Published April 18, 2019

North American Forest Soils Conference - International Symposium on Forest Soils

Soil Compaction Effects on Root-Zone Hydrology andVegetation in Boreal Forest Clearcuts
Linnea Hansson*

Swedish Univ. of Agricultural Sciences
Dep. of Soil & Environment
Box 7014
750 07 Uppsala, Sweden

Jirka Šimůnek

Univ. of California Riverside
Dep. of Environmental Sciences
Riverside, CA 92521

Eva Ring

Skogforsk
The Forestry Research Inst. of Sweden
Uppsala Science Park751 83 Uppsala, Sweden

Kevin Bishop

Swedish Univ. of Agricultural Sciences
Dep. of Aquatic Sciences & Assessment
Box 7050
750 07 Uppsala, Sweden

Annemieke I. Gärdenäs

Swedish Univ. of Agricultural Sciences
Dep. of Soil & Environment
Box 7014
750 07 Uppsala, Sweden
andUniv. of Gothenburg
Dep. of Biological & Environ. Sciences
Box 461405 30 Göteborg, Sweden

Core Ideas• Impact of forwarding on soil waterand vegetation was investigated after
4 to 5 years.
• Soil water content was highest inwheel tracks and lowest between
tracks.
• Bare soil was more frequent intracks and between tracks than inundisturbed soil.
• In wheel tracks, soil aeration may
be restricted in periods with highprecipitation.
• Simulated root-zone pressure heads
showed little variation between
microsites.
Soil Science Society of America Journal

Soil compaction is a common consequence of forestry traffic traversing unprotected, moist soils; it decreases porosity and affects hydraulic conductivity
even in coarse-textured soils. The aim here was to study root-zone hydrology
and vegetation in three microsites (in, between, and beside wheel tracks) 4
to 5 yr after forwarder traffic, on stony and sandy till soils in two clearcuts innorthern Sweden. Measurements of soil volumetric water content (VWC), vegetation indicators and one-dimensional hydrological modeling (Hydrus-1D) ofwheel tracks and undisturbed soil were conducted. Soil VWC was monitored
hourly during 2017 and 2018 in three or four plots along a slope on each site.
Soil VWC was also measured once with a portable sensor in 117 plots along
two slopes at each site, where the vegetation was recorded and analyzed using
Ellenberg indicator indexes. Soil VWC was highest in wheel tracks and lowest
between tracks; this was corroborated by the species composition in the wheel
tracks (Ellenberg indicator for soil moisture). Bare soil was more frequent inwheel tracks and between tracks than in undisturbed soil. The model simulations indicated that the changed soil hydraulic properties influenced the VWC
results in the wheel tracks. However, the differences in average pressure heads
in the root zone were small between the microsites and only apparent during
dry periods. In the wheel tracks, air-filled porosity was <0.10 m3 m-3, indicating insufficient soil aeration during 82% (Site T) and 23% (Site R) of the 2017
growing season. Insufficient aeration could be one explanation for the presenceof some still unvegetated areas.
Abbreviations: VWC, volumetric water content.

Soil compaction and rutting (wheel tracks) are common disturbances associated with forestry operations on unprotected, moist soils and may result
in decreased porosity and hydraulic conductivity, even on coarse-textured
soils (Cambi et al., 2015; Hansson et al., 2018a; Toivio et al., 2017) and decreased
aeration and infiltration capacity (Meek et al., 1992; Startsev and McNabb, 2009).
Hansson et al. (2018a) found that soil compaction increased the risk of poor aeration by means of reduced total porosity, pore connectivity, hydraulic conductivity as
well as longer periods of high water content, due to a higher water holding capacity.
Soil recovery after off-road traffic is usually slow and disturbances have beenfound to persist for many decades (Cambi et al., 2015; Ebeling et al., 2016; Klaes
et al., 2016; Wei et al., 2016), even when freeze–thaw cycles occur (Corns, 1988).
The growth and decay of roots and mycorrhiza promote the recovery process(Bottinelli et al., 2014; Meyer et al., 2014) and are important when no or few
earthworms or other soil-mixing fauna are present in the soil or when early andthick snow cover reduces soil frost.
Soil compaction may reduce tree seedling survival and growth (Kabzems,
2012; Murphy et al., 2004), but growth can also be unaffected (Ares et al., 2005;
Kamaluddin et al., 2005; Page-Dumroese et al., 2006), or increased (Brais, 2001;
Ponder et al., 2012), depending on a number of factors, including soil moisture
Soil Sci. Soc. Am. J. 83:S105–S115
doi:10.2136/sssaj2018.08.0302
Received 17 Aug. 2018.
Accepted 5 Feb. 2019.
*Corresponding author (linnea.hansson@skogforsk.se; linneahansson@yahoo.com)© 2019 The Author(s). Re-use requires permission from the publisher.

