Regolith properties under trees and the biomechanical effects caused by tree root systems as recognized by electrical resistivity tomography (ERT)

    Regolith properties under trees and the biomechanical effects caused by tree root systems as recognized by electrical resistivity tomography (ERT) Łukasz Pawlik, Marek Kasprzak PII: DOI: Reference:

S0169-555X(17)30417-8 doi:10.1016/j.geomorph.2017.10.002 GEOMOR 6188

To appear in:

Geomorphology

Received date: Revised date: Accepted date:

13 March 2017 4 June 2017 1 October 2017

Please cite this article as: Pawlik, L  ukasz, Kasprzak, Marek, Regolith properties under trees and the biomechanical effects caused by tree root systems as recognized by electrical resistivity tomography (ERT), Geomorphology (2017), doi:10.1016/j.geomorph.2017.10.002

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ACCEPTED MANUSCRIPT Regolith properties under trees and the biomechanical effects caused by tree root systems as recognized by Electrical Resistivity Tomography (ERT) 1,2,*

Łukasz Pawlik, 3 Marek Kasprzak

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Pedagogical University of Cracow, Institute of Geography, ul. Podchorążych 2, 30-084 Kraków, Poland 2 University of Silesia, Faculty of Earth Sciences, ul. Będzińska 60, 41-200 Sosnowiec, Poland 3 Wrocław University, Institute of Geography and Regional Development, pl. Uniwersytecki 1, 50137 Wrocław, Poland

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*Corresponding author: [email protected] (Łukasz Pawlik), +48782859870

ACCEPTED MANUSCRIPT Regolith properties under trees and the biomechanical effects caused by tree root systems as recognized by Electrical Resistivity Tomography (ERT)

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Abstract: Following previous findings regarding the influence of vascular plants (mainly trees) on weathering, soil production and hillslope stability, in this study, we attempted to test a hypothesis regarding significant impacts of tree root systems on soil and regolith properties. Different types of impacts from tree root system (direct and indirect) are commonly gathered under the key term of “biomechanical effects”. To add to the discussion of the biomechanical effects of trees, we used a non-invasive geophysical method, electrical resistivity tomography (ERT), to investigate the profiles of four different configurations at three study sites within the Polish section of the Outer Western Carpathians. At each site, one long profile (up to 189 m) of a large section of a hillslope and three short profiles (up to 19.5 m), that is, microsites occupied by trees or their remnants, were made. Short profiles included the tree root zone of a healthy large tree, the tree stump of a decaying tree and the pit-and-mound topography formed after a tree uprooting. The resistivity of regolith and bedrock presented on the long profiles and in comparison with the short profiles through the microsites it can be seen how tree roots impact soil and regolith properties and add to the complexity of the whole soil/regolith profile. Trees change soil and regolith properties directly through root channels and moisture migration and indirectly through the uprooting of trees and the formation of pit-and-mound topography. Within tree stump microsites, the impact of tree root systems, evaluated by a resistivity model, was smaller compared to microsites with living trees or those with pit-and-mound topography but was still visible even several decades after the trees were windbroken or cut down. The ERT method is highly useful for quick evaluation of the impact of tree root systems on soils and regolith. This method, in contrast to traditional soil analyses, offers a continuous dataset for the entire microsite and at depths not normally reached by standard soil excavations. The non-invasive nature of ERT studies is especially important for protected areas as it was shown in the present study. Keywords: biomechanical weathering, tree roots, soil, regolith, electrical resistivity tomography, ERT, hillslope, Beskidy

ACCEPTED MANUSCRIPT 1. Introduction

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Trees are powerful geomorphic, pedogenic and ecological features (Šamonil et al., 2010a, b, 2014; Pawlik, 2013; Pawlik et al., 2016a, b; Sullivan et al., 2016; Brantley et al., 2017; Hasenmueller et al., 2017), and since at least the Devonian, trees have had significant influence on many components of the natural environment e.g. slope hydrology, soil properties and stability, climate, sediment flux, etc. (Rettalack et al., 1985; Algeo et al., 1995). Based on the existing literature and our own experience we distinguished four fields of interest linked to geomorphic and pedogenic role of trees and their root systems (which are directly or indirectly connected to our present study):

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1. Biomechanical and biochemical weathering through deeply rooted vascular plants. According to the Devonian Plant Hypothesis, it is assumed that biomechanical and biochemical weathering through deeply rooted vascular plants drove the initial soil development and climate change, primarily through intense mineral weathering (mainly silicates) and the bounding of large amounts of atmospheric CO2 followed by climatic cooling in the Late Devonian (Algeo and Scheckler, 1998; Retallack, 2001; Beerling and Berner, 2005; Algeo et al., 2001; Le Hir et al., 2011; Goudie and Viles, 2012). At present this issue in relation to soil and regolith evolution is intensively investigated under the Critical Zone paradigm (Pawlik et al., 2016a; Shouse and Phillips, 2016; Brantley et al., 2017; Hasenmueller et al., 2017).

