Soil physical properties degrade further on skid trails in the year following operations

Ramin Naghdi·Ahmad Solgi·Eric K.Zenner·Farshad Keivan Behjou

Introduction

Soil compaction is the physical consolidation of the soil by an applied force that destroys structure,reduces porosity,limits water and air in filtration,increases resistance to root penetration and nutrient acquisition(Greacen and Sands 1980;Quesnel and Curran 2000;Zhao et al.2010),and ultimately restrains plant growth(Meyer et al.2014;Kozlowski 1999).The magnitude of soil compaction depends on many factors,such as traf fic intensity,slope,site characteristics,harvesting machines,planning of skid roads and production season(Naghdi and Solgi 2014;Solgi and Naja fi2014;Naja fiet al.2009;Demir et al.2007;Eliasson and W?sterlund 2007;Laffan et al.2001).

Traf fic intensity(the number of passes of a harvesting machine on a skid trail)plays an important role in compaction,because soil deformations typically increase with the number of passes and can lead to excessive soil disturbance(Solgi and Naja fi2014;Mosaddeghi et al.2000).Bulk densities are non-linearly related to traf fic intensities(Naja fiet al.2010;McNabb et al.1997),indicating that the majority of the surface compaction occurs during the first few passes,but the amount and depth to which bulk density may increase grows with more intensive traf fic(Brais and Camiré1998;Williamson and Neilsen 2000).Impacts of traf fic intensity can deteriorate soils even more on steeper slopes(Naja fiet al.2009),presumably because a given load develops a more uneven weight balance on the axles(usually the rear axle is weighted more heavily)during skidding operations on steeper terrain.Similarly with increased traf fic intensity,traf fic on steeper slopes can increase soil disturbance in both extent and depth(Naja fiet al.2009).

While much is known about the negative effects of machine traf fic on the physical structure of forest soils(e.g.,Greacen and Sands 1980;Kozlowski 1999;Williamson and Neilsen 2000),much less is known about soil resilience to machine traf fic.Soil resilience,de fined as the ability of the soil to recover after a disturbance(Greenland and Szabolas 1994),depends primarily on natural soil processes(Froehlich and McNabb 1984).Processes such as swelling and shrinking as a result of moisture changes(humid-dry cycles),movement of soil particles due to cycles of freezing and thawing(including frost heave),and biological factors such as activities of soil biota and vegetation(particularly root penetration)aid the recovery of soil physical properties toward pre-disturbance conditions(Reisinger et al.1988).Three factors appear to be necessary for effective recovery processes:(1)the soil must be sensitive to the processes;(2)the climate must produce the temperature and moisture regimes necessary for the processes to occur;and,(3)the cycles or processes must occur with suf ficient frequency and duration(Froehlich and McNabb 1984).

Despite the potential for recovery,natural recovery of compacted soil is a very complex and slow process that can take between 5 and 140 years depending on the soil type,degree of compaction,and climate(Greacen and Sands 1980;Froehlich et al.1985;Webb et al.1986;Rab 2004;Ezzati et al.2012).Under cool temperate conditions,the consequences of soil disturbance by harvest machinery may persist for decades in clay to silt loam soils(Froehlich et al.1985;Greacen and Sands 1980;Rab 2004).In addition to the necessary factors that drive effective recovery processes,recovery rates of soil physical properties are strongly in fluenced by soil type,vegetation,soil biota,and climate as well as by logging-related factors such as the degree of compaction and the depth of compacting layer(Ezzati et al.2012;Zenner et al.2007;Rab 2004;Webb 2002).For example,if the soil is not highly compacted,repetitions of alternate dry and wet periods can reduce compaction in a clay soil to a much higher degree than in sandy soils(Farrakh Nawaz et al.2013).Further,natural recovery of the top 10 cm of physically disturbed soil is possible at a more rapid pace than at lower depths where recovery processes are much slower due to reduced burrowing activities of macro-invertebrates associated with dung deposition that are most pronounced between the soil surface and a depth of about 5 cm(Drewry 2006).Because the short-term,natural recovery of the soil physical condition is typically restricted to surface layers(Drewry 2006),in this study we concentrated on the short-term recovery of these layers.Speci fically,we focused on the recovery of bulk density and porosity that closely re flect the effects of soil compaction and are regarded as the most important indicators for soil recovery(Heinonen 1977;Farrakh Nawaz et al.2013).