availability and aeration: a decreased soil gas permeability in theupper 5 cm of the mineral soil has been found to decrease root
density of oak (Gaertig et al., 2002). Soil moisture availability isalso important for predicting species composition of the groundvegetation (Härdtle et al., 2003). Wei et al. (2015) found thatsoil moisture in skid trails was positively correlated with thenumber of species that prefer moist or wet conditions.
Ground vegetation responds rapidly and at fine scales tomanagement practices and disturbances, and thus, it is suitablefor detecting changed soil conditions caused by off-road traffic (Duguid and Ashton, 2013). For example, the Ellenberg
indicator values (Ellenberg et al., 1991) can be used to revealchanged soil moisture conditions or disturbances. These sitespecific values (usually between 1 and 9) are commonly used toindicate environmental conditions based on the species present(Bartelheimer and Poschlod, 2016). Species diversity (the number of species and their distribution) can also be used for detecting changes in vegetation between different microsites.
Air-filled porosity (i.e., total porosity minus volumetricwater content [VWC]) is considered to be the main limiting
factor for soil aeration and VWC is commonly used for quantifying soil aeration (Ben-Noah and Friedman, 2018). After water and nutrient availability, aeration is the most important soil
factor affecting plant growth (Ben-Noah and Friedman, 2018).
Oxygen is consumed by plant roots and microorganisms, CO2
is produced by respiration, and, if soil conditions become anaerobic, toxic gases are formed by chemical reduction processes(Ben-Noah and Friedman, 2018). Gas diffusion essentially stops
when the air-filled porosity decreases below 0.10 m3 m-3 (Xu etal., 1992). This value has also been found to be a threshold for
reduced seedling growth (Wall and Heiskanen, 2003).
Hydrological models can be used to analyze how changes
in soil physical properties caused by off-road traffic may affectthe root-zone soil water dynamics. Hydrus-1D (Šimůnek et al.,2008, 2016) is a public domain, one-dimensional modeling environment that can be used for the analysis of water, heat andsolute movement in soils. The program solves numerically theRichards equation for both saturated and unsaturated water flow
and the Fickian-based advection-dispersion equations for soluteand heat transport. In this study, Hydrus-1D was used to assess
the impact of changes in hydrological properties such as total porosity, water retention, and saturated hydraulic conductivity, on
root-zone water dynamics.
Our overall aim was to study how soil compaction affected
the soil water conditions in the root zone, as well as the vegetation cover, at three microsites (in, between, and beside wheel
tracks), 4 to 5 yr after forwarder traffic on coarse-textured tillsoils in two forest clearcuts in northern Sweden. We wanted
to test whether differences in soil physical properties between
wheel tracks and the undisturbed soil, measured at the same sites
in the upper parts of the slopes (0–5 cm in the mineral soil) byHansson et al. (2018a), influenced the soil water dynamics in thewheel tracks and thus, prolonged the time when air-filled porosity was below the threshold for sufficient soil aeration for roots
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and microorganisms. We also wanted to study the vegetation
species composition and cover to determine whether there weresigns of non-optimal growth conditions, such as restricted aeration. We formulated the following research questions:
1. Does the VWC differ between the microsites (wheel
tracks, between tracks, and undisturbed soil) and, if so, are there
any differences along the slopes or between sites?
2. Are there differences in the species present and/or theircover between microsites and do these indicate differences in thesoil moisture or disturbance level (as expressed by Ellenberg indicator values)?
3. Are there any areas where soil aeration could be restricted
due to high VWC?
4. Do the previously measured differences in soil physicalproperties in the upper mineral soil between wheel tracks andthe undisturbed soil (Hansson et al., 2018a) change the simulated root-zone hydrology, and if so, are the results sensitive todifferent weather conditions?

Materials and Methods

Study Sites and Experimental Design
This study was performed at the field sites 294 Rotflakamyran(hereafter Site R) and 296 Trågalidsberget (Site T) (Hansson
et al., 2018a; Ågren et al., 2015). Site R (64°32.5¢ N, 20°4.5¢ E;
305 m AMSL) and Site T (64°19.3¢ N, 20°35.7¢ E; 145 m AMSL),
are situated in northern Sweden on podzolized glacial till soils(Cryods) with a high stone and boulder content; approx. 50% of thesoil consists of rock fragments with diameters >20 mm (Stendahl etal., 2009). Soil textures are mainly sandy loams. When only particles
smaller than 20 mm are considered, the average mixture in the upper 0 to 5 cm of the mineral soil consists of 14% pebbles, 51% sand,
31% silt, and 3% clay. The silt and clay fractions increase in downhill
positions at Site T (Hansson et al., 2018a). The depth of the humus layer is ~3 to 7 cm at Site T and ~7 to 12 cm at Site R, withhigher values in downhill positions. Each site contains two slopes(A and B) with inclinations <10%. At Site R, Slope A is facing southand Slope B west, while at Site T, Slope A is facing south-west andSlope B north-east. The mean annual temperature is ~1 to 2°C andthe precipitation is ~600 to 700 mm yr-1 (ref. period 1961–1990;
SMHI, 2019a, 2019b).
The mixed Scots pine [Pinus sylvestris L.] and Norway
spruce [Picea abies (L.) H. Karst] stand at Site R was harvested in
December 2011 and the Norway spruce stand at Site T was harvested in February 2012 (winter conditions at both sites). The harvester and forwarder drove over slash mats in 4-m wide strip roads
located between the study plots. As a result, the study plots were unaffected by traffic during clearcutting. Each slope contained three orfour study plots subjected to different driving treatments. The plots
were about 15 m wide and ~100 to 150 m long, depending on thelength of the slopes. In this study, however, all measurements were
undertaken within the study plots where the laden forwarder (John
Deere 1410D, total weight approximately 34 Mg, including timberload of ~10 Mg) drove down and up the slope three times, in total
six passes, without applying any soil protection (logging residues reSoil Science Society of America Journal

moved). Due to severe rutting and risk of getting stuck on the lower
part of Slope B at Site R and Slope A at Site T, the part of the slope
subjected to driving had to be shortened after the first two passes(and for Site T even more during passes 5 to 6). The driving was
performed in June during the summer following clearcutting (2012)at Site R and the following summer (2013) at Site T (Hansson etal., 2018a; Ågren et al., 2015). Three types of soil microsites wereidentified: wheel tracks (WT; each ~0.7–1 m wide), between thewheel tracks (BT; ~1.3–1.5 m wide) and undisturbed soil at least 2
m beside the wheel tracks (U; Fig. 1).

Vegetation Survey and Soil Water Measurements(Portable Sensor)
A vegetation survey was performed at the end of May 2017
to assess whether there were any differences in the vegetation coverand species composition at different microsites and if the soil water conditions were reflected in the species present. Plots for vegetation survey (0.4 m by 1 m) were laid out at 10-m intervals at
10 (9 at Site T, Slope A, due to a shorter slope length) sampling
locations (numbered 1–10) along each of the four slopes. Eachlocation contained three plots, in total 30 or 27 plots per slope,and they represented undisturbed soil (microsite U), wheel track(microsite WT, located in the left wheel track) and the area between the left and right wheel tracks (microsite BT). The first plot(U, within Location 1) was laid out at a distance of 1 to 6 m fromthe lowest end of the traversed plots on the undisturbed soil, ~4m to the left of the wheel tracks (Fig. 1). The following data wererecorded for each of the 117 vegetation plots: a track depth (microsite WT), the area of ponded water (%), the area with bare soil(%), heights of planted pine seedlings (if present) and self-seeded
seedlings of pine or other tree species. Finally, the occurrence ofspecies and their cover (at estimated full seasonal development)
were recorded according to the Braun-Blanquet abundance-dominance classification (as it is described by Wei et al., 2015): i, one
individual, the cover <5%; +, very few individuals, the total cover<5%; 1, few to many individuals, the total cover <5%; 2, many individuals, the total cover 5 to 25%; 3, the total cover 25 to 50%; 4,
the total cover 50 to 75%; and 5, the total cover >75%. The sevencoefficients were later transformed into the following percentile
cover values: i, 0.1%; +, 0.5%; 1, 5%; 2, 17.5%; 3, 37.5%; 4, 62.5%;
and 5, 87.5% (Wei et al., 2015). The species were divided into thefollowing groups: self-seeded tree seedlings, Ericaceae, graminoides (species with grass-like morphology), mosses, and other species(Appendix A). In the middle of July, the plots at both sites werebriefly resurveyed to see whether there were any new species thatemerged late in the season. Only a few new individuals were foundand they were added to the data.
Each species’ Ellenberg values were obtained from the literature for the following indicators (Appendix A): soil moisture (F,where 1 indicates that the plant prefers xeric conditions, 5 mesic,
9 wet and 12 underwater conditions), disturbance (D, where 1
indicates that the species can colonize and compete in alreadyclosed forests and 9 that the species quickly colonizes bare soil
and disappears quickly), light (L, where 1 indicates that the spewww.soils.org/publications/sssaj

