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Soil Science Society of America Journal 65:224-231 (2001)
© 2001 Soil Science Society of America

DIVISION S-7-FOREST & RANGE SOILS

Skidder Traffic Effects on Water Retention, Pore-Size Distribution, and van Genuchten Parameters of Boreal Forest Soils

A.D. Startsev and D.H. McNabb

Forest Resources, Alberta Research Council, PO Bag 4000, Vegreville, AB, Canada T9C 1T4

Corresponding author (andrei{at}arc.ab.ca)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Compaction by harvesting equipment has a potential to decrease the quality of root environment through alteration of soil pore space. Effects of skidder traffic on soil water retention and pore-size distribution were studied on medium-textured Inseptic and Oxyaquic Haplocryalfs, and Typic Dystrocryepts at 14 sites in Alberta foothill and boreal forests. Treatments included 3, 7, and 12 cycles by skidders equipped mostly with wide tires. Soil cores were collected at four random points of each treatment at an average depth of 5 and 10 cm. Soil water content of each core was measured at six potentials. Tempe cells were used to measure water retention at potentials from -2 to -30 kPa; a conventional pressure plate was used in the range of -100 to -1500 kPa. The four-parameter water retention function developed by van Genuchten (1980) was fitted to the data. The pore space of trafficked soil was not significantly affected when soil was drier than field capacity. At higher soil water contents, the first few skidding cycles caused a decrease in {theta}s and {alpha} parameters, which reflected flattening of water retention curves in the high potential range and a simultaneous shift of the steepest part of the slope to a lower potential. The result of compaction was a decrease in air-filled porosity below 0.10 m3 m-3, which restricted aeration without changing field capacity or available water holding capacity. Most modifications of soil pore space by wide-tired skidders can be avoided if the soils are drier than field capacity.

Abbreviations: ANOVA, analysis of variance


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
VEHICULAR COMPACTION affects pore-size distribution and water retention because the decrease in soil volume can only occur as a result of the compression of pore space. Most of the decreases in water retention occur at high potential where changes in different groups of macropores affect air-filled porosity and water availability to plants (Warkentin, 1971; Dickerson, 1976; Froehlich and McNabb, 1984; Allbrook, 1986; Bruand and Cousin, 1995). Soil water retention at lower potentials may not be affected or may increase due to an increase in smaller pores at the expense of larger pores that are compressed during compaction (Hill and Sumner, 1967).

The relationship between soil compaction and water content is usually included in models predicting soil compaction (Amir et al., 1976; Raghavan et al., 1977; McNabb and Boersma, 1993). However, the dependence of compaction-induced changes in soil water retention and pore-size distribution on water content are not well documented.

Most soil modifications, such as compaction, puddling, or rutting, occur during the first few trips of a skidder (Hatchell et al., 1970; Froehlich and McNabb, 1984; Greene and Stuart, 1985; Rollerson, 1990; Meek, 1994). Further trafficking causes a progressively smaller decrease in soil volume and, consequently, a smaller decrease in total pore space. The optimal conditions for compaction often occur at a water content near field capacity (Akram and Kemper, 1979; Soane et al., 1981; Gent and Morris, 1986). Forest machine trafficking may have little effect on drier soils (Greene and Stuart, 1985).

