkamp et al., 2006; Phillips et al., 2008b; Gallaway et al., 2009), many forest environments have not yet been fully studied and thus further work is required for a more complete evaluation of forested hillslope dynamics. Tree uprooting (also tree throw, tree tip, tree saltation; Faliński and Falińska, 1986; similar in meaning is root throw, but see the explanation in Osterkamp et al., 2006, p. 7) (Fig. 5) is not only one of the most important active biogeomorphological processes taking place on forested hillslopes, it is also considered an important link between atmosphere, biosphere and pedosphere (Small et al., 1990), undoubtedly contributing, at least to some extent, to the Critical Zone paradigm (Anderson et al., 2008; Lin, 2010). Its efficiency in breaking ground (Wilkinson et al., 2009) has been widely evaluated and exists under several

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256 Table 1 Reported mean root plate volumes. Mean root plate volume (in m3)

Place

Tree species or forest type

Altitude (m a.s.l.)

1.8–3.6⁎

Tatra Mts., Poland

lower and upper belt spruce forest

1000–1500 Kotarba, 1970

1.2–2.8⁎

New York, USA

different species in second-growth forests

–

Beatty and Stone, 1986

4.0

Washington, USA

western hemlock (Tsuga heterophylla) and Pacific silver fir (Abies amabilis)

70–1020

Reid, 1981

2.0

Arkansas, Ouachita Mts., USA Tatra Mts., Slovakia

75–530

Phillips et al., 2008a, 2008b

923–1284

Dąbrowska, 2009

mixed shortleaf pine (Pinus achinata) and hardwood forest 1.9 mixed spruce-larch forest (Lariceto Piccetum) 0.1–0.7 British Columbia, Canada lodgepole pine (Pinus contorta Loudon var. latifolia Engelm.), Engelmann spruce (Picea engelmannii Parry ex. Engelm.) 1.7 Tatra Mts., Slovakia mixed spruce-larch forest (Lariceto Piccetum) 0.3 Puerto Rico dry, moist, wet and rain forest with broadleaf, needleleaf and palm trees 4.0 Sudety Mts., Poland lower belt spruce forest 4.0 (beech)–5.6 (spruce) Outer Western Carpathians, natural fir-beech forest Czech Republic

Reference

1330–3086 Gallaway et al., 2009

800–1300 0–1000

Rojan, 2010 Lenart et al., 2010

700–900 600–800

Pawlik, 2013 Šamonil P. (unpublished data)

Notes According to Norman et al. (1995) calculated volumes are twice too large Root plate volumes were calculated as cylinders (V = ∏r2h) Calculation made with “a model of cylinder with a segment missing”

Survey was carried out in post-fire conditions

For the calculation a formula for half an ellipsoid were used as proposed by Norman et al. (1995)

⁎ Mean values obtained from different plots.

terms, e.g. biomechanical weathering, bioturbation, pedoturbation, floralturbation or floralpedoturbation (Pawluk and Dudas, 1982; Johnson, 1993; Gabet et al., 2003; Phillips and Marion, 2005; Schaetzl and Anderson, 2005), as well as biogenic transport (Swanson et al., 1982a; Burns and Tonkin, 1987; Schaetzl et al., 1990), which has been defined as “the movement of debris as a result of biological activity” (Swanson et al., 1982a). Tree uprooting is also known as biological saltation (Birot, 1966) and bioporting – transport by biota (see Phillips and Marion, 2005) – and can be divided into two categories: 1) catastrophic local transport (caused by e.g. tree throw, animal burrowing) and 2) slow transport (e.g. root growth, wind stress on trees) (Swanson et al., 1982a). The average volume of soil mass transported in root plate form is between 0.1 and 5.6 m3 (Table 1). Tree uprooting is a major disturbance factor in most natural forests (Schaetzl et al., 1989a; Huggett, 1995; Everham and Brokaw, 1996; Brázdil et al., 2004) and is considered to be an important variable in so called disturbance ecology and gap-phase dynamics (White, 1979; Pickett and White (Eds.), 1985; Ulanova, 2000; Linke et al., 2007). Simultaneously, it is also an agent altering hillslope stability and affecting other geomorphological processes, changing both their course and intensity (see reviews by Schaetzl et al., 1990, p. 285, and Viles, 1990, p.12). For instance, after severe windthrow in the Swiss Alps caused by extra-tropical cyclone ‘Vivian’ in 1990, countless erosion scars were formed (Gerber et al., 2002). Additionally, due to heavy rainfall in subsequent years, the combination of construction works carried out after the storm and the lack of vegetation led to further scar development. The authors concluded that the areas affected by windthrow were highly susceptible to erosion and shallow landslides, a reciprocal effect also considered in previous work examining hillslope stability and evolution (e.g. Schaetzl et al., 1990; Selby, 1993; Johnson and Wilcock, 2002). The uprooting of trees contributes to the downslope movement of soil (seasonal biogenic creep; Kirkby, 2004; Wilkinson et al., 2009) and rock mantle, and locally may initiate gully erosion (Lutz, 1960; Wilkinson et al., 2009). Published results, which range from 0.02 to 1.3 t ha−1 year−1, demonstrate that tree uprooting should be seen as an important factor influencing sediment transport in a watershed (Schaetzl et al., 1990). As shown by Swanson et al. (1982a), sediment transfer driven by tree uprooting can be close in magnitude to soil creep rates (see also Clément, 1993). Interactions between the tree