Speci fic objectives of this study were:(1)to quantify the extent of compaction associated with different harvest traf fic intensities and different slope gradients using four important soil physical characteristics,(i.e.,bulk density,total porosity,macroporosity and microporosity);and,(2)to investigate whether the recovery of these soil characteristics 1 year following operations differed among plots subjected to different traf fic intensities on different slope gradients.

Materials and methods

Study site

This research was conducted between June 2013 and June 2014 in the Shenrood Forest,Guilan province,northern Iran(36°59′N and 37°1′N and 50°3′E and 50°7′E).The harvested stand was mixed Oriental beech(Fagus orientalis Lipsky)and hornbeam(Carpinus betulus L.)with a canopy cover of 85%,a stand density of 180 trees/ha,an average diameter at breast height of 32.5 cm,and an average height of 21.8 m.The elevation of the research area is approximately 1400 m above sea level with a northerly aspectand an average annualrainfallof 1200 mm.Themaximum meanmonthlyrainfallof 120 mm usually occurs in October,while the minimum rainfall of 25 mm occurs in August.The mean annual temperature is 15°C,with the lowest values in February.At the time of skidding with an ‘Onezhets 110’crawler tractor(Table 1),weather conditions had been damp,with an average soil moisture content of 28%.One year after skidding(June 2014),the average soil moisture content was 21%.Texture was analyzed using the Bouyoucos hydrometer method and was determined to be clay loam along the skid trail.The soil had not been driven on before the experiment.

Experimental design and data collection

The initial effects of combining g frequency of skidder passes and trail slope gradients on soil physical properties were studied in comparison with undisturbed nearby areas.Recovery ofphysicalproperties was quanti fied bycomparing 1 year post-skidding soil bulk densities and porosities sampled in proximity to the original locations on skid trails to immediate post-skidding values and to values from undisturbed areas.Sampling was done along a skid trail that covered a range of longitudinal gradients,including portions without any lateral slope and a maximum slope of 31%in the steepest portion.To compare our results to previous findings,we combined our data into two classes(≤20%and＞20%).Slope class≤20%included skid trail sections ranging from 3 to 15%,whereas the slope class＞20%contained sections from 24 to 29%.In addition to the two slope gradients,treatment plots included three levels of skidder traf fic intensity:light,moderate and heavy(Fig.1).Each combination ofsoilgradient×traf fic intensity was replicated three times.

Table 1 Features of the ‘Onezhets 110’tractor

Moreover,for control purposes,samples were taken from an undisturbed area 50–60 m away from the skid trail(two tree lengths to reduce side impacts)not exposed to skidding impact.Hence,24 sampling plots(6 control and 18 sample plots)were established.Each was 10 m long by 4 m wide,with a minimum buffer of 5 m between each plot(Fig.1).Plots included four randomized 4-m wide samples placed across the wheel tracks perpendicular to the direction of travel,with a 2-m buffer zone between lines to ensure subsampling covered the entire plot.Along each line,one sample was taken from a depth of up to 10 cm at three different points:left track(LT),between track(BT),and right track(RT).Sampling was repeated 1 year later in locations that were within 30 cm of the previous year’s sample locations.

Soil samples from 0 to 10 cm depth were collected,placed in polyethylene bags and brought to the laboratory for weighing.Samples were oven-dried at 105°C for 24 h and moisture content measured gravimetrically after drying(Kalra and Maynard 1991).

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Soil bulk density was calculated as:

where Db is dry bulk density(g cm-3),Wdthe weight of the dry soil(g),and VC the volume of the soil cores(196.25 cm3).

Fig.1 Sampling layout with four sample lines placed perpendicular to the direction of skidder traf fic.Sample plots were placed on top of the left wheel track(LWT),between the wheels(BWT),and on top of the right wheel track(RWT)

Total soil porosity was calculated as:

where AP is the total porosity(%),Db the dry bulk density(g cm-3),and 2.65(g cm-3)the particle density.

Macroporosity was calculated as:

where MP is the macroporosity(%),Db the dry bulk density(g cm-3),2.65(g cm-3)the particle density,and θmthe water content on a mass basis(%).

Microporosity was calculated as:

where MIP is the microporosity(%),AP the total porosity(%),and MP the macroporosity(%).