Fig. 1. Schematic representation of the experimental design with (i)locations of TDR sensors at Site R (Slope A) and Site T (Slope B); (ii)close-up of TDR installation in microsites undisturbed (U), wheel
tracks (WT), and between wheel tracks (BT); and (iii) close-up of thelayout of the vegetation plots in microsites U, WT and BT from above.
At Location 2, Site T, there was an extra 15-cm-long TDR in the wheel
track and an extra 30-cm-long TDR in the undisturbed soil. Plots forvegetation survey were established at ten locations along each of thetwo slopes at both sites, except Site T, Slope A, with nine locations (intotal 117 plots).

cies prefers deep shade and 9 full light), reaction of soil (R/pH,
where 1 represents extremely acidic conditions and 9 alkaline),
nitrogen/fertility (N, where 9 indicates the most fertile conditions) (Ellenberg et al., 1991; Hill et al., 2004, 2007; Sundberg,
2018; Tyler and Olsson, 2013). For each vegetation plot (p), a
weighted mean indicator value (yEIp) was calculated for eachEllenberg indicator (EI) by averaging the indicator value (e) of
each species (i) and weighting these with the respective covervalue (xi), using the following equation:xi∗e ii =1 x totn=yEIp ∑

The weighted mean indicator values for each plot andEllenberg indicator were used to determine whether there wereany differences between microsites, sites and slope positions. Asa measure of species diversity in the plots, the Shannon diversity
index (Shannon, 1948) was calculated. This is the most commonly used measure of diversity (Spellerberg and Fedor, 2003)
and a relatively low number indicates that few species dominate
most of the area and other species are not abundant (even if there
are many different species), whereas a relatively high number indicates that there are many species and they are equally abundant.
In each of the vegetation plots, the soil VWC was measured at three points (evenly distributed within the plot) witha portable time domain reflectometry (TDR) sensor (TRIMEPICO64, HD2, IMKO Micromodultechnik GMBH, rod
length 16 cm) using the standard calibration for universal soil,
provided by the manufacturer. At Site T, VWC measurements
were performed on the same day as the vegetation survey (27May 2017). At Site R, due to a rod breakage, VWC measurements were made 6 wk later (12 July 2017). In 45 of the 117
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plots (mainly in the upper parts of the slopes), no measurements
could be recorded, due to the stoniness of the soil preventing acomplete insertion of the rods.

ing values were 0.49 and 0.35 m3 m-3 in the undisturbed soil
and the wheel tracks, respectively (Hansson et al., 2018a).

Soil Water Content at the Permanent Plots

Hydrus-1D version 4.16.0110 (Šimůnek et al., 2013) wasused for modeling hourly water flow and root uptake in the wheel
tracks and the undisturbed soil microsites, separately, in the upper
part of the slopes (Locations 2 and 3, Site T, Fig. 1), the same partthat was studied by Hansson et al. (2018a), including sampling andanalysis of 36 soil samples from each site. We selected Site T forthe modeling, as the wheel tracks in the upper 50 m of the slopes
there were shallow (0–5 cm), and the traffic impact was mostly inthe form of soil compaction, that is, the rutting profile was negligible for the hydrology and thus, the groundwater level (the lower
boundary condition, measured at the site) could be assumed to be
the same in both microsites. The upper boundary conditions, thatis, the atmospheric conditions based on measurements from thesites, were also the same and thus the only differences between thetwo modeling setups for the two microsites, WT and U, were thesoil hydraulic properties in the upper 0 to 5 cm of the mineral soil
(Table 1; Hansson et al., 2018a).
In Hydrus-1D, measurements can be compared with simulated values when the model is run in the inverse mode and themodel parameters can be optimized by improving the correspondence between simulated data and measurements. To evaluate
whether the modeled water contents were comparable to themeasured ones, mean values for the 15-cm-deep TDRs (n = 3
in WT, n = 2 in U) in Locations 2 and 3 were linked to the observation node 3 (8 cm depth) in the undisturbed soil and wheel
tracks, respectively, and the 30-cm TDRs (n = 3) were linked tosubregion 1 (0–30 cm) of the undisturbed profile. See Appendix
B for more details about the model set-up.
Weather conditions, groundwater, and VWC measurements from June 2018 were also used with the same model setup,
to validate it, and test the impact of different weather conditions
on the differences in water flows and pressure heads between thetwo microsites.