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2. Bioprotection which currently is considered as one of the most important functions of trees. It means protection of soils against erosion, surface wash and mass wasting, and therefore contribute to the stabilization of hillslope surfaces (e.g. Nilaweera and Nutalaya). Bioprotection is important in terms of geohazards and applied geomorphology (e.g. Vergani et al., 2017).

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3. Soil spatial complexity as a derivative of tree/soil feedbacks. Several studies mention soil’s spatial complexities, which are at least partly influenced by the presence of trees through, for example, stemflow and stemwash (Levia Jr. and Frost, 2003), and/or biomechanical and biochemical processes associated with their living functions, disturbances, mortality and decomposition (Zinke, 1962; Daněk et al., 2016; Shouse and Phillips, 2016; Stutz et al., 2017). Trees are able to modify the structure of soil and regolith by displacing material with their root growth (Phillips and Marion, 2006; Pawlik, 2013). Tree roots also influence the formation of root channels and root casts (Gaiser, 1952). Several aspects of soil complexity (physical redistribution, chemical changes) were considered as a result of tree uprooting (Šamonil et al., 2010b; Pawlik et al., 2013; Šamonil et al., 2014; Daněk et al., 2016). For instance, through tree uprooting trees lead to formation of pit-andmound topography, and through stem-flow and stem-wash form so called “basket” or “egg cup” Podzols (Bloomfield, 1953; Schaetzl, 1990). 4. Circulation of matter and energy. By binding large amounts of atmospheric CO2 in their bodies through photosynthesis (Anderson-Teixeira et al., 2015) and by releasing root exudates, enzymes and organic acids below the ground in the root zone (Hodge and Berta, 2009), trees modify the circulation of matter and energy. Although many of the potential effects of trees and their root systems have been recognized, their monitoring and documentation is still limited, especially in the case of roots – the hidden part of trees (Gregory, 2006). Beside the lack of indirect observations, another constraint is environment protection laws which prevent soil studies and root excavations in protected areas. In this paper, through the application of electrical resistivity tomography (ERT), we aim to contribute to the discussion on the abovementioned four fields of interest with the main focus on biomechanical role of tree root systems. ERT is one of the most popular geophysical methods applied to geological, geomorphic and pedogenic studies (Samouëlian et al. 2005, Van Dam 2012, Furman et al. 2013). Although geoelectrical imaging methods are not new, having originated at the start of the 20th century (Bevan 2000, Meunier 2012), the

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application of ERT in various studies has significantly increased in recent years. The primary reason for this use has been the rapid development of computer software dedicated to ERT data processing. Electrical resistivity imaging is more often applied to the study of small scale structures of geologic bedrock. For example, if the imaging resolution is suitable, it can be used to study bedrock outcrops (Roqué et al., 2013), monument deterioration (Mol and Viles 2010; Saas and Viles 2010), the stability of flood banks (e.g., Jones et al., 2014) and to recognize the internal properties of patterned grounds (Kasprzak, 2015). The method was also pioneered and satisfactory applied to study pit-and-mound microtopography (Pawlik and Kasprzak, 2015).

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Within the framework of the present study, we tested a hypothesis regarding the influence of tree root systems on soil and regolith properties due to biomechanical effects (Johnson, 1993; Phillips and Marion, 2006; Pawlik et al., 2016a). Here, biomechanical weathering is considered as a primary factor of change and the process can include: 1) root growth in bedrock and regolith (in fissures and rock openings) (Phillips, 2016), 2) soil material pushed out by growing roots (sometimes tree root mounds are formed) (Hoffman and Anderson, 2014). Other sources of direct and indirect impact of roots can also be mentioned: 1) water content due to stemflow and water conductivity through root channels (Phillips et al., 2017) and 2) soil organic matter content from fine root turnover, mycorrhiza and microorganisms in the rhizosphere. Biomechanical effects were analyzed by application of ERT in the following three configurations:

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1. no disturbance: large living trees with potentially healthy root systems (healthy trees without visible signs of disease and/or decay),

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2. disturbance of mature trees: decaying tree stumps of broken or cut large trees with roots left in situ in the ground,

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3. disturbance of tree and soil: pit-and-mound microtopography formed by tree uprooting and the subsequent decomposition and deterioration of a root wad.