Statistical analysis

Two-way ANOVAs were performed using the SPSS 11.5 software to analyze the effects of traf fic intensity and gradient on soil physical responses.Mean values of soil physical properties of each plot were compared with those in undisturbed areas using Duncan’s multiple range test(Zar 1999).One-way ANOVA(signi ficance test criterion α≤0.05)compared soil physical properties amongst the three traf fic intensities(main effects)with those in undisturbed areas.Paired t-tests analyzed soil property data in the two sampling periods and in the two slope gradients for each period at an α-level of 0.05.

Results

Dry bulk density,total porosity,microporosity,and macroporosity showed statistically signi ficant differences between samples taken from skid trails at a depth of 0–10 cm and those from undisturbed areas(Table 2).

Dry bulk density

Average dry bulk densities across three traf fic intensities and two slope gradients increased from 0.7 g cm-3in undisturbed plotsto 1.26 g cm-3(0.96–1.49 g cm-3)immediately after skidding, and to 1.40 g cm-3(1.08–1.64 g cm-3)1 year after skidding(Tables 2,4).After 4,10,and 16 passes,average dry bulk density increased by 52,86,and 103%,respectively,compared to undisturbed areas.In both time periods,dry bulk density increased statistically signi ficantly with traf fic intensity(p＜0.05)and skid trail slope(p＜0.05),but traf fic intensity×skid trail slope interaction was not statistically signi ficant(Tables 3,4).Compared to values of dry bulk density immediately after skidding,no signi ficant reduction was detected on any portion of the skid trail 1 year after skidding.In fact,bulk density values in the top 10 cm of the soil pro file were greater(+11%)1 year after skidding than immediately after skidding,indicating that no recovery in had taken place since the initial disturbance(Figs.2,3).

Total porosity

Mean total porosity declined from 67%in undisturbed plots to 50%(40–59%)immediately after skidding and to 44%(33–57%)1 year later(Tables 2,4).In both time periods,total porosity was signi ficantly reduced with increasing traf fic intensity and slope gradient(Tables 3,4;Fig.4).In each traf fic intensity class,mean total porosity was consistently lower on steeper than on shallower slopes;the reduction in total porosity was more severe on steeper slopes for the same traf fic intensity.Compared to undisturbed and traf ficked areas immediately after skidding,no signi ficant recovery of total porosity was observed after 1 year in any of the traf fic intensity×slope gradient combinations.In fact,1 year after skidding,total porosity in the top 10 cm of the traf ficked pro file was a further 11%increased than immediately after skidding.

Macroporosity

Mean macroporosity decreased from 47%in undisturbed plots to 24%(13–34%)immediately after skidding and to 17%(5–32%)1 year after skidding(Tables 2,4).In both time periods,macroporosity was statistically signi ficantly reduced with increasing traf fic intensity(p＜0.05)and slope gradient(p＜0.05),but traf fic intensity×slope interaction was not signi ficant(Table 3).Macroporositywas considerably lower in traf ficked areas compared to undisturbed areas on both slopes(Fig.5)and on steeper ones regardless of traf fic intensity(Table 4).Compared to undisturbed and traf ficked areas immediately after skidding,macroporosity in the top 10 cm of the traf ficked soil pro file did not recover after 1 year in any of the traf fic intensity×slope gradient combinations and was a further 28%lower than immediately after skidding.

Table 2 Average bulk density,total porosity,macroporosity,and microporosity in undisturbed areas and on skid trails,immediately after skidding operations(IAS)and 1 year later(IYAS)

Table 3 P values based on analysis of variance on the effects of traf fic intensity,slope,and their interaction on different soil physical properties for two time periods

Table 4 Mean values of different soil physical properties by time period(IAS:immediate after skidding;IYAS:1 year after skidding)for trails exposed to different traf fic intensities on two slope classes letters indicate statistically signi ficant differences(α ≤ 0.05)among group means based on Duncan’s multiple range test

Fig.2 Mean bulk densities on slope classes averaged across traf fic intensity classes.Letters indicate statistically signi ficant differences(α ≤ 0.05)among group means based on Duncan’s multiple range test

Fig.3 Mean bulk densities of trail sections exposed to different traf fic intensities averaged across gradient classes immediately termination and 1 year later.Letters indicate statistically signi ficant differences(α ≤ 0.05)among group means based on Duncan’s multiple range test

Fig.4 Mean total porosity on skid trails exposed to different traf fic intensities averaged across slope classes immediately after termination of operations and 1 year later.Letters indicate statistically signi ficant differences(α ≤ 0.05)among group means based on Duncan’s multiple range test