uprooting process and soil creep have been studied in the Stołowe range of the Sudety Mts., SW Poland (Pawlik et al., 2013). Probably less frequent but more important is the transport of large boulders. Published data indicate that a wind-uprooted tree may move rocks with a volume of as much as 1.4 m3 and a weight of up to 4 tons (Lutz, 1960). Additional results show that uprooting may contribute to sediment transfer via several other mechanisms (Schaetzl et al., 1990): 1. Tree-throw pits absorb water which leads to a reduction in soil shear strength and, as a consequence, can result in debris avalanches or debris flows; 2. During tree fall a sort of vibration is induced which can cause soil materials to reach their liquid limit and fail, triggering mass movement. Tree uprooting is a very common forest process (e.g. Stephens, 1956) that significantly influences the structure and layering of slope covers (e.g. Phillips and Lorz, 2008), and especially applies to mountain areas for the following reasons. Firstly, wind, which is the primary factor of tree damage (Everham and Brokaw, 1996; Brázdil, 1998; Peterson, 2007; Phillips et al., 2008b), speeds up with altitude (see review by Mitchell, 2012), while another important factor of tree damage is the occurrence of ice storms (see, for instance, Bragg et al., 2003). The proportional area covered by pit-mounds after a large catastrophic blowdown may be up to 11% (Peterson et al., 1990), suggesting that a substantial part of a hillslope may be affected by the tree uprooting process. Secondly, due to the high slope gradients, up to 50% of soil material from the exposed root plate falls outside the pit (Burns and Tonkin, 1987; Gallaway et al., 2009), partly because trees tend to fall downhill on steeper slopes (Burns and Tonkin, 1987; Norman et al., 1995); this phenomenon has been assessed at 90% probability for a 45° slope (Gabet and Mudd, 2010). Thirdly, soil attached to the root system is subject to further deterioration due to subsequent superficial processes such as rain splash, wash or mass wasting (Small et al., 1990; Pawlik, 2013; Pawlik et al., 2013), thus prolonging its downslope transfer. The material removed from root balls covers an area lying directly below and affects, for instance, undergrowth vegetation. However, it has to be kept in mind that rock fragments of different sizes can be trapped at some height above the ground within the root system, and remain there for many years after the toppling of the tree (Fig. 6).

Ł Pawlik / Earth-Science Reviews 126 (2013) 250–265 Table 2 Tree uprooting and its influence on soils, pedogenesis and transport of sediments (based on Phillips and Marion, 2005; Phillips et al., 2008b). Effects Direct

Indirect

• Soil mixing • Soil profile inversion • Local redistribution of soil substance • Formation of pit-and-mound microtopography (windthrow morphology)

• Action of erosion and mass wasting processes on exposed root plate. Microscale variations within windthrow morphology in terms of: - Weathering intensity, - Moisture flux and water availability, - Dynamics of organic matter (preferable litter accumulation in pits), - Chemistry, - Microclimate. • Intensified organic activity in pits (e.g. by earthworms)