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Up till now between the three cases mentioned above the most studied is the case of living trees with potentially healthy root systems. However, the existing studies were predominantly focused on agricultural functions of soils and roots, and, in that context such important properties as moisture content and root biomass were evaluated most frequently (Amato et al., 2008; Zenone et al., 2008; Rossi et al., 2011). Pit-and-mound topography is a special case of imprinting made by trees within soils or on hillslope relief and is induced by external disturbances that lead to tree uprooting (Šamonil et al., 2010a; Pawlik, 2013). Tree-throw pits mirror, through their volume and size, the morphometric properties of the former root systems (Kotarba, 1970; Gabet et al., 2003). These change over the long term as these microsites are filled with soil material and organic matter (Pawlik et al., 2016b). By conducting field studies in areas characterized by limited human intervention in order to minimize anthropogenic impact to soils, we attempted to answer the following research questions: 1) what are the geoelectrical properties of soils under trees and within tree-affected microsites?, 2) can we map the biomechanical effects of roots based on ERT?, 3) is ERT an appropriate method for surveying the impacts of tree root systems on soils, regolith and bedrock? In a separate section we also discuss the advantages and disadvantages of the ERT method in tree root system studies.

3. Methods 3.1. Description of the study sites All measurements were made in three distinct mountain ranges that belong to the Polish section of the Outer Western Carpathian Mountains. The highest peak of the mountain range is Babia Góra (1725 m a.s.l.), and the dominant geological substratum is flysch (Stupnicka, 1989). The entire area of the Flysch Carpathians is known for its susceptibility to landslides which, together with floods, pose the most serious threat to humans in the area (Starkel, 2006; Jania and Zwoliński, 2011). Within this region, we chose three sites with forest ecosystems that are under strict protection as nature reserves and have not been affected by forest management for at least the past 20-60 years:

ACCEPTED MANUSCRIPT 1) Żebracze in the Sądecki Beskidy Mountains (part of the Poprad Landscape Park), 2) Turbacz in the Gorce Mountains (part of the Gorce Mountains National Park) and 3) Oszast in the Żywiecki Beskidy Mountains (part of the Żywiecki Landscape Park) (Fig. 1, Table 1). Figure 1. Locality of the study sites within the Outer Western Carpathian Mountains.

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3.1.1. Żebracze Nature Reserve The Żebracze Nature Reserve (hereafter just Żebracze) was established in 1995 to protect remnants of the Carpathians old-growth forest and covers almost 45 ha at present (Fig. 2; Table 1). This reserve is situated on steep south-western hillslopes of Mt Wielka Bukowa (1104 m a.s.l.) between 670 and 1010 m a.s.l. and above the village of Szczawnik. The dominant tree species is beech (Dentario glandulosae-Fagetum) with an admixture of fir. The bedrock is predominantly built of sandstones and shales of the Magura Nappe (Paleogene), and the regional relief is strongly influenced by the structure of the Magura Nappe resulting in steep slopes, highly elevated ridges and deeply incised river valleys. The most recent relief has also been influenced by numerous landslides (Chrząstowski et al., 1993). The climate is humid with a mean annual precipitation of 720 mm (years 1986–1995) and mean annual air temperature of 6.4°C (www.imgw.pl). Figure 2. Żebracze study area with the main ERT profile (black line) and the zone of detailed investigation (white box). 3.1.2. Turbacz Nature Reserve The Turbacz Forest Reserve (hereafter just Turbacz) has an area of 319 ha and was first established in 1927 and again in 1964 after World War II (Loch, 2016). This reserve belongs to the Gorce Mountains National Park, which was established in 1981. Turbacz is situated in the Outer Western Carpathians region and from a geological viewpoint it belongs to the Magura Nappe and is built of sandstones and shales (Cieszkowski et al., 2015). The entire region of the Gorce Mountains has been affected by landslide events and our study site was partly disturbed by a large rotational landslide (not studied, of unknown age) (Fig. 3, Table 1). The dominant tree species are fir (Abies alba) and European beech (Fagus sylvatica L.) (phytosociological association Dentario glandulosae-Fagetum typicum). Soils are predominantly Cambisols. The climate of the Gorce Mountains is harsh with mean annual temperatures between 3°C and 6°C and an annual precipitation range of 800 mm to 1200 mm (www.gorczanskipark.pl). Figure 3. Turbacz study area with the main ERT profile (black line) and the zone of detailed investigation (white box). 3.1.3. Oszast Nature Reserve The Oszast Forest Reserve (hereafter just Oszast) was established in 1971 (fully protected from 2008), and it currently covers 46.27 ha. It is located in the Żywiecki Beskidy Mountains in the Wielka Racza mountain range. The Oszast reserve is located on the northern side of Mt Oszast (1147 m a.s.l.) (Jaworski and Pach, 2014; Wilczek et al., 2014). The entire reserve has been remodeled by a large complex landslide (Fig. 4, Table 1). According to climatic regionalization, the mean annual temperature ranges from 2°C to 4°C and the annual precipitation can reach 1300 mm (Hess, 1965). The dominant plant association in Oszast is Dentario glandulosae-Fagetum, with the main tree species being Abies alba (age range: 120-220 years), Fagus sylvatica (65-200 years) and Picea abies (170-220 years). The soil type has been classified as Endoeutric Cambisol (Jaworski and Pach, 2014). Figure 4. Oszast study area with the main ERT profile (black line) and the zone of detailed investigation (white box).