Fig.5 Mean macroporosity exposed to different traf fic intensities averaged across gradient classes immediately after and termination and 1 year later.Letters indicate statistically signi ficant differences(α ≤ 0.05)among group means based on Duncan’s multiple range test

Microporosity

Mean microporosity increased from 21%in undisturbed plots to 26%(24–27%)immediately after skidding and to 27%(25–31%)1 year later(Tables 2,4).In both time periods,microporosity increased signi ficantly with traf fic intensity(p＜0.05)and slope gradient(p＜0.05),but traf fic intensity×slope interaction was not signi ficant(Table 3).Microporosity increased in traf ficked compared to undisturbed areas on both slopes(Fig.6)and was signi ficantly greater on steeper than on more shallow slopes,regardless of traf fic(Table 4).Compared to undisturbed and traf ficked areas immediately after skidding,microporosity in the top 10 cm did not recover after 1 year in any of traf fic intensity×slope gradient combinations and was 5%greater than immediately after skidding.

Fig.6 Mean microporosity exposed to different traf fic intensities averaged across gradient classes immediately after termination of operations and 1 year later.Letters indicate statistically signi ficant differences(α ≤ 0.05)among group means based on Duncan’s multiple range test

Discussion

Soil physical changes of increased bulk density,decreased total porosity and macroporosity,and increased microporosity observed in this study are neither unexpected nor new findings.When a crawler tractor with a weight of 11.2 t drives over a soft forest soil with an average moisture content of 28%,physics will inevitably lead to increased compaction with the accompanying features of reductions in total and macroporosity.The main result of this study,however,i.e.,the continued degradation of physical properties documented 1 year following the termination of operations,was not expected.This indicates that the recovery of soils following skidding is at best delayed,if not altogether compromised,by failing to properly retire abandoned skid trails,particularly on steeper slopes.

Overall magnitudes of increase in soil compaction with increasing traf fic intensities,with most of the increases in compaction/bulk density or decreases in total/macroporosity occurring after the first few passes,are consistent with other studies that have similarly documented a＞50%increase in bulk density after three passes and with continued but lower rates of increases thereafter(Ampoorter et al.2007;Zenner et al.2007;Naghdi and Solgi 2014).The reduction of total porosity is typical and occurs at the expense of the large air- filled pores in soil surface layers due to a conversion of soil macropores to micropores(Williamson and Neilsen 2000).The reduction in macropores is a typical effect of harvest traf fic(Rab 1996;Blouin et al.2005;Ezzati et al.2012).It is noteworthy that the＜50%reduction of macropores and the＜25%increase of micropores at moderate and severe traf fic levels is considerably lower than the 68–89%reduction in macropores and much more modest than the 75%increase in micropores reported for a similar soil exposed to moderate and severe compaction treatments(Shestak and Busse 2005).Nonetheless,signi ficant decreases in macroporosity and corresponding increases in microporosity have been shown to cause poor aeration,reduce permeability to water,produce waterlogged conditions,and inhibit gaseous exchange between soil and air(Greacen and Sands 1980).

The signi ficantly more negative effects of harvest traf fic on bulk density,total porosity,and macroporosity on steeper slopes observed in this study have also been documented in other studies(e.g.,Naja fiet al.2009;Ezzati et al.2012;Jourgholami et al.2014;Solgi et al.2014).These negative effects are is most likely due to a different machine operation on steeper terrain that causes higher stress,particularly under the front chain track when the weight of the vehicle is transferred to a lower contact area(Marsili et al.1998).In addition,lower average travel speeds of machines on steeper slopes typically cause more transfer of engine vibration to the top soil and enhance adverse effects on soil physical properties(Naja fiet al.2009;Ezzati et al.2012;Solgi et al.2014).

Without ameliorative treatments,natural recovery of a compacted soil is a slow process that can take many decades to return to pre-disturbance levels(Greacen and Sands 1980;Froehlich et al.1985;Webb et al.1986;Rab 2004;Webb 2002;Ezzati et al.2012).This is of particular concern in regions with milder climates such as the temperate forests of Iran that lack signi ficant freeze–thaw cycles that speed up recovery processes(cf.Froehlich and McNabb 1984).Freeze–thaw cycles may have contributed to a partial recovery in the upper 10-cm layer and a full recovery of the surface layer(to 5 cm)on trails affected by four or fewer passes in a climate with hot summers and cold winters in as few as 3 years after skidding(Zenner et al.2007).Fine-textured clay-loam soils in more temperate areas failed to recover to pre-harvest conditions even after 10 years(Page-Dumroese et al.2006).Such delayed recovery may occur because compaction breaks down soil particles and/or realigns them into more of a platy structure that allows them to fit closer together after traf fic(Johnson et al.2007).Therefore,our expectation that the effects of machine-induced compaction would persist for some years and would not exhibit a signi ficant recovery of physical propertieswithin 1 yearwaswell-supported by our research.The further deterioration of bulk density,total porosity,and macroporosity,and the magnitude of this deterioration(10–30%)over 1 year following closure of skid trails,was unexpected an most important result of this study that warrants further attention.

Even though our expectation was that of a very modest recovery,there may in fact be several reasons to the further deterioration of soil physical properties.Compacted soils are often layered,with one layer being more compact and less permeable than others(Greacen and Sands 1980)and it is conceivable that the crawler tractor used for skidding in this study may have loosened the top layer to a depth of 2–3 cm from the lower,denser layer.This may have resulted in the erosion of the soft layer following rain events.Field capacity,which is to a great extent affected by microporosity,unsaturated hydraulic conductivity and their ordering and distribution between layers,can greatly affect water movement(Warkentin 1971).With continued compaction,a point is reached where the total porosity declines and is converted into relative increases in the proportion of micropores.This results in reduced volumetric water content at field capacity(Hill and Sumner 1967).Thus,soil compaction and reduced macroporosity can increase surface runoff due to reduced in filtration rates(Greacen and Sands 1980).In addition to changes in soil permeability,the loss of organic matter after canopy removal may also enhance the physical impact of raindrops on the exposed soil,which may explain the further deterioration of soil physical properties following traf fic of harvesting machinery(Page-Dumroese et al.2006).

It has long been known that the speed of recovery of soil physical properties is primarily affected by the magnitude of compaction that occurs during skidding,and that the prevention of widespread compaction is best achieved through the designation of skid trails,the use of low impact equipment(i.e.,machines with low ground pressure),and the temporal limitation of operations to dry or frozen conditions(Greacen and Sands 1980).The reduction of adverse initial impacts of logging operations on soils,and especially soil displacement on skid trails is often accomplished by covering the soil with slash(Jourgholami et al.2014).In light of the findings in this study,covering skid trails with tree tops and slash might be viewed as a means to protect soils from further deterioration by reducing the physical impact of raindrops where organic matter has been removed and that are prone to surface erosion.Alternatively,mechanical treatment such as ripping and disking could be considered as the ultima ratio to reduce soil compaction and initiate the process of soil recovery(Farrakh Nawaz et al.2013).The latter approach would,however,be most successful if accompanied by the addition of organic matter through arti ficial seeding of deeprooting vegetation such as legumes or lucerne grasses that may speed recovery and improve the nitrogen status of compacted soil(Talha et al.1973).In light of our research results,it is important that foresters expand their views on cover crops following logging operations to include not only the long-term bene fits of improved soil nutrient status,but more importantly to consider cover crops as a means to act as a physical buffer against the physical force of rain drops on bare soil.We thus reiterate the recommendations by Greacen and Sands(1980)that several simple and inexpensive techniques to prevent and ameliorate soil compaction exist that could be readily incorporated in a post-logging management strategy.Though techniques for prevention and amelioration of soil compaction and proper retirement of skid trails are available,forest managers seem frequently unconvinced that the effort is worth-while.To change this perception,future studies need to investigate the mechanisms,magnitudes,and time frames over which unmanaged post-logging skid trails deteriorate and how this contrasts with properly retired skid trails.

Conclusion

As anticipated,harvest machinery increased soil bulk density and microporosity,and decreased total porosity and macroporosity in the top 10 cm of a clay-loam soil.Increased traf fic frequency and slope gradient increased adverse effects on bulk density,total porosity,macroporosity,and microporosity.Interestingly however,1 year after the skid trails were decommissioned,soil physical properties exhibited degradation levels in excess of those observed immediately after the cessation of skidding.Although we can only speculate on the mechanisms that led to this deterioration which we largely attribute to the physical force of rain drops on exposed soil surfaces,our research nonetheless demonstrates the consequences of the failure to properly retire the abandoned skid trails,which are most serious on steeper slopes.We conclude that forest engineers might want to utilize organic material to avoid enhanced levels of compaction in the first place and use cover crops,to not only aid in the recovery process of compacted soils,but to prevent further deterioration after the termination of skidding operations.

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