An important parameter which allows for better evaluation of tree uprooting frequency is the ratio of uprootings to broken trees, both as a consequence of long-term variation and a single strong wind event such as an extra-tropical cyclone. Yamamoto (2000) discovered uprooting to be the least recorded cause of tree mortality in Japan, a finding which is consistent with studies from Europe, including the following: 1) In the old-growth fir-beech forests of the Czech Republic, the ratio between bole breakages and uprootings was 2–3:1 (Šamonil et al., 2013), 2) in the beech-fir old-growth forests of Bosnia and Herzegovina, figures of 14% uprooted trees, 60% snapped stems and 26% standing dead trees were recorded (Bottero et al., 2011). Both given examples come from mountain forests and document a phenomenon associated with gap phase dynamics. The proportion of uprootings is distinctly different after catastrophic bora and foehn wind events. In the High Tatra Mts. of Slovakia after a bora wind event in November 2004, a ratio of 41% windsnaps to 59% uprootings was recorded (Rojan, 2010), while similar figures were observed on the northern side of the Tatra Mts. after a strong foehn event in 1966 (48% windsnaps and 52% uprootings) (Bzowski and Dziewolski, 1973). These latter results likely reflect the presence of tree species such as the generally shallow-rooted Norway spruce which dominate in the Tatra Mts. massif. Peterson (2007) found a good relationship between vulnerability to uprooting and tree species/features (mainly tree diameter, wood properties) in several natural forests in North America. Generally, coniferous trees are more susceptible to windthrow, with tree resistance to uprooting negatively correlated with their diameter at breast height. For data regarding other influencing factors such as topography or forest stand structure, see also reviews by Everham and Brokaw (1996) and Mitchell (2012). After tree uprooting, soil horizons are extensively disturbed by: (1) the overturning of the tree and soil mass upheaval above the ground in root plate form, (2) the mixing or partial inversion of horizons during mound formation (Schaetzl, 1986) and (3) the subsequent erosion of the tree-throw mound and concentration of rock fragments within the topsoil and/or the surface (e.g. so called gravel armours; Small et al., 1990; Phillips et al., 2005). Rock fragment veneers derived from coarse rock fragments falling forwards from the root plate beyond the pit area are of similar origin (Osterkamp et al., 2006). An equally important modification made to soil mantles caused by tree throw is the uneven redistribution of soil thickness across a hillslope (Gabet and Mudd, 2010) in the form of pit-and-mound microtopography, which can persist for more than 2000 years (Schaetzl and Follmer, 1990), although more frequently for only 100–200 years (Šamonil et al., 2009; Šebková et al., 2012) and up to 300 years (Denny and Goodlett, 1956, see review by Šamonil et al., 2010a). So far, the highest documented age based on 14C dating was recorded in Michigan, USA, and exceeds 6000 years (Šamonil et al., 2013). The reported variation in pit-mound longevity depends on climatic conditions and site characteristics; on well-drained sandy Podzols such topography can persist for more than

257

a thousand years (Šamonil et al., 2010a). All the potential effects of tree uprooting on soils are summarized in Table 2. A final issue to consider is the contribution of tree uprooting and sediment transport as a self-reinforcing mechanism. Compared to pits and undisturbed sites, tree-throw mounds are more frequently and easily occupied by the next generation of trees and if uprooted lead to further downslope transport of soil material from the mounds (Lyford and MacLean, 1966; Kabrick et al., 1997). The same factor, i.e. tree growth on mounds, causes their conservation (stabilization), whereas the adjacent pits are filled much more rapidly. 4. Trees and superficial processes on hillslopes 4.1. Trees and accumulation processes Trees, either alive or dead – standing (snags) or lying (logs) – can prevent and delay the downslope movement of mineral and organic matter via the physical obstruction of particle transport on hillsides (Fig. 7). In a similar manner they also affect surface runoff (Harmon et al., 1986; Maser et al., 1988), protect hillslope surfaces against erosion (Raška and Oršulák, 2009) and contribute to soil stabilization by controlling the flow of water, soil and litter across the forest floor (Stevens, 1997). Such functions, according to biogeomorphic nomenclature, can be considered bioprotective (Naylor, 2005). An important variable accelerating soil material accumulation within a hillslope is the number of snags and logs per unit area. The amount of logs accumulated on the forest floor is a function of tree uprooting and tree snapping intensity (Harmon et al., 1986; Maser et al., 1988; Stevens, 1997) – modulated in most cases by strong wind events (Everham and Brokaw, 1996; Schelhaas et al., 2003) – and is thus positively correlated with the amount of trapped sediment, assuming that the tree trunks preferentially fall and/or lie perpendicularly to the slope profile, i.e. along the contour. However, according to this author's knowledge, the latter relationship has not yet been verified, and indeed virtually not studied. Soil particles in transport accumulate on the upslope sides of trees due to various processes such as sheet wash, soil creep, debris flow and shallow landsliding. Although the amount of sediment can be calculated (Measeles, 1994), interpretation is probably restricted to the cumulative effect of all potentially active processes within the forested hillslope. Coarse woody debris which lies parallel to contour lines plays a similar function; however, because there is no root anchoring in bedrock and/ or regolith, soil creep cannot be measured in this case. Another important source of weathering material deposited on the upslope side of trees, snags and logs is the contemporaneous development of frostriven cliffs enhanced by vegetation (tree roots causing the disintegration of frost-riven cliffs) (Raška, 2007). Recently, the geomorphological role of trees has been analyzed using the concepts of sedimentation traps (Matyja, 2007) and dam-like effects (Raška, 2007; Raška and Oršulák, 2009); however, detailed description of the damming effect of trees in fact dates back to 1968 (LaMarche, 1968). In general and conceptual models, both upper and lower forest belts have traditionally been considered as accumulation zones where sediments from upper mountain zones (subalpine and alpine) are trapped and stored. Such a conclusion was reached for the Karkonosze range of the Sudety Mts., SW Poland, after a study of mechanical denudation in the region (Bieroński et al., 1992). However, the authors did not consider the biotic components of hillslope environments; for instance, the tree uprooting process (as a source of mechanical denudation and biogenic transport) was not included in their analysis. In contrast, Swanson et al. (1982a), after referencing published rates of transport due to tree throw (between 1.5 and 2.0 mm year−1), concluded that the latter are comparable in magnitude to soil creep measurements on forested hillslopes and should thus be taken into account in sediment budgets (p. 8). Accumulation processes were recognized in the Tatra Mts., Western Carpathians, where they were described as biological aggradation by

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Fig. 7. Examples of the configuration between trees, relief and surficial processes causing transport of rock particles (figure not yet published).