The VWC was measured hourly between 30 May and 6
Oct. 2017, and during June 2018 at four locations along Slope
A (16 sensors) at Site R and at three places at Site T, Slope B (14sensors; Fig. 1). The measuring period in 2017 covered most of
the growing season (17 May to 6 October), which was defined as
the period starting with the first 6 d in a row in the spring when
the diurnal mean air temperature was >5.0°C and ending at thefirst 6-d period with temperatures <5.0°C (Frich et al., 2002).
The instruments used were CS616 TDRs (Campbell ScientificLtd); in the wheel tracks, they were shortened to 15 cm as we anticipated that insertion of 30-cm rods would be very difficult in
the compacted and stone-rich soils. In undisturbed soil, both 15and 30-cm long TDRs were installed (Fig. 1). Only 30-cm TDRs
were inserted in the between-track microsites. The sensors wereconnected to a CR1000 datalogger with an AM16/32B multiplexer and a CR10X datalogger with an AM416 multiplexer(Campbell Scientific Ltd.) at Sites R and T, respectively. In thelaboratory, site-specific calibrations, as described in Campbell
Scientific (2016) for the 30 and 15 cm TDR-sensors, were performed at 12 different water contents, using soil collected in twopits from each site and then all mixed. At each water content,measurements with the TDRs were compared with water contents derived from oven-drying for 24 h at 105°C (three cylinder
samples [diameter 7 cm, height 5 cm] collected for each watercontent). The VWC measurements were corrected for variations
in soil temperature by the temperature correction function of theoutput time signal (Campbell Scientific, 2016). Soil temperature
was measured next to each TDR with thermocouples (T107,
Campbell Scientific Ltd., length 10 cm) inserted vertically into
the soil from the soil surface (from the top of the humus layer).
Estimated insufficient soil aeration at the different microsites along the slopes was assessed by counting the number ofdays when the daily mean VWC reduced the air-filled porosity
to <0.10 m3 m-3 (Hansson et al., 2018b; Wall and Heiskanen,
2003; Xu et al., 1992). At Site T, 0.10 m3 m-3 air filled porosity
corresponded to a VWC of 0.45 m3 m-3 in the undisturbed soil
and 0.38 m3 m-3 in the wheel track. At Site R, the correspond-

Hydrological Modeling with Hydrus-1D

Statistical Analyses
To analyze the results of the vegetation survey, linear mixedeffect models were constructed in R (version 3.4.2, package“nlme”), using the function “lme” to examine the effect of theTable 1. Soil hydraulic parameters used in Hydrus-1D for the undisturbed (U) and wheel track (WT) microsites. When the values
for the two microsites differ, the wheel track value is given in italics. The van Genuchten parameters (Qr, Qs, a, and n) and thesaturated hydraulic conductivity (Ks) in the mineral soil are based on measurements of soil samples from the sites published in
Hansson et al. (2018a), except for Layer 3, which is unpublished but sampled on the same occasion as the other samples. The poreconnectivity factor, l, was obtained using the inverse solution parameter optimization.
Soil hydraulic property
Layer†Depth

1 (F/H)†

cm
0–5

Qr

Qs

----------- cm3cm-3 ----------0.07
0.76

a
cm-1
0.56

n

l

Reference

Lundmark and Jansson (2009),
Laurén and Heiskanen (1997)
0.55/0.48
0.12/0.03
1.28/1.29
25.2/7.1
6/2.1
Hansson et al. (2018a)
2 (E)
6–11
1.00×10-8
3 (B and C)
12–1001.00×10-8
0.53
0.04
1.26
3
0.5
Hansson, unpublished data
† F/H, partly decomposed litter and organic layer; E, eluvial horizon; B, illuvial horizon; C, unaltered horizon with parent material.
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1.27

Ks

cm h-1
1208

3

Soil Science Society of America Journal

www.soils.org/publications/sssaj

1.8 a
1.9 a
2.3 a
2.6 b
132 a
128 a
10 a
3a

3>7
Location diff.#† The response variable was square root-transformed in the model.
‡ VWC, volumetric water content; The response variable was log-transformed in the model.
§ Values with different letters (by columns) are significantly different (p < 0.05) in the post hoc Wald tests.
¶ Weighted Ellenberg index values were light (L), soil moisture (F), reaction of soil (R/pH), nitrogen/fertility (N), and disturbance (D).
# Numerals of the locations that differ significantly (e.g., Location 3 had a higher proportion of bare soil than Location 7).

1 > 4–9
2 > 8–9

1, 8 > 3

1 > 7,9,10

1 > 7–10

2.4 a
2.5 a
5.6 a
5.4 b
6.0 a
5.8 b
1.58 a1.39 b0.27 a0.39 b
61 a
52 a
47 a
17 b

2.5 a
2.5 a
2.3 a
5.4 a
5.7 b
5.4 a
5.9 a
6.0 a
5.9 a
1.57 a1.41 a1.47 a0.33 ab
0.45 b
0.28 a
75 a
47 b
49 b
43 a
13 b
42 a
35 a
42 a
41 a
153 a
106 b
134 a
0.03 a§13 b
6b

Microsite
Undisturbed
Wheel tracks
Between tracks
Site
R
T

23 a
56 b

1.8 a
1.9 a
1.9 a
2.3 a
2.5 a
2.5 a

D
N
m3 m-3------------------------ % cover --------------------------------------- % ------------------

Weighted Ellenberg index¶

R/pH
F
L
Shannon index
VWC‡
MossesEricaceae†
Graminoides†

The measurements with the portable soil water sensor in thevegetation survey plots indicated that 4 to 5 yr after forwarder
traffic, the VWC was highest in wheel tracks and lowest between
the tracks (Table 2); this was also corroborated by the species
composition in the wheel tracks (the Ellenberg indicator value,
F; Table 2). The wheel track depth in the vegetation plots varied
between 0 and 70 cm, with average values of 31 cm at Site R (n =20) and 19 cm at Site T (n = 19). Bare soil was more frequent in
the wheel tracks and between the tracks than in the undisturbed
soil when all locations were included (Table 2). Two plots (SiteR, Slope A, WT, Location 1, and Site T, Slope A, Location 3)
were covered by ponded water (and were excluded from the statistical analysis of VWC). Another four plots on the lower partsof the slopes were partially ponded (2–10% of the plot area).
The wheel tracks and between-track microsites had lessmoss cover than the undisturbed soil and the wheel tracks had
less cover of Ericaceae than the other two microsites. However,the species composition and cover indicated no difference in di-

Total cover

Soil Water and Vegetation along the Slopes

Bare soil†

Results

Table 2. Microsite and site mean values (across all slopes, sites, and locations) of the response variables in the linear mixed effect models. Vegetation plot locations are numberedfrom 1 to 10 (situated 10 m apart), starting at the lower end.