3.2. ERT – general description of the applied method ERT is an indirect method which gives proxy information about soil and rock properties including: 1) particle size distribution, 2) mineralogy, 3) porosity, 4) water content, 5) solute concentration and 6) temperature (Samouëlian et al. 2005). Because the ERT method is very sensitive to the amount of

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water in soil it is highly suitable for root zone mapping and monitoring (Furman et al., 2013). It combines depth-sounding and resistivity profiling (traversing). This type of induced polarization measurement involves the passing of an electric current through electrodes inserted into the ground (C1, C2), and the detection of the voltage difference using two other measuring electrodes (P1, P2). Knowing the current’s strength and the difference between the electrical potentials (voltages), it is possible to calculate the resistivity at a given point according to Ohm’s law: R = U / I, where R – resistivity of electrical conductor, U – voltage between two ends of conductor and I – current intensity. The calculated resistivity value is the average obtained from the distribution of the true resistivity of the analyzed rock body: ρa = k (ΔU P1P2 / I C1C2), where ρa – apparent resistivity [Ω∙m], ΔU P1P2 – difference in electrical potentials between electrodes P1 and P2, I C1C2 – current intensity emitted by electrodes C1 and C2 and k – geometrical factor (in meters) determined by electrode arrangement. The bedrock is not a homogenous body, thus the measured resistivity, being the ratio of voltage to current (taking into account a coefficient, k, which depends on the electrode array), is just the apparent resistivity (Reynolds, 2011; Loke, 2013).

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Some of the principles of the ERT method include (Samouëlian et al. 2005; Furman et al. 2013): 1. it uses artificially-generated electric currents and measures differences in the apparent resistivity of the soil.

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2. during measurements relatively low frequencies of the electric current of less than 100 Hz are applied.

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3. measured values of soil electrical resistivity can change from 1 Ω∙m to several 105 Ω∙m for dry soil overlying crystalline rocks.

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4. clays and shales because of higher surface of electrically charged particles are characterized by the best conductivity.

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3.3. Pattern of fieldwork measurements Each study area was surveyed using two methods, which enabled the examination of the general properties of the analyzed hillslopes, as well as a detailed study of the microsites affected by trees. With the first approach we aimed to recognize the main properties of the shallow geological bedrock at a scale of a large section of the analyzed hillslopes. With the second approach, we aimed to create a detailed geoeclectrical image of the bedrock and regolith at the specific locations. We therefore used different spatial resolutions in the ERT survey: 1. One long profile covering a significant part of a hillslope, reaching 189 m in length. In this case, we used 3 m spacing between the electrodes and employed the Wenner-Schlumberger method, which is considered universal, fast and enabling good detection of horizontal and vertical structures (Milsom, 2003; Reynolds, 2011). We made three long profiles, using one per study area. 2. Six short profiles with high spatial resolution, reaching 19.5 m in length and with 0.5 m spacing between the electrodes. In this case, the dipol-dipol method was applied in order to increase the number of measuring points (Milsom, 2003). Another advantage of the dipol-dipol array is the complete separation of the current and voltage circuits which reduces the vulnerability of inductive noise. Moreover, this type of configuration allows for deeper penetration of the current’s path (Reynolds, 2011). These profiles were made in two configurations at each study plot: along and perpendicular to the hillslope’s angle. Our study plots were of three different types: 1) large living healthy trees, 2) stumps of dead trees in different stages of decomposition and 3) pit-and-mound topography resulting from tree uprooting events.