Jahn (1979), who discovered a change in the type of sedimentation in front of a dwarf pine clump from debris to sand and supposed it to be caused by the regeneration of plant cover on the upslope side. Another case study focused on a 2002 debris flow that occurred in the Babia Góra Massif, Beskidy Mts., S. Poland. This study revealed that trees (standing or decaying) and the coarse woody debris of dead tree trunks lying on the hillslope influenced transport and depositional processes. These natural elements of the slope surface not only caught part of the debris flow material, reducing its energy and range (Matyja, 2007), the coarse woody debris also formed a step-like surface on the slope, increasing surface roughness and again affecting flow. Assessing the time necessary for complete decomposition of a tree trunk, such an effect could last for up to 100–150 years (Matyja, 2007) and potentially even more than 400 years (the time of decomposition of the Douglas Fir; Maser et al., 1988). The above examples permit the formulation of the general conclusion that trees temporarily modify transport and sediment flux on hillslopes, simultaneously affecting their long-term transfer between hillslope and river valley geomorphic systems. Furthermore, trees are also associated with the direct addition of mass to river channel systems via coarse woody debris, litter and organic matter of various types (Reid, 1981; Marston, 1982; Swanson et al., 1982b). For instance, coarse woody debris can control erosion-sedimentation processes and thus changes in river channel morphology. 4.2. Trees and slope stability It is widely accepted and proven that vegetation, especially forest, can stabilize steep slopes (Ziemer, 1981; Marden, 2004; Rickli and Graf, 2009). This issue has been comprehensively reviewed by O'Loughlin (2005), who pointed to: 1) the modification of soil moisture and porewater pressure by trees, 2) the formation of a protective permeable organic forest floor layer and an increase in soil conductivity, enhancing soil drainage (e.g. via the rapid transmittance of water through decayed tree roots) and 3) the mechanical reinforcement of soil by tree roots. The latter function, which is enabled through root anchoring in bedrock, crossing zones of weakness in the soil and providing long fibrous binders within a weak soil mass (Ziemer, 1981; Roering et al., 2003), is recognized as the most important beneficial effect of vegetation on slope stability (Kuriakose and Beek, 2011). Such beneficial effects of trees are mostly recognized in shallow soils (Montgomery et al., 2000; Gabet and Dunne, 2002). Rickli and Graf (2009) documented more than 500 shallow landslides (b2 m deep) after heavy rainfall events in Switzerland, concluding that shallow landslides were less frequent in forest than in open land. However, when forest condition had decreased, for instance due to a past disturbance event (e.g. windthrow, fungi disease or bark beetle outbreak), a markedly higher intensity of landslides was observed. Although especially visible on steeper slopes, these landslides were caused mainly by a reduction in soil cohesion due to loss of root strength

(Selby, 1993; Johnson and Wilcock, 2002; Rickli and Graf, 2009). Clearcutting has a similar effect on root strength and slope stability, after which landslide frequency also increases (e.g. Montgomery et al., 2000). For instance, it has been hypothesized that 90% of root reinforcement is lost within 9 years after logging (Ziemer, 1981) (Fig. 8). Notwithstanding this, in some natural environments (i.e. not anthropogenically altered) trees can lead to the destabilization of slopes while the latter are oversaturated with rainwater. Such conditions frequently appear in the tropics where trees and dense root mats enhance infiltration and cause an increase in pore-water pressures (Thomas, 1994). Another stabilizing phenomenon is connected with tree uprooting, which leads to surface or near-surface rock fragment layering. Layers such as gravel armours or rock fragment veneers limit erosion and thus stabilize hillslopes (Schaetzl and Follmer, 1990; Osterkamp et al., 2006). Similarly, coarse woody debris inhibits downslope sediment transport and erosion below tree trunks. However, some authors have documented a very interesting dualistic effect of forests, through which the tree uprooting process also contributes substantially to hillslope sediment flux (e.g. Hughes et al., 2009; Constantine et al., 2012). Another two-fold effect is known to promote the occurrence of mass wasting due to tree uprooting in short-term intervals, but in some cases this can be hindered by the widespread and continual thinning of soils by downslope transport in the long-term (e.g. Gabet and Mudd, 2010). One of the first reports documenting windthrow-induced debris avalanches (caused by a rapid decrease in the positive effect of root strength) came from Alaska (Swanston, 1967). Such results contradict the common opinion that forest cover stabilizes hillslopes and inhibits erosion. 5. Trees and the development of landforms and surface rock structures Although Dietrich and Perron (2006) were unable to define a topographic signature of vegetation or fauna on the Earth's surface, Gabet and Mudd (2010) have pointed out that pit-and-mound microtopography is one of the most spectacular forms resulting from biological activity and is clear evidence of the tree uprooting process. As the size of individual forms (Table 3) and the density of pit-mound pairs vary between sites, it is difficult to draw any firm conclusion regarding the general validity of this theory. However, such microrelief seems to be more pronounced and to last for longer on steeper slopes (Schaetzl and Follmer, 1990; Norman et al., 1995) and in temperate forests than in tropical forests (Putz, 1983). The density of pit-and-mound microtopography ranges between 50 (Cremeans and Kalisz, 1988) and 1200 (Lyford and MacLean, 1966) pit-mound pairs per hectare, with the features potentially persisting for up to 2500 years (Schaetzl and Follmer, 1990) or even more than 6000 years (Šamonil et al., 2013). Pit-and-mound topography has been found on mesoforms of various morphogenesis, including glacial features such as drumlins (e.g. Kabrick et al., 1997) and kames (Norman et al., 1995). The uprooting of trees,