microsite (U, WT and BT), site (R or T) and sampling location(numbered 1–10) on different response variables such as VWC,
fraction of bare soil, diversity index, cover of different groups ofspecies and weighted Ellenberg indexes. The fixed effects in themodel were: site, location along the slope and microsite. The
random effect was microsite within each slope (=subject). An
autoregressive covariance structure was introduced for the relationships between different locations (=Distance.from.lower.
end) of the microsites along each slope [random=~1|subject, cor
relation=corCAR1(form=~Distance.from.lower.end|subject)].
All models were tested with the interaction effect of locationand microsite, but it was never significant, and was thus excluded
from the model. The response variables bare soil, graminoides(plants with grass-like morphology) and Ericaceae were squareroot-transformed and VWC was log-transformed to obtain
more normally distributed residuals. The Wald test was used asa post hoc test (function “emmeans”) for differences between
microsites, locations and sites, including the Tukey method for
adjustment of p-values. Effects with p-values below 0.05 wereconsidered significant in all analyses.
The seasonal means and medians of the VWC measurements were tested with the same model as described above.
However, in this case, the interaction effect of location and microsite was significant and retained in the model. Sampling locations were 1 to 3 at Site T and 1 to 4 at Site R (Fig. 1). Fisher’s
exact test was used to evaluate the significance of differences
between microsites in numbers of days when the daily mean airfilled porosity was below 0.10 m3 m-3.
The Hydrus simulations were evaluated by comparing
the root mean square error (RMSE) and the coefficients ofdetermination (R2) for the predicted versus observed values
from 2017. The results of 2017 represent a calibration and theresults of 2018 a validation of the model setup.

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1, was wetter than the upper locations (VWC, Locations 4–9; F,
Locations 7–10). Location 1 also had more light tolerant species
than Locations 7, 9, and 10 (Table 2).

Seasonal Soil Water Measurements
There were statistically significant differences in seasonalmeans and medians of VWC values between sites (p < 0.01) andlocations along the slope (p < 0.01), but not for microsites (p =0.29; Fig. 2), and not for the interaction effect between locations
and microsites (p = 0.06). However, the differences between sites
and locations were not significant in the post hoc test.
The number of days during the 2017 growing season whenaeration could have been restricted was highest at both sites in thewheel tracks (except at Location 2, Site R, where no days were over
the threshold value) and the lowest at the between-track microsite, BT, at Site T (Table 3). At Site R, there were no days withpotentially restricted aeration in either the undisturbed soil or in
the between-track microsites. In total, the air-filled porosity may
have been too low for sufficient aeration of roots in microsite WT
during 82% and 23% of the 2017 growing season, at Sites T and R,
respectively (Table 3).

Fig. 2. Mean seasonal volumetric water content (VWC) values foreach microsite of wheel tracks (WT), between wheel tracks (BT)and undisturbed soil (U) at Sites T (dark gray) and R (light gray).
The error bars indicate standard deviations. The rod length of thesensors (measurement depth from the surface of the humus layer
in centimeters), is given in parentheses. The post hoc test of thestatistical model (including both sites) did not indicate significant
differences between microsites.

versity (Shannon diversity index), light tolerance (L), fertility(N), pH (R) or disturbance (D) between microsites (Table 2).
Neither were there any statistically significant differences in the
total number of species or the number or cover of self-seeded
tree species for microsite, site, or location. There were no planted
seedlings within any of the vegetation plots in the undisturbed
soil. The average height of planted seedlings was 29 cm in thewheel tracks (standard deviation [SD] = 9.5 cm) and 31 cm between tracks (SD = 9.2 cm, n = 21 for both microsites). The
average height of self-seeded seedlings (Betula spp., Sorbus aucuparia L., P. abies, P. sylvestris and Salix spp.) was 33 cm (SD= 35 cm, n = 8) in the undisturbed soil, 17 cm (SD = 17 cm,
n = 22) in the wheel tracks, and 27 cm (SD = 20 cm, n = 14)
between wheel tracks. The difference in the cover of other species (e.g., herbs) could not be tested using the statistical analysis,
as no transformation was found to make the residuals normally
distributed: the standard deviations for the cover of other species
at the different microsites were larger than the mean values.
Ericaceae were more common at Site R than at Site T,
where graminoides were more common. Furthermore, Site R
had a higher Shannon diversity index than Site T (Table 2). The
weighted mean Ellenberg index for light tolerance (L) and soil
moisture (F) was also higher for Site R than Site T, whereas thefertility was higher for the latter site (Table 2).
The slope position (Location in the statistical models) was
significant for both measured VWC and the Ellenberg indicatorof soil moisture. The location at the bottom of the slope, Location

Hydrological Modeling with Hydrus-1DThe coefficient of determination, R2, for predicted versus
observed water contents was 0.86 for the undisturbed microsite
and 0.69 for the wheel track in 2017. Root mean square errors,
RMSE, were 0.010 and 0.015 for undisturbed soil and wheel
tracks, respectively. When validating the modeling setup with theweather, groundwater level, and VWC data for June 2018, the R2
values were 0.97 and 0.54, and RMSEs were 0.026 and 0.040 for
the undisturbed soil and wheel tracks, respectively. These values
indicated that the models could describe sufficiently well the water
fluxes and pressure heads in the soil profiles. The simulated pressure heads at the two microsites U and WT were similar, except
for the surface of the humus layer where it was drier on some occasions in the undisturbed soil (Fig. 3). The average pressure headof the root zone was also influenced during the driest periods withsomewhat lower pressure heads in the undisturbed soil (Fig. 3).
The groundwater level was lower in June 2018 (0.9 m from thesoil surface on 1 June), than in June 2017 (0.5 m), and thus thesurface layer became even drier in the undisturbed soil, comparedto the wheel tracks (Fig. 3). At the 8-cm depth, in the middle of theupper mineral layer, the simulated VWC was higher in the wheel
track compared to the undisturbed soil (Fig. 4). There were no dif-

Table 3. Numbers of days (30 May to 6 Oct. 2017) with daily mean volumetric soil water contents above the threshold value ofan air-filled porosity of 0.10 m3 m-3. Time domain reflectometry (TDR) sensors were positioned at three locations at Site T and atfour locations at Site R in undisturbed soil (U), in wheel tracks (WT), and between tracks (BT).
Site T
TDR locationU

WT

Site R
BT

U

WT

BT

1
45.5 a†
129 b
2c
0a
95 b
0a
2
2a
83.5 b
33 c
0a
0a
0a
3
7a
106 b
0c
0a
19 b
0a
4
0a
7b
0a
† Different letters indicate significant differences between microsites, row-wise comparisons within sites (p < 0.05; Fisher’s exact test).
S110

Soil Science Society of America Journal

Fig. 4. Simulated volumetric water content (VWC, m3 m-3) in the
middle of the upper 5 cm of the mineral soil of the undisturbed soil(U, black lines) and the wheel tracks (WT, gray lines) at Site T during
the growing season 2017 (a) and June 2018 (b).