The measurements were treated using the inversion procedure, which reduces the difference between the observed and resistivity curves to a minimum and is determined by a trial-and-error approach (Loke, 2013). The inversion procedure requires several iterations in order to minimize the errors of fit. We used ARES devices (GF Instruments, Brno, Czech Republic) and after the fieldwork stage, the

ACCEPTED MANUSCRIPT results were then processed using RES2DINV software (Geotomo, Malaysia). During processing a default smoothness-constrained inversion formulation was used (last squares inversion), and after applying iterative modeling techniques, the inverse models of the geological strata were produced. To visualize the inverse results graphically, logarithmic contour intervals were applied (Loke, 2013). For the sake of direct visual comparability of the values on the inverse models in our three study plots (trees, stumps, pit-and –mound topography), we used a unified color scale.

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4. Results

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During the current study, we made 21 ERT profiles: at each of the three study sites, there were one long profile measuring 189 m in length and three pairs of short profiles, with each measuring 19.5 m in length. Inverse models of the longer profiles consisted of 14 layers with resistivity points and 588 model blocks. The upper layer was located 1.5 meters below ground level, and the model did not image near the subsurface. The inverse models of the short profiles were composed of 14 layers and 356 model blocks. The electrical imaging started from a depth of approximately 0.2 m.

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4.1. ERT analyses in Żebracze The inverse model for the entire hillslope clearly indicated geoelectrical differences and distinctness of the near-surface regolith (Fig. 5A). Two separate layers could be drawn: an upper layer with higher resistivity values and a lower layer with low resistivity of the substratum material. The layers’ reciprocal arrangement in the middle of the hillslope suggests that the area might have been reshaped by gravitational movements (solifluction). The hillslope is very steep (32°) and is deeply incised by the Szczawniczek stream flowing at its base, which perhaps served as an additional factor of instability. A map of landslide hazards proposed by the Polish Geological InstituteNational Research Institute indicates several landslides that occurred in close vicinity of the study area. The opposite hillslope, SW of the study plot, was disturbed by two large complex landslides (ca. 6 ha each) of unknown age (www.geoportal.pgi.gov.pl). It seems, however, that the section of the hillslope that was surveyed by the ERT method was not affected by mass movements over the past several decades, as it does not show any recognizable imprints of landslides in the terrain either during our direct observations in the field or while examining the digital elevation model (Fig. 2). The study site is covered by old-growth forest, and the hillslope surface is densely disturbed by uprooted trees forming pit-and-mound topography. These features in combination with logs of broken and uprooted trees, stumps of broken and cut trees, bedrock outcrops and blocks of sandstone result in highly variable microtopography that can be seen on the elevation model as a rash-like surface. Figure 5. Inversion results of electrical resistivity tomography at Żebracze: A – slope cross-section; a – upper (dry?) layer of regolith, b – lower (moist?) layer of regolith; B – detailed cross-sections through root system of living tree (P1, P2), stump (P3, P4) and pit-and-mound topography (P5, P6). In the first microsite, we surveyed fir trees (F. sylvatica) growing on a steep surface (27° slope angle upslope of the tree and 31.5° measured downslope of the tree to the end of the profile). This site consisted of two trees but the larger fir (diameter at breast height, dbh, 70 cm) had overgrown its younger neighbor (35 cm dbh). The smaller tree was ingrown into a larger tree stem. The extent of their root systems was clearly visible on the resistivity image at 0.5–1 m depths. Without trees growing at this location, the soil/regolith would have a higher resistivity of at least 4 kΩ·m (see Fig. 5B, P1 and P2). The root system was better developed in the downslope direction, but this can also indicate a vector of water migration: either above the ground due to stemflow or below the ground in the root zone along root channels of living and decayed roots. This large contrast in resistivity seems to be an effect of the living tree root system only because, as seen on figure 5B (P3 and P4), it almost vanishes under tree stumps of similar size (60–70 cm of diameter, 30–40 cm high) (Fig. 6 and 7). However, it has to be highlighted that a weak effect is still visible on the ERT profile even after a minimum of 20 years after the tree was cut (this is a minimal approximation as the Żebracze nature reserve was established in 1995). Irregularities of apparent resistivity can be seen within pit-

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and-mound topography with contrasting values between the tree-throw mound and the pit. The pit, although partly infilled by litter, did not present low resistivity values similar to soil under a living tree. Soil material in the tree-throw mound (size 4×2x0.5 m; length × width × height) is of low bulk density because it came from the root wad of the uprooted tree and was subsequently deteriorated by erosion, slumps and other trunks falling on it. Figure 6. Example of a tree stump surveyed at Żebracze study site (see Fig. 5B, P3). (Photo: Ł. Pawlik) Figure 7. Locality and configuration of short ERT profiles at each study site. Note that arrows indicate the main direction of surveying. (All photos: Ł. Pawlik)

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4.2. ERT analyses in Turbacz The analyzed hillslope is steep with variations in the slope angle from 20° within the ridge area up to 36° at the bedrock outcrop, which forms a step-like morphology directly before the ridge area. The hillslope along the ERT profile is straight for the first 137 m of the profile length, where a massive sandstone outcrop begins. From 153 m to 164 m, the profile is through the sandstone outcrop. Below the outcrop, the slope surface is densely covered with rock fragments, which form a sort of rock-fragment veneer. The inversion model for the studied hillslope indicates two near-surface layers characterized by different geoelectrical features, similar to the situation observed at Żebracze (Fig. 8A). The upper layer is distinguished by higher values of resistivity and, below it, a layer of low resistivity (with better electrical conductivity). In the deeper geological bedrock, in the middle of the surveyed hillslope, there is an almost vertical structure with substantially lower resistivity values, which can be interpreted as a tectonic fault or a clear lithological boundary. However, geological maps of the Gorce Mountains indicate homogeneous bedrock within this site, without faults or boundaries of large folds . The hillslope might have been reshaped by mass movements in the past, but no clear evidence has been found in the field; however, this assumption is supported by the presence of a significant rotational landslide west of the study site (Fig. 3), as well as other locations in the Gorce Mountains, which are densely covered by different types of landslides.

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Figure 8. Inversion results of electrical resistivity tomography at Turbacz: A – slope cross-section; a – upper (dry?) layer of regolith, b – lower (moist?) layer of regolith, F – fault (?); B – detailed cross-sections through root system of living tree (P1, P2), stump (P3, P4) and pit-and-mound topography (P5, P6). The characteristics of the short profiles supported the conclusions drawn from the study plots at Żebracze. The extent of living tree’s root system (fir, F. sylvatica, dbh = 65 cm) is clearly visible due to its low resistivity and indicates better developed roots downslope of the tree. These features were not as distinguishable under the tree stump, which still influenced soil properties, although the stump is almost completely decomposed (Fig. 8B, P3 and P4). Additionally, in this study, the pitand-mound microsite is so well-preserved that it clearly impacts the soil properties both in the treethrow mound and in the pit. The long term impact is especially visible in the tree-throw pit. The mound and part of the ground downslope of it consist of relatively dry soil material of greater thickness compared to the pit and the area above it (Fig. 8B, P5). 4.3. ERT analyses in Oszast Field and digital elevation model examinations confirmed the information derived from early geological maps and geological web services dedicated to geohazards (http://geoportal.pgi.gov.pl/SOPO/) that almost the entire area of the Oszast reserve has been remodeled by a large complex landslide. Our long ERT profile was established on the left flank of the landslide, which can be considered to be the middle of the colluvium accumulation zone. The landslide’s depth was partly assessed by the examination of a stream valley north of the study plot, which was incised into the material created from the formation of the landslide body. In the nearsurface, a layer of colluvium measuring approximately 15 m deep could be distinguished (Fig. 9A).

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A hummocky terrain is widespread at this site and can be attributed first to landslides and, secondly, to uprooted trees, which were easily visible on the terrain. The model indicates a distinguishable failure plane with almost isoclinally formed layers of bedrock, imaged by different electrical resistivity bodies (Fig. 9; c, d, e, f) with their upper segments forged and translocated downslope. All ERT profiles indicate good conductivity of regolith and bedrock. Figure. 9. Inversion results of electrical resistivity tomography in Oszast reserve, Mt. Oszast : A – slope cross-section; a – upper (dry?) layer of slope cover, b – colluvial deposits (moving down), c–f – bedrock (stratified), SP – landslide shear plain; B – detailed cross-sections.

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Based on the general comparison of all profiles from the present study we can conclude that the range of resistivity values is very broad (ca. 10-103 Ω∙m). Similar values were recorded for the long and short profiles at Żebracze and Turbacz but only when the first microsite is considered (with living tree) (Fig. 10). In other short ERT soil profiles the range of resistivity values is smaller and starts from 100 Ω∙m. Higher variability in resistivity values at the Oszast study plot could be caused by different regolith properties which in this place is of colluvial origin and is characterized by a lower bulk density and probably higher moisture content (the area belongs to the first order stream watershed). Soils in all profiles can be characterized by particle size distribution dominated by silt and sand fraction (Fig. 10). Figure 10. Results of the present study compared with the general resistivity values of different rock types, sediments and soil fractions (partly based on Palacky, 1987).