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Fig. 8. A conceptual model of forested hillslope stability before and after a disturbance event. Source: after Selby (1993), Ziemer (1981) and Sidle (2008), modified.

subsequent deterioration of root plates and levelling of tree-throw mounds can lead to the formation of structures and forms traditionally attributed to other geomorphic activities, e.g. frost processes (see examples given e.g. by Embleton-Hamann, 2004). For instance, the erosion and levelling of mounds have been found to leave rock fragment structures similar to patterned ground (Denny and Goodlett, 1968). Whereas this is a direct imprint of mound flattening by surficial processes, pitand-mound microtopography itself was in the past incorrectly interpreted as the result of contemporaneous congelifluction (Dylikowa, 1956) or frost processes of Pleistocene or Holocene age (e.g. Denny and Goodlett, 1956; Embleton-Hamann, 2004). Structures similar to patterned ground were documented by Phillips and Marion (2006), Fig. 8, p. 245) and attributed to tree root expansion, rock fragment displacement and stump hole infilling. Some authors have suggested that whereas the development of gravel armour reflects the complete erosion and elimination of any topographic expression of the treethrow mound (Small et al., 1990), rock fragment veneers can develop via root throw on immature soils (Osterkamp et al., 2006). Finally, although tree uprooting in the short-term disrupts layering and horizonation, on a longer time scale it produces distinct surface or near-surface layering due to continual mining of rock fragments from the subsoil (Phillips and Lorz, 2008).

A number of authors have suggested that trees can contribute to the development of larger landforms. For instance, it has been hypothesized that due to the biomechanical and biochemical action of their roots, trees can influence tor formation in karst regions. Gams (1966) proposed a two-stage formation process for tors in the Dinaric and Alpine Karst of Slovenia, comprised of 1) deep subsurface weathering followed by 2) denudation of weathered material and excavation of tors. This hypothesis has probably never been thoroughly studied, nor has that of weathering pit formation (known as oriçangas in Portuguese) under trees, as proposed by Freise (1938). Moreover, trees, through their root systems, disintegrate rock fragments and widen fissures in rock faces and bedrock (see Fig. 3). As Jackson and Sheldon (1949) observed: “There are good reasons for believing that these woody plants, (…), play an important part in detaching rocks from the edge of the cliff, so causing recession of the cliff face”. The authors also pointed out that the action of the trees was quickened by the properties of the rock cliff in question (soft limestone with numerous joints and bedding planes). However, as with tor and weathering pit formation, this process has not been intensively studied (despite the fact that it is frequently observed). This general remark may also be applied to all potentially active biomechanical processes in mountain environments (see also Yatsu, 1988). In trying to explain such neglect, one could point to: 1) problems regarding the

Table 3 Reported mean treethrow mound and/or pit volumes. Mean mound volume (m3)

Mean pit volume (m3)

Place

Tree species or forest type

Altitude (m a.s.l.)

Reference

-

0.7

Colorado, USA

2900

Osterkamp et al., 2006

0.6⁎ 0.2⁎

0.2⁎ –

Wisconsin, USA New York, USA

– –

Kabrick et al., 1997 Denny and Goodlett, 1968

– 2.2–3.0⁎ – 1.7 2.9

3 ± 1.3 1.3–1.9⁎ 0.8 1.6 2.2

USA N Iran Blue Mountains, Australia Stołowe Mts., Poland Outer Western Carpathians, Czech Republic

Pine (Pinus ponderosa and P. contorta), Douglas fir (Pseudotsuga menziesii) and aspen (Populus tremuloides) Sugar maple-basswood forest The area was forested in presettlement times only. Douglas-fir Temperate forest of Mazandaran Province Eucalypt Fertile mountain beech forest Natural fir-beech forest

– 100–1700 – 600–760 600–800

Gabet and Mudd, 2010 Kooch et al., 2012 Richards et al., 2011 Pawlik et al., 2013 Šamonil P., unpublished data

⁎ Values calculated using data from the cited reference. For this purpose the equation from Norman et al. (1995) were used.