Fig. 3. Simulated pressure heads (meter) at the surface of the humus
layer (a, b) and the average value of the root-zone (depth of 25 cm;c, d) in the undisturbed soil (U, black lines) and the wheel tracks(WT, gray lines) in the upper 50 m of Site T during the 2017 growing
season (a, c) and June 2018 (b, d). Note the different scale on the yaxis in panel (b).

ferences between the two microsites in evaporation, transpiration
or bottom cumulative fluxes.

Discussion
At these boreal low-productivity sites, the reestablishment ofvegetation and regeneration of trees are slow processes. Four to five
years after clearcutting and off-road traffic, there were still areas ofbare soil, especially in the wheel tracks, not only in the discharge
areas on the lowest part of the slopes but also in the recharge areas up-slope. The reestablishment of vegetation varied between
microsites with more graminoides and less Ericaceae and mosses
in the wheel tracks, a similar pattern as noted in other soil disturbance studies (i.e., stump harvest) in Fennoscandia (Andersson
et al., 2017; Hyvönen et al., 2016). However, no difference wasfound between the number or cover of self-seeded tree seedlingsin the present study. The slow natural regeneration of trees is themain reason why clearcuts like these need to be planted to assure asufficient regeneration of trees as specified in the Swedish Forestry
Act (Swedish Forestry Agency, 2017).
Our results indicate that restricted soil aeration may be a
problem in the wheel tracks, especially in normal to wetter years.
During the 2017 growing season, the estimated air-filled porosity in the wheel tracks was below 0.10 m3 m-3 on the majority of days at Site T (82%) and one fourth of the days at Site R(23%; Table 3). In the study by Fründ and Averdiek (2016) on
skid trails in Germany, no recovery of soil aeration in the 12- to
24-cm depth was found during the first 3 yr after harvester andforwarder traffic. In Finland, the growth of Norway spruce seedlings was reduced when air-filled porosity was below 0.10 m3 m-3(Wall and Heiskanen, 2003); also, when studying mature stands of
Scots pine, the growth-rate was reduced when this threshold was
exceeded (Wall and Heiskanen, 2009).
www.soils.org/publications/sssaj

Wetter conditions in the wheel tracks were indicated by boththe measurements with the portable TDR along the slopes andthe Ellenberg indicator value F (soil moisture). This is commonly
found in soil compaction studies on forestland (Ares et al., 2005;
Fründ and Averdiek, 2016; Wei et al., 2016). However, in dry
years, the higher water-holding capacity in the wheel tracks may
be beneficial for seedlings, as long as the soil is not too compacted
for root penetration. Steber et al. (2007) found that high-porosity
soils often show improved root–soil contact, water holding capacity, thermal regimes and/or nutrient uptake after compaction, if
the forest floor cover is also removed. The mean porosity of theupper 0 to 5 cm of the mineral soil at Site R was 0.59 m3 m-3 inthe undisturbed areas (Hansson et al., 2018a), and moderate soil
compaction at this site may have been beneficial. However, the soil
compaction measured by Hansson et al. (2018a) was substantial
in the upper parts at Site R (mean porosity in WT was 0.45 m3
m-3, i.e., it was reduced by 34%), and the mean dry bulk density(1.44 g cm-3) fell within the range 1.3 to 1.5 g cm-3, at which it
could start to impede growth of various conifer species after theseventh year, according to Zhao et al. (2010). In some places at
Site R, in the study by Hansson et al. (2018a), the dry bulk density was as high as 1.6 to 1.7 g cm-3. These areas are probably toocompacted for optimal root growth (Daddow and Warrington,
1983; Zhao et al., 2010) and this may be one of the explanations
for there still being areas without vegetation on the upper partsof the slopes. At the low slope positions, where the VWC is highall year round, the slow reestablishment of vegetation in the wheel
tracks is probably due to the restricted aeration and because thewheel tracks sometimes contain standing water during snowmelt
and high precipitation events. In mid-slope positions, however, thevisual impression was that both the establishment and growth ofboth planted and self-seeded seedlings were good, especially in the
between-track microsite; this, however, could not be proven withthe experimental design and statistical approach used in this study.
Root and mycorrhizal growth is important for soil structure
recovery after soil compaction (Flores Fernández et al., 2018;
Kabzems and Haeussler, 2005; Meyer et al., 2014), especially innaturally acid podzols as at Sites R and T, where worms and othersoil mixing fauna do not thrive, and an early, thick snow cover often inhibits soil frost. Thus, it is especially important not to exceed the compaction levels where root penetration is reduced or
inhibited, and also to avoid reducing porosities to levels where theS111

air-filled porosity readily falls below 0.10 m3 m-3. However, morestudies are needed to investigate how soil scarification can mitigate
soil compaction effects in the topsoil on these types of sites that are
naturally compact, beneath the topsoil, due to the pressure of theice during the last glaciation.
Differences in the simulated root-zone pressure heads between
the wheel tracks and the undisturbed microsite were small but thedifferences increased during dry periods (Fig. 3). A smaller range ofpressure heads in the wheel tracks compared to the undisturbed soil
is in line with the results presented by Fründ and Averdiek (2016).
However, there were no differences found between the water balances of the two microsites, which was expected for the present
modeling approach when the same weather data, plant descriptions
and groundwater level data were used for both microsites, and theonly difference was in the hydrological properties of the upper 0 to 5
cm of the mineral soil. Nevertheless, the simulations did corroborate
the higher VWC in the wheel tracks than in the undisturbed soil in
the upper parts of the slopes. An explanation for this is that the shapeof the water retention curve caused a higher simulated VWC in thewheel tracks. The water holding capacity was higher in WT for pressure heads more negative than -0.2 m (statistically significant below-0.5 m; Hansson et al., 2018a).
Our study could not detect any statistically significant differences between microsites, locations, and sites with respect tothe number of species present. Generally, there were very few(median 7; range 0–11) species in each plot of 0.4 m2. There
were no differences in species diversity between the microsites,
but regeneration (clearcutting) itself probably increased species diversity compared to the diversity of the mature forest(Widenfalk and Weslien, 2009).
Forest till soils with a high content of rock fragments are difficult to sample, as pebbles, stones, and coarse roots aggravate theinsertion of the sensors. Once a point is found where a TDR canbe inserted to its full length, the sensor cannot be removed and installed again at the same place without causing gaps of air close to
the rods, with underestimation of the VWC as a result. Thus, thehumus layer could not be removed after a proper installation point
was found. The relationship between the dielectric constant and
VWC is different in humus compared to mineral soil (Pumpanen
and Ilvesniemi, 2005; Schaap et al., 1997), and as a consequence,
there is an uncertainty in the VWC results, as the calibration was
performed on mineral soil but the VWC reflects both the humus
layer and the mineral soil.
Combining TDR measurements, vegetation data, and hydrological modeling for estimating soil water conditions in this study
has the advantage that even if measured differences by one method
are not clear, the ensemble of results can add to our knowledge
about the conditions in the soil after off-road traffic on clearcuts.
The VWCs measured with the portable TDR on a single occasion
give a snapshot of the VWC differences in the three microsites
along the slopes, whereas the Ellenberg indicator for soil moisture reflects conditions over time. The number of permanently
installed TDRs may be too few to get clear statistical results withthe linear mixed-effect model, but instead they indicate the VWC
S112