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5. Discussion

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5.1. Biomechanical effects of tree root systems This study investigates a limit of the Critical Zone, where abiotic and biotic factors interact and Earth’s living matter impacts the abiotic properties of regolith and bedrock (Riebe et al., 2016). To this end, we used electrical resistivity tomography in a rather uncommon application to study the extent of root systems and the properties and features of the ground which might have been influenced by the presence of roots (mainly by moisture content) (Bast et al., 2014).

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In soil geomorphology, biogeomorphology, paleobotany and geology the best known effects of tree roots is the biomechanical and biochemical alteration of soils and regolith, although this nomenclature does not always apply (Pawlik et al., 2016a). For instance, in the so-called Devonian Plant Hypothesis, the biomechanical and biochemical effects of tree roots are a key issue for speculations about climate change in the past epochs (Algeo et al., 1995, 2001; Algeo and Scheckler, 1998; Beerling and Berner, 2005; Le Hir et al., 2011). Their global influence has been suggested in many recent papers which usually point to rhizospheric processes and mycorrhizal associations with plant roots (Morris et al., 2015). It is argued that trees, through the action of their roots, can change physical and chemical characteristics of regolith and soils and they can have repeating influence on the same portion of substrate and bedrock subsequently leading to soil deepening (Zinke, 1962; Limbrey, 1975, after Wood and Johnson, 1978; Phillips and Marion, 2006; Phillips, 2008; Shouse and Phillips, 2016) (Fig. 11). Figure 11. Different conceptual views on various aspects of the tree root system impact on soils, regolith and bedrock. The figure on the left side illustrates the formation of root casts (Limbrey, 1975, from Wood and Johnson, 1978). It is also a good example of the role of bedrock structure (the pattern of joints and fissures). Roots change the properties of bedrock by driving the formation of root casts. It is slightly different example than a sequence proposed on the figure on the upper right side. Here, tree roots operate within soil horizons and do not penetrate fresh bedrock (Phillips and Marion, 2006). Roots through their growth and penetration of the soil make a space for themselves which is infilled by transported surface debris after roots are completely decayed. This can affect the next generation of trees because such microsite is rich in organic matter and fresh debris. It has also a lower balk density and these features can favorite fast growing of saplings. The last example (lower right side) is the most extreme one demonstrating that trees and their root systems are able to weather bedrock and thus extend downward the thickness of soil mantle and the limit of the Critical

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Rarely used high resolution geoelectrical imaging (electrodes spacing 0.5 m) enabled measurement of the real impact of tree root systems on soil and regolith, which was manifested by a decrease of the electrical resistivity of the upper part of the soil mantle up to 1 m to 1.5 m depth. However, this zone of decreased resistivity disappeared after tree death and decomposition of the root wood. These changes are also visible within pit-and-mound topography where tree-throw mounds give higher values of resistivity, as they are probably much drier compared to tree-throw pits which are locations where water and organic matter concentrate. Similar results were obtain by the present authors at Mt Rogowa Kopa, SW Poland (Pawlik and Kasprzak, 2015). In the case of tree-throw pits, we expect that long-term effects are probably detectable in their geoelectrical properties even after complete infilling by debris and organic matter (due to leaching and translocation of organic carbon into lower horizons). Our results are consistent with the results of Rossi et al. (2011) who found marked variability in soils caused by root density. Based on the present results we can conclude that the soil variability is less distinguishable or disappear during and after tree decay. However, when soil is disturbed by tree uprooting sudden changes in soil resistivity are detectable on a very shor distances, firstly because of abrupt changes in soil thickness, and, secondly, due to contrastic hydrological properties of the tree-throw mound and pit.

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To compare profiles affected by tree roots it is possible to distinguish sites which can be considered as “control” profiles with only soil and bedrock at two ends of the ERT profiles made transversely to the slope. Potentially, they represent “normal” conditions not affected by trees. In each case the upper horizons of the soil and regolith are characterized by the highest electrical resistivity values. But at each of the microsites this sequence has been modified by the presence of living trees, decaying tree stumps or uprooted trees which resulted in the formation of pit-and-mound topography. For instance, at resistivity profiles under living trees higher values were shifted 1-2 m down the soil profile (Fig. 5B-P1-P2, fig. 8B-P1-P2, fig. 8B-P1P2). It may suggest changes in the moisture conditions but also structural changes in the soil body (dense net of tree roots, soil compaction, hydromorphic alterations, etc.). To summarise, there is still an open question, especially in the geomorphic context (biomechanical weathering), what size of tree roots are detectable by ERT. It is difficult to conclude and to present any solid data because most of the studies were mainly conducted for agricultural purposes and focused on the detection of moisture content and/or root biomass quantification based on ERT. In terms of the tree root biomass (root dry mass per unit soil volume), both Paglis (2013) and Rossi et al. (2013) showed positive correlation between root biomass and soil electrical resistivity. Soil electrical resistivity inreases together with the increase of root biomass but only to a certain boundary value after which resistivity increases but root biomass is constant. This suggests that other factors cause further increase in soil electrical resistivity.

5.2. Advantages and disadvantages of ERT as a tool for non-invasive studies of tree roots Tree roots, studied in situ, are the most difficult part of the tree to analyze, and investigative attempts for a variety of purposes commenced only recently (Stokes et al., 2002; Nadezhda and Čermák, 2003; Amato et al., 2008; Zenone et al., 2008; Paglis, 2013; Furman et al., 2013; Jayawickreme et al., 2014; Rodriguez-Robles et al., 2017). The traditional approach, which relied on soil profile excavation, is normally very time consuming and difficult to apply under large living trees because of the presence of thick, hard vascular roots (which are only possible to cut with a chainsaw). Additionally, with this method we are able to obtain only discrete point data from one profile making it impossible to obtain a complete picture of the root architecture with the soil excavation method. The last limitation can be reduced with the use of an air spade (Nadezhda and Čermák, 2003); however, this method is inappropriate for soil production studies because the internal structure of the soil is damaged. Conversely, the advantage of geophysical methods is their non-invasive character which becomes a key issue in strictly protected areas (Tabbagh et al., 2000). These methods (ground penetrating radar, GPR, electrical resistivity tomography, ERT, near-surface seismic refraction) greatly overcome the limitations of the traditional approach because they offer continuous datasets which can be collected relatively quickly (Roering et al., 2010; Łyskowski et al., 2016). For instance, both the GPR and ERT techniques can produce 3D images but only GPR is

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able to produce an image of very small roots under different signal frequency (450 or 1500 MHz antenna) (Hruška et al., 1999; Nadezhda and Čermák, 2003; Zenone et al., 2008). In the GPR technique root direction is important and when it is not perpendicular to the profile detection of the root position is lost (Zenone et al., 2008; Rossi et al., 2011). With the ERT method detailed results for several profiles can be obtained within 1-2 days of work. However, the inversion models acquired during ERT studies show only one physical feature of the ground i.e., electrical resistivity which can be directly contrasted to electrical conductivity. Electrical features depend on various factors, among which the mineralogical composition, the structure and texture of rocks are only examples. In the case of analyzed near-surface formations, in most cases water content is responsible for differentiation in their electrical features. In spite of obvious limitations, the applied method enabled us to obtain satisfactory results. Conclusions One of the recent areas of study is the modelling of root system architecture and root zone extents, which have significant meaning for studies on hillslope stability and soil production. Recently, several studies focused on the biomechanical effects of tree root systems. However, it is not known exactly how trees interact with the bedrock and regolith and what the rate of biomechanical and biochemical weathering is. Many constraints, especially technical ones, made full evaluations of the active role of tree roots difficult. In this study, geophysical studies are a first step toward a better understanding of these important interactions. Non-invasive, electrical resistivity tomography is especially important in strictly protected areas, where soil profile excavations are prohibited. Another positive aspect of ERT studies is the creation of a continuous image of the soil/regolith mantle and bedrock. Based on ERT results, we concluded that tree roots change the soil properties of the ground for several decades, possibly even more than a century. The most tremendous changes follow tree uprooting, which cause so-called pit-and-mound topography with subsequent long-term fluctuations of moisture content within the tree-throw mounds and pits. Acknowledges The study was entirely supported by the Polish National Science Centre (UMO2014/15/D/ST10/04123). We are thankful to the authorities of the Piwniczna Zdrój Forest Division, the Gorce Mountains National Park and the Regional Directorate of Environmental Protection in Katowice for permissions to conduct our research in the strictly protected areas of the Beskidy Sądecki Mountains, the Gorce Mountains and the Beskidy Żywiecki Mountains. We would like to express our great thanks to Katarzyna Marciniec and Kacper Marciniec for their field assistance, Tomasz Bryndal and Paweł Kroh for making available spatial data for the Polish Carpathian Mountains, and Łukasz Longosz for his help with English language editing.

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Dominant tree species

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sandstones (flysch) (Paleogene)

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N slope of Mt Oszast (1147 m a.s.l.)

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ACCEPTED MANUSCRIPT Highlights: Biomechanical effects of tree roots evidenced by electrical resistivity tomography for the first time.

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Tree roots have surprisingly long-term influence on soil and regolith properties.

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Tree roots add to the complexity of soil/regolith continuity.

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Electrical resistivity tomography has many advantages when applied to tree root zone studies.

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The study documents limits and architecture of the Critical Zone.

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