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availability of tools allowing for quantification of process magnitude e.g. for each type of rock and tree species and 2) measurement difficulties. In terms of the latter, it may be difficult, for instance, to distinguish the effects of tree root action on rock fissures from co-existing frost weathering. 6. Geomorphic processes and trees — attempts at quantification The main aim of many analyses is not only the qualitative description of processes, but also an attempt at their quantitative expression. As trees are involved in many different hillslope processes, various ways (both measurement methods and physical models) of quantifying these processes have been developed. The most important of these methods include: 1. Calculation of the root plate volume of uprooted trees; 2. Calculation of soil displacement due to tree root growth; 3. The damming effect of trees, erosion and exposure of roots. 6.1. Calculation of the root plate volume of uprooted trees So far, relatively few researchers have tried to measure and calculate rates of transport caused by tree uprooting, i.e. the amount of soil material upheaved along with the root system of a fallen tree onto the ground surface (e.g. Kotarba, 1970). However, such information plays a crucial role in any attempt at the measurement of watershed sediment balance. Recently, established methods developed for measuring root plate volume were reviewed by Richards et al. (2011). The most frequent method of calculation was based on the formula for half of an ellipsoid in which the axes are not equal, which is a close approximation of the shape of a root plate (Dąbrowska, 2009; Rojan, 2010; Pawlik, 2013). This equation can be written as follows:

so-called partitioning method. Along a baseline, the pit is hypothetically partitioned into rectangular prisms (the dimensions of which are noted), with the latter's volumes then calculated and summed (p. 1244). However, this method cannot be applied if: 1) there is no typical pit but only a shallow uncovered surface, 2) the root plate is partly or entirely placed within a pit (e.g. complex treefall with backward displacement; Schaetzl et al., 1989a) and 3) treefall was incomplete (Beatty and Stone, 1986). Others have proposed a method of root plate volume estimation from DBH (diameter at breast height), based on a linear regression model (Burns and Tonkin, 1987). The authors found statistically significant relationships between DBH, root plate volume and the dimensions of the resulting pits and mounds. In a similar manner, as shown in Figs. 8 and 9, the volume of upheaved soil material can be approximated by measuring pit and mound dimensions, with the latter's volume then calculated using the equation for half of an ellipsoid Eq. (2) (Norman et al., 1995; Kabrick et al., 1997) (Fig. 11) or quarter of an ellipsoid (not shown here; Putz, 1983). This method can be simplified if we assume that width and height are equal (Gabet and Mudd, 2010), then: V¼

2π 2 r d 3

ð4Þ

where w is width, h is height and t is depth of the root plate (Fig. 10). Richards et al. (2011) also proposed a new method with which to estimate the displaced soil volume by measuring pit volume, using the

where r and d are pit radius and depth, respectively. Although pits are normally filled to some extent with organic and mineral matter, and mounds already partly levelled, this method seems to be much more precise in estimating upheaved soil volume than does the direct measurement of soil material in root plates, because it does not take into account the volume of root wood (normally already decomposed). Although this part of a root plate is commonly not excluded because of difficulties associated with its volume assessment, calculation attempts have been made. According to some authors, the root volume can be derived from tree diameter measured at breast height (DBH) or stump height (Santantonio et al., 1977; Thies and Cunningham, 1994), while others have attempted to calculate the volume of tree wood that penetrates into the root plate or continues through it as a taproot. This latter parameter was determined based on the cylinder volume equation, i.e. Vw = πr2p, where r is the radius of the tree trunk and p is the penetration depth of the trunk into the soil-root plate (Richards et al., 2011). The authors also calculated the volume of both large (N1.5 cm in diameter) and fine roots; together with Vw these three values were then subtracted from the pit volume calculated using the partitioning method described above. The above information is essential in order to use the model of sediment transport via tree uprooting proposed by Gabet et al. (2003), which was later developed into a bio-geomorphic model of soil production by Gabet and Mudd (2010) and incorporated into a biologically-

Fig. 9. Main dimensions of a root plate used for volume calculation. Author's original work.

Fig. 10. Definition of root plate dimensions proposed by Reid (1981, p.153).