Analysis of effects of compaction on water retention of forest soils is often limited to comparing water contents at specific levels of pressure or differences between them, for example, the macropores, micropores, field capacity, air-filled porosity, and available water holding capacity. A parameterization of the curve allows a more integrated understanding of changes in pore-size distribution. Lenhard (1986) related the pore-size distribution index of Brooks and Corey (1966) to the number of vehicular passes on volcanic ash soil. Jorge et al. (1992) found a significant difference between linearized water content and water potential relationships for a compacted and undisturbed sandy loam soil using covariance analyses. The objective of this study was to determine changes in soil water retention and pore-size distribution functions that result from trafficking medium-textured soils by skidders.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Fourteen study sites were selected in mature conifer stands across west-central Alberta where forest harvesting and silvicultural operations in summer are most common (Table 1). The area is part of the Southern Alberta Uplands ecodistricts of the Lower and Upper Boreal Cordilleran ecoregions (Strong, 1992). The sites are dominated by lodgepole pine (Pinus contorta Dougl. ex Loud.) or white spruce [Picea glauca (Moench) Voss] with a small component of black spruce [Picea mariana (Mill.) Britton et al.] or aspen (Populus tremuloides Michx.) depending on soil wetness. Aspen is more typical of the Lower Cordilleran ecoregion (Corns and Annas, 1986). Soils were predominantly coarse and fine-loamy mixed superactive Inseptic and Oxyaquic Haplocryalfs that vary in degree of reduction and depth to redoximorphic features. Soils at the two southern sites were coarse and fine-loamy mixed superactive Typic Dystrocryepts. Soils were of similar texture due to their formation in similar till materials (Strong, 1992).


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  		Table 1. Characteristics of soil and types of equipment used at 14 study sites in west central Alberta

 
Most sites were skidded with rubber-tired grapple skidders (John Deere 648E, 748E, Deere & Company, Moline, IL; or Timberjack 480B, 450C, Timberjack, Helsinki, Finland), but two sites were skidded with 3-axle forwarders (Valmet 540, Valmet Corp., Jyvaskyla, Finland; or Timberjack 520A) and one site was skidded with a tracked crawler (Caterpillar D4H TSK, Caterpillar, Peoria, IL) (Table 1). Most grapple skidders were equipped with tires 1.1 m wide, at least on the rear axle. At each site, a skidding machine or forwarder made 3, 7, or 12 cycles (one empty and one loaded pass) in a designated skidding corridor that was marked in felled timber prior to skidding (Table 1). Each corridor was 40 m long and 6 m wide and was separated by a 10-m-wide untrafficked control area. This treatment block was replicated four times at each site. All blocks were skidded as part of normal harvesting operation.

Immediately after skidding, four sampling points were randomly selected within each treatment. Soil cores, 3 cm high and 7.6 cm in diameter, were collected at the midpoint of the 5-, 10-, and 20-cm depths at each point in the four treatment blocks for determination of bulk density. The block with the most representative soil was selected at each site for sampling soil for measurement of water retention. In this block, undisturbed soil cores, 3 cm high and 5.2 cm in diameter, were collected at each sampling point from the midpoint of the 5- and 10-cm depths. All core samples were collected in thin-walled metal rings that were pressed into the soil by hand (McNabb and Boersma, 1993). Cores for water retention were sealed in plastic wrap and stored at 4°C to reduce fungal and bacterial growth and to maintain soil water content until analyzed.

Water retention was measured on these cores at -2, -5, -10, -30, -100, and -1500 kPa potential. Tempe pressure cells (Soil Moisture Equip. Co., Santa Barbara, CA) were used for water potentials between -2 and -30 kPa to reduce swelling effect (Reginato and van Bavel, 1962). The pressure in the Tempe cells was maintained within ±0.02 kPa using a pressure transducer and solenoid valve connected to a 7X datalogger (Campbell Scientific, Logan, UT). A pressure plate extractor was used at lower water potentials (Klute, 1986).

A four-parameter equation (van Genuchten, 1980) was fit to the soil water retention data using the Marquardt (1963) algorithm. Volumetric soil water content ({theta}, m3 m-3) as a function of water potential ({psi}, kPa) is given by

(1)
where {theta}r (m3 m-3) is the residual water content and {theta}s (m3 m-3) is the saturated water content; {alpha} and n are empiric parameters. The residual water content is defined as the water content in the range of low potentials for which the d{theta}/d{psi} becomes indefinitely small. In practice, it is sufficient to define {theta}r as the water content at the lowest water potential measured, for example, -1500 kPa (van Genuchten, 1980). Values of {alpha} and n are obtained for each individual core during fitting procedure.