dynamics over the growing season in different parts of the slopes.
Finally, the hydrological model may give us some explanations forthe recorded differences in VWC.
The results of our study support the conclusion of previous
studies that severe soil compaction should be avoided or reduced inorder not to impair regeneration (Daddow and Warrington, 1983;
Zhao et al., 2010). At the same time, intermediate soil compaction
may promote regeneration (Brais, 2001; Cambi et al., 2015; Ponder
et al., 2012), and the rutting in some parts of the slopes may serve as aform of thorough soil scarification. By careful planning, both before
and during the forest operation, the area of severe soil disturbancecan be reduced and driving on slash can further reduce the impact(Han et al., 2006). It is important to follow the growth and development of planted and self-seeded seedlings as well as the ground andfield vegetation at these sites to assess the overall long-term impacts
of off-road traffic. Skid trails or wheel tracks normally cover about
12 to 30% of the clearcuts (Brais, 2001; Eliasson, 2005; Mohtashami
et al., 2017; Solgi and Najafi, 2014), and reduced soil aeration insome parts may be of less importance for the future stand, especially
if they are subjected to soil scarification. However, more investigations are needed on these matters.

Conclusions
To conclude, differences between the three microsites createdby off-road traffic remained 4 to 5 yr after forwarder traffic, mainly
apparent in the cover of different species groups, proportion of baresoil and soil water contents. The risk of restricted aeration was highest in the wheel tracks and was more pronounced during wetter periods on the lower parts of the slopes, where both the VWC and theweighted mean Ellenberg value for soil moisture were higher.
The changed soil physical properties after off-road traffic
in the upper parts of the slope influenced VWCs, having higher
water contents in the wheel tracks than in the undisturbed soil.
However, according to the simulation results, the changed soil
physical properties only influenced the average root zone pressure
head during dry conditions. During dry years, with low groundwater levels, differences will be more pronounced in the top soilof the two microsites, and the increased VWC in the wheel tracks
may then be beneficial for seedling establishment and growth.

Acknowledgments

We would like to thank Erik Bergqvist, Mikael Holmlund and Viktor
Sjöblom for fieldwork assistance; Mikael Andersson (Skogforsk), Johan
Boström (Insitu AB), Maria Blomberg and Örjan Berglund for technicalsupport; Ulf Olsson and Claudia von Brömssen for statistical advice;
Ola Löfgren for help with species identification, Sebastian Sundberg
for adapting the Ellenberg values to Swedish conditions, Sees editingfor proof-reading, and finally Per-Erik Jansson for the idea of looking at
the vegetation to capture the soil water regime and for perspectives andsuggestions on hydrological modelling. The study was funded by the
Swedish Univ. of Agricultural Sciences and its Climate fund, Swedish
Research Council Formas (2010-1168, IMPRESS 2011-1747), and themultidisciplinary research program Future Forests.

Soil Science Society of America Journal

Appendix A

Appendix B

Table A1. Species present in vegetation plots at Sites R and T and their
Ellenberg indicator values, arranged in the groups used in the statisticalanalyses. Light (L), soil moisture (F), pH or soil reaction (R) and nitrogen/fertility (N) values are based on Hill et al. (2004, 2007), and Ellenberg
et al. (1991) and modified for Swedish conditions by Sundberg (pers.
comm., 2018). The soil disturbance (D) values are from Tyler and Olsson(2013). Species without Ellenberg indicator values are denoted (–).
The scientific names are from the Swedish Taxonomic Database (www.
dyntaxa.se).