V¼



 1 4 w h  π  d 2 3 2 2

ð1Þ

V¼

πwhd 6

ð2Þ

where w is width, h is height and d is depth of the root plate (Fig. 9). Other methods have also been used, such as the equation for the volume of a cylinder (e.g. Beatty and Stone, 1986) or for a cylinder with a missing segment (Reid, 1981) (Fig. 10). In this case the equation is written as shown below: 8 0 1 2 39 > ffi > = < w2    qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi  2  ðw−hÞC w −1 B 6 w 2 7 −ðw−hÞ  w −hw −ðw−hÞ2 5 −4 cos @1− w A− > 2 2 ; : 2 2

V ¼ t>π

ð3Þ

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6.3. The damming effect of trees, erosion and exposed roots

Fig. 11. Main dimensions of a pit and mound essential for their volume calculation as illustrated by Norman et al. (1995), modified.

based forest-gap model (Constantine et al., 2012). Additional relevant data include event frequency (number of trees toppled per hectare per year) and slope angle. 6.2. Calculation of soil displacement due to tree root growth

Trees are natural obstacles to washed or gravitationally-transported rock particles of different fractions. The highest amount of trapped sediment on the upslope sides of coarse woody debris or standing trees is typically observed in the close vicinity of rock faces on scree slopes, especially when the supply of fresh material is quite intense (Fig. 12). However, because they are constantly under physical stress, trees may exhibit lower growth rates or much faster decay rates than under normal conditions on a forested slope. The amount of material accumulated against standing trees or coarse woody debris can be quantified. For logs lying perpendicular to the slope profile such an attempt was made by Raška and Oršulák (2009), who employed a method developed for the quantitative assessment of total accumulation rate (TAR), with results of 1 to 4 m3 observed. A similar attempt at quantification by Measeles (1994) used standing trees and stumps. However, in this case the amount of trapped sediment was interpreted only as an effect of soil creep, with values of up to 0.12 kg m−1 year−1 recorded. The author also provided no detailed information regarding the formula used. Tree roots are frequently observed above ground, a phenomenon often interpreted as an effect of their erosion and simultaneous exposure. Although this assumption allows for assessment of the amount of eroded material (e.g. Dunne and Dietrich, 1982), certain constraints should be taken into account (Dunne et al., 1978). A detailed description of the method was provided by LaMarche (1968), who pointed to the following limitations: 1) near-surface roots may be uncovered, which is the result of an increase in diameter over time, 2) the depth of soil removed before the roots are uncovered is not known and 3) asymmetrical root exposure. The last limitation corresponds to a gently sloping terrace on the uphill side of a tree, as well as a concave hollow on the downhill side. Such a configuration is especially common on steep slopes covered by old trees, and causes large discrepancies between the maximum depths of root exposure and actual slope degradation (LaMarche, 1968). Some trees are able to develop aboveground roots (aerial roots) as a response to site conditions and the effect of morphological and structural adaptation e.g. within floodplain forest (Wittmann and Parolin, 2005). All the above-mentioned problems must be taken into account and caution must be kept when choosing an appropriate site and trees for measurement.

Another attempt at quantification of tree growth and mortality is based on the observation of tree roots that expand horizontally and vertically, causing physical displacement of soil mass. After root decay and disappearance, the soil material collapses vertically (Gabet et al., 2003, Fig. 1, p. 253) and thus contributes to net downslope soil flux (qsx) Eq. (5). The variables included in the calculation are as follows: 1) rooting depth, 2) root turnover time (growth and dieback of roots), 3) density of root material and 4) slope angle. A general slope-dependent equation calculating soil mass displacement via root growth and decay was proposed by Gabet et al. (2003):

qsx ¼

xrτ ρr

ð5Þ

where x (m) is the net horizontal displacement of soil, r (kg m−2) is the root mass per unit area, τ (year−1) is the root turnover rate and ρr (kg m−3) is the density of root material. Not only do the root turnover rate and the density of root material vary between different ecosystems and tree species, these values also change significantly throughout the year, especially for fine roots, the growth and decay times of which can be very rapid. For instance, in an oakwood ecosystem, fine-root biomass increased from nearly 13 t ha−1 in April to 21 in July, then decreased to 10 t ha−1 in November (Santantonio et al., 1977).

Fig. 12. Pile of loose material accumulated on the upslope side of a tree. This tree is growing on a scree slope in close vicinity to a rock face built of mudstones in the Stołowe Mts., Sudety, SW Poland. The material is derived from weathering processes deteriorating the rock face.

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2010), 8–10 times (1250 years; Šamonil et al., 2009) or up to as many as 30 times (300–500 years, Denny and Goodlett, 1956) (but see Norton, 1989, for a different methodological approach). A similar attempt at calculation of turnover time has been made with respect to the formation and infilling of stump rot depressions (pits) and soil displacement via root growth (Phillips and Marion, 2006). The authors found that given an average disturbance density of 23 m2 ha−1 in the case of stump rot pits, the time required for the entire surface to be affected was around 43,000 years, compared to around 6000 years for soil displacement with a disturbance density of 160 m2 ha−1 (Phillips and Marion, 2006). These data suggest that the above processes can have a cumulative effect on the geomorphology of forested hillslopes and should not be neglected if we expect our results to be close to reality. 7. Potential directions for future research — conclusions and final remarks Fig. 13. General scheme of feedbacks and interactions between a forest ecosystem and the geomorphic domain of a forested hillslope (previously unpublished).