Differentiation of Eq. [1] provides a quantitative measure of the change in the slope of the soil water retention curve that is otherwise difficult to assess because of the scale over which data are collected. Differentiation of Eq. [1] gives

(2)

The {alpha} parameter is inversely related to the maximum value of d{theta}/d{psi} (Wosten and van Genuchten, 1988).

The largest effective diameter D (m) of pores retaining water at suction h (m) was calculated by the capillary equation of Vomocil (1965)

(3)
where {sigma} (kg s-2) is the surface tension of water, {gamma} (degrees) is the contact angle between pore wall and water, {rho} (kg m-3) is the density of water, g is acceleration due to gravity (m s-2), and C is a constant which equals [(4{sigma} cos{gamma})/({rho}g)].

The volumetric water content of soil at -10 kPa was assumed to be field capacity. Available water holding capacity and air-filled porosity were calculated as differences between water contents at -10 and -1500 kPa and between saturation and -10 kPa, respectively (Vomocil, 1965).

Effects of skidding traffic on bulk density, the four parameters of the water retention function, field capacity, available water holding capacity, and air-filled porosity were analyzed using analysis of variance (ANOVA) with depth as a repeated factor (SAS Institute, 1991).


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Soil wetness, as indicated by soil water potential, varied widely across the sites at the time of skidding and was the dominant factor determining whether bulk density increased significantly during skidding (McNabb et al., 2001). A significant increase in bulk density occurred after three skidding cycles at eight sites where water potential was higher than about -15 kPa. The six sites where soil water potential was less than -15 kPa were not significantly compacted after 12 skidding cycles (Table 1).

Data for the four parameters (Eq. [1]) are given in Table 2. Site had a significant effect on all four parameters (Table 3). The number of skidding cycles only had a highly significant effect on the {alpha} parameter. Depth had a significant effect on two of the four parameters but its interaction with treatment was not significant, indicating that the effect of compaction was similar at the depths measured (McNabb et al., 2001).


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  		Table 2. Parameters of the water retention function fitted to van Genuchten's equation (Eq. [1]) for 14 sites subjected to different levels of skidder traffic

 

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  		Table 3. Summary of repeated measure analysis of variance (with depth as repeated factor) of parameters of the water retention function fitted to van Genuchten's equation (Eq. [1])

 
Site accounted for about one-half of the variation in the ANOVA when all sites were combined. The sites were then coded for whether they were significantly compacted or not for subsequent analyses (Table 1); classification by compaction accounted for about two-thirds of the variation among sites. Compaction had a significant effect on three water retention parameters. Only the effect on {theta}r, which is related to the water content at the lowest water potential, did not change (Table 3). The number of skidding cycles only had a significant effect on {alpha}. The main and interactive effects of depth did not change from the previous ANOVA of all sites because depth was the same repeated factor.

Reanalysis of data for sites classified as significantly compacted from those that were not found that site remained significant for {theta}r for both groups (Table 3). The {theta}s and {alpha} parameters were both significant for the group of sites that were significantly compacted, but only the {alpha} parameter for the nonsignificantly compacted sites maintained the same level of significance as the original analysis. The significance of {theta}r for water retention at the lowest water potentials reflected the dominant influence that soil texture has on this parameter, which is similar for our two groups of sites (Table 1). The {alpha} parameter remained significant for the noncompacted group of sites because the level of statistical nonsignificance increased with decreasing water potential (Table 1). For example, compaction at Site 9 was significant at P < 0.07 in the original ANOVA and is barely different from Sites 5 and 12, which are the driest soils in the nonsignificantly compacted groups of sites (Tables 1 and 3).

Data were combined by the presence or absence of significant compaction and by depth, and these data were used to develop a common set of parameters for the two groups of sites (Table 4). The differences in the parameters of the water retention function for sites with significantly compacted soil vs. noncompacted sites is apparent in the generalized water retention curve for the two groups of sites (Fig. 1) . The difference in the slope of the water retention curve as a result of skidding is even more apparent in the slope of the derivative function curve.