The 1 m-deep soil profiles were discretized into a
101-node finite element mesh and observation nodes
were inserted at depths of 2, 5, 8, 15, and 30 cm. The van
Genuchten-Mualem single-porosity model with an air entry value of -2 cm was used to describe the soil retention
and hydraulic conductivity functions (van Genuchten,1980). The soil hydraulic parameters (porosity, organic
matter content and the van Genuchten model parameters)
for the 5 cm deep humus (F/H) layer were taken from
Lundmark and Jansson (2009) (“Swedish till” 0–10 cm)
and the saturated hydraulic conductivity, Ks, from Laurén
and Heiskanen (1997) (Table 1). In the upper 0 to 5 cmof the mineral soil (5–10 cm in the model) the properties
are based on the results of the soil physical analyses at Site
T presented in Hansson et al. (2018a) (Table 1). Soil sampling beneath this depth was not possible in the upper partof the slopes at Site T due to the high stone content, buta soil pit was dug out in the lower part and samples wascollected at a depth of 25 to 30 cm and assumed to be representative of the mineral soil in both the undisturbed soil
and the wheel track in the upper part.
The van Genuchten parameters were derived by fitting the van Genuchten function (van Genuchten, 1980)
using nonlinear regression and least squares fitting to themean of the water tension measurements (n = 36, at 0.5,
2, 5, 10, 60, and 1500 kPa) of the soil samples from Site T(diameter: 7 cm, height: 5 cm). The residual water content
parameter, Qr, was not allowed to be lower than 1 × 10-8
(cm3 cm-3) in the fitting process. As a starting point, thepore connectivity factor, l, was set to 0.5 (Mualem, 1976)
for all layers. The selected l-values are based on manual
tuning and inverse solution calibration of both profiles
separately: the values that improved the fitting of the simulated versus observed VWC the most for both profiles
were selected for the first and the third layer (Table 1). For
the second layer (the upper 0–5 cm of the mineral soil),
we separately calibrated the l-value of the microsites as
the x-ray image analysis by Hansson et al. (2018a) showedthat the pore connectivity was reduced in the wheel tracks
compared to the undisturbed soil.
Air temperature at the height of 2 m (T107
Campbell Scientific Ltd.) and precipitation at the groundlevel (tipping bucket ARG100) were measured bihourly
at the site. In addition, global (solar) radiation (pyranometer 2000SZ, Li-Cor Inc., USA); relative humidity(Hygroclip probe, Rotronic AG, Switzerland), and windspeed (A100R Contact Closure Anemometer, Vector
Instruments, UK, at 2 m 2017 and 1.75 m 2018) measured
at Site R were used (the sites are situated 34 km apart). The
wind sensor broke in July 2017 and data from the closest
meteorological station measuring wind speed (Petisträsk)of the Swedish Metrological and Hydrological InstituteScientific name and groupin the analyses
Self-seeded tree seedlings
Pinus sylvestris L.
Picea abies (L.) H. Karst.
Betula spp. (Betula pubescens Ehrh.)
Salix spp. (Salix caprea L.)
Sorbus aucuparia L.
Ericaceae
Rhododendron tomentosum Harmaja
Empetrum nigrum L.
Calluna vulgaris (L.) Hull
Andromeda polifolia L.
Vaccinium myrtillus L.
Vaccinium vitis-idaea L.
Vaccinium oxycoccos L.
Vaccinium uliginosum L.
Graminoides
Avenella flexuosa (L.) Drejer
Luzula pilosa (L.) Willd.
Calamagrostis phragmitoides Hartm.
Eriophorum vaginatum L.
MossesHylocomium splendens (Hedw.) Schimp.
Pleurozium schreberi (Brid.) Mitt.
Ptilium crista-castrensis (Hedw.) De Not.
Pohlia nutans (Hedw.) Lindb.
Aulacomnium palustre (Hedw.) Schwägr.
Dicranum spp.
Polytrichum commune Hedw.
Polytrichum strictum Menzies ex Brid.
Sphagnum spp.
Tiny Bryopsida/Polytrichopsida spp.
Marchantiophyta Stotler & Crand.-Stotl. spp.
OthersJuniperus communis L.
Lycopodium annotinum L.
Gymnocarpium dryopteris (L.)
Equisetum sylvaticum L.
Rubus idaeus L.
Linnea boeralis L.
Melanpyrum pratense L.
Veronica officinalis L.
Chamaenerion angustifolium (L.) Holub.
Epilobium palustre L.
Epilobium adenocaulon Hausskn.
Rumex acetosella L.
Oxalis acetosella L.
Solidago virgaurea L.
Lysimachia europaea (L.) U. Manns & Anderb.
Maianthemum bifolium (L.) F.W. Schmidt
Cladonia Hill ex P. Browne spp.

www.soils.org/publications/sssaj

Site

Ellenberg indicator values
L
F
R
N D

RT
RT
RT
R
RT

7
7
7
7
6

6
6
7
7
6

2
3
3
7
3

3
4
3
7
5

1
1
5
5
5

R
R
R
R
RT
RT
R
R

6
7
7
8
6
6
8
7

9
6
6
9
6
5
9
6

2
2
2
1
2
2
2
2

2
1
2
1
3
2
1
2

5
2
5
1
2
2
1
2

RT
RT
R
R

6
5
6
8

5
5
8
8

2
5
5
2

3
4
3
1

2
4
1
1

RT
RT
RT
RT
R
RT
RT
RT
RT
RT
RT

6
6
5
5
7
4
6
8
8
–
–

5
5
6
5
8
6
7
7
7
–
–

4
2
3
2
3
3
3
2
1
–
–

2
2
2
2
2
2
2
1
1
–
–

2
1
1
–
2
2
1
2
1
–
–

R
T
T
RT
T
RT
RT
T
RT
T
T
T
T
T
T
RT
T

8
6
4
5
6
5
5
6
6
7
7
7
4
5
5
3
–

5
6
5
8
5
5
5
5
5
8
6
5
6
5
6
5
–

4
3
4
5
5
2
2
4
6
5
6
4
5
4
3
3
–

3
3
4
4
7
2
3
4
6
3
6
3
5
4
3
3
–

2
1
1
2
6
1
1
4
5
4
6
4
1
3
1
1
–

Set-Up in Hydrus-1D

S113

(SMHI) were used for the rest of 2017. A variable pressure headboundary condition, based on bihourly groundwater measurements(CS450 Campbell Scientific Ltd.) from the site, was used as thelower boundary condition for water flow. The groundwater level,measured about 12 m up from the down-slope end of the plots in acontrol area, was assumed to follow the topography along the slope
and thus, mimic conditions higher up the slope. However, during 7
to 8 Sept. 2017, the drastic increase in groundwater level down-slope
indicated that water was also coming from up the hill and, to adjust
the dynamics to up-hill conditions, the groundwater level was lowered by 10 cm during the rest of the period. A surface water layer, i.e.,
ponded water, of 1 cm was allowed in the simulations.
The albedo, used in Hydrus to reduce the incoming shortwave
radiation, was set to 0.17, which is representative of old Vacciniumtype clearcuts with Avenella flexuosa (L.) Drejer (Jansson andJutman, 1975). Solar radiation was selected for Hydrus calculationsof cloudiness for estimating the net longwave radiation. The leaf
area index (LAI) was set to 2 m2 m-2 at both microsites. Roots wereassumed to be distributed down to 25 cm in both soil profiles witha distribution based on Persson (1983). The S-shape water up-takereduction model with parameters P50 = -800 cm and P3 = 3 (-)
was used. A default interception constant of 1.5 mm was selected.
The spatial root distribution in both profiles was kept the same, eventhough the roots might have been affected by soil compaction, as we
wanted to focus on the soil water differences caused by the different
soil physical properties in the upper 0 to 5 cm of the mineral soil.
The initial conditions were selected so that the pressure heads
at all depths were in equilibrium with the measured groundwaterlevel of that day. If not otherwise noted in the description above, thedefault settings in Hydrus-1D were used.

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