Recently, Raška and Oršulák (2009) presented the following formula allowing for the quantification of eroded material: TER = [(ER + x) ∗ rr] − (x ∗ rr) / 2, where ER = [hd − (td ∗ tgα)] / 2, x = rr * tgα − er, TER is total erosion rate, ER is erosion rate, hd is the difference in height between downslope and upslope ground level at the tree trunk, td is trunk diameter, α is slope inclination and rr is root radius. Another method of solving the same problem was described by Gärtner (2007), who employed a dendrochronological approach focused on changes in the anatomical structure of the annual rings of exposed roots. This latter technique seems to be unburdened by the limitations associated with the traditional approach. With the use of dendrochronological methods, it is also possible to detect episodes of faster soil creep, which is frequently evident in the shape of bent trees (Braam et al., 1987). Via treecoring, a set of proxy data regarding the moisture conditions of the hillslope can be obtained, since soil creep is more intense during periods of higher rainfall (e.g. Pawlik et al., 2013). 6.4. Trees and palaeogeographical reconstructions After collecting basic information regarding the present-day magnitude and frequency of processes caused or modified by trees, certain palaeogeographical reconstructions can be carried out. The palaeogeographical significance of trees can be considered on two levels: 1) individual trees and 2) forest ecosystems. It has been suggested, for instance, that forest soils have been extensively mixed during the Holocene and historic time periods (Phillips et al., 2005). If so, the following features of montane forests must be taken into account: 1) the spatial extent of trees and migration of the timber line and 2) changes in forest type and tree species, with a potentially different impact on slope cover (e.g. deeply rooting species vs. trees with shallow root systems). Together with the migration of trees onto previously unvegetated areas, a slope may be affected simultaneously by the new factor of mechanical and chemical weathering directly attributed to the root zone. It seems that during the Holocene, the most important process has been tree uprooting leading to the rapid transport of rock mass. Some authors have pointed to the possibility that uprooting-derived whole slope surface remodelling may have occurred not only once, but even several times in the last 10,000 years at some sites, up to a depth of at least 1 m. The rate of such forest turnover (the time required for an area equivalent to 100% of the ground surface to be uprooted; Phillips et al., 2008b) for the Holocene could be between 2–3 times (every 3751–5000 years; Brewer and Merritt, 1978; 2777 years; Lenart et al.,

Traditionally, trees are viewed as a stabilizing factor for slopeweathering cover against mass movement, with forests considered a vegetation formation inhibiting surface wash and erosion. However, given the contribution of individual trees to rock weathering, sediment transport and accumulation are still rather apparent, even if quantitative methods are limited and field study poses many difficulties. This thesis is supported by a body of literature that is particularly focused on biomechanical and biochemical weathering, with the accumulative and bioprotective functions of trees less well-studied. At present, the most important issue is the integration of the efforts of different disciplines towards achieving an explanation of forested hillslope dynamics based on variation in forest ecosystem changes, incorporating forest ecology, palynology, anthracology, dendroecology etc. and research methods adequate for each (White, 1979; Pickett and White (Eds.), 1985). The general assumptions of the proposed approach are shown in Fig. 13. A similar effort has already been made to bridge the gap between pedology and forest ecology (Šamonil et al., 2010a), as well as between climate change and sediment transport via tree uprooting (Constantine et al., 2012). The patterns and interactions shown in Figs. 1 and 13 point to integration on different levels of complexity, with the problems analyzed in terms of forest ecology having a fundamental effect on forested hillslope dynamics. At the same time the biology of individual trees has to be considered, as this factor plays a key role in the development of e.g. rock faces, weathering front migration and changes in soil biomantles within upper and lower forest belts. In this context, forms and sediments depend in large measure on the extent (horizontal and vertical), volume and structure of the root system, as well as the processes active in the root zone and rhizosphere. As a general overview, it is suggested that the end product of root activity, together with the presence of trees in different states of growth and decay within a forest community, should be viewed as a potential source of matter supplying the geomorphic system as a whole. Finally, one of the most urgent objectives is to utilize knowledge (even if still limited) regarding contemporaneous processes and forms influenced by trees for palaeogeographical reconstruction, with special attention focused on the Holocene. Acknowledgements I would like to thank Dr Ian E. Evans (Durham, U.K.), Dr Pavel Šamonil (Brno, Czech Republic) and M.S. Krzysztof Urbaniak for their many invaluable comments and suggestions which considerably improved the quality of this paper. Insightful comments from two anonymous reviewers are gratefully acknowledged. I also thank Dr Pavel Šamonil (VUKOZ, Brno, Czech Republic) for giving me permission to use his database from the Razula National Nature Reserve, Czech

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