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  		Table 4. Effects of different levels of skidder traffic on parameters of the water retention function fitted to van Genuchten's equation (Eq. [1]) and generalized for two groups of sites that differ by the presence or absence of significant at P < 0.05 compaction following three skidding cycles (Table 1)

 


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  		Fig. 1. Effects of different levels of skidder traffic on the soil water retention curves generalized for two groups of sites separated by the presence or absence of significant compaction following three skidding cycles (Table 1). Curves represent van Genuchten's equation (Eq. [1]) and its derivative function (Eq. [2]) with the parameter estimates as in Table 4

 
Little change in the shape of either curve with increasing level of traffic on the nonsignificantly compacted soil confirms the lack of significant increase in bulk density at these sites (Fig. 1). The soil water retention of significantly compacted soil decreased between saturation and approximately field capacity (-10 kPa). The greatest change occurred in pores at a water potential -5 to -6 kPa; this corresponds with effective diameters of 48 to 58 µm as calculated using Eq. [3]. The peak of the derivative curve is the steepest part of the slope of the water retention curve and is most sensitive to minor changes in pore distribution resulting from compaction. These changes in pores are not evident when the data are plotted on a logarithmic scale. Soil compaction compressed the peak of the derivative curve and shifted the peak to a lower water potential. The shift in the peak is associated with the significant difference in the {alpha} parameter between the control and three skidding cycles, and between three and seven skidding cycles (Table 4). The peak of the control soil is somewhat higher and its water potential is lower at the sites where compaction was not significant, but could not be attributed to any specific differences in soil among sites.

The d{theta}/d{psi} peak corresponds with the part of the pore-size distribution curve where the pore volume starts to increase sharply with decrease in pore diameter (Fig. 2) . This is the point where most of the changes occur during compaction. The decrease in the slope of the water retention curve and shift of its steepest part reflected a pore volume reduction in the range between 20 and 200 µm with maximum reduction in the 30- to 60-µm pores. Smaller pores were filled with water, according to the water potentials measured at the sites at the time of skidding. There was no reduction in pores larger than 200 µm because few pores of this size are present in undisturbed soils.



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  		Fig. 2. Effects of different levels of skidder traffic on the cumulative water content and its derivation as a function of the effective pore diameter (pore-size distribution) depending on the presence or absence of significant compaction following three skidding cycles (Table 1)

 
Most of the changes in the shapes of water retention curves and pore-size distribution occurred at a water potential higher than field capacity. Therefore, trafficking did not change field capacity or available water holding capacity of these soils (Table 5). Air-filled porosity decreased significantly after the first three cycles at the sites where the soil was significantly compacted. The differences in air-filled porosity between 3, 7, and 12 skidding cycles were not significant.


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  		Table 5. Effects of different levels of skidder traffic on field capacity, available water holding capacity, and air-filled porosity of soil generalized for two groups of sites that differ by the presence or absence of significant at P < 0.05 compaction following three skidding cycles (Table 1)

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
CONCLUSIONS
 REFERENCES
 
The effective pore diameters and pore volumes of soil calculated from water retention curves are often used to subdivide soil porosity into classes such as macro-, meso-, and micropores that are indicative of the soil hydrologic and biological environments (Luxmoore, 1981). In our boreal forest soils, few macropores occur naturally, as evident by the flat slope of the water retention curve between saturation and -0.3 kPa water potential (Fig. 1). The presence of few macropores in these soils is considered to be consistent with their genesis and other soil properties. The soils in the region are young, developing in tills that were deposited during the last glaciation <10000 yr ago (Mayewski et al., 1981). Soil biological activity is suppressed because of the cold northern climate (Startsev et al., 1998), and organic matter content of mineral surface soils is commonly <0.02 kg kg-1 (McNabb, 1994). As a consequence, aggregates formed in these soils are generally small and weak (McNabb, 1994, unpublished data), suggesting that physical and mechanical processes dominate their development rather than biological and chemical processes (McNabb, 1994).

Essentially all of the air-filled porosity of the significantly compacted soils were in the mesopore size class, pores drained of water between -10 to -0.3 kPa water potential (Fig. 1). Soil compaction at these sites caused a significant increase in bulk density and reduced mesopore space by more than one-third after three skidding cycles and two-thirds after 7 and 12 cycles (Table 5). The reduction in mesopore space occurred because the pores collapsed during compression. Compaction of wet soil is much more likely to cause deformation of soil aggregates that collapse the large pores than the fracturing of aggregates, which occurs in drier soil (Hodek and Lovell, 1979). The fracturing of aggregates produces fragments that fill the mesopores, creating additional micropore space. The collapse of the mesopores produces relatively few new micropores compared with the number in undisturbed soil; hence compaction did not cause a noticeable change in the shapes of the water retention curve and its derivative function at water potential less than -10 kPa. The air-filled porosity remaining in these soils after compaction is attributed to air being trapped in the soil that could not escape during trafficking, and the gas diffusion in compacted soil essentially stops at an air-filled porosity <0.10 m3 m-3 (Xu et al., 1992). Anoxic soil environment can adversely affect root growth directly through deficient O2 supply and indirectly as a result of anaerobic processes that develop in these soils in the postharvest period (Startsev et al., 1998).

Soil compaction has a variable effect on the water holding capacity of forest soils (Froehlich and McNabb, 1984). A significant increase in bulk density did not affect the parameters of field capacity, permanent wilting point, and available water holding capacity in these soils (Table 5) because the changes in soil porosity were essentially confined to the mesopore space while the micropore space remained unaffected (Fig. 1). The stresses transferred to soil by the equipment used in this study were insufficient to cause significant compaction or a change in the water retention curve of these soils when the water potential was less than about -15 kPa.

The analyses of water retention curves of noncompacted and compacted soil provide a more detailed interpretation of how soil changes as a result of compaction than is evident from measuring bulk density. Additional information can be gained from studying the water retention curve, particularly the effect on the macro- and mesopore space (Fig. 1). But much of the change in the water retention curve as a result of compaction occurs at the steepest point on the curve of undisturbed soil, which makes simple analyses difficult. A more quantitative analysis of water retention data and associated changes in pore-size distribution is possible if parameters of models fit to the data or the derivatives of the model are analyzed (van Genuchten, 1980; Kosugi, 1996). In our study, the {alpha} parameter in Eq. [1] was the most sensitive to the effects of soil compaction, and the derivative of the water retention function was particularly sensitive to where the treatments changed the shape of the curve. Both analyses are powerful tools for analyzing the effects of soil compaction on porosity and soil structure.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
REFERENCES
 
Wide-tired skidders operating on poorly structured medium-textured boreal forest soils did not affect significantly the shape of water retention curve and pore-size distribution when the soils were drier than field capacity. At higher soil water contents, the effects were limited to the air-filled pores, which suggests that damage to soil structure is lessened and integrity of the aggregates is maintained despite the compaction. However, the loss of air-filled pore space below the critical level of 0.10 m3 m-3, which is indicative of deficient soil aeration, is a serious consequence of soil compaction by harvesting equipment, which can adversely affect soil processes, root environment, and forest regeneration in the region.

The van Genuchten (1980) equation provided a useful means of assessing differences in soil conditions resulting from compaction. Differences in water retention and pore-size distribution functions of compacted and noncompacted soils were more apparent through analysis of van Genuchten parameters than through evaluation of bulk density, available water holding capacity, or aeration porosity.


   ACKNOWLEDGMENTS
 
The authors gratefully acknowledge financial support from Alberta Department of Environmental Protection, Alberta Forest Development Research Trust, Foothill Model Forest, Canadian Forest Products LTD, Weldwood of Canada LTD-Hinton Division, Weyerhaeuser Canada LTD-Grande Prairie, Sundance Forest Products LTD, Sunpine Forest Products LTD, Millar Western Industries LTD, and Alberta Newsprint Company. The authors thank Mrs. Susan Paquin for collecting soil samples and measuring soil water retention and Mr. Hai Van Nguyen for his assistance in conducting statistical analyses.

Received for publication September 21, 1999.